Higher Voltage: Fool Your Alternator?

Hi there,

It has been said that charging a car battery at a voltage that is higher than usual for a short time period will help the battery store charge better. This means a longer useful life as well as a longer allowable idle period before it needs to be recharged. I think the auto industry has recognized this fact over the years as newer alternators are said to put out a higher voltage than older ones. Some of the older ones may have only put out 13.8v but that's not enough.

Using a charger circuit to do this is no problem, simply raise the voltage level. But what if you want to use the alternator to do this instead? This would mean you would not have to remove the battery or have the car close to a 120vac or 230vac line to power the charger.

So have you tried this, and if so how did you modify your alternator or associated circuit in the automobile? We dont need a lot more voltage, maybe from 14.5 up to a max of about 16v, but i'd be more apt to limit it to 15v.

 

Are you sure you get 'longer useful life'?
This article suggests battery life would be shortened.

 

"It has been said" by whom?

 

First, let us make sure we understand how an automotive alternator works. Here is a block diagram of the typical automotive alternator charging system:



This shows an alternator with a built-in Voltage Regulator (VR). Basically, the voltage regulator senses the battery voltage on the S wire, and controls the current in the rotating field winding (Rotor) via F1 and F2 . The IG wire simply switches on/switches off the VR circuit so that it doesn't drain the battery when the vehicle is parked. The current produced by the alternator goes to the battery via the B wire.

When the alternator is spinning, the rotating magnetic field created by DC current flowing in the Rotor through the slip rings induces three-phase AC currents in the Stator windings (stationary part of the alternator) which are rectified to DC by the six-diode bridge inside the alternator. The important take away is that a small DC current in the rotor winding (F1 F2) makes a much larger DC current (with a bit of ripple on it) at B. Another important point is that the VR senses battery voltage and controls the average field (rotor) current.

The wiring is similar if the VR is in a separate box, mounted external to the alternator. In this case, F1 is sometimes connected to the B wire inside the alternator, and the VR controls F2 by sinking current, or F2 is grounded inside the alternator, and the VR controls F1 by sourcing current.

To demonstrate the key elements of how a charging system works, I made this simple LTSpice simulation. It has much simplified behavioral models of the VR, alternator, battery, and some static loads, but it behaves similarly to the real thing..



The alternator is modeled as a current-controlled current source with Iout = 30*Irotor . This is a simplistic, but quite accurate model of the 14V 60A Prestolite Alternator in my Cessna 182. It assumes that the Alternator is spun up at an rpm where it can produce full output (not at idle). The typical resistance of the Rotor of a 14V automotive alternator is about 8 Ohms. The inductance of the Rotor is quite large, measured at 1H.

If you bypass the regulator, and apply full battery voltage directly to the Rotor (between F1 and Ground), the Rotor current is ~2A, and if you spin up the alternator to ~5000rpm, the alternator puts out it's full rated near 60A. It acts very much like a current source. If you open-circuit the alternator output, the voltage will soar to over 150V (its compliance). In normal operation, it is the battery that holds down the alternator output voltage (and filters the ripple). Think of "Load-Dump".

The VR is modeled as a voltage-controlled switch S1 (which is what it is) with a bit of hysteresis. In this example, I set the trip voltage to 14.22V with 4mV of hysteresis. To make the switch turn off when the battery voltage is above 14.24V, I set Ron=10meg. When the battery voltage is below 14.20V, I close the switch by making R0ff=10mOhm. Note that this is a classic bang-bang house thermostat type of control system. The VR either applies full battery voltage to the Rotor, or zero volts to the Rotor. The average Rotor current is the result of the VR acting as a Pulse Width Modulator (PWM). You need to understand this if you are mucking about with the VR.

The battery is modeled as a huge capacitorC1 , with some internal resistance. The static load (represents the things in the car that draw current) is shown as R2.



Now look at the first plot pane, which shows what typically happens right after motor start. Cranking the starter pulls some charge out of the battery, making its initial voltage ~12V or lower. This is sensed by the VR, and it applies the battery voltage to the Rotor. As the engine comes up to fast idle or faster, the alternator cranks out nearly its full rated output of 57A, about 14A of which goes into R2, and the rest is available to charge the battery. The time scale on this sim is arbitrary for illustration only, but see V(bus) green trace does during the first 20 sec. The "battery" is being charged at a rate of ~57-14 = 43A.

