I have a small 3 year-old 6V 1.2Ah SLA battery that is used in a solar powered burglar alarm sounder box. The sounder recently stopped operating and the battery shows a voltage of about 4V so I am recharging it (using a charger designed for this purpose). I will clean the solar cell and replace the unit's 9V PP3 battery which is really only for initial start-up.
However I know that SLAs have a finite lifetime and are typically replaced every five years.
I also have a spare 6V 1.3Ah battery that is probably of similar age but which has not been used for 3 years (1V shown).
After charging them back to 6V, is there some way I can test these batteries using a multimeter, a load such as a light bulb of known wattage (and possibly other components) to see which battery is in best condition or whether I really should buy a new one?