Charge battery more by turning on accessories?


You said an electrical engineer is going to comment… yes he is!

One of you made a statement that was so wrong, you should be ashamed of yourself. You said the ammeter showed how much the alternator was supplying. No, that is incorrect. It actually shows how much is flowing into or out of the battery. That is the important thing to measure, as you want to know if you still have proper charging. Otherwise, what would a negative reading mean? The alternator only puts out current in one direction, never sucks it back in.

As for the issue of an additional load having an effect on the battery charging, yes it does, but it is not a simple matter of the more load, the more charging. In the charging circuit, you have the alternator, the voltage regulator, battery and the electrical load. For a particular engine speed, there is a maximum current the alternator can produce. Additionally, the voltage regulator limits the alternator output, depending on the voltage it reads. Ideally, the regulator would read the battery terminal voltage, but for ease (lower cost) of manufacture, it actually reads the voltage at the alternator output.

For the particular case of the question, we have the alternator charging the battery, but the voltage regulator has sensed that there is enough voltage at its terminals, and is limiting the output. When a moderate load (the fan) is put on, the voltage regulator senses a voltage drop, and increases the alternator output to compensate. This increases the voltage, and hence the charging current at the battery. However, further increasing the electrical load (lights) now hits the maximum current the alternator can produce at this engine speed, and therefore has less current available for the battery, so the charging level drops, or even goes negative. More load will only make this condition worse.

While the act of putting on some accessories may possibly help to bring up the voltage regulator output, it is a tricky thing to find the maximum charging point. One would be best off in driving the vehicle, or having the engine run at some moderate speed for for a while.


Another EE here! I agree with RickNY. However, there is another factor that may be at play here. Perhaps the listener, in addition to turning on his fan, also turned on his AC system, thus turning on his compressor. This would of course raise his engine idle speed, raising the available output from the alternator.


I am not sure about "You said the ammeter showed how much the alternator was supplying. No, that is incorrect."
It may vary from model to model.
If you have a failing battery, the ammeter may show 14 volts or so. That is what the alternator is putting out. You turn off the engine and the voltage indicator may show 10 volts for a battery charge. Start it up again and it reads 14 volts. I say it indicates the output voltage of the alternator.


Somewhere in this show you mentioned the ‘bogus’ claim that turning the lights on for a while can help to crank the engine at very low temperatures when the battery is weak. This does actually work (sometimes). The reason is that the battery capacity is a function of temperature. Having the headlight current (about 10A) flowing through the battery for a few minutes warms up the battery (power dissipation on the battery internal resistance) and can increase the capacity more than what is lost in the headlights.


I would like to add to what RickNY and richw42 had to say. One other positive of the higher load on the electrical system is that some vehicles operate on the battery with the charging system keeping the battery charged. Remove the battery and the vehicle will not run. The higher loads help to keep the batter in top condition allowing the battery to accept a deeper charge.


“the ammeter may show 14 volts or so”

Um… you’re describing a voltmeter.

I haven’t seen a run-of-the-mill car come from the factory with an ammeter since the late '60’s.

As @rickny said the ammeter showed what current went in or out of the battery, with the exception of the current to the starter, which connected to the battery through a separate (thick) wire.
The ammeter was wired in series with a line that connected everything else to the battery.
They read zero at center and typically +/- 30A full scale.


Well, in the length of time it took me to also compose a rebuttal to the click and clack diagnosis, others have clearly beat me to it. But I’ll confirm the general sentiment of the others.

You told a guy with a 1978 (?) Land Cruiser that today’s story about “turn on some electrical load to speed weak battery charging” was bogus.

Probably not bogus; you didn’t listen to the data he gave you. He said “ammeter reads flat, turn on fan blower, ammeter jumps up, turn on headlights, ammeter jumps back down”.

The ammeter in the Land Cruiser is a simple voltmeter connected across a fusible link (of known resistance) connected immediately between the alternator and battery, so it is a true measure of total current into or out of the battery (cranking current not included). So first of all, the data he gave you should cause you to consider the possibility that the effect he describes is true.

But how could it be so? One must consider carefully both how the relatively simple regulators of the 70’s worked and how the battery voltage depends both on state of charge and battery current (either charge or drain).

From the standpoint of control theory, a car battery charging system is actually rather complicated and so to avoid stability issues that can occur in such a control system if either high loop gain is used or tight voltage control is demanded, a rather simple system is used. The key to the success of the system is /hysteresis/ in the operation of the regulator.

An old-style auto regulator senses battery/system voltage and, when the voltage drops too low, it connects the field winding in the alternator to the battery so as to increase the power output of the alternator. But it does so in one discrete step, not finely controlled. So the charging rate of the battery will increase by a minimum “chunk”, so to speak, and the battery voltage will rise in a similar “chunk” because the battery voltage is dependent on charging rate.

