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'[OT]: Solar panel, battery sizing, calculations ch'
2011\04\27@094401 by

I need to size a solar install to remotely power a 3G Modem and a
Intenna (Intenna is an enclosed wifi antenna with  90 mW UltraWAP).

Total current draw at 12v is 0.5amps.  I will base my calculation on 1 amp.

Below are my calculations which include some assumptions which maybe
incorrect.  Therefore I welcome any corrections or suggestions.

Location Lat=-28°  Long=+122°

Winter Solstice 21/06/2011  Rise 0642  Set 1701 = 10:18 hours daylight
= 7 hours usable hours.

Worst case battery drain for 1 day = 1 amp current draw for 17 hours =
17 amp hours.

Considering 50% charging efficiency requires 34 amp hours to recharge battery.

Total Amp Hours = 34 amp hours + 7 amp hours = 41 amp hours

Over a 7 hour period = 41/7  ~ 6 amps.

Power = 6 x 12 = 72 watts.  Considering solar panel not working at MPP
need to add margin guess 10% = 80watt panel.

>From above, 17 amp hours discharge during dark period.  Batteries
Availabale in 18amp hr and 40 amp hr.  Choose 40 amp hour.

Summary

To power 1 amp continuous at 12 volts requires 80watt solar panel
mounted at 28° with 40 amp hour battery.

Does this appear to be a reasonable selection.

Cheers Justin
Hi Justin,

How long does this remote device need to operate without maintenance?
Each day your battery will see some amount of charge and discharge and
this will put some "cycle life wear" on the battery which will cause
the battery capacity to decline over time and it will eventually need
to be replaced. What kind of battery are you thinking of using?

Sean

On Wed, Apr 27, 2011 at 9:43 AM, Justin Richards
<justin.richardsgmail.com> wrote:
{Quote hidden}

>
Another thing I forgot from my last email - I am no expert in solar
power but I don't see any consideration of the average insolation
(both due to latitude and weather) at your location. Have you taken
that into account?

On Wed, Apr 27, 2011 at 9:43 AM, Justin Richards
<justin.richardsgmail.com> wrote:
{Quote hidden}

>
> Another thing I forgot from my last email - I am no expert in solar
> power but I don't see any consideration of the average insolation
> (both due to latitude and weather) at your location. Have you taken
> that into account?
>
Hi Sean,

thanks for the input.

I am not really sure about insolation or how to calculate it from our
lat/long and the math looks complicated and appears to deal with a
surface that is parallel with the ground.  I had planned to mount the
panel at 28 deg using our latituded as a guide. If at the equator it
would make sense to mount at 0 deg which is the lat at the equator and
it would seem to make sense to mount at 90 deg at the poles so
interpolating i figured 28 deg for a lat of 28 deg.

Sealed Lead Acid was a choice of battery but have nothing to base this
on.  Various solar suppliers appear to supply gel and lead acid.

The site is only 10 km from base so battery replacement is not a big
issue and I was expecting no other maintenance unless anyone has any
recommendations.

Days are mostly sunny and with a 40amp hour battery I would suspect a
minimum of 40 hours supply with no sun.  And even cloudy days I
susprct will provide some solar power.

I was hoping that some people might have some rule of thumb as it
appears a black art.  100% uptime is not crucial as we can tolerate
the occasional outage.

Chers justi
On 28/04/2011 15:16, Justin Richards wrote:
> Sealed Lead Acid was a choice of battery but have nothing to base this
> on.  Various solar suppliers appear to supply gel and lead acid.

The semi Sealed "Calcium Technology" low maintenance lead acid used on Cars and Lorries seems to last best. The voltage seems to rise sharply from 13.6V to 14.2V at fully charged allowing cut off of charging. Gel cells seem poor for repeated deep discharge and have sightly lower terminal volts.

Important is a good "smart Charger" that can deliver current equal only to load current when battery is charged. For long life of battery, it's likely you only normally want less than 50% discharge. So maybe a 80 Ah. 56Ah is common car battery size.

I'd choose the Solar panel to be able to charge battery at discharge rate x 2 at least on an overcast day and supply the equipment current at your location, assuming there is no automatic panel cleaning and long term use. Here the winters are much shorter days (about 8hrs so much less light, maybe need twice as much panel here)

The Electronics payload should work over 10V to 16V range and the load should be disconnected to protect battery if the battery is below 10.7V approx (or according to datasheet).

It's important if Performance quoted for Battery or Solar panel is average or minimums. i.e. 10,000 average life is useless on a 4,000 hr requirement if significant % of units only last 1000 hrs.

On 28/04/2011 15:50, Michael Watterson wrote:
> I'd choose the Solar panel to be able to charge battery at discharge
> rate x 2 at least on an overcast day and supply the equipment current at

Also I'd set the elevation to be optimal for midwinter rather than the "average".

Russell is likely the resident Solar panel Expert.

