Renewable Energy Questions/Discussion > Automation, Controls, Inverters, MPPT, etc

DIY auto alternator charge controller advice

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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

oztules:
1. yes usually less than 5A

2. The ones I build to replace the originals in auto alts use a BD647 to switch the low side. In all alts I have worked with, the small diodes that feed the control and brush drive,go directly to the b+ side of the brushes, as they/me switch the brushes to ground.The darlington tranny drives like a fet (hi impedance).


............oztules

madlabs:
Oz,

What alts have you worked with? I haven't played with too many, the Delco 7122 and the one I use now, a Leece-Neville 555. The Delco I believe is high side switching only and the 555 could go either way. Great heavy duty alt by the way, the thing is a beastn and it's easy to remove the stock regulator.

The few DIY charger schematics I have seen have been high side switched. Are you saying that most alts you have encountered need to be low side switched? I want to make this thing work with as many alts as possible.

As you can tell, I'm no EE. In fact, I'm always amazed that my things usually work OK.

Thanks!

Jonathan

Cornelius:
Without any idea, i'm guessing it depends on the make of the alt, and where in the world those alts are common...
Being an old Ford fan (Fords in Europe in the '70 and '80's...), they used Bosch alts with regulators on the negative side...

... But what do i know?! :)

Very interesting project! I might replicate your findings, but with Picaxe's instead. ;)

oztules:
I think Cornelius may have hit the problem on the head,

All the alts are from Australia. Mostly bosh and lucas over here on the older cars. .... The newer ones don't seem to have needed my attention at this stage.... maybe 10 over the last few years.

I have only needed to do the top side switching on some old caterpillar generators (for the fields) and a Fiat tractor.

Edit:
I have thought further on this, and find that thinking about the idiot light on the dashboard and how it works... makes sense for neg switching.

When you turn the key, +12v is pushed through a 5w panel light on the dash, and provides 12v less it's resistances to the pos brush.... which is wired straight to the three diodes from the stator (the little 3A ones). All the cathodes are joined at the pos brush, as is the power now delivered by the panel lamp.

As soon as we put the series lamp power to this point, the circuit wakes up, and being less then 14.2v, the neg brush is grounded by the transistor, (bd647).... now the lamp glows, and the rotor sees about 5w of dc through it.

When we start the engine, this small current is used as the magnetising current and speeds up the output from the stator to get going quickly (it will may work with just remnance, but takes lots of revs and sometimes dose not).

As the emf builds in the stator, it delivers +12v from the three phase windings to the pos brush via the three cathodes.... now the panel light has +12v from the key on one side of it, and +12v from the stator on the other..... so it goes out..... the alternator is working normally.

The stator continues to drive the three small diodes to provide the "switched" magnetising current to the rotor. It no longer needs the excitation from the key switch.

The panel lamp is still left in circuit, so if the voltage from the stator drops to well less than 12v, it may glow weakly on very low idle rpm and heavy load.

Thats why/how they work over here (all I have seen anyway).


...............oztules

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