Different kinds of batteries have different levels and ranges of usable voltage. To make things even more complicated, different devices have different voltage demands to run them properly. Batteries considered dead in a high-power demand device such as a radio-control car will often still work in a low-power demand device such as an analogue wall clock, and for a reason The most common sizes for AGM batteries are 7-8 Amp/hours, so I have a good selection to choose from. Charging Your Battery. I chose the Cabela’s 12V 8 Amp/hour Battery, which comes with a charger rated at 500 mA. Calculating the charge time of your battery is the reverse of calculating the capacity.
On my machine, this means that the computer usually gets enough power to stay on (without sucking the battery), but not enough to also charge the battery. Sometimes, if I'm not doing anything that's so power intensive, the battery will charge very slowly. Sometimes, the computer isn't getting enough electricity, so it draws power from the battery.