At 20 sec, the battery V(Bus) green trace voltage exceeds the setpoint of 14.24V, and the VR begins doing its thing. Note the alternator output current I(B1) red trace decreases asymptotically approaching the 14A static load current. The battery voltage is maintained within a few tens of mV. The VR is now regulating the battery voltage. If more loads are turned on, the VR increases the average Rotor current to cause the battery (bus) voltage to stay constant.



Now look at the second plot pane. I plot the detail of the voltage at F1 (output of the VR) as V(f1) green trace at the 20 second point in the previous plot. I also show the current through the Rotor as I(L1) red trace. Note the low-pass filtering effect of the inductance of the Rotor, and the catch diode D1. Does this remind you of what happens in a Switch Mode Power Supply? Note that the VR makes PWM that is not constant frequency; it is whatever it has to be to make the average Rotor current be what it has to be to create enough current at the B output to just keep the battery voltage near the VR's set point. Remember that I(B1) is 30 times I(L1).

I have used a 'scope to look at the PWM at the field terminal in my airplane and various cars. Once the battery recovers after the initial charge up, the PWM rate varies between ~30Hz and ~150Hz. This is the ripple attributable to the PWM; nothing to do with the 3-phase, full-wave rectified ripple which I am ignoring in this simple simulation.

In the next posting, I will show the details of some real VRs.

 

Alec, if you are going to quote from Battery University, then at least pick the appropriate article...

and this other one...

 

I thought I had . Here's what it says:-
"The correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V per cell. Setting the voltage threshold is a compromise, and battery experts refer to this as 'dancing on the head of a needle. On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, an over-saturated condition causes grid corrosion on the positive plate and induces gassing."

 

Did you read the two articles I linked to?

 

Hello Alec and Carl (i'll reply to Mikes post in a separate reply next),

The thing is, this process is a little more complicated than simply applying some particular method every time we charge the battery, and i must say that my original post did not correctly indicate the time line involved here.

Normally when we charge a battery we apply some current and watch the voltage and limit it, and then wait for a while. Once the current drops to a certain level, we turn off the charger and start using the battery again. This is more or less how the automobile charges the battery too, after we start the car.

But what i am talking about is a secondary charge method, that works in addition to the 'normal' charging profile. This is where, every so often, we apply a much higher voltage than usual for a limited time period. This is not done for every charge cycle, just once in a while, depending on the battery condition i would think.

We have to be careful how we word this, because it is not that we do this kind of charging the same way we normally charge, just once in a while, and i can go into detail a little more with this in a minute.

I did not want to word my first post as if this was something that was tried and proven, but if you look at Mike's links, you'll see some very informative and true information. Also, i have seen this in action myself recently but never saw it worded out quite so well like that.

Ok, so what exactly does it take and how can this improve the battery life...

First, the battery is charged normally, but because the car may not charge it all the way up for various reasons, the battery starts to suffer voltage depression. This is where the voltage starts out at a reasonable level, but drops down faster than it should. This i believe is because the battery over time develops a higher series resistance, and that prevents complete charging. Each time the car is used, it gets worse and worse. If left alone for 2 days at a time after each use, we would see the battery voltage drop more and more until it was almost dead. If we are lucky, the resistance still allows at least some charge so we can start the next time we go to use it.
After this happens though, if we apply a higher voltage (something like 14.5 to 16v but adjusted as needed) the series resistance drops down to a lower value, and it can be quite significant. Now when we use the car once every three days, it charges up much better each time, until after a couple months or so it happens again and we have to do a higher charge again.
So it looks like once every two months or so.

If the alternator voltage could be turned up a little, it may be possible to eliminate this effect, but that would probably require measure the temperature of the battery internally (not ambient temperature) to get it right each time. It's not possible to do this with the average battery, so we go with the secondary charge.

How can i be so sure this works?
I have several months of data now that shows that this is what happens. I only had two test batteries, but the same thing happens with both. I had discarded one a couple years back but kept it around for a while and found that if i charged it with a home charger it would work again, to my surprise. The new one started getting flaky too, but once i charged it with the secondary method it started to work better as well, showing the same basic thing: the internal resistance goes down with the secondary charge and therefore it allows the normal charge to put more charge into the battery during each charge, even though it's not perfect. This second battery was much worse however so i had to bring it back for a replacement anyway, but the technique did show this idea to work with that one too even though it was seriously defective.
Alternately like i said, if we could charge the battery perfectly each time then we would probably not have to do this, and if we err on the high side (too long of a charge each time) then we ruin the battery in another physical way so we cant 'restore' it, so we err on the low side and then it requires a secondary charging technique.
I do measurements every day, about one to three times per day. I get the measurements from a sending unit installed in the car that sends the data to the house where i log it (voltage, temperature, time). If i look at the profile, i see the voltage dropping slowly over time (a couple months) and it is clear that after each use it is not being charged completely, at least not how much the battery is capable of taking.

