Choose the battery charger that slowly recharges at low amps. But if the battery is dead, fast charge the battery with high amps to start the vehicle now.
A 10 amp charger will charge an auto battery in 6 hours.
A 50 amp charger will do in an hour.
Battery size means the number of amps and volts. To compute charge time for each battery, generally, take the amp rating of the battery, divide by the charger amp rating, and add 10% for full charge.
It is best to slow-charge the battery, and rates depend on the battery type and size.
For an automotive battery, 10 amps or less is considered slow, and above 20 amps is fast charge.
For an average car battery, a 10 amp charger is good enough.
A trickle charger, or maintenance charger, keeps the motorcycle, classic car, ATV or boat battery charged during the off-season. But the low current is only about 2 amps. So charging a car battery with a 2-amp battery takes 1-2 days because the small unit only generates about 1 amp an hour.
Smart chargers have microprocessors. These battery chargers charge regular car lead acid and other types of batteries. They “test” the battery first and adjust the charge current and voltage based on that battery. That lets the battery charge fast, correctly, and entirely. They say the smart chargers won’t overcharge and can be left on the battery constantly without damage.
The 40 and 50 amp chargers are workhorses. A dead marine deep cycle battery is 100 amp hours, and would take 3 hours to charge a dead battery.
Some people wanting quick recharge should look for a charger with more amps.
If you have time, use a smaller charger. Remember, you need enough power and time to do the whole job.
Others need to quickly restore a battery ASAP.
A 10 amp charger needs 5 hours to fully charge the battery. But a 2 amp charger could take 2-4 days.
The difference between a fully charged battery and a discharged battery is only 0.25 volts per cell. A full12 volt battery is 12.9 volts, and 11.4 volts when discharged – a difference of 1.5 volts. All 6 cells drop .25 volts for a total of 1.5 volts.
When recharging your battery, if it gets too hot, stop. Don’t let the battery heat above 125F.