My guess is that your friend's system is not set up to guarantee that those batteries were given an acceptance voltage of at least 14.4V when they were recharged from a deep discharge condition (more than 3% discharge). When this happens they keep getting sulphated from always sitting in a state of discharge, never having been brought up to "full".
The method of bringing them back from the dead involves the use of a variable current
, voltage limited power supply. What I do is apply a constant current
supply having a compliance voltage of 20 V (for a 12V battery). For the group 31 batteries you might use a current limit of 12A initially. You need to monitor
the current and voltage. Ititially you might observe no current with the 20V applied if they are really bad. Watch to see if the current begins to flow within 1/2 hour. Make sure that the ambient is not below about 65 deg F to get the current to flow. Eventually current will flow, hundreds of mA then a few amps.
Feel the case of the battery
to make sure that no single
place is getting hot when the rest of the case is not. If that happens the battery
is probably junk. If you are able to bring it back from the dead the case will warm up and when you reach the current limit the voltge will be dropping. If the battery is charge accepting it is O.K. to be warm all over but not hot else lower the current.
If you are fortunate at 12A the voltge will drop to about 14.6V or so. When the voltage starts to rise again drop the voltage to 13.7 to 13.9V and leave it overnight at that voltage watching the current and noting that the battery uniformly cools down to ambient.
Use a power inverter
driving a 100W bulb to load the battery and measure the internal resistance by dividing deltaV by deltaI, the change in standing voltage from no-load to inverter
load divided by the inverter load current. Do this for 5 minutes. Recharge the battery band do the discharge test again. Keep doing this until no more improvement is observed in the battery resistance decrease.