# How long does it take to charge a battery?



## JonW (Jan 6, 2002)

On my '93 325is, the battery is about 4 years old, 25,000 miles I'd guess. The car sits around a lot, so I got myself a Battery Tender Junior, it's a trickle charger of sorts. I've had it hooked up there for a little more than 2 days and it's still charging, not yet switched over to the trickle mode. I'm wondering how long it typically takes to get charged up. No hurry at all, just curious.


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## Steve D (Oct 10, 2002)

It depends on the initial state of battery charge and the charger output current. Typically you want to keep the charge rate to less than 1/5 of the rated battery amp-hours to prolong battery life. Most home chargers are 10A or less and most common automotive batteries are 80 amp-hours or less so a typical charge rate would be about 10% of the rating.

So if a 60 amp-hour battery is completely discharged and you charge it a 6A rate it theoretically should take 60/6 or 10 hours to charge fully. However most home charger's current rate reduces as the battery voltage rises so it could take up to twice as long for a full charge. The cell voltage will rise to as high as 2.7V/cell as the battery charges and after full charge is reached will actually falls back slightly. This is the point where chargers go into “float” mode.

The Battery Tender is not intended to recharge a heavily discharged battery. I am not that familiar with the specs but would guess that its max. output current is no more than1A. If your battery is say 50% charged and is 60A/H then it would take 30/1 = 30 hours to recharge and even more is the current folds back as the cell voltage rises. So 2 days could be normal and indicates that your battery is in a low state of charge. You may want to get it charged fully with a 10A charger and then see if there is an abnormal current drain in your car causing it to discharge. 

Another way to confirm the state of charge is to check the specific gravity of each cell with a hydrometer. This will also give a better indication of the condition of each cell. One or more cells that won’t achieve full charge generally means that the battery is near the end of its life.

Steve D


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## rruiter (Feb 10, 2004)

Steve D said:



> It depends on the initial state of battery charge and the charger output current. Typically you want to keep the charge rate to less than 1/5 of the rated battery amp-hours to prolong battery life. Most home chargers are 10A or less and most common automotive batteries are 80 amp-hours or less so a typical charge rate would be about 10% of the rating.
> 
> So if a 60 amp-hour battery is completely discharged and you charge it a 6A rate it theoretically should take 60/6 or 10 hours to charge fully. However most home charger's current rate reduces as the battery voltage rises so it could take up to twice as long for a full charge. The cell voltage will rise to as high as 2.7V/cell as the battery charges and after full charge is reached will actually falls back slightly. This is the point where chargers go into ?float? mode.
> 
> ...


I always thought not to charge a lead-acid battery with more than 1/10th of the capacity and that slower charging is always better.

On the subject, yeah a battery tender will take it's time, but it will get there, and get there safely. (unless your batery is shot)
It's not good at all to discharge this battery completely. Keep that battery tender hooked up.


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## rwg (May 10, 2002)

JonW said:


> On my '93 325is, the battery is about 4 years old, 25,000 miles I'd guess. The car sits around a lot, so I got myself a Battery Tender Junior, it's a trickle charger of sorts. I've had it hooked up there for a little more than 2 days and it's still charging, not yet switched over to the trickle mode. I'm wondering how long it typically takes to get charged up. No hurry at all, just curious.


After market/4 years? There's a good chance that the battery is shot.


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## JonW (Jan 6, 2002)

Thanks for the info guys. :thumbup: It sounds like Steve is "Mr. Battery." 

It took about 3 days worth of charging and it's full now. I'm not too concerned about the battery being shot just yet. It seems to be fine, no problems starting the car. I just got the battery tender to lengthen the battery life, especially for over the winter when I won't be driving the car. When I bought the tender, I had the battery checked and it had 550 CCA's left in it. Although I don't know what that really means. The guy said the battery was going to die soon. No problem, really.


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