# How calculate total Capacity (Ah) for batteries.

Discussion in 'General Electronics Chat' started by mustafa2, Dec 24, 2013.

1. ### mustafa2 Thread Starter New Member

Dec 24, 2013
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0
how calculate autonomy time (back up time) for 4 Bat 12V170Ah load (24V,50A).

2. ### GopherT AAC Fanatic!

Nov 23, 2012
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Welcome to the Forum.

Since your load appears to be 24 volts, I assume yo have two batteries in series. Please confirm.

If they are in series, you can add the voltage but you cannot add the amp-hour capacity. Therefore, your time would be 50 amps divided by 170 AmpHour = 3.4 hours nominal.

I say nominal because the battery voltage would be falling over this time and, depending on your load, the current may also be falling (because of the falling voltage) and cause the battery to last longer.

Good luck.

3. ### Alec_t AAC Fanatic!

Sep 17, 2013
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You would also not want to run the batteries flat or it would damage them.
Unless 50A is the current which the manufacturer used (unlikely) when specifying the Ah rating, your actual run time will be different from the time calculated from the rated AH capacity.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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Sounds like the OP is running two parallel strings of two batteries in series, a total of four batteries...

The time to stop discharging is when the first battery terminal voltage reaches ~11.0V, so the capacity is determined by the weakest battery in the chain. Knowing when to stop discharging is difficult. Here is some good advice...

A lot of good information in the Tech manual SLA, downloadable here

Last edited: Dec 24, 2013
5. ### mustafa2 Thread Starter New Member

Dec 24, 2013
3
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Yes 4 Bats: 1st series set (1+2) parallel with 2nd series set (3 +4) to get 24V

6. ### mustafa2 Thread Starter New Member

Dec 24, 2013
3
0
yes thanks 4 Bats: 1st series set (1+2) parallel with 2nd series set (3 +4) to get 24V