# Battery life indicator help

Discussion in 'The Projects Forum' started by evilgreenius, Feb 13, 2013.

1. ### evilgreenius Thread Starter New Member

Dec 22, 2012
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I have this project in zilog microcontroller that measures the remaining battery life of a 6v 4Ah battery.

when i measured it using Digital Multimeter. the reading is 6.48V but in zilog adc its 0.xx.

can someone enlighten me about battery lives. i just need to get its percentage value as my project.

2. ### bountyhunter Well-Known Member

Sep 7, 2009
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Isn't the whole point of this class assignment so you can research this topic and learn about batteries?

I'll start you off with one big hint:

measuring cell or battery voltage is virtually useless for determining state of charge. It's a lot more complicated than that.

3. ### crutschow Expert

Mar 14, 2008
13,011
3,233
Depends upon the battery chemistry. Lead-acid batteries, for example, have a fairly reliable correlation between charge and voltage as long as the battery temperature is known.

4. ### evilgreenius Thread Starter New Member

Dec 22, 2012
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so, do you think to measure its current rating.. im using 6v 4ah lead acid battery, i measured its voltage and its 6.51V

5. ### crutschow Expert

Mar 14, 2008
13,011
3,233
You need to measure the voltage with no load and wait an hour or so after any charging. Nominally a fully charged battery would be 6.4V. Here's some further info on that. (You divide the voltages by half, of course, for a 6V battery).

Nov 30, 2010
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7. ### evilgreenius Thread Starter New Member

Dec 22, 2012
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thanks for that info.
im currently using the battery(superbright 37 leds) and tabulating the voltage of it every 1 hour. after20 hrs. from 6.50v - 5.94v.. im tabulating it to produce it in percentage format. any ideas.? thanks.

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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If this is for a microcontroller class, I don't think the object of the lesson is to calibrate an ADC to be a voltmeter, unless it is an intro project.

You need to combine a few measurements: current, time, and cutoff voltage to stop deep-discharge.

A very small sense resistor in series with the low side of the load will give a voltage proportional to the current through that resistor, from the known value of the resistor, and the measured voltage, current can be found with Ohm's Law.

Summation of dI/dT over the discharge period will result in Amp Hour units prior to battery terminal voltage reaching cutoff voltage.