Hi Owen,

I took a look at the datasheet of your charger. Problem could be the
equalize mode of that charger. 15 min. equalizing every 21 hours is a lot.
When batteries are full charged or in equalize mode voltage schould not be
more than 14.4 volts. Check that voltage by measuring it at full charged
batteries. If voltage is more, your charger has failed.
Very high temperature of batteries may cause drying them, but that takes a
lot of time. If Water is "boiling out", the charger will be the problem.

Juergen