Car battery chargers all have a load that they charge on.. when the battery is weak / dead they will up the amperage needed to charge the battery, once it's reached a level it deems appropriate it will reduce the current to a very slow "trickle" and keep it at that level.

People leave car battery chargers on whilst the car is not been driven, and it could be weeks... Although I don't recommend doing this with the $30.00 as they will prob blow up from being left on for that long LOL.

but no.. you will not OVER charge the battery.. it will just stop charging when it needs to.

This is how the cheaper $30-$40 supercheap auto chargers look like - - CLICK

If you can spend a bit more like $70+ you will get a lot better quality try looking for an Arlec one, or go to Jaycar / Repco etc..