solar cells with old battery- safe?
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.