Battery charge faster if revving engine?

I’ve heard different answers on this. If you’ver recently got your car jump-started because ofa dead battery, will revving your engine charge the battery more quickly then keeping it at idle?

In very simple terms (I’m sure sailor or Crafter_Man will come by to expand it) your alternator produces power as a function of the rotational speed. So the faster it goes, the more amps it can produce - and in some shop manuals, they will actually show a chart of amps versus rpm for you.

Now…whether or not this leads to a better charge on the battery depends a little on battery conditions and how much other auxiliary drain you have at the time. If you have a very high drain at the time (lights and wipers, fan full blast, rear-window defogger), then you may want to be running the engine at a higher speed to overcome the drain and make sure that there is enough energy left for the battery to soak up. If it’s midday and dry and there is no real drain, then most likely normal driving will be able to allow it to recharge and keep its charge.

This assumes, of course, that your battery is decent. Many batteries seem to have a hard time taking being run down flat, while others bounce back nicely - and it’s not only a function of new versus old or auto versus deep-cycle marine - different makes seem to work much better.

I always recommend that people get the largest, meanest, “Conan the Electrucutor” battery they can physically fit into their car, from a top brand and make. But what comes from the factory with new cars is sometimes pathetic.

That’s a good question. I rev the engine (a little) out of paranoia to keep the RPMs up so the engine won’t die. If left at idle with a dead battery jumped back to life sometimes the engine dies. Possibly a automobile electrical system maven can give us the answer.

astro - I know with some alternators (all? I don’t know) there is a critical rpm below which they do not operate. In fact, on my Honda Civic, because the engine was so small and I had it set to idle at about 500 rpm, I could slowly increase the engine speed until I would hear a “click” from the alternator, and the engine pitch would change, as it now had the extra load of turning the alternator. Scary stuff, to see the engine slow from it…but yes, on some cars, there is a minimum speed below which the alternator is not on - however, I confess ignorance as to knowing how this is set or activated.

The voltage regulator in the car senses the amount of electricity the car needs, and signals the A/C Generator, or Alternator, which provides that electrical energy to the car systems. If the alternator does not provide all the required electricity, the battery will also provide some to make up the difference. The alternator is run by a belt, which is run by the engine’s crankshaft. Based on this, I’d say it was important to get the alternator running fast enough, through engine RPMs, so that there would be no drain on the battery, and the excess voltage from the alternator would recharge the battery. Keep the drain from the electrical systems low (turn off headlights, airconditioning, radio, unplug cell phone chargers, etc…).



I am not a mechanic, though.

Anthracite: Have you checked the battery guage to see if it’s charging below 500 RPM? When I hear a click in my Jeep, it’s the radiator fan switching on.

“gauge”. :o

Increasing the engine RPM above a certain number doesn’t change charging rate very much. At some specified RPM, I don’t know what it is but I would guess somewhere a little over 6-700 RPM, the voltage output of the alternator is regulated to a relatively fixed voltage. So, no matter how fast above that number of RPM the engine turns, the charging current will increase very little. From that point on the current is determined by the voltage difference between the fixed alternator output voltage and the battery voltage divided by the resistances of the alternator and battery in series. As the battery voltage rises, the chargeing current gradually decreases.

It only happened when the car was first started, and it was definitely the alternator. The car was very simple, and had no power steering, no emissions controls (hey, it was how I bought it), or other computer controls.

This is not complicated but let’s see if I can explain it well or if I make it sound more complicated than it really is (which if often do).

Old cars with DC generators had a regulator composed of a couple of relays. It was very simple and tended to make batteries not last long because it could overcharge them. Taxis and other cars that drove continually could easily overcharge their batteries so the drivers would switch on lights or other devices just to use up electricity.

Let’s forget about those old cars and talk solely about “modern” (<30 years) cars with alternators and solid state regulators. The solid state regulator is about as simple in concept as the old relay regulators but, rather than having two states (off/on) regulates linearly through a whole range. So, in simple terms (I hope) here’s how the charging system works now:

The alternator’s output is rectified with diodes so, for the rest of this we can just assume the output is DC. The output current is proportional to the field input current and to the RPMs (in the linear range). What the generator does is multiply your field (excite) current by a factor which depends on the RPM.

At low rpm you do not enough output voltage to get to the 12 - 13 volts you need. The rectifier didodes prevent reverse flow through the alternator but, as you idle you can see the red pilot light telling you there is no output from the alternator to the rest of the system.