I've used Truck battery and smart charger for Voice repeater PSU. We powered it off the mast site power, which has its own major array of batteries, meaning we got 8 to 10hrs Transmit time (and 100hrs RX) after everything else on site was dead..
Rushing. Very brief comment only so far.

Insolation estimate is too high.

www. gaisma.com is superb.
Often easier than their menus is to gargoyle    gaisma city_name

eg gaisma nairobi

yields    http://www.gaisma.com/en/location/nairobi.html

6.34 sunshine hours (SSH) best month.
4.40 sunshine hours worst month

kWh ~= Wmp x SSH

Most sites are worse than Nairobi.
(Kabul is amazingly good in summer)

You have not allowed for multi day holdup. How long can it rain / snow
/ look glum and still work?

Deep cycling deep cycle batteries is bearable - better if not done.
Deep cycling "ordinary" batteries is an invitation to very very short lifetimes.

Consider a LiFePO4 battery. Much higher capital cost but lower whole
of life cost.

Many sites offer panel tilting formulae.
Moving twice a year can be useful.
More often has diminishing gains.

More ...

Russel
Justin Richards wrote:
> I am not really sure about insolation or how to calculate it from our
> lat/long and the math looks complicated and appears to deal with a
> surface that is parallel with the ground.

The real issue is not so much the decrease of energy hitting the collector
due to angle as due to weather.  That can vary significantly from place to
place and have nasty worse case values over a few days to a week.  I had to
do some rough calculations like this in college, and I found that airports
keep good weather records that can give you useful expectation of what
fraction of the time the sun is shining in the nearby area.

The loss due to latitude is pretty small, even for a fixed panel.  For the
best yearly average, tilt the panel down from straight up by your latitude.
If you are at 28 deg south, then point the panel down 28 deg from vertical
towards north.  For a pure black body, the power received due to angle
compared to facing directly to the source is the cosine of that angle.
Since the earth is tilted 23.5 deg, the sun will appear to change in height
by +-23.5 deg over the year at noon.  Cosine(23.5deg) = 92%, so at noon at
each solstice you loose 8% due to angle alone.  Other times of the year it
will be better.  At the equinoxes at noon the panel will be facing the sun
directly and therefore there is no angle loss.

The loss due to angle during the day is more severe, since it can go to 90
degrees (cosine = 0) and beyond at times.  Tracking panels seek to recover
some of that loss.  They do, but also add expense and complexity.  Last time
I looked into this, it made sense to just get a bigger fixed panel than a
tracking panel for most applications.

That's all just angle and a pure black body.  Solar panels are not black
bodies and are covered with stuff that reflects a larger portion of the
light at low angles.

You can go nuts trying to figure the total efficiency to the last percent,
then get hit with 50% variation over a week due to weather anyway.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000
Michael Watterson wrote:
> Also I'd set the elevation to be optimal for midwinter rather than the
> "average".

That's backwards.  Solar cells are more efficient when cold.  At 28 deg
latitude, the sun isn't going to be that much weaker in winter due to
traversing more atmosphere on the way to the panel.

Apparently the power will be running some electronics.  He hasn't said
anything that indicates the load will vary seasonally.  If the power were
used for heating, then optimizing for winter might make some sense, or if
you knew that location was significantly more cloudy in winter.

Considering all the competing effects together, just pointing it down from
vertical by latitude is simpler and never too wrong.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000
On 28/04/2011 16:42, Olin Lathrop wrote:
> He hasn't said
> anything that indicates the load will vary seasonally.  If the power were
> used for heating, then optimizing for winter might make some sense, or if
> you knew that location was significantly more cloudy in winter.

Unless you are on the equator, midwinter has the least hours of daylight. Thus you want the "best" output to charge the battery for the hours of dusk - dark - dawn .

Even if the equipment takes same power all year round (which I assume it does).
On 28/04/2011 16:34, Olin Lathrop wrote:
> The loss due to angle during the day is more severe, since it can go to 90
> degrees (cosine = 0) and beyond at times.  Tracking panels seek to recover
> some of that loss.  They do, but also add expense and complexity.  Last time
> I looked into this, it made sense to just get a bigger fixed panel than a
> tracking panel for most applications.

how well does having two panels, one angled slightly west and one angled slightly east compared to one flat panel of same cost, angled for winter time?

in this application optimising the available energy gathered isn't important, it's not selling excess to grid, but just ensuring enough (minimum) charging of a battery with a fairly fixed load
> how well does having two panels, one angled slightly west and one angled
> slightly east compared to one flat panel of same cost, angled for winter
> time?

http://www.gaisma.com

Illuminating

Russel
> Many sites offer panel tilting formulae.
> Moving twice a year can be useful.
> More often has diminishing gains.

