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

[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

[ghurd]

I am confused about the application.

Stand alone, 2-stage control, on a modified golf cart.
And "Float charging with a gas motor is nuts!"

Is the regulator just for the starting battery?
G-

[madlabs]

Hi All,

Was away for a 48 hour shift.

Ghurd, it's a classic lawnmower/alternator battery charger. In my case a 10 hp Honda 5kW genset with the AC genhead removed (it was blown so I got the unit for free) and replaced with a truck alt, the LN555 mentioned earlier. My charge controller is actually part of my DIY power controller that tracks watts in and out, monitors the well and the windmill and so on.

What I am working on now is a stand alone version, meaning that I'll have the controller mounted on and dedicated to the alternator charger instead of being part of my whole system. The golf cart project will have a 5 hp Honda with a 60 amp alt, for charging the cart if there isn't enough sun and so on.

As to the high/low side switching dillema, it's easy enough to make the board accomadate either, just populate the right parts in the right places. Oz, of course I should use a Darlington tpye like the bd647 or it's PNP counterpart. Duh. Just the sort of thing I don't think of because I don't really know what I'm doing (and I was playing with what I had in the parts bin). That gets rid of the big power wasting resistor.

Cornelius, should be easy beans to convert to a picaxe. Those have a hardware PWM, right? I'm using the F88 for the simple reason that I have a few and they have lots of code space. I have it all working on the bench using some 12V light bulbs for dummy loads. Probably wont get a chance to try it out proper for a couple of weeks, have some other things that need to get done. Do you have a rig that you can test it on?

Thanks for the help!!

Jonathan

[ghurd]

OK, lets think about this a second.

10HP 5KW type gen motor.
And it is running to charge a golf cart battery bank?

Somewhere, I missed something.
G-

[madlabs]

The 10 hp is charging a 1200aH battery bank. It's still too much motor for the 160 amp alt, I have a bigger alt to put on it and am going to modify the govenor to match the load. It's charge controller is built into my DIY power controller.

However, that is what I already have. What I am building now is a controller for a couple of other projects. The golf cart will have a 5 hp engine and a 60 amp alt. It will have 240 aH bank. A couple of friends want similar chargers. So my plan is to make a charger with the specs listed above and make some PCBs so I can make them all at once, as well as for use in future projects.

Jonathan

[ghurd]

NOW I get it.   
G-

[Cornelius]

Cornelius, should be easy beans to convert to a picaxe. Those have a hardware PWM, right? I'm using the F88 for the simple reason that I have a few and they have lots of code space. I have it all working on the bench using some 12V light bulbs for dummy loads. Probably wont get a chance to try it out proper for a couple of weeks, have some other things that need to get done. Do you have a rig that you can test it on?

I was thinking of using the newer PicAxe 08M2 with 8kb mem and 32MHz. It has only 5 in/out total, but those pins can be configured to suit my needs. (2PWM, 3ADC, 1DAC, 4 in, 4 out, SerIn/SerOut, etc... (Not all at once ofcourse. ))
 
I have the perfect candidate as a test-rig:

 
It's a 5HP B&S, and a 12V, 70A Bosch Alt. on a lousy stand, and i've already ripped of the internal regulator and mounted it on that 'instrument-panel', together with a manual rheostat and a switch between Manual/Auto.

[madlabs]

Corenlius,

You might want to consider a device with more pins. So far my pin count looks like this:


ADC - 3 channels, volts, current, temp.
PWM - 1
LCD  - 1-4
PWR - 1, one pin for the power relay
LED  - 1, status indicator
IN    - 3, inputs to adjust settings
OFF - 1 Engine shutdown relay

As you can see I need a minimum of 11 I/O pins. I'd like to use a 4 bit LCD instead of a serial because they are cheaper and plentiful. If I do, then I'll have to use a different PIC with more pins, as the F88 only has 13 available pins.

Jonathan

[Cornelius]

Well; i do have a 20-pin PicAxe (20M2) laying here, waiting for a project.

But it could be fun to start with a simple version with the 08M2:

ADC - 2ch.: volts, current
PWM - 1
OFF - 1 Engine shutdown relay
TEMP - Switch: Summer/Winter Temp comp.: add 0.3V to 1.0V in Winter mode (depending on avg. winter temp.).

I have to admit that i'm at a beginners level regarding pic's; though, i've managed to get a PWM dump load controller with mosfets working rather nicely with the 08M2...

[madlabs]

Cornelius, I wonder if you could dual purpose the temp. switch I/O to also run a serial LCD. Have a weak pulldown to ground, say 100k. Then have a MOM switch connected to VDD. On startup, make the pin an input and see if it is hi or low. If hi (you held the switch down during startup) it sets it to summer mode, otherwise winter. Then make the pin an output and use it to run the LCD during operation. I'd want the LCD to take full advantage of the current sensor and see what the output is. Heck, you could dual purpose the PWM pin the same way if you wanted to for more startup options or temp. settings.

Got parts ordered and worked on some of the housekeeping code, like storing user settings in eeprom and loading them on startup. Started drawing out the full schematic and starting to figger out how to put it in Eagle. Buddy of mine has a CNC router that I'll use to make the test boards.

Can you program the picaxe in circuit? That seems like a must.

[Cornelius]

Seems that my memory of the 08M2 spec wasn't entirely precise... 2k mem (not 8k), and 6 in/out. Here's the layout:
http://www.picaxe.com/Site_Resources/Media/Site_1/pinout/pinout08m2.jpg (big pic.)
 
The pin (pin4/C.3) i intend to use as temp switch are a digital in only, so its use are limited. I agree it would be nice to have the display, but to manage that with this small pic, i'd have to drop the engine shutoff relay. That would be fine by me, since my use for this charger would be emergency charging, and that means closely monitoring. (Especially with a display...)
 
All the PicAxe can be programmed in circuit; which also means that monitoring/logging directly to a pc are easy.