How to find out power capacity of an automotive lead acid battery?

Question

I have a few 12V lead acid batteries in good condition which I would like to use to power a surveillance camcoder on a remote site. Ultimately that will be solar-powered, but in the meantime I would regularly take battery out for recharge and put a freshly charged one instead.

The hitch is that only up to 25% of the battery capacity should be used before it is taken out for top up, because otherwise it will be dead after several cycles.

My assumption is that if the camcoder power consumption is X and the battery can hold Yh amount of power, then the maximum time the battery should be used before recharging is 0.25Y/X hours. Is this roughly correct?

Now, the main question is how to find what Y is. The only other electrical metric labelled on the battery (apart from 12V) is 730 CCA (cold cranking amps). The battery product code is N70ZZMF if this helps.

Answer 1

The easiest way is simply to ask the manufacturer. Battery capacities are measured in amp hours (Ah) and Exide quote 90 Ah for one of theirs, Century quote 90 Ah for one and 85 Ah for another and Challenge quote 90 Ah for theirs so yours is likely to be similar - find out the manufacturer and get the data sheet for full details of capacity and how it's affected by discharge rate, temperature etc.

In simple terms, 90 Ah means the energy it stores is equivalent to that delivered by a current of 90 amps flowing for one hour. Or one amp for 90 hours. But in practice, the voltage will drop as the battery becomes discharged so it's never that simple, and if you regularly discharge the battery by about 50% its life will be drastically shortened unless it's a deep-cycle type that is designed to cope.

Your 25% design goal is far more realistic, and would give you 1 amp for 22.5 hours. You'll need to know the actual consumption of your camera to get an accurate figure.

Answer 2

Since you already have the battery why don't you determine things by experimentation. The ending point for usage of a lead acid battery can be simply when the battery voltage drops to some predetermined voltage level. So the experiment can be pretty easy.

1. Charge up your battery fully.
2. Connect up a volt meter across the battery and attach your remote surveillance equipment.
3. Monitor the setup till the battery has discharged to the predetermined voltage level.

If you want to be a bit more sophisticated you could obtain a data logger device that can accumulate voltage over time data so that you could later plot it in a graph. This may prevent you from having to babysit the test setup for a long period of time.

I have used these USB data logger devices and find them particularly useful for this type of experiment.

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