The trials I've done with panels show that it's worth tracking. Whether
you would depends on your latitude and if you feel the effort required
to build a tracking mechanism is necessary to chase the extra power

NZ is at a fairly low latitude and the difference between mid-day sun
height during the year is significant

Mid-summer solar noon is 76.6 degrees above the horizon, mid-winter
solar noon is 29.7, almost 46.9 degrees difference

Today, late autumn, it's 39.2 degrees

http://www.timeanddate.com/worldclock/astronomy.html?n=22&month=4&year=2011&obj=sun&afl=-1&day=1

Compare Quito (equator) and Nome (way north)
> NZ is at a fairly low latitude

Only relatively :-).
Invercargill antipodes is Bay of Biscay. Land's end is in the southern ocean!.
Auckland ~ Barcelona.
Kaitaia is in Saharan Africa
Australia is the Bermuda triangle of the south pacific.
Figures :-).

and the difference between mid-day sun
> height during the year is significant
>
> Mid-summer solar noon is 76.6 degrees above the horizon, mid-winter
> solar noon is 29.7, almost 46.9 degrees difference

Half of the difference =~ 23.5 degrees
Cos(23.5) = 0.917.
ie at extremes output is down by about 8% for static versus angle changing.

Move it twice  annually so you are within say 12 degrees and Cos(12) =
0.978 or down by about 2% at extremes of the ranges.

That's seasonal tracking.
Daily tracking is different again.

http://www.gaisma.com shows you horizontal and vertical sun angle by time of
day nd day of year.
Informative.
Part of year at many sites the sun covers more than 180 degree arc in
a day. The 'must get it all' people must look over their shoulders at
days end.

Sunniest place on earth? - South and North polar regions. About 4 x as
sunny as tropical midsummer equatorial Africa :-).

In some places at some times the sun rises yesterday or sets tomorrow.

Russel
IVP wrote:
> NZ is at a fairly low latitude and the difference between mid-day sun
> height during the year is significant

The yearly angle variation of the midday sun is the same everywhere on the
globe.  However, in the polar regions part of the time it is below the
horizon so the full angle spread can't be seen.

> Mid-summer solar noon is 76.6 degrees above the horizon, mid-winter
> solar noon is 29.7, almost 46.9 degrees difference

The earth is tilted about 23.5 deg, so you will see a variation of twice
that everywhere (outside the polar regions as noted above).

For example, I'm at about 42.5N, so the height of the midday sun varies from
42.5 + 23.5 = 66 deg at the summer solstice, to 42.4 deg at each equinox, to
42.5 - 23.5 = 19 deg at the winter solstice.
> The yearly angle variation of the midday sun is the same everywhere
> on the globe

> The earth is tilted about 23.5 deg, so you will see a variation of twice
> that everywhere (outside the polar regions as noted above).

If you look at the equator example, Quito, you'll see that the variation
is only 23 degrees

89.8 [Mar 19], 66.8 [Jun 20], 90.0 [Sep 23], 66.9 [Dec 21] and day
length [sunrise - sunset] of 12 hours and a few minutes

> For example, I'm at about 42.5N so the height of the midday sun
> varies from 42.5 + 23.5 = 66 deg at the summer solstice, to 42.4
> deg at each equinox, to 42.5 - 23.5 = 19 deg at the winter solstice

In Boston, according to

http://www.timeanddate.com/worldclock/astronomy.html?n=43&month=3&year=2010&obj=sun&afl=-1&day=1

24.3 [Dec 21] to 71.1 [Jun 20] = 46.8 difference
IVP wrote:
> 89.8 [Mar 19], 66.8 [Jun 20], 90.0 [Sep 23], 66.9 [Dec 21] and day
> length [sunrise - sunset] of 12 hours and a few minutes

Yes, but that neglects to mention the polarity of the declination.  Quito
being nearly on the equator, the sun should be at 90 deg at each equinox,
which is what they are showing.  However, it's going to be 66.8 north in
June and 66.9 south in December, for a total swing of 46.3 according to
those numbers.

>> For example, I'm at about 42.5N so the height of the midday sun
>> varies from 42.5 + 23.5 = 66 deg at the summer solstice, to 42.4
>> deg at each equinox, to 42.5 - 23.5 = 19 deg at the winter solstice
>
> In Boston, according to
>
>
http://www.timeanddate.com/worldclock/astronomy.html?n=43&month=3&year=2010&obj=sun&afl=-1&day=1
>
> 24.3 [Dec 21] to 71.1 [Jun 20] = 46.8 difference

Boston is about south from me, but close enough.  I don't know why they
think the sun is 4 deg higher there than what I came up with for here.  Some
charts consider the angle to be to the bottom of the sun's disk as seen from
the ground because that's how it was measured with a sextant long ago.
However if they were using the bottom instead of the center their numbers
would be lower.  The figures you quoted make more sense for Washinton DC to
me than Boston.  I can't explain the discrepancy.

In any case, the spread is still about 47 deg, which is twice the tilt of
the earth's axis.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

>> 89.8 [Mar 19], 66.8 [Jun 20], 90.0 [Sep 23], 66.9 [Dec 21]
>> and day length [sunrise - sunset] of 12 hours and a few minutes
>
> Yes, but that neglects to mention the polarity of the declination

Ah, I see what you mea
> www. gaisma.com is superb.

