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My car had been sitting for a while without being driven. The battery now has a weak charge but it can get the engine started (barely). I would like to charge the battery by running the engine, but because my brakes are currently not working, I cannot take it for a drive.

Assuming the battery is still viable, while in neutral, what RPM and for how long should I run the engine to top up the battery? Does the RPM and duration depend on the battery type (amps, for example) or the type of vehicle? This is an automatic transmission car.

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This is a terrible idea for your alternator. Charging a dead battery using the alternator puts tremendously more strain on the internal components... It can particularly burn through a set of brushes in the alternator very quickly (which here in the US you typically can't replace separately) –  Drake Clarris Oct 10 '13 at 13:13

5 Answers 5

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There are a few variable involved.Namely the size of the battery and its general health. and to what extent you want it charged.Do you want it charged 100% or enough to start it the next time.The charging capacity of the alternator and the quality of the connections.The engine RPM won't really matter as anything from just above idle will have the alternator at full output.You also have to consider the power the engine is using to keep itself running,fuel pumps,computer,electric cooling fans are all consuming power while running the engine.Another consideration is that this is making the alternator work beyond what it is made to do and you are going to shorten the batteries life by fast charging it.If you had a 100 amp alternator and and a 500 amp battery that was dead it could take 5 hours under ideal conditions.But you don't have ideal conditions,you are taking some of the power away to run the engine along with the losses at connections etc.As an estimate I would guess that an hour should charge it enough to start it the next time as long as the engine can start without extended cranking time.

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+1 Thanks mikes. This answer currently does the best job of actually answering the question on RPM and duration without getting sidetracked. –  logicalscope Apr 14 '12 at 1:26

Perhaps an alternate suggestion: purchase, borrow or rent a battery charger. I bought mine for about $20 US and it will easily charge a nearly dead battery up to an acceptable level in about an hour (going from memory).

If you do the math, the car is a terribly inefficient battery charging system. It's pretty good in terms of transportation but that's a lot of overhead just to run an alternator in your driveway.

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Fair enough -- but I wrote the question because I don't always have a battery charger on me. I was in a similar situation a few years back with a borrowed campervan (it was nighttime and others were sleeping in the back, so I couldn't up and drive around). This is the intent of the question. Your point is well taken, however. –  logicalscope Apr 13 '12 at 17:48
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Charging a battery with an alternator is a good way to toast the alternator too. They're not all that fond of running at full output for extended periods of time. –  Brian Knoblauch Oct 10 '13 at 11:32

This site does a great job in breaking down a basic charging system, as described below:

Current Demand and Flow: If you have an alternator that can produce 120 amps of current (max) and the the total current demand from the electrical accessories (including the battery) is only 20 amps, the alternator will only produce the necessary current (20 amps) to maintain the target voltage (which is determined by the alternator's internal voltage regulator). Remember that the alternator monitors the electrical system's voltage. If the voltage starts to fall below the target voltage (approximately 13.8 volts depending on the alternator's design), the alternator produces more current to keep the voltage up. When the demand for current is low, the full current capacity of the alternator is not used/produced (a 120 amp alternator does not continuously produce 120 amps unless there is a sufficient current draw).

So the car would not be considered a great tool to charge the battery, as it would take forever... Depending on what the battery is demanding. RPM and loads are irrrelevant, as teh alternator will make do as needed as long as it is not overloaded. The reason a battery charger works best, as Bob described, is because it forces a set amount of current through the battery. This allows for faster, and better charging.

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It would not take forever. When your particular car is just started and running, this is what will happen. Since battery is discharged it will demand a lot of current, say 60 amps, or possibly more. But after a while, the charging current will taper off and go down continuously until the battery is full, since the alternator is a constant voltage, high current capacity charging source. Basically, at idle you can probably get 80 amps out of your alternator. You can charge a flat battery to 80 percent full in about 2 hours, so long as you alternator can manage produce around 14 volts at the battery terminals this whole time.

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While an idling car with an alternator will generally make sufficient power to run all its systems, my experience with using one car to jump start another is that at least 2x-3x idle RPM is necessary to get into the range where the alternator is able to make a good solid charging current.

To know for sure, you should be able to get a spec sheet for the alternator which will give the output levels for input RPM. You then calculate the ratio of engine RPM to alternator RPM (due to pulley sizes), and you have an exact answer. :-)

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