Were the control circuit poorly designed, the regulator would sense the rise in voltage, and say “now the battery is fine”, shut itself off, the battery voltage would discontinuously drop, and the process would repeat; the system would be unstable.

So to solve this problem, regulators are designed to have /hysteresis/, that is, they kick in at say 11.8 volts, but don’t cut out until the battery voltage has risen to, say 12.6 volts.

When your caller says that he set up the idle a bit, and saw flat charging current, this is consistent with low current demand, relatively high battery voltage, but not very good charge state. Then he turned on the blower. The added current drain on the battery/alternator system pulled the system voltage down just a bit, below the “on” threshold of the regulator, partly because of the low charge state of the battery (in a low charge state, the battery voltage will fall quickly with only a slight added system load). The field coil of the alternator is energized, the power output of the alternator increases (the voltage across the alternator increases) and the charging rate is increased, as per the evidence supplied by the caller. But the regulator will not kick out until the battery charges enough to get to the, say, 12.6V I mentioned earlier.

Now he says that he turned on the lights. This is an added system load. But the regulator has already kicked the alternator up. The alternator, at modest engine rpm will not provide enough current for both charging the battery and running lights and blower. So the alternator voltage output falls (because of the increased current demand) and therefore the charging rate of the battery drops, again as reported by the caller.

The key to understanding the behavior of the whole mess is the coupling between the regulator hysteresis and the dependence of the battery voltage on both charge state /and/ charge current.


Ammeters, as installed, were meant to detect if the battery is discharging. They are directly in line with the positive battery lead. The only load that does not get current thru the ammeter is the starter motor itself. Otherwise, if you see a positive reading, the battery is charging, no matter what you think should be happening. This is in agreement with many other comments.

I have this to add. I still own a motor vehicle, specifically a Yamaha XS400D from 1977, with a mechanical regulator. It is a two-state device. In the active state, when the system voltage is enough to hold the relay, current is supplied to the alternator field coil through a resistor, thus the alternator makes less juice. If the system load remains low enough, the output will remain low.

Once system voltage falls below a certain threshold, the relay drops, and meets a contact that supplies full power to the field coil. At that point the alternator output voltage jumps to a higher level. Of course, as system load increases further, the system voltage will go back down. If your car is old enough to have a mechanical regulator, a certain system load, just enough to drop that relay, but no more, will result in the highest system voltage, and thus the highest charging current.


Quote from RickNY: “The alternator only puts out current in one direction, never sucks it back in.” Unquote

False. It is possible for an alternator/regulator under the right conditions to fail to produce charging current yet absorb about 3 amps for the rotating field or rotor as it is commonly called.

The remainder of your post is pretty much on the mark.

By the way, a battery is not particularly sensitive to voltage imposed on it by an alternator. Using two of my shop manuals for reference, 13.5 to 14.8 volts at equalization (steady state charging) are sufficient.


It is often the case that the blower connection for high speed is connected directly to the battery (through a relay controlled by the fan switch). Bypassing the ammeter, the blower current appears as a high charging rate–when in fact it is not.


Water boy, FYI the ammeter measures amps, not volts.
All, I agree as an ee and from experience that
A modest load helps warm up a battery and therefore gives more cranking amps (by reducing internal resistance of battery)
And a modest load can cause the voltage regulator to increase voltage and therefore charge faster
Also, these were two separate questions from two different calls


Mike: As an EE you know that a regulator that increases voltage under increasing load is inherently unstable. So if this condition exists at all, then it must exist over a very narrow range of currents. Ultimately, increasing load must cause the voltage to drop–I doubt there is much to be gained by adding load to the alternator. In the caller’s case, the increase in the ammeter reading when the blower is turned on high ( but not the lights!) has more to do with wiring than regulation.


Point taken. the vr will pass more current to maintain the voltage, but should not increase the voltage. So yeah, the battery won’t charge any faster, because of vr. If the computer senses the extra mechanical load and increases the rpm, that might increase the voltage at the battery. And the engine will be doing more work,so temp under hood will rise, reducing internal resistance of battery. And so on. How much of a difference this all makes would require some serious investment in test equipment and time, but I’d guess it’s not much.


Batteries are funny things and depending upon how they are used, the capacity of the battery will vary. One of the factor is temperature. It is possible that under cold circumstances, there is nor enough juice to start the car, but that if you turn the lights on, you can drain the battery a little and also warm it up. One reduces capacity and the other will increase it. Drain too much and it drain more than you gain with the temperature. Don’t drain enough and it won’t warm up the battery and still drain. So, under special conditions, I can see how you could increase the capacity with the heat more than you drain it and you could reach a point where the capacity increased enough to allow the starter to start the car… but it would be impossible to put into equation and say exactly how much you need to drain for how long to get there.
Anyway, this is my input on it. My guess is that you can always give it a try because you have nothing to lose even if the chance of succeeding are 1%.