Closest town listed is aprox 1200km away at Perth which gives average
4.9 (kW·h/m2)/day

Wiki gives my area approx 220 w/m2  (it looks like a small "w" from
the chart) = 5.2 (kW·h/m2)/day

> 4.40 sunshine hours worst month
>
> kWh ~= Wmp x SSH

Perth appears to be 10 SSH worst month (from gaisma)

Therefore kWh = 80W x 10hr  (assuming Wmp = Maximum power the panel is rated at)
= 800 kWh ??? or 800Wh

Does this equal 66 amp hour.  This appears to be ample so i am suspect.

>
> Your battery charge percentage is low for a lead acid system.

Accepted, changing to a approx 50% charge requires 130 watt panel.

>
> You have not allowed for multi day holdup. How long can it rain / snow
> / look glum and still work?

I would nomally say that it rarely rains, rarely overcast, never snows
but for the last 2 days it has been raining.  The desert is begining
to green.

40 amp hr battery I would expect to give 48 hours holdup  @ 1 A cont
drain assuming that even glum days still provide some useable solar
output.

2 days is acceptable for us as we can always drag out a truck battery.

>
> Deep cycling deep cycle batteries is bearable - better if not done.

Accepted: To avoid deep cycling, increase battery to 80 amp hour.

Thanks to all for the input.

Cheers Justin
At 07:43 AM 4/27/2011, Justin Richards wrote:
>I need to size a solar install to remotely power a 3G Modem and a
>Intenna (Intenna is an enclosed wifi antenna with  90 mW UltraWAP).

Some other information is needed.

The most important thing is: Does the temperature drop below freezing at any time?

My preferred battery technology is still Lead-Acid.  I use gell-cells (AGM) for small projects and flooded batteries for larger installations.

Most of my stuff has to work under Arctic conditions.  That means that the battery boxes are EXTREMELY well insulated and have tiny heaters to keep the battery above 10C even when its -30C to -50C outside.  It takes surprisingly little power to keep the batteries warm if the insulation is good enough.

The freezing point of a fully-charged Lead-Acid battery is significantly better than -50C (all that I am worried about) but decreases as the battery is discharged.  The freezing point drops to about -40C when the battery has been depleted to about 80% (20% charge used) and steadily gets worse: about -20C at 50% depletion, somewhat below 0C when the battery is fully discharged.

The battery is usually considered to be destroyed if it should ever freeze.

So: you have to size the battery so that it never becomes discharged to the point where it might freeze at your lowest expected temperature.

This often results in the situation where you have a huge battery and a relatively small solar array.  Insulating the batteries reduces this disparity.

dwayne

-- Dwayne Reid   <dwaynerplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing
At 09:42 AM 4/28/2011, Olin Lathrop wrote:

>That's backwards.  Solar cells are more efficient when cold.  At 28 deg
>latitude, the sun isn't going to be that much weaker in winter due to
>traversing more atmosphere on the way to the panel.
>
>Considering all the competing effects together, just pointing it down from
>vertical by latitude is simpler and never too wrong.

Most of our customers tell me that they set the solar panels to be perfectly vertical.  These are unattended huts in the far North and the idea is to make it difficult for snow to stick to the surface of the panel.

Again - your local conditions will dictate what you should do.

dwayne

-- Dwayne Reid   <dwaynerplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing
> The most important thing is: Does the temperature drop below freezing
> at any time?
>
Not in my 10 year exposure.  Annual rain approx 230mm, av max 27 deg C
av min 13 deg C, lowest recorded 4 deg C, highest recorded 45 deg C.

Cheers Justi
On Fri, Apr 29, 2011 at 12:22 PM, Dwayne Reid <dwaynerplanet.eon.net> wrote:
> My preferred battery technology is still Lead-Acid.  I use gell-cells
> (AGM) for small projects and flooded batteries for larger installations.

Just one point - AGM and gel are not the same thing, both in
construction and behavior. The DO both fall into the category of
Valve-Regulated Lead Acid (VRLA) batteries, where Hydrogen production
during charge is reduced and Oxygen which is produced can re-combine
with H+ ions to form water again - thereby eliminating the need to add
water and allowing the battery to be sealed except for one-way valve
vents to allow occasional escape of gas during over-charge.

AGM stands for Absorptive (or Absorbative) Glass Mat. The electrolyte
is truly a liquid in AGM cells, but it is retained within the
separator material which is a like a thin "rat's nest" of glass fibers
of varying length and diameter. The adhesion of the electrolyte to the
glass keeps the electrolyte from leaking out as long as the height of
the cell is limited.

Gel cells actually use a gelled electrolyte, made from sulfuric acid
and usually fumed silica. The separator material is usually much
thinner and similar to that used in flooded lead-acid batteries (a
porous plastic sheet).

