The usual ones just consist of a mains lead, a step-down transformer, and a half-wave rectifier in series with the transformer secondary output leads, Deejay. One of the secondary output leads will have a red clamp on the end, and the other a black clamp. Pretty high tech, huh? (A slightly better one will use a bridge rectifier and maybe a thermal interrupter on the secondary side.)
There are much better ones that have a switching supply that is Pulse Width Modulated to produce a programmed output voltage. The program senses output current and voltage and applies three charging phases, commonly called Bulk or Current Limited, Absorption or Voltage Limited, and Trickle or Float. The Current Limited phase is based on the charger's maximum continuous output current rating, the Voltage Limited phase should be determined by the type of battery you are charging, and to a lesser extent so should the Float phase. The old lead-antimony open-cell batteries liked about 14.4 Volts for the Voltage Limited phase, but that would kill a gel cell or glass mat battery, and would overheat a lead-calcium battery (about 14.1 Volts suits those). The float voltage should be 13.0-13.1 Volts for a lead-antimony battery, and something fairly similar for the other types, though I think a lead-calcium type would like about 13.3 Volts even better.
The battery chargers you can buy range from a few bucks for a simple transformer type, up to the sky's the limit for a programmed type. Of course the manufacturer's blurb wants you to think most of them are the programmed type when they actually aren't. So, to some extent you get what you pay for. Of course you still have to use some sense: if you connect a big battery charger to a small battery, the charging rate will be excessive during the Constant Current phase, which will cause overheating and plate erosion. If you set the charger for normal or car battery mode and connect it to a gel cell, it will apply too high a voltage during the Constant Voltage phase, and much the same applies to an AGM (Absorbed into Glass Mat)battery. Above all, don't fast charge any battery unless it is an emergency. After a short time the cell electrolyte temperature will rise above the maximum allowed and the plates will be eroded.
So, you may have made the right decision by not having a battery charger at all.
