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 Battery Info and Questions
 Calculating Capacity of an Used / Unknown Battery
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Posted - November 19 2012 :  11:31:37  Show Profile Send kcarring a Private Message  Reply with Quote
So I did a first round of pulsing a battery with a TSO, took it to 15.2V (almost), unfortunately my circuit quit, so I couldn't hold it at a cold boil as long as I would have liked to, but...

After discharging at approximate C40 rate, the battery, a trojan 1275 C20 rated at 150 aH, was reading (loaded) 11.79V after 13.5h of running the 50 watt, or 4.3A load.

This equates to 58.05 amp hours. After resting 20 minutes, it recovered to 12.1V (one should wait 2 hours or more, though, for it's final evaluation)

State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75

On the chart, which is only a guideline, this would mean that the battery was at approximately 50% discharged. Rather than take it right down to 10.5V, I can estimate that 12.1V is close to 50% discharged. that'd mean about 116 Ah. Given that the C20 is 150, C40 maybe as high at 175 aH, and we could call this battery 66-70% of it's rated capacity.

That's how I do it... and remember, this is just for deep cycle batteries.

If you have a better way, let me know! :)

You wouldn't laugh at my igloo if you knew how cold my beer is!

Edited by - kcarring on November 19 2012 11:34:08

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Posted - December 13 2012 :  15:09:14  Show Profile Send ron_o a Private Message  Reply with Quote
Here's a link to a circuit that gives a guide to battery condition ( also included is a desulphator circuit )

You must be logged in to see this link.

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