Battery life

[shorteddiode]

I have 8 batteries with a 12 volt output. The device they are powering draws 57 ma. How do I figure out how long the device will run?

[selldoor]

Re: Battery life

you need to know the capacity of the battery/s

Divide the energy storage capacity of the battery (watthours) by the battery voltage (volts) to get Ah (Amphours). Multiply by 1000 to get mAh.

then divide by 57

I think.

[mariushm]

Re: Battery life

Pretty much as selldoor says.

However, also consider the following.

It depends on the battery... if you mean car/motorbike batteries, those are about 13.5-14v when charge, and they're discharged completely at about 10v.

The device may need more current as the voltage drops and may use less current when the voltage is about 12v so it may not be a constant, linear discharge... just dividing the mAh by the device consumption won't give you the exact time.

This site: http://www.batteryshowdown.com/ has graphs with AA batteries discharging at 100mA and 1000mA

100mA : http://www.batteryshowdown.com/results-lo.html
1000mA : http://www.batteryshowdown.com/results-hi.html

If you scroll down to the table, you can click on whatever battery you want and see the discharge graph, and you can see the 1.2v battery is 1.4v new and discharges pretty much completely at 0.8v

[shorteddiode]

Re: Battery life

Thanks Guys,