A charger which charges at 1/2 amp is OK for a battery with a capacity of 5amp.
But not OK for a battery with a capacity of less than 5amp
The ideal charge rate for a sealed lead acid battery (Gel Cell) would be 10% of the batteries capacity IE: 5amp should be charged at 500ma, 7amp at 700ma and so on.
Try charging them any faster and you stand the chance of damaging the battery due to overheating >:( If you try charging them at a much lesser rate than 10% you won't damage the battery but it will take longer to charge and you will not create the correct chemical reaction within the battery to charge it efficiently.
Use a good quality charger ! :) your batteries will last longer and give you better service. Some of the cheaper chargers will not bring a battery up to spec after a deep discharge.
Regards Gary