 

Hello again Mike,

That's a very good post there and shows a lot of information. Very nice of you to share that with us. Some of those links confirms what i have been seeing now for the past year or so after making daily measurements on my automobile battery. Nice to finally see it worded out so well.

Now the question still on my mind is, how did you actually fool the alternator into putting out a higher voltage? I dont want to guess because this is something i want to do myself and i want it to work the first time without too many problems.

 

Interesting results MrAl.

Indeed I did. Looks like it's a fine balance between lengthening and shortening the battery life .

My concern there would be that any downstream voltage regulators in the vehicle's electronic systems would have to drop more volts and so cope with dissipating more power. Could that compromise the lifespan of the ECU for example?

 

W

No ECU in my boat, Jeep or airplanes. The avionics in the airplanes operate on any voltage from 10V to 35V (smps power supplies designed for either 14V or 28V aircraft). The Voltage Regulators in my newish GMC truck, and Toyota are already factory set to ~14.6V. We are talking about raising the battery/bus voltage from 14.6 to 15.5ish. I'm not worried about the car's electronics...

 

Mike,
Are you going to answer the question about how you fooled your alternator or not?

Alec:
The voltage difference i am talking about is maybe 14v to 14.5v, maybe a little higher, that's it. There should be no problem there.
Mike may be raising his higher.

 

MrAl; Have you looked at desulfation?

 

The answer depends your specific type of Voltage Regulator. Some are potted, some are pot adjustable, some you can open by drilling out rivets, some are inside the alternator, some are inside the ECU, some are in a separate box mounted on the firewall...

Basically, the VRs I have worked on and the ones I designed have used similar methods to implement the voltage-controlled switch I described in the previous post. They all use a voltage reference (like a Zener, or an IC voltage reference, or a TL431), a comparator (made from transistors, IC, Opamp), a power switch (NPN, PNP, Darlington, FET), a catch diode (Silicon rectifier) and a resistive divider to sample the bus voltage. Adjusting the output voltage involves tweaking either the upper or lower resistor in the divider...

In the case of my boat alternator, there is some history. The cabin cruiser is powered with a (Swedish) Volvo I/O with a Chev V8. The OEM alternator was originally a French POS made by Paris-Rohne (reputation sort of like Lucas). It had an internal potted VR that failed, so quit charging. I took the alternator apart, and everything but the VR was ok. I called the Volvo boat parts dealer and they could not sell me just the VR; I had to buy a whole new alternator to the tune of $750. I said B.S. and went an bought a external solid-state regulator that fit a 1970's Ford at NAPA for $15 and wired it to the existing alternator. It has been working for over twelve years...

The Ford regulator is the same as the one used in my Cessna, so I was familiar with it and I knew it would work with the French alternator. It was sealed with rivets, so is not "servicable". However, it was easy to drill out the rivets. I found a trim-pot inside. It was an easy matter to shunt the lower part of the trim-pot with a separate resistor in-series with an external switch. I empirically found the resistance value which would boost the battery voltage from a nominal 14.25V up to ~15.5V. The switch is mounted by the engine cover, so I can "equalize" either my starting battery or the "house" battery at will while cruising down the lake...

 

Hi,

Yes, but apparently i dont need that, or at least not a pulsing thing.

 

Hi Mike,

Oh yes now we're getting down to the long and the short of it

That's very interesting. So you are saying that you took apart your alternator and modified the voltage regulator inside. That makes sense. I thought you could just add an external resistor though, in series with the voltage regulator, but i dont know your voltage regulator very well.