Gel cells usually cannot handle very high charge and discharge rates
as well as AGM can. Gel cells are also more subject to failure from
punctured separators since the separator is thinner than in AGM.
However, depending on the battery design, Gel type batteries often
have a longer deep cycle life and sometimes a longer float life, too.
The failure modes are different since the gel can become detached from
the plates whereas for AGM, the liquid electrolyte will generally
maintain contact with the plates even after dimensional changes. AGM
cells also can have a high clamping force to hold the plate stack
together which tends to reduce active material shedding.

Just reading up on this lately and couldn't let the point of AGM vs
Gel difference go :)

Sean

On Apr 29, 2011, at 9:22 AM, Dwayne Reid wrote:

> My preferred battery technology is still Lead-Acid.

Does charging efficiency vary a great deal between different battery  technologies?
Different implementations/vendors within a technology?

BillW
> At 09:42 AM 4/28/2011, Olin Lathrop wrote:
>
> >That's backwards.  Solar cells are more efficient when cold.  At 28 deg
> >latitude, the sun isn't going to be that much weaker in winter due to
> >traversing more atmosphere on the way to the panel.

It depends which parameter you design for and whether you
electroically drive the panel to maximum power point AND efficiently
convert the output to suit the load conditions.

Most systems do not use an MPPT (maximum power point tracking)
controller. Depending on type of system, insolation , battery state
and more the above assumption may be less relevant than expected or
wrong. For modern crystalline silicon Vmpp/Vos is about 80%. Higher
efficiency calls give higher percentages.

For crystalline silicon (poly and mono) max power and max voltage have
negative temperature coefficinct BUT max current has a slight positive
tempco. IF you are running the panel as a current source - and this
tends to be what you do when you load a panel below its MPP (max power
point) using eg a battery, then you may depend only on the current
tempco, and it may be positive. ie current per insolation rises with
rising temperature.

So an eg 18V panel is optimised for about 0.8 x 18 = 14.4V which suits
the load is actually 12V and no energy converter is used.

A well designed system without MPPT will try to operate the panel near
Wpp at typical insolation, load and battery states.
For small portable devices a battery will almost always load its panel
to below its peak power voltage, resulting in something more like a
current source. So Isc is closer to meaningful than Wpp in many cases.
For 3 x NimH batteries industry practice is to use 12 PV cells. I've
mulled over using 11 rather than 12 which in some cases will give
12/11 output or almost 10% more power into the battery for the same
active PV area. The disadvantage is reduced Voltage at lower
insolation so reduced charging in lower light conditions.

Russell
Hi Bill,

It DOES vary significantly among different technologies. It varies
less from vendor to vendor or implementation to implementation but can
still be significant.

However, as a general rule of thumb, Lithium rechargeable batteries
are better than 80% efficient. Lead acid varies more but is about 70%
efficient and gets worse over the service life of the battery.

For Lithium, basically all of the inefficiency is due to charging
voltage being slightly higher than discharge voltage. For lead acid,
this is still true, BUT it is also true that you don't even get out as
many amp-hours on discharge as you put in during charge, especially
when you fully charge the battery or when it is aged.

I'm not as familiar with NiCd or NiMH but my understanding is that
they fall in-between Lead Acid and Lithium in terms of efficiency.

Sean

On Fri, Apr 29, 2011 at 2:17 PM, William "Chops" Westfield
<westfwmac.com> wrote:
{Quote hidden}

>
> It DOES vary significantly among different technologies. It varies
> less from vendor to vendor or implementation to implementation but can
> still be significant.

There are some very important things to be known here which can
greatly affect end results.

In eg small solar applications many people charge batteries from PV
panels without a good understanding of what is happening to energy and
current sent via battery storage. Even a major solar lighting
evaluation organisation  (which I won't name)  closely linked to supra
government attempts to improve developing country solar lighting
provides product assessment guidelines based on incorrect
understandings on how solar charging and batteries work together.

{Quote hidden}

A new LiFePo4 has about 99.5%current recovery efficiency.
eg if you put in 1000 mAh of charge you will get about 995 mAh back.
As Sean says, this is not energy recovery efficiency as Vin and Vout dffer.

Better yet, as a LiFePO4 battery ages the current recovery efficiency
INCREASES.

I haven't got exact figures to hadn for std LiIon (Lithium Ion) but it
is high - probably 90% + - maybe near as good as LiFePO4.

I'll call current recovery efficiency Zi.
Energy recovery or Watts recover = Ze or Zw (same thing).
This is what we usually care about BUT not always.

Most other battery chemistries have noticeably poorer Zi due to
secondary chemical reactions set up in parallel to the one being used
to store and retrieve energy

A lead acid battery (will vary with sub technology) has a better Zi
than most alternatives other than Lithium. AFAIR it's in the 80%-90%
range.

NimH and NiCd are typically lower than lead but not as low as may be expected.
Again from memory, I'm seeing reported rel world ratios of better thn
80% Zi for NimH.

