# Battery life indicator help

#### evilgreenius

Joined Dec 22, 2012
6
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.

#### bountyhunter

Joined Sep 7, 2009
2,512
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.

#### crutschow

Joined Mar 14, 2008
26,974
measuring cell or battery voltage is virtually useless for determining state of charge. It's a lot more complicated than that.
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.

#### evilgreenius

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

#### crutschow

Joined Mar 14, 2008
26,974
so, do you think to measure its current rating.. im using 6v 4ah lead acid battery, i measured its voltage and its 6.51V
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).

#### evilgreenius

Joined Dec 22, 2012
6
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).
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.

#### thatoneguy

Joined Feb 19, 2009
6,359
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.