I had 2 Big deep Cycle batteries that had been on trickle charge that we pulled from Bros boat to put back into my Boat ... One battery was 3 years old ,, the other 8 years old .... The 3 year old battery was just a little low on voltage and the 8 year old had only 6 volts .. Having 2 battery chargers I charged both overnight. I had it somewhere in my Pea Brain that they were Maintenance free... They are not ---- When I pulled the cover from the cells on the older battery I could see the plates exposed in all 6 cells ---- I used a 16 oz water bottle and a funnel to hit the holes... To just barely cover the plates it took 60 OZ of distilled water ----- this battery is TOAST ---- The second battery that is just 3 years old also had the tops of the plates exposed and took almost 30 oz to fill the cells to the correct level. My Battery charger has a feature to recondition a battery by sending pulses through the battery to knock the Sulphide from the Plates.. Hopefully I caught this one in time.. These batteries had a solar collector keeping 1/16 of an amp trickle running through which is way less than those used to Maintain your battery ... I had 5 old batteries from stuff like lawn mowers , auto batteries, and deep Cycles ... Every single battery I looked in had the Plates Exposed. When a Battery charges from the Alternator the current flow will make the water/ Sulphuric acid boil, over time leaving the Plates exposed to Kill the Battery before its time --- Go check yours you may be Surprised