Charging lead acid batteries in parallel?
Batteries don't often fail low voltage or short circuit, but if they do, then a 'very parallel' arrangement could be bad as all the good batteries gang up on the bad one to force a high current through it. Protect each battery with a fuse in series.
Before connecting them in parallel, make sure one or more of them aren't duds, check each individually into a load. You may have to charge them individually to do this.
Before connecting them directly in parallel, roughly equalise the voltages. This is more important with lithium than with lead or nickel chemistries, but it's still worth making sure that you don't have large imbalances in the voltage, even with lead batteries.
Once you've connected them in parallel, you can treat them like a high capacity battery.