How to Choose a Lead Acid Battery Charger?
The Amp hour (Ah) rating of your batteries will make a difference on what size charger you should use to charge your cells. Any charger providing a current with a value of more than (Ah/4) is considered a fast charging unit for that battery. If you need your batteries to last, you should not use this method. 

The method of parallel charging is acceptable, but will slow down the charge rate, essentially splitting your current by the number of batteries in parallel. This method, combined with the knowledge above could bring down the current from a 'fast charging' unit to an acceptable charging range.

The biggest problem with using a regular power supply to charge your batteries is that the charging will have to be very closely monitored. If the batteries become over-charged, electrolysis of the water in the cells will start to degrade the batteries. A battery charger will have the necessary circuitry to maintain a constant voltage, limiting the possibility of over-charging.