DIY auto alternator charge controller advice

[madlabs]

Hi all,

I'm working on an automotive alternator charge controller. The one I use for myself is built in to my DIY power system controller, but I have a couple of neighbors that want a stand alone version and I am working on a golf cart to ranch utility cart conversion that will have one as well. I am going to go all DIP parts and have a PCB layout that can be done easily by DIY'ers. The object is to keep it as simple and cheap as possible. Once it's done this will be an open source project.

Specs so far:

PIC16F88 for brains. There are cheaper and maybe I'll switch this later.
Allegro 100 amp current sensor. Might use an Amploc instead.
Programming in Pic Basic Pro. .hex and .bas files will be available.

Features:
Will be a two stage charger. Float charging with a gas motor is nuts!
User will be able to set max charge current, min charge current, charge voltage, and compensate for voltage drop between the alt and the battery bank.
Optional serial LCD output. Might do 4 bit interface instead if I have the I/O.
Optional automatic shut off relay.
Optional temp. compensation.

So, I am fiddling about with this project at the bread board stage and have a few questions.

1) Typical field coil currents. Looking around, what I can see is that most seem to draw less than 5 amps. Is that true? Both mine draw less than that, more like 3 amps at max current.

2) Right now I am using a couple of transistors to PWM the field coil. I am switching the high side for compatibility with more automotive alts. I could switch to FET's which would increase efficiency but also increase cost and make board layout more critical. I am kinda leaning towards sticking with transistors, you can get 'em from Rat Shack on a Sunday. I hate the big 10W base resisitor burning up juice but in the big scheme of things it's a pretty small thing.

3) Best PWM frequency?

4) Any other ideas/comments/observations?

Thanks for the input!

Jonathan

[kevbo]

The field has fairly high inductance, so your PWM frequency does't need to be high at all.  A few KHz, or even less will do...heck the old mechanical voltage regulators did it at 10 Hz or less..not quite the same as true PWM, but gives the idea.

Thoughts: Avoid darlington transistors, slow and high forward drop.  If you limit yourself to 90-95% max duty cycle, you can probably use a bootstrapped base/gate drive supply and use N channel or NPN transistor for high side drive.  These are significantly more efficient than the complementary devices.   Make sure you use a Schottky or very fast Si freewheel diode.  A slow diode will stress itself and the switching transistor.  Alternatively, you can use some of the raw AC off the stater to run a voltage multiplier and use that as a rail to drive the switches.

Layout:  This takes experience.  Here are the rules: Pay attention to the entire path for the field current, including the freewheel, return, and through the bypass capacitor. Make sure none of your sensitive electronics is connected so as to be sensitive to either DC drops, or transients on that path.  This path will be 2-3 closed loops.  Make sure your sensitive stuff is not inside any of those loops, or you will have magnetic coupling.  You will still have some magnetic coupling, so make those high current loops have the smallest possible area...including the return from the alternator.  If the field is grounded then run all grounds as a star with the alternator case as the center of the star.  Run the field drive line in a twisted pair (or use coax) with the field return line.  It is often worth slowing down the switching _a_little_ , and taking inefficiency hit in order to soften the switching transients.

Ways to reduce drive power:

Set your base/gate resistor(s) for reasonable steady state drive, then parallel a capacitor for faster switching. 

The Maxim gate driver ICs are awesome...just make sure to follow the PS bypass cap recommendations.  You can screw around a long time with discreet drive circuits and these chips will still work better and probably cheaper.

If you are willing to take the loss of a very simple switch drive, then recognize that linear drive of the field works very well.  You can have your uC adjust the reference voltage to get the charge voltage you want.


If you run at more-or-less constant speed, you can tune your control loops for good performance.  Loop gain will vary with speed, so if you have to run over a wide speed range, you will have to compromise the loop response in order to make it stable at all speeds.

[David HK]

Jonathan,

Please take a look at this URL PDF because it may be of some use to you.

http://homepower.com/files/webextras/mark8.pdf

The circuit may save you the need to work with programmable logic.

David in Hong Kong

[Treehouse]

Not sure you have seen this but xantrex makes something like this. xxx.xantrex.com/power-products/power-accessories/alternator-regulator
they are a little pricey.