As RPMs increase, the regulator adjusts the field current so that the output current will maintain about 13.8 volts. Or, looking at it another way, the regulator circuit (solid state) will begin to reduce field current if the sensed output goes above 13.8 volts. At medium RPM the max current is going to be determined by the RPM and not the regulator. AS RPM increase further, the regulator starts reducing output.

This works much better at avoiding battery overcharge than the old relay regulators which is better in cars but it presents a separate drawback for people with boats and deep cycle systems because it will start reducing the charge as the battery rises in voltage and it is almost impossible to fully charge the battery. You are running the motor just to charge the battery and the battery is being charged very slowly. To get around this, some people, including me, modify the regulator circuit so that, with the flip of a switch, it will put out the max current. This is good but dangerous in the sense that now you have the original problem of overcharging batteries if you are not careful.

So, knowing all this, here is the answer to the OP: (What was the question again? . . . Oh yes.)

When you have a dead battery and you jumpstart the engine, if you let the car idle too low, then the charge system will not charge and any electricity is coming from the battery so the engine can easily stall as the ignition has no power (different story on a diesel wich you cannot kill by shutting off any ignition). So, at this stage you definitely need to keep a high idle which will ensure the charging system is putting out juice. Higher idle at first and gradually decreasing as the battery is charged.

But the question is how high?, how many RPMs? Every alternator, every car and every situation is different but, in general, revving up the engine too much is not going to do much good for the simple reason that the alternator has a max curent it can put out and it is not going to put out more just because you make it go faster. In any case, you have started the car and you are going to be driving it so the question is what to do at traffic lights or when stopped in traffic. My answer is todrive normally and keep a reasonably high idle, higher if you are using headlights or other electrical devices but do not rev up the engine too much. And the best thing you can do is switch off everything you can. Every amp you use on some device is one less amp going to charge the battery. The only reason to keep the engine going is if you are not moving the car, just running the motor to charge the battery. In that case twice the time at half the RPM is much betetr than half the time at twice the RPM. It is difficult to give any hard numbers but my guess is that a gasoline car which idles at 700 - 800 RPM should be idled at 1200 - 1400 if the battery is dead and maybe up to 2000 RPM if you have a ton of electrical things drawing current and/or the drive is going to be very short.

Looks like I was incorrect in saying that current increased as a fucntion of rpm, as that would inply that it continually increased. I knew there was a limit, but did not know at what point. I swear I’ve seen a nice graph showing amps steadily increasing to 5500 rpm or so, but perhaps that would be for a DC generator?

Anthracite, I do not have any grphs at hand so I cannot say but we should realize several things:

Those graphs probably show Alternator RPMs, not Engine RPMs. generally the pulley ratios is so that you can get a good output at low to mid engine RPMs even if you gain nothing at higher RPMs.

Also those graphs would probably be for max field current, i.e. with the regulator disabled. In other words, they show you the performance of the alternator isolated from external factors. They would also be a a certain temperature, output voltage etc. Once you start factoring in temperature, voltage, regulator, etc, then you get a huge family of curves.

An alternator rated at 55 amps can give 55 amps for a while but not for a long while as it will overheat.

i found some graphs on the net and most alternators (I am assuming with the regulator functioning) seem to have a logarithmic output. They all refer to alternator RPM. They vary a lot but as something representative: If we call 100% the max current output then we get 5% output at 1000 RPM, 50% output at 2000 RPM, and 100% output above 4000RPM, flat to 6 or 7000 RPM. The alternator generaly turns fater than the crankshaf so you would have to divide those RPM accordingly to get crankshaft RPM.

As you can see, the best results are probably in the 2 - 3000 alternator RPM range, which may be in the 1200 - 2000 RPM for the car engine. You can get more juice above that by with a decrease in efficiency.

As I said, I would just drive normally and keep the idle at about 1200 rpm during the first few traffic lights or stops. I would try to keep the lights and other electrical consumption to a minimum but if I had to have them on then I would keep the idle at 1200 for the first 30 - 60 mins. Otherwise I may be discharging the battery during those stops. What I do in these circumstances is turn the lights off when I am stopped and turn them on again when I move again.

One other thing: the red pilot light goes off just as the alternator begins to put out juice but that does not mean the battery is being charged. If the alternator is putting ot 20 amps and the consumption is 10 ams then anotehr 10 are going into the battery. But if the alternator output is 20 and the consumption is 30 then 20 come from the alternator and 10 come from the battery.