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?
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.