[dinges]

Please take a look at this URL PDF because it may be of some use to you.

http://homepower.com/files/webextras/mark8.pdf

A few years ago I had built a batterycharger with an  old (but good) Briggs & Stratton 3.5 hp gasoline engine and a 55A car alternator, of which the controller was removed and for which I built that Mark-8 controller. I wouldn't recommend building that controller.

In my case, it was impossible to properly adjust the engine-alternator combination to a stable charging current; the engine would rev up and down at about .5 Hz, as the engine governor and Mark8  were both controlling the system. Or trying to.....

It could possibly be solved by adding some delay, filtering, or even a real PID-controller, but I gave up on that circuit. Hm. Have still got the thing laying in the attic. Was built in a nice cabinet and all..... I also recall it getting pretting warm, despite plenty of heatsink.

If I were to do it again, I think I'd go the lowtech route, with a rheostat or a bank of resistors that could manually be switched in or out of the alternator's field winding. But that may not be the right solution for your (possibly very a-technical) neighbours.

Edit: just had a look for it and found it. Think I'll be taking it apart again to recover the components and cabinet.





And to answer one of your questions: the maximum field-current in my particular alternator (a Delco Remy 55A version) was 4 A. Can't recommend a suitable PWM frequency, but I figure 1-2 kHz would be fine. Don't think I'd go much higher. Like I said before, if I were to do it again I'd simply use resistors to control the field. Easy, robust (near-impossible to destruct), easy to repair. I wouldn't worry too much about the 10W or so lost in te resistor, as the total system efficiency (gasoline -> mechanical energy -> electrical energy -> chemical energy (battery) ) is lousy anyway. A few watts extra won't make much difference, but will greatly simplify the setup. When running it would need an occasional check by the operator to see if process parameters are within limit, but I wouldn't run such a generator unattended anyway.

Come to think of it, if I were to do it again, I'd use a very simple voltage comparator to check whether a set battery voltage was reached, and then use a big clunky relay to shut everything off or sound a loud bell to alert the process operator to manually switch it off. 'Sophisticaasjion' definitely isn't my middle name....

[richard]

Try a 6 to 8  H.P. B&S engime.    richard

[David HK]

Dinges,

In your comments you mention "that Mark-8 controller". Do you mean it was a controller built as per the PDF circuit mentioned in my Post #2 above?

Where does the term "Mark 8" come from?

Regards,

David in HK

[dinges]

In your comments you mention "that Mark-8 controller". Do you mean it was a controller built as per the PDF circuit mentioned in my Post #2 above?

Yes, it was.

Where does the term "Mark 8" come from?

No idea, the designer/publisher of that circuit is probably in a better position to answer that question than I am.

[David HK]

Dinges,

Thanks. I see it now. The builder callled it Version 8.3, whereas your referred to it as Mark 8.

It was easy to misunderstand.

David in HK

[madlabs]

Hi Everybody!

Been away for a few days, so just catching up on this thread. Thanks for all the replies!

@kevbo - I think I'm going to accept the loss on Darligintons. Less parts, layout not so critical, cheap and easy to get, wide range of substitutes that will work. Slow should be ok, I can use a fairly low frequency, no? It's not meant to be an "end all be all" but cheap and easy to DIY while working fairly well.  Got the fast freewheel diodes. I'll look into the parallel cap for faster switching. Assuming a base resistor of 1.8k on a 1000 HFE darlignton, being driven with 5V from a uC pin, do you have a reccomended cap value? As to layout, when I get it down in Eagle I'll post it, and if you have the time, perhaps you could look at it.

@ David HK and dinges - I did build that circuit and wasn't happy either. Besides, with the uC it is easy to add temp. compensation, voltage drop, current read out etc. As well as the auto-shutdown, a feature I LOVE with my present controller.

@treehouse - Yeah, there are some commercial products out there. But I want a simple, cheap, effective DIY alternative that I can make a few of. I like the one I use now, but it is built into my general power controller (that does my solar, windmill, monitors water and power usage etc) so I am building a stand alone version.


@richard - The controller will be used with motors from 5 to 10 HP.

Thanks!

Jonathan

[madlabs]

An update on this project:

I'm out in the shop watching the new controller circuit run my generator! It's running well so far. Right now I'm cranking 140 amps into the bank and am approaching 14.4V, so I'll get to see it ramp down the current soon. Current and voltage sensors working smoothly so far. The circuit is still on the breadboard and lacks a few things, like zener protection for the ADC input connected to the battery bank, but I can really get to work on the board layout. Lots of code stuff left to do, but I'm going to focus on the PCB and worry about the software side later.