NBNBNB this iz Zi and not Ze.

As ean says, the trouble witn Ze is that Vin and Vout differ. One
reason is internal resistance so that on charging Vchrge is i_charge x
Rinternal higher than internal cell voltage an on discharge output
voltage is i_discharge x Rinternal lower than available Voc_battery.

Discharge/lad level curve sets available from most battery suppliers
give a good indication of the voltage drop that occurs at discharge at
various C discharge rates (where C=1 = 1 hour discharge at rated
capacity.) While internal impadenace is non linear and is affected by
eectro chemical second order effects, the curves give a good
indication that the voltage drop is related to resistance.

SO other things being equal, battery Ze can be improved by operating
at relatively low charge and discharge currents so that internal IR
losses are low.

In chemistries with internal cheical losees, a slow charge time may
cause greater losses overll, but in chemisrries where losses are
mainly IR (eg Lithium) you get better efficincies at say C/10 than at
C1.

With LiFePO4 Vin and Vout are reasonably close for about 2/3 of the
charge cycle - ~ while in the constant current charge part of the
charge cycle. In this reason Ze appears almost as good as Zi and it
seems that Ze's of 90% + should be achievable if battery capacity was
limited by onlty using this range.

I say "appears" and :should be" as I have not yet tried this and have
not read of anyone else's results,but based on the quite good
available data sheet dat it appears feasoble.

I have not looked at Lead acid but it too may have a similar region -
but it does not have the same current then voltage charging cycle of
LiIon.

If a solar panel is operated as a current source and the battery then
operates eg a switch mode supply, of interest is probably Ichg and W
discharge.
For many systems Vdischarge is relatively constant so current ratios
give a fair idea of available efficiency overall. In which case lead
acids high current efficiency may be an advantage.
Panel Wattages are then misleading with Ichg and Idischarge or W
discharge at mean Vout being of more relevance.

the experts always seem to assume.

Russell

....Lead acid... may have a similar region...

Lead-Acid lore claims that a battery maintained at less than full-chg
for any great length of time will lose the unused capacity to sulfation.

I'd be interested to know if this is so.

Jac
There is  voltage above which sulfation will not occur. An
electropotential issue afair.

Notional figure is slightly under 2.1 V/cell.
No doubt varies with temperature, phase of Moon and more.

____________

www.batterystuff.com/tutorial_battery.html
Sulfation of Batteries starts when specific gravity falls below 1.225
or voltage measures less than 12.4 for a 12v battery, or 6.2 for a 6
volt battery

R

> Lead-Acid lore claims that a battery maintained at less than full-chg
> for any great length of time will lose the unused capacity to sulfation
This is true, but at least for VRLA (valve regulated lead acid)
batteries, sulfation is not an unrecoverable condition, depending on
battery design. Fully charging at some interval (on the order of weeks
for batteries in the 10s of AH) and equalization charging (a periodic
controlled overcharge) can reverse most of the sulfation. This type of
operation is known as partial state-of-charge or PSoC.

Sean

On Sat, Apr 30, 2011 at 8:07 AM, John Gardner <goflo3gmail.com> wrote:
> ...Lead acid... may have a similar region...
>
> Lead-Acid lore claims that a battery maintained at less than full-chg
> for any great length of time will lose the unused capacity to sulfation.
>
> I'd be interested to know if this is so.
>
>  Jack
>
On Sat, Apr 30, 2011 at 3:49 AM, RussellMc <apptechnzgmail.com> wrote:
> I have not looked at Lead acid but it too may have a similar region -
> but it does not have the same current then voltage charging cycle of
> LiIon.
>

Russell,

Why do you say that lead acid does not have a "current then voltage"
charging cycle? In my experience, it is quite typical for chargers to
apply their maximum current until the lead acid battery reaches some
voltage threshold, and then that voltage threshold is held until
charge is complete. Better chargers will temperature compensate that
CV (constant voltage) setpoint, but still, the overall charge is
CC-CV, which is quite similar to what you do with Lithium chemistries.

Sea
> Why do you say that lead acid does not have a "current then voltage"

I'm not a battery or chemistry expert.*
But I've read much on batteries and have substantial practical experience
with some subsets.
I'm certainly not as experienced with LA as a number here
LA seems to be both a blacker and less exact art than LiIon.
It is indeed run in a CC-CA mode of sorts, but some of the parameters are
variable with plate construction, manufacturer electrolyte system and more.
The CC rate  limits on LA seem in many cases to be thermal, with some
'authorities' suggesting that rates 5 or 10 x usually accepted are possible
with due attention to temperature or other factor.  Statements about what
can or can't be done with a given LA cell may be very implementation
specific. Much of this uncertainty may be present in LiIon s well but, if
so, it is not expecially evident from the commonly available literature.

Probably best summarised by my line above:

"LA seems to be both a blacker and less exact art than LiIon."

