posted 05-07-2009 08:56 AM ET (US)
The term "maintenance free" battery is not a precise descriptor of the battery type. I am not sure exactly what type of battery you have. I will assume that your maintenance free battery is really a sealed valve regulated lead-acid battery with conventional flooded cell construction, or an SVRLA battery.When the charging voltage applied to a lead-acid battery is higher than about 14.5-volts, the chemical reaction that occurs causes hydrogen gas to bubble out of the electrolyte. In a non-sealed battery the gas vents to the atmosphere. In a sealed battery the gas is contained, and materials in the battery try to move the gas back into solution with the electrolyte. This is a recombinant technique. If too much gas is produced, it exceeds the capacity of the recombinant material to handle it, and internal pressure in the sealed case builds up. A relief valve eventually opens to vent the gas to the atmosphere. If enough venting occurs the result will be loss of electrolyte. If enough electrolyte is lost, the battery loses its capacity to hold a charge.
Some "maintenance-free" batteries have caps which are removable, and they provide access to the cells and their electrolyte. If you have a battery of this type you can replenish the electrolyte by adding distilled water to the cells. These caps are not intended for routine service, but can be removed if needed.
If you have a SVRLA battery that is completely sealed and provides no access to the cells, you cannot replace any lost electrolyte. If you overcharge such a battery you will eventually boil off the electrolyte. The battery will slowly lose its capacity as it loses its electrolyte. Also, more of the lead plates will become exposed to air. This may also cause damage.
It is hard to know exactly how much gassing will be produced by your charging situation. If the recombinant feature of the battery can keep up with the gassing, the battery will repair itself. If the charging keeps causing the battery to vent gas, and you cannot replace it, the battery will eventually fail.
I can't offer any first-hand experience with this because I have not tried it.