Formula goes something like this:
Numbers for volts and letters for lead post position and/or poles swapping w/amp rating. So if we buy a 12N12a battery, move the decimal point to the right of 12 and then this represents 1.2a. This says you need at least a 1.2 amp rated charger. To charge the battery, the remaining formula says to charge the battery for literally 12 hours with a 1.2a charger.
If say you buy a 12N14a battery, you won't fully charge a 1.4a battery with a 1.2a charger. So you run formula to buy the charger needed for said battery. And then you use a 1.4a or higher [1.5a rated say] charger and charge it for 14 hours with a 1.4a rated charger.
Now say you have an 8 or 10a car charger. The problem with this is the chance of overheating the battery if charged too long. So you feel the body case for heat. Problem being: the bubble size. Meaning, the difference between a boiled water bubble as opposed to champagne bubbles. The large bubbles will blow out, buckle the plates with that kind of heat [rating].
Make sense?