I have a couple problems that you probably dont have with yours.
First, i dont think my alternator has a 'sense' input. I have a feeling (but not sure) that it is just and 'on/off' function, so that when the ignition turns on it tells the VR to go ahead and start pumping current to the winding. That terminal however i dont think has any ability to regulate the current, just allow it to flow, period.
Second, the other input is a 'bulb' output that runs the failure indicator bulb (standard small incandescent). I doubt that senses voltage too although i have no idea for sure.
Third, my alternator has a third set of three diodes:
first and second sets (6 diodes total) are high current diodes as usual, but the third set (3 more) is low current (1 amp maybe) that might provide the feedback to the VR. So i suspect those three diodes supply the feedback signal and that's what varies the total output of the alternator, but i cant be sure they might even supply the current to the winding (via slip rings).

Have any idea how this might be altered, and is there any change that can be done externally to the alternator?
Mine comes apart, but it's very cramped inside.
Also, my VR is totally potted and a new one is $60 or more USD. That means i probably can not get to the VR circuit itself.

Also, i find that the ground connection to the body of the alternator might be critical. Any idea how critical this really is?
I suspect that any drop in the ground circuit can lower the max charge voltage, as the regulator then sees a higher than real battery voltage and thinks it's charged already. Would be nice to hear if you've had any experience with this too.

Above all, thanks for your feedback here.

 

To trick the VR without taking it apart might be tough.

The extra "diode-trio" was used on the original Motorola Alternator Patent and on some of the one-wire alternators. The purpose of the three extra diodes is to make the alternator self-exciting, without using the battery as the source of the rotor current. Most of the newer designs have done away with the diode-trio and the idiot lamp. The newer regulators have a low-voltage alarm LED but it has nothing to do with the old Volvo burned-out lamp, my alternator won't bootstrap issue... In this design, only a few ma flows in the sense wire, so adding some resistance (few tens to hundreds of Ohms) to the sense wire might raise the output voltage. It is likely that the wire coming through the Ignition switch could be both a turn-on signal as well as the sense wire...

There are some VRs that use the same wire for both supplying the Rotor current (~2A) from the battery and for sensing the battery voltage. The one used in Piper aircraft (Chrysler alternator) is of this type. This can create oscillation or instability because of the voltage drop due to the large current flowing in the sense wire. With this type, you can also jack the voltage by adding some resistance in this common wire, but here it might be a fraction of an Ohm, and adding the resistance may make the system unstable (oscillate).

 

Hi Mike,

That's interesting. It's interesting that they would add three tiny diodes just for that when they basically perform the same function as the other set of three big diodes. Three of the six big diodes are wired to the three stationary coils exactly the same as the three tiny diodes, just that the tiny diode cathodes all go to the regulator and i think one brush, the 'positive' brush, and the three big diode cathodes go to the battery positive terminal.

I have seen another mod that requires adding an extra series diode between the three tiny diode cathodes and wherever the cathodes went. That is what gave me the idea that the VR might be sensing from the cathodes of the three tiny diodes, possibly in addition to driving the winding. But i have no idea if this works or not, and it would be hard to mount a diode into that packed alternator, so i was hoping for another solution.

So i guess the only thing i can do then is test it with some external resistance? That's a little hard to do too but i think i can swing it if i make up an adapter like plug connector for the terminal that needs the connection.

What do you think about the diode solution, think it works or no?

 

I have seen an alternator that is as you describe, where the extra diode-trio is used as a source of the Rotor current (wired directly to F1), AND is used as the sense input to the electronic Voltage Regulator. The big B terminal comes from the common-cathode of the three-phase rectifier is wired to the battery. This means that the VR is sensing not the actual battery voltage, but a faux voltage which is supposed to represent the battery voltage, but isn't. You would have to break into the connection between the diode trio cathodes and where it goes into the VR to add resistance or a diode-drop to raise the battery voltage...

This system is what I call the Motorola system, which was only used for a few years when US cars first came out with alternators in the 1960s. This was copied by the Russians, and a lot of cold-war era Yugos, et al are running around with this system. What are you driving?

BTW, your alternator may not bootstrap if that trouble lamp in the dash ever burns out. To light that lamp, current flows from the battery, through the lamp backwards into the alternator. That small current provides the initial rotor current to get the alternator going right after engine start. If the bulb burns out, there may not be sufficient residual magnetism in the rotor for the alternator to bootstrap.

 

Hi Mike,

This is in a Hyundai automobile. The funny thing is, the last alternator i had i could have sworn was measuring 14.2 or 14.5, something like that, but this (new) one measures only 14.0 at the battery even without running any lights or anything else.
I wasnt taking readings so often back then so i dont have any written data to compare.