How long do I charge battery

I have a batter charger with 10 amp, 25 amp, and boost settings. How long should I charge an average battery (2006 VW Jetta) in order to obtain a full charge at 10 amp and 25 amp settings. (Assuming the battery is dead or near-dead.)

Charge it just long enough so it starts, then drive the car at least 20 minutes on the highway and let the car do it’s job. I have heard the boost setting is harmful to batteries but have no experience verifying that. I have a battery maintainer that will get a dead battery going in about 20 minutes with no fear of overcharging.

If the battery is completely discharged, it could take 6-10 hours at a 10 amp charging rate to fully recharge the battery. At a 25 amp charging rate it would take a shorter time but could result in a surface charge of the battery which is useless.


Ideally you would have a manual for the charger that would tell you what you need to know although the rule of thumb info given works. If you don’t have the manual many manufacturers keep them online so I would search. Presumably this is not an automatic charger that knows how to charge & maintain on its own? One other general rule of thumb is that lower amps and more time is normally much better for a battery than higher amps with less time.

How old is the battery? It might be a better idea to replace it. Running a battery totally down is very hard on it and often they don’t recover well.

If it is cold (like below freezing, I would bring the battery inside if possible over night. Trying to charge a discharged frozen battery is not a good ideal. If you must, then I would start with the 10 amp setting and do maybe 5 minutes, then 5 minutes rest, repeat once and then go to 10 minutes, repeat, then 30 minutes at 10 amp. Check the voltage between each charge. After that the battery should be ready to accept a slow charge (10 amp) for say an hour. then maybe switch to the 20 amp rate for an hour.

Note not all chargers are created equal so I have tried to be conservative. If the charger has instructions with it, follow those instruction with the possible adjustments for a possibly frozen battery. If you bring the battery into your home for a day, then it should not need special treatment due to cold.

Hopefully it will be able to start your car by then

I would charge it at the 10 amp setting. Slower is almost always better.
It takes at least 8 hours to fully charge a near-dead battery.
During that time the current should gradually fall to below ~2 amps.

I don’t believe that anyone can say unless they have run experiments plus knowing if the battery is good or damaged. A starting battery, which you have, can recover from only a few complete discharges.

If you don’t need to drive the car soon, use the 10 amp setting and, as was said, the charging will be very near or at completion when the current is at or below 2 amps.

If you must drive soon, use the 25 amp setting long enough for the engine to start; then remove the charger after the engine starts.

Most car batteries have 50-60 amp/hour capacity. So six hours at 10 amps and 2 hours at 25 amps. But your charger will probably self regulate the charging currant, tapering it off as the battery approaches full charge. Most chargers have an amp meter so you can see the actual charging rate. Batteries can be charged at a high rate when they are low, but as they near full charge, the rate should be reduced to avoid over-heating the battery…

Many thanks to all for responding. The charger is at least 15 years old and didn’t come with a manual. I doubt if there is anything online, although I will look. Unfortunately, the battery is about a week old, and the discharging is being caused by another problem (brake lights are turning on at random at all times of the day or night), so I hope it hasn’t had its life shortened by too much. Hopefully will get that problem ironed out this week, so it won’t be an issue much longer. Thanks again.