R
* Long ago I looked at an  ME thesis topic in which the then sole NZ
telephone company  proposed measuring cell impedance modification by bubble
formation as a means of holding large LA batteries at optimum charge point
under heavier than normal charging conditions, while not shortening cell
life. This was based on then current research elsewhere It never eventuated..
had it done so I may be able to tell you a lot more about the subject than I
now can
At 10:19 AM 4/30/2011, Sean Breheny wrote:

>Why do you say that lead acid does not have a "current then voltage"
>charging cycle? In my experience, it is quite typical for chargers to
>apply their maximum current until the lead acid battery reaches some
>voltage threshold, and then that voltage threshold is held until
>charge is complete. Better chargers will temperature compensate that
>CV (constant voltage) setpoint, but still, the overall charge is
>CC-CV, which is quite similar to what you do with Lithium chemistries.

Flooded Lead-Acid can accept astonishingly-high charge currents without being damaged or destroyed.  Most people don't need to charge that quickly, so they use a much smaller charger that is then run in constant-current mode until the battery terminal voltage reaches the appropriate threshold.

In other words, the reason the charger runs in constant-current mode is simply because of the limits of the charger.

A real-world example of rapid Lead-Acid charging occurs in practically every internal-combustion engine automobile on the planet.  The alternator dumps as much current into the battery as it can - on large trucks, that can be upwards of 100A.  The battery doesn't complain at all.

dwayne

-- Dwayne Reid   <dwaynerplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing
At 11:34 AM 4/29/2011, Sean Breheny wrote:
>On Fri, Apr 29, 2011 at 12:22 PM, Dwayne Reid <dwaynerplanet.eon.net> wrote:
> > My preferred battery technology is still Lead-Acid.  I use gell-cells
> > (AGM) for small projects and flooded batteries for larger installations..
>
>Just one point - AGM and gel are not the same thing, both in
>construction and behavior.

<snip>

>Just reading up on this lately and couldn't let the point of AGM vs
>Gel difference go :)

I understand the differences but from my point of view, I treat them very similarly and essentially consider them to be interchangeable.

Most of my outdoor projects using a sealed battery have limited charge current available and thus could accept either type of battery.  Basically, I design for the lowest common denominator so that both battery types can be used.

On the other hand, I have a couple of large AGM batteries that I use for portable power.  These are true AGM and one of the reasons I chose them was because of the increased charge current capability (I can charge them quickly if needed).

dwayne

-- Dwayne Reid   <dwaynerplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing
FYI:

You can purchase solar cells on EBay and make your own panels. You can buy the tabbing and buss wire as well. DIY instructions are available at no cost on the Internet. I bought 72 watts worth for under \$50 US, plus a few dollars for the wire. I Aldo bid on some cracked cells to practice on as they break easily.
Sent from my iPhone

On Apr 30, 2011, at 4:21 PM, Dwayne Reid <dwaynerplanet.eon.net> wrote:

{Quote hidden}

> -
I think we're saying the same thing. My point is that a fully-depleted
lead acid battery usually will be able to draw more current than the
charger can apply, therefore the beginning of the charge is a constant
current charge at the max that the charger can supply. Even chargers
that can supply 100s of amps usually would not cause a
fully-discharged LA battery to reach the CV voltage right away. Either
the charger or the wires or the interconnects inside the battery are
usually the limiting factor for charge current at low SOC, but that
still results in a CC charge as the fastest way to charge the battery
within this region.

Sean

On Sat, Apr 30, 2011 at 4:15 PM, Dwayne Reid <dwaynerplanet.eon.net> wrote:
{Quote hidden}

>
> I think we're saying the same thing.

If that's what you originally meant then it's quite different from
what is neant by CC in a typical LiIon call, which was the point I was
originally dealing with.

Common or garden [tm] LiIon have a 1C max charge current. Exceeding
this rate substantially tends to lead to a JLLB (Jerry Lee Lewis
Battery) situation. (Great balls of fire).
Lead Acid tends to just die at greater or lesser rates depending on
how the high rate affects the cell. LiFePO4 aretypically rates at 10C
charge and som newer Lithium chemistry variants are claiming around 1
minute charge times. (60*C)

> My point is that a fully-depleted
> lead acid battery usually will be able to draw more current than the
> charger can apply, therefore the beginning of the charge is a constant
> current charge at the max that the charger can supply.

Yes. So its a AC-CV mode (Available current, constant-Voltage) unlike
the CC-CV mode fr LiIon where battery characteritics set both limits.

> Even chargers
> that can supply 100s of amps usually would not cause a
> fully-discharged LA battery to reach the CV voltage right away. Either
> the charger or the wires or the interconnects inside the battery are
> usually the limiting factor for charge current at low SOC, but that
> still results in a CC charge as the fastest way to charge the battery
> within this region.

Not wanting to be picky , but that's really just saying that th
fastest way to charge the battery prior to it reaching a present
voltage is 'the fastest way you can manage'.

