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PICList Thread
'Battery Level Monitor'
2003\06\17@164700 by Michael Milner

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Hi,

I'm currently making a battery powered device to run off a 3V lithium
battery.  My application requires a constant 3.3 volt source, so I'm
using a charge pump (MAX1759) to keep the voltage constant (the MAX1759
will accept an input in the range of 1.6 - 5.5 volts and generate a
regulated 3.3V output).

The Question:
I'd like to use one of the AD converter inputs to monitor the original
battery voltage so I can determine how much battery capacity is left.
Is this possible?

How much load would it put on the battery?

What would happen if the battery voltage were higher than the PIC supply
(3.3V)?

What sort of interface circuit would I need to connect the ADC to the
battery?

Thanks,

Mike

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2003\06\17@165944 by David VanHorn

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>
>Is this possible?

Sure

>How much load would it put on the battery?

Depends greatly on implementation.
I would disconnect it from the battery, with a mosfet, when not looking at the battery.

>What would happen if the battery voltage were higher than the PIC supply
>(3.3V)?

Depends on implementation, you'll either rail the A/D, start drawing lots of current, or use a resistive divider which will draw a small constant current unless disconnected when not in use.

>What sort of interface circuit would I need to connect the ADC to the
>battery?

A resistive divider and mosfet to disconnect when not in use?



Now for the bad news.
Lithium batteries have a very flat discharge "curve", making it very difficult to determine state of charge from voltage.

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2003\06\17@170608 by Herbert Graf

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> The Question:
> I'd like to use one of the AD converter inputs to monitor the original
> battery voltage so I can determine how much battery capacity is left.
> Is this possible?

       Certainly.

> How much load would it put on the battery?

       Pretty negligible, the only time the PIC A/D input uses energy (aside from
leakage) is during acquisition, which is basically charging a small
capacitor in the PIC.

> What would happen if the battery voltage were higher than the PIC supply
> (3.3V)?

       Well, there is your problem. PIC inputs (most of them at least) have
protection diodes, one to GND and one to VCC. They clamp the voltage at the
pin to be between GND-Vdiode and VCC+Vdiode, where Vdiode is the forward
drop of the diode (assume around 0.7V). As long as the battery voltage isn't
high enough to cause the protection diode to conduct, if it is then you've
got a problem. An easy solution is to use a rail to rail, CMOS op amp, that
can run directly from the battery, configured to divide it's input by two,
this will limit the voltage the PIC sees. Hope this helped, TTYL

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2003\06\17@170818 by Bob Axtell

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Use a voltage divider to prevent the possibility of having the battery
voltage higher than the PIC voltage. You could, of course, use a protective
diode, such as a 1N4148 to protect the input, in which case just a series
resistor would be fine. I'd prefer the protective diode method, frankly,
because it will allow a more precise measurement.

When fiddling with the A/D converter, remember that the input has a nominal
10K impedance when it is being sampled and measured. I use a cap on its
input, say 0.01uF, to ensure that the level doesn't sag when the
sample/hold circuit switches in.

--Bob

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2003\06\19@103508 by Moises Cambra

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-----Original Message-----
Sent: Tuesday, June 17, 2003 10:55 PM
Subject: Battery Level Monitor

>I'd like to use one of the AD converter inputs to monitor the original
battery voltage so I can determine how much battery capacity is left. Is
this possible?


For Li-Ion batteries it would be more accurate to use those battery
gauges chips, like the TI BQ2018 or newer ones.


Moises

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