NOT from me (RNinMunich)! I just edited it to delete a previous comment of mine!
Lead acid batteries should not be discharged and kept charged up as much as possible. Discharging these batteries will reduce their capacity. if they are kept for any length of time without a charge, they will not charge up again.
To charge up these batteries use the same formula.
However lead acid batteries can cope with heavy currents and it is sometimes a good idea to charge them up with a higher current. Now and then charge them up with double the current as usual and for half the time.
So for example, a 5000mA/h battery, should be charged on 1 Amp for 5 hours.
To ensure they will be fine when next used make sure they have a top-up charge every couple of weeks. Half the length of time of a normal charge.
DO NOT HAVE THE BATTERY ON A CONSTANT TRICKLE CHARGE!
Large 12 v lead acid batteries (e.g. old home security batteries) have a time constant of approx 4 hrs so need to be charged 5 x 4 hrs = 20 hrs. I leave mine on overnight which usually does the job, offload charged battery volts are approx 13.8 v.