Electronics gurus: Will an AC->DC treadmill motor controller work as battery charger?

[MetroMPG]

Background...

I picked up a "broken" electric treadmill from freecycle out of curiousity. "It wasn't working properly", they said, "running too slowly/weakly." Mainly I got it for the DC permanent magnet motor inside. Always nice to have little motor/generators for projects...

Well, it looks like what was wrong was one of the motor brushes was sticking in its gummed up "sleeve". It was arcing up a fireworks show because it wasn't making good contact with the commutator. So... fixed! Comm doesn't look horrible. Seems to run nicely again with that brush making good contact.

The thing takes household AC, and the on-board motor controller converts it to 0-80v DC to run the .4 hp rated motor, adjusted via a physical potentiometer/slider.

Anyway, to my question:

Can I use that motor controller as an adjustable voltage battery charger? I suppose I'd have to stick a fat diode between the controller and the battery to keep the current on a 1-way street?

If not, I guess I'll just keep it around as a variable voltage bench power supply. I'm sure I'll find reasons to play with it.

[saand]

metrompg,
it might do what you want but there may be a few things you want to check or understand first.
the 0 - 80v may not be a steady voltage at 40v it may be a pulsed 80v at half duty cycle, this is a common method of achieving variable voltage, the best way to check this is with a oscilloscope
battery chargers typically have multiple stages or modes of charging which this power supply wont have. For example a normal battery charger would limit current if your battery charge is very low because very quick charging will heat up the battery and power supply and may cause damage. The power supply will inherently limit current somehow (because you cant get infinite current out of something) but the natural current limiting may be far higher than what you want and it may blow a internal fuse before it gets to the natural current limiting level.
regarding the diode, it will have to be a really really big one, batteries can take up to hundreds of amps (not sure what the power supply can supply) so you will need to size your diode appropriately. Note: you may not even need a diode.

hope that helps, good luck

[MetroMPG]

Thanks for the reply, saand!

Originally Posted by saand

the 0 - 80v may not be a steady voltage at 40v it may be a pulsed 80v at half duty cycle

Would a battery really care about this, assuming its charging rate & voltage was adjusted to keep the battery within specs?

battery chargers typically have multiple stages or modes of charging which this power supply wont have.

Understood. The plan wasn't to use this for unattended, automated charging. My thought was simply that I could slowly dial up the voltage to recharge a small battery. Not planning to try recharging the ForkenSwift pack with it

regarding the diode, it will have to be a really really big one, batteries can take up to hundreds of amps (not sure what the power supply can supply)

It's rated at "3 Amp treadmill rating / 8 Amp breaker". I assume that's the AC side.

Note: you may not even need a diode.

Any easy, non-destructive way to determine this?

Thanks for the feedback!

[saand]

yup assuming the voltage and charging rate the pulsed nature of charging is not an issue, at worst it may reduce the battery life slightly but then again i think "battery desulfaters" work by pulsing at high frequency so maybe it will be good for the battery.

regarding your diode current, now that i think about it more, if you have 110 or 240v mains supply then the most you will get at 80v will be 3 times the current rating of the circuit breaker or fuse on the mains side. So a 30A diode should work perfectly for you, they aren't that expensive. If you routinely plan to charge at voltages down at 12v that current rating may need to be somewhat higher. Also take note of the power rating of the diode, if your dropping 0.7v at 30A that is 21W which will need to get out of the diode somehow otherwise it will heat up and stop working.

unfortunately no there isn't an easy non destructive way to tell if you need a diode or not, if i was in your position i would chance it and try it out with the diode or just buy a cheap 30A diode and not worry about it.

The worst that will happen is you will blow up your power supply so you will be no worse off.

In a pinch a while ago when my car battery had discharged i charged it up with a laptop power supply so if you are after a battery charger that will only need to charge a 12v battery a laptop power supply or even rip out a desktop power supply and you might get what you want as well.

[MetroMPG]

Excellent info! (I love this forum ) Thanks again.

[Nevyn]

Where did you find a .4 hp? The two treadmills I have had 1.25 and 2.0 HP permanent motors! The problem is, the controllers are SCR, not PWM - so you have to have AC for them to work; you can't use an SCR controller with a battery pack unless you invert to make AC, and then let the SCR convert it to DC.....which is stupid. So I'm not doing that.

If your project is staying near AC, then go for it. Unless you got lucky and got a PWM controller in your treadmill.

[MetroMPG]

Yup, it's an SCR based controller.

But I'm not looking to use the controller as a motor controller . I was looking to use it as a bench variable power supply / small battery charger only.

The motor, on the other hand... I can think of a few little projects I might use it for as either a motor or generator.

I'm pretty sure the motor tag said .4 hp. I'll double check and get back to you.

The treadmill is a "Weslo cadence 815" WCTL8159

EDIT: another cool thing is there was a good wiring schematic taped inside the cover over the motor/controller/guts area.

[MetroMPG]

Yup, it's rated 0.4 hp

[saand]

Pleasure to help, after reading many of your useful posts its good to return the favour

Just note that as its SCR based your likely to get the peak of the mains voltage on your batteries so if your on 110v AC that is 156v. If you had just measured the 80v with a multimeter in DC mode it may be off for 1/3 of the time and rectified mains for the rest of the time so it may have gone up to 156v. The battery may smooth this out for you but it may cause you some issues.