Ralph- It's a good idea to unplug the charger after they are up to full charge. Battery chargers are like all other pieces of electronic gear- they will eventually fail!  Generally speaking you could see two potential failures of a charger relative to the battery, 1.) The charger control fails and over-charge's the battery or 2.) the charger fails and isn't charging or under-charge's the battery. Either failure mode can kill your batteries. A battery "state of charge meter" or a digital volt/multimeter is a good investment.  By being aware of the state of charge, the functionality of the charger and in control of the charging cycle/time you have a better chance of preventing severe damage to your batteries.  A typical refresh charge of your batteries shouldn't take more than 8 hours. Talk to your battery supplier or charger supplier & see what they recommend.

Any one who recommends "plug it in and forget it" is a playing the odds and has been very lucky. It's a nice concept- but not practical from a protect your investment perspective. Here's one little factoid most folks don't even know about. Over-charging lead-acid batteries can cause fires, i.e., the battery actually can self ignite from excessive internal heat buildup!!! This is especially true of VRL:A (sealed no maintenance). However,  the same can happen to wet-cells once the electrolyte has boiled away from overcharging.

Al Warner

http://www.thenethouse.biz

www.welcometothechumbucket.com