I have an ordinary secondhand car battery, that I charge through the lighter socket with nothing more than a plug, 2 crocodile clips and a 10A fuse the car's own lighter socket has a 16A fuse, so the "charger" fuse should blow first.
They work best in warm climates, and need to be constantly fed with energy to keep those internal chemical reactions balanced—even when the car it's attached to isn't being driven often.
If there is current flowing, you have a draw, but where? Is this another bad battery, or is my car a battery killer? There are multi-phase charging steps and temperature sensitivity for more efficient charging.
Always ran fine, charging voltage was within the acceptable range, but if I didn't disconnect the battery it would be dead by morning.
Â Tip: Check the vehicle owner's manual to guide you in choosing and buying a replacement battery.
Just don't cheap out and get a "dumb" trickle charger; models that have a float, storage, or maintenance charge mode are preferred.