Forklift 100 amp charger- regulating output

[madlabs]

Hi All,

I scrounged up an old forklift charger. It works, but the regulation isn't quite what I want. It floats up to 15.1V when it starts ramping down to about 30 amps. It doesn't have any regulator circuitry, just transformer, diodes and caps. How the heck does this thing regulate? I'm guessing it has to do with the transformer core, but can anyone shed a little light on how it works?

I was wondering if I could use SSRs to regulate the output? I use a scheme like this on my buddys hydronic floor heating. He has a backup electric flash heater, and what we do is PWM a SSR that is connected to the second element in the heater. Now, I know that with an inductive load, I may generate some spikes. These are zero crossing SSRs though, so would I really make any more noise? If the SSR route will work, should I PWM both legs (I'm running it on 220V from a generator) or just one?

I'd rather not control the DC side, 100 amps is a lot of juice to switch.

Thanks!
Jonathan

[joestue]

How does it regulate? it doesn't. that 15.1 volts is your nominal line voltage with a little bit of core saturation.  take the input up to 130 and see if you get 16.3 on the output, i doubt it, but you might.

anyway, it would be a lot easier just to wind 10-15% more turns on the primary (if you have room) and use 2 or 3 ssr's to switch between two or three taps.
you can't just pwm a transformer and rectifier load by feeding pwm into a zero crossing SSR, unless your pwm signal is locked to the 120hz line, and then you have a  http://en.wikipedia.org/wiki/Phase_fired_controllers . 
but if you think that thing is efficient now... just wait until you double the copper losses with a phase modulated ssr.

[ghurd]

Best do it with a regular old relay in some kind of bang-bang set up.

SSRs 'leak'.  Large transformers will buzz or hum when they should be off.  It can be impressively loud and irritating.

Transformers sort of self regulate like a solar panel self regulates.
If the voltage is higher, the amps are lower.  Somewhere, hopefully around the battery's higher voltage range, the amps output are low enough to keep the battery from going very far over voltage.  
If the battery bank is large enough it can work out OK.  It sounds like your battery may be a lower capacity than the battery it was designed to work with.  (Stray thought-  It could be your battery cables are heavier, and the charger was expecting more wire losses?)

Leaving them unattended or unmonitored for very long WILL cook the battery, often even if the thing claims to be good for that type of use.
My old 'automatic' charger cooked more than its share of batteries.
G-

[madlabs]

Thanks for the replies.

Joe, you are right that effeciency-wise it just ain't worth it. Especially as I am driving it with a 5kW genny.

I guess I should have seen that it would self regulate, like a solar panel, but the transformer is so big I was sure it would be bigger than my battery bank but I guess not. Ghurd, I don't know what it is rated for, the data plate is unreadable, I think it got run into by the forklift it was charging.

I think what I'll do is make a quick controller that just shuts it off when the volts go above 14.4V and just use it for bulk charging. That will be simple, easy and cheap.

Jonathan