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I am charging an old (probably 90s or older) security system battery with 4 3.3v solar cells compounded and partly covered up to reduce the voltage. the standby charging voltage is 13.5-13.8v, and the solar cells are putting out 13.65v. first, is this safe? second, can I determine the battery life by the voltage outputted by the battery after a certain time of charging? third, is it dangerous to charge the battery with less than 13.5v?

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  • \$\begingroup\$ Safe in terms of exploding batteries .. for lead acid, yes it's pretty safe. Safe in terms of not prematurely aging batteries ... does that really matter with old batteries? If you're worried, solar charge controllers for lead acid batteries are pretty cheap. \$\endgroup\$ – Brian Drummond May 26 '18 at 12:00
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Lead-acid batteries (whether flooded, Gel or AGM) of that vintage are almost certainly heavily sulfated, particularly if they have been unused for some time, and will not accept a charge - they should be discarded.

However, you can try to charge them if you wish. If you apply say 15 volts, and measure the charging current, you will probably find that the current is approximately Zero - you can charge for weeks without getting a useful charge into a battery that old.

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  • \$\begingroup\$ Would it not be a better solution to regulate the output of the solar cells, than covering them up? That way you would have voltage overhead for cloudy days etc. Also That battery as mentioned by @Peter is a sure bet for needing a replacement. \$\endgroup\$ – Sara Heart May 26 '18 at 4:28

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