Car and motorcycle battery charger

Connecting a lead-acid battery to a dumb charger and leaving it there for a long time is a bad idea.

If your goal is to charge a starter battery to the point where it will start an engine, then the other answers are correct: you can use a smaller charger to get enough energy in there to do the job of starting. Be careful not to overcharge. Once the engine is started, you can let the alternator do its job and charge the rest of the way.

An alternator's regulator controls what voltage comes out, to satisfy the needs of the electrics in the vehicle and to charge the battery. However, when the alternator is turning very slowly, the voltage will be low regardless of the regulator. Thus, in most vehicles the battery won't charge at idle. Hence, the advice to drive at highway speeds to recharge the battery.

Lead-acid batteries like to be charged in a very specific way. They can absorb an enormous amount of energy, very fast, up to around 80% charge, then you have to back off.

From :

Instead of building a small computer to control charging rates, most regulators take a very simple approach to charging, which means you'll never quite get the maximum charge and maximum life out of a battery. It is possible to install a smart regulator, but at $500 it's not worth it in a car. (It may be worth it for off-grid boats, RVs, and houses.)

Related posts: