# How long the battery would last for given load?

Discussion in 'The Projects Forum' started by raviprakash.hegde, Jan 4, 2015.

1. ### raviprakash.hegde Thread Starter New Member

Nov 14, 2011
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How to calculate how long a DC battery would last for given load, with single charge?
What are the DC battery parameters to look for?

For ex. I have 12V 10A load. Given the battery specifications, how to calculate how long the battery(say lead-acid) would last for given load?

And, can a low Ah battery used for high amperage load?
i.e 5Ah battery can be used for 10A load?

Last edited: Jan 4, 2015
2. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
Look for the energy rating listed in units of Ah, that is Ampere-hours.

For example, a 12V 7Ah battery will last for 7Ah/10A = 0.7 h = 42 minutes while supplying 10A.

Edit: before someone starts quibbling, Ah is actually a measure of charge.
Energy would be 12V x 7Ah = 84Wh = 0.084kWh.

Last edited: Jan 4, 2015
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3. ### WBahn Moderator

Mar 31, 2012
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5Ah is the amount of energy (using charge as a proxy) while 10A is a current. The two are fundamentally different.

Your battery will not only have a capacity (the Ah rating), but also a max current draw rating (though this can be harder to find than it should be).

Assuming your battery can deliver 10A, then if it had a 5Ah rating it could maintain that for (5Ah)/(10A)=0.5h.

One thing to note is that battery capacity is NOT a constant and any battery's effective capacity is a function of several things, most notably the current level, whether the current is continuous or pulsed, the temperature, and the cutoff voltage. The cutoff voltage is the voltage at which the battery is considered "dead" and, depending on the chemistry, may be 80% or more of the fresh battery voltage. For most chemistries, the cell voltage drops fairly steady as the battery is drained until it reaches the "official" cutoff voltage and then it drops rapidly after that. But some circuits will stop working well before that point and so you may need to use a higher cutoff voltage. Other circuits may continue to work until well below the normal cutoff voltage, but that usually doesn't buy you much additional time.

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4. ### raviprakash.hegde Thread Starter New Member

Nov 14, 2011
26
0
Thanks WBahn.! This answers most of my doubts.