I work at a car auction and we use deep cycle marine batteries with booster cables attached to hand trucks to wheel around the lot for boosting cars. I've been told by our battery distributor the best way to charge the batteries individually is at a low (2-5 amp) charge setting for 12-24+ hours.
We use a wheeled automotive battery charger that has several amp settings (2 amps up to 200 amps) in 6- and 12-volts. We have had problems in the past with batteries gassing and even exploding from being hooked up to the charger improperly (luckily no one has been injured).
The latest failed attempt at charging them all simultaneously involves two copper pipes hooked to the positive and negative leads of the charger (running on a 2amp setting) and the positive and negative booster cables of 4 batteries attached to the corresponding copper pipe. They don't seem to charge this way, even after several days. My guess is that the amperage is being divided among the 4 batteries, so they're only getting a very small trickle, if anything. I've been afraid to crank the charger up to 20amps or so to test this theory without finding out for sure if it is safe.
How can we safely charge multiple deep cycle batteries (we currently use 4) simultaneously using a wheeled automotive battery charger?