Suitable amperage for charging 12V 12Ah DC lead-acid battery
For flooded lead-acid batteries, it is generally recommended that you not charge at more than 20 - 25% of the Ampere-hour rating - for your 12 Ah battery, that would be about 3 Amps.  Gell and AGM batteries can often be charged faster than flooded types, but you should check the manufacturer's recommendations.