But it seems that the only difference between LA and Lithium Ion in
what you are saying is which piece of equipment requires that the
charge current be limited. Even for lead acid, it may well be that it
is not safe (or good for the life of the battery) to allow it to
charge at the highest current it will accept at the CV point. All of
the battery books I have read, as well as all of the charger
documentation I've seen, refers to typical lead acid charge profile as
CC-CV (they sometimes talk about CC-CV-CC to describe putting in a
fixed fraction of a C at the end to make the tail of the charge curve
more deterministic, to protect against thermal runaway, and to ensure
a controlled amount of overcharge).

Sean

On Sat, Apr 30, 2011 at 10:54 PM, RussellMc <apptechnzgmail.com> wrote:
{Quote hidden}

>

'[OT]: Solar panel, battery sizing, calculations ch'
2011\05\01@185038 by
At 09:17 PM 4/30/2011, Sean Breheny wrote:
>But it seems that the only difference between LA and Lithium Ion in
>what you are saying is which piece of equipment requires that the
>charge current be limited.

For me, the greatest charging differences between Lead-Acid and Lithium Ion are:

1) extremely accurate voltage required for Lithium-ion: end of charge voltage is either 4.10V or 4.20V, depending upon which anode the battery has.  L-A is happy with ball-park voltage levels, relatively speaking.

2) Lithium-Ion charge current is greatly reduced while battery is significantly discharged (unlike L-A).

dwayne

-- Dwayne Reid   <dwaynerplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing
> For me, the greatest charging differences between Lead-Acid and
> Lithium Ion are:
>
> 1) extremely accurate voltage required for Lithium-ion: end of charge
> voltage is either 4.10V or 4.20V, depending upon which anode the
> battery has.  L-A is happy with ball-park voltage levels, relatively speaking.

Taking 0.1V off the specified endpoint voltage for LiIon significantly
reduces capacity.
Taking 0.2V off, very much more so.
Also increases cycle life greatly.

Russell
I wasn't saying that there aren't significant differences in charging
lead-acid vs Lithium chemistries - simply that the most common means
for fast charging lead-acid is also referred to as CC-CV.

You really can't lump all of the Lithium-Ion types together when it
comes to charge rate. Lithium Iron Phosphate, (for example, A123's
cells) are happy with high charge rates even when deeply discharged.
Lithium Titanate are also capable of high charge rate from 0% right up
until about 90% SOC. Lithium Cobalt or Lithium Manganese Cobalt (if I
remember correctly), which are the "traditional" Li-Ion cells, are, I
think, mainly what you are talking about. Overcharge in these types
will easily cause deposition of metallic Lithium which will then react
violently with the electrolyte.

Lithium end of charge voltage is critical, but I would say that
Lead-Acid charging, if you want maximum life from the batteries, is
actually far more complex than Lithium. LA battery life in float
service, for example, requires very accurate voltage regulation based
on temperature and even battery age to avoid the two extremes of
undercharge (which is more than just not being 100% charged - you
actually can develop a situation where the potential of one electrode
goes up and the other goes down so that the SOC of the limiting
electrode is falling while the terminal voltage is staying the same)
and excess float current leading to heating and thermal runaway.  Even
in cycle service, you have to have accurate voltage AND current
measurement to properly fully charge lead acid unless you are trickle
charging them.

Sean

On Sun, May 1, 2011 at 6:50 PM, Dwayne Reid <dwaynerplanet.eon.net> wrote:
{Quote hidden}

>
On 01/05/2011 23:50, Dwayne Reid wrote:
> For me, the greatest charging differences between Lead-Acid and
> Lithium Ion are:

Established & simple 90 year old charging technology (14V Power supply + resistor works!), also newer low/zero maintenance "Calcium" technology appears to have a sharp knee at "fully charged" enabling cease of charge and thus avoiding electrolysis of water to H and O. Getting 4 to 5 years from Automotive batteries seems feasible.

vs

Great balls of fire... Even if the design is correct and either you accidentally  install wrong model cell, cell is punctured (the LiPoly ones in bags) or cell is faulty. Lithium life seems suspect too, though may depend on exact type and usage (I never get more than a couple of years on laptop, but my Sony Camera 8mm digital tape battery is very old and still does 5hrs).

In my experience you only have to high current or deep discharge Gel Lead acid once or twice and it's scrap. NiMH and NiCd don't mind being left flat, it destroys lead acid. The longest life Lead Acid for actual regular use seem to be regular automotive "Calcium" zero maintenance type. Gels will last maybe twice as long as automotive if charged  and not actually used! (i.e. good for Alarm panel backup etc).  The Gel Lead Acid do appear to have very low self discharge, better than Automotive Lead Acid. Modern cars of course take power all the time even when off, so for long periods off the road the battery may need to be disconnected.

High capacity NiMH seem more finicky than Lead Acid or NiCd or "low capacity" NiMH as they appear to require near zero trickle charge and seem to have significant self discharge

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