I have a similar problem with a Hawker Armasafe Plus 100 Ah battery from 2007 (
this thread). After reading up on battery technology and maintenance, I am looking into buying a
Pulsetech charger - desulfator. The link shows the three models mentioned below on the same page. Pulsetech has a nice 12 volt Maintenance charger and desulfator (XP-100C, $ 99) with very good reviews or the same in a 2 x 12 volt parallel configuration for a few $$ more. The cheapest 24 volt solution seems to be using their quad-link added to their XP-100C, setting you back
$ 199 in total.
Anybody any experiences with these models? They are relatively expensive but still a lot cheaper than a new battery.
Edit later: they also have a
dual station type for the same $ 199, better for 24 volts. Prices in online shops are about $ 30 to $ 60 cheaper.