Thanks for all the help!

Jonathan

[GreatBallofFire]

An update on this project:

I'm out in the shop watching the new controller circuit run my generator! It's running well so far. Right now I'm cranking 140 amps into the bank and am approaching 14.4V, so I'll get to see it ramp down the current soon. Current and voltage sensors working smoothly so far. The circuit is still on the breadboard and lacks a few things, like zener protection for the ADC input connected to the battery bank, but I can really get to work on the board layout. Lots of code stuff left to do, but I'm going to focus on the PCB and worry about the software side later.

Thanks for all the help!

Jonathan

Did you end up using the PIC16F88?

-Mark

[madlabs]

Yes I did. This project got a little sidetracked but as soon as I finish the nixie clock that took its place I'll be back on this project. The board design is almost done and the circuit is working well on the breadboard.

Jonathan

[Bruce S]

Madlabs;
Forgive my tangent.
Nixie clock? nice! I can remember the old NEC computers using these for displays.
How are you driving the clock? direct HVdc ?
Bruce S

[madlabs]

I'm using a 200VDC power supply that many Nixie geeks use, a simple switcher. To actually drive the Nixies I am using the venerable 74141 which was made for driving these tubes. I've been saving these tubes for years, pulling them out of dead equipment and so on. At my old on grid place I used them for everything - displaying windspeed, temp, humidity, hot tub temp, kitchen timer, anything I could think of. Now that I live off grid I've limited myself to two clocks so far...

Jonathan

[bob g]

i realize this is late in the day for your project, but something i have been kicking around that might be interesting to you or others.

years ago, delco made an alternator for heavy trucks, it was very popular the number was 1100080,

it used an internal regulator that looks just like the one they used in just about everything they built for cars and trucks for years and years, however the regulator had one difference.

the two spade terminals that came out the side of the case, such as most others used to power an idiot light and such, had a square rubber resistor block and a red plastic cap that you could unplug and push it back in 4 different orientations...

inside the rubber block was a resistor network, wherein the red cap was a jumper that connected each level of resistance in steps to adjust the output voltage.

now, what i think would be cool is using the regulator on any other alternator one likes, replace the rubber resistor network with one of those electronic stepped resistors and use a microcontroller to step the resistance across those two spade terminals and in doing so one could then make the alternator do any charging scheme one could think up code for.

iirc the regulator can deliver 5-6amps of field current, and are relatively cheap at less than 20 bucks, the regulator has all the driver circuitry all ready built in, pwm control of the field and all that, leaving
one to only have to connect the stepped controlled resistor and a programmed micro to finish the job.

one could add a couple of temp sensors to the micro so that you could program for temp compensation, and protection of both the alternator and the battery bank easily enough.

there would be full isolation of the micro so if something fails it would be a simple swap of an off the shelf part and your back in business.

fwiw
bob g

[madlabs]

Hey Bob,

Yeah, it's too late, the controller I have is already working well. BTW, I've been using the Leece-Neville 555 alt that you recommended a couple of years ago on your forums. That thing is a beast! With some extra cooling I can pull a little over the max rating of that thing for an hours if I want.

Jonathan

[bob g]

Jonathan

good  to hear you are liking the 555 alternator, yes it is a beast.

as far as i am concerned, it is the best design, of anything on the market over the last 30-40 years, and one of the best bang for the buck going.

are you using it for a 12 volt application?

i have used one in a 24volt application, wherein it will do 100amps at 28.8volts all day long, and will do 130amps or more if i were to turn it the right direction (it has a directional fan as you know, and my application has it backwards so i limit the output)

not many alternators one can buy for ~165 bucks new, that will make 3kwatts  output into a 24volt bank, and do it all day long.

good to hear your regulator/controller is doing the job!

bob g

[madlabs]

Bob,

Yes, I am using it on a 12V system. Mine is rated for 160 amps and I often pull 165 out of it for extended periods if I'm welding or something. I do have extra good cooling though. The alt is attached to a 4.5kW Honda generator that had a blown gen head. I mounted the alt direct drive style, inside the old gen head housing. That way the fan for the old gen head pulls air past the alt too and of course the alt still has its own fan.

The other nice thing, aside from beaing tough, is that the field wires are super easy to get to and the stock regulator easy to remove.

Jonathan