Sorry fellas, but I have to respectfully disagree. Turning the lights on will not do any more to warm up the cells than a few cranks of the starter. I submit that the best approach is to forget the lights, turn all the accessories off, and try to start the car. The accessories should be turned off because as you cycle the key from start to “on” trying to start the car, you’s be cycling the accessories on and off at the same time, and that’ll take a bit of needed juice.


I also disagree. Turn off everything and try the starter. If it starts…fine. If not, get a jump, new battery or replace your alternator whichever is required.


Getting back to the subject of the OP, I can see why turning on the blower fan to high would help charge the battery faster.

The one problem with any alternator is that it produces pulsating DC. The voltage regulator will cut back the field as the voltage peaks to the set point at the alternator so the charging system will not be able to supply a steady current to the system.

If the lead acid battery is sufficiently discharged that the electrolyte is deficient in sulfate ions, the battery will have a high resistence to the charging current allowing the voltage pulse of the alternator to rise quickly without passing much current.

Now, if the heater fan is running the DC motor acts as a dynamic capacitor i.e. it speeds up as the voltage rises drawing current and slows down as the voltage drops. The DC motor draws power as the pulse appears and its momentum feeds power back into the supply system between pulses. So the DC motor running helps hold the charging system voltage constant. It would be almost like having a true generator, i.e. a constant voltage source, charging the battery.

Since the battery now sees a more constant voltage, it can take more current. This is reflected in the ammeter needle moving more toward the charge side.

Having the head lights ‘on’ will not help as that load would be resistive and just consume a portion of the alternator’s power.



In the technical world of electricity, there is no electrical term called “Juice”. There are volts, amperes (amps), watts, volt-amps, coulumbs, ampacity, watt-seconds and plenty more but not “Juice”. If you will, please translate the coloquial “Juice” for me and others of like minded curiosity into a conventional electrical term.

I suspect that “Juice” means adequate volt-amperes


From Researcher: Now, if the heater fan is running the DC motor acts as a dynamic capacitor i.e. it speeds up as the voltage rises drawing current and slows down as the voltage drops. The DC motor draws power as the pulse appears and its momentum feeds power back into the supply system between pulses. So the DC motor running helps hold the charging system voltage constant. It would be almost like having a true generator, i.e. a constant voltage source, charging the battery.

Sir: You seem to have a tremendous knowledge of electrical phenomona as it relates to motor vehicles. Can you say then if a motor vehicle alternator that produces DC voltage with some ripple, regulates at peak, average or RMS voltage?

I can check you on this as I have an RMS and a non RMS voltmeter. I do not have an oscilloscope, much less a calibrated oscilliscope to measure peak voltage but I could possibly do this with a capacitor and a diode. Waiting for your answer.


To Wha Who: The automotive alternator produces a three phase AC voltage. Current flow is rectified by a 6 diode bridge (sometimes 7 with the seventh at the tie point of the three phase Y). Also I have seen Zener diodes used in the grounded diodes but that is a subject for another time.

If the alternator were unloaded, the ripple would be similar to a rectified, unfiltered three phase AC line voltage. With the battery loading down the B+, the ripple is quite a lot less. It would be an interesting experiment to observe the voltage waveform on a dual trace, triggerable oscilliscope with the alternator unloaded, loaded resistively, loaded with a full charged battery, loaded with a severly discharged battery, and loaded with a severly discharged battery and free spinning DC motor.

The voltage regulator of the alternator is peak voltage reading. When the sensed voltage point exceeds the reference voltage (internal to the regulator), the power transistor supplying the voltage to the field is switched ‘OFF’. The field current continues to circulate through the bucking diode across the power transistor. When the sense voltage drops below the reference voltage, the power transistor switches back ‘ON’ and 12 volts is impressed across the field winding. So the field winding will see pulses of voltage supplying the power to the field coil to replace that lost due to resistive dissipation.

One point to consider regarding RMS meters is that most RMS meters assume that the wave form is sinusoidal. A lot of multimeters that are not RMS, rectify the AC voltage and are calibrated assuming that the peak reflects the peak of a sinusoidal wave from. Fluke etal. have multimeters which are true RMS i.e. the read the effective voltage compared to a DC voltage. One way to check that a meter is true RMS is to read the voltage of a square wave. If the voltage reads the same as the flat top peak of the square wave it is true RMS. Some of the more advanced digital scopes will calculate the true RMS value of a wave form.

Hope this gives you something to mull over. The next subject to discuss is how automotive alternators limit the maximum current without current sensing. Review the schematic of a single wire alternator and try to determine how overcurrent is folded back.