Building High Amperage Chargers from dead UPSs.

[srnoth]

Hey there,

I recently blew the MOSFETS on two identical UPSs by running them for too long (even with cooling fans, they over heated and gave me some Magic Blue Smoke). They were very cheap, very poor UPSs, and I am almost glad to get rid of them, since they obviously could not handle extended use connected to my battery bank.

After reading the article on the battery charger built from an old inverter, I started thinking. Would it be possible to wire the two transformers in parallel to the 120VAC power, which would then convert to ~12VAC. Then put this 12VAC into the AC side of the rectifier (i think that is what it is) of a car alternator, which would convert the 12VAC to 12VDC, which I could use to charge some batteries?

These are the figures:

2. x 250W (500VA) UPS transformers, which if wired in parallel, should be capable of converting 500W @ 120VAC to 500W @ 12VAC. This should mean it would output 500/12 = about 40 amps.
   
1. x 65amp Alternator Rectifier, which converts 12VAC to 12VDC, at about 40 amps.
   
   

Thus charging my batteries with 40 amps of power.

The reason for this is that I want to be able to run 2 x 400W inverters from AC power, in effect creating an inline UPS:

120VAC          ---->          12VAC          ---->          12VDC          ---->          120VAC

(wall)    (UPS tranformers)                 (rectifier)                       (inverter)

I currently see several possible problems with this design:

*Old UPS transformers may overheat with constant use, since only built to run for about 5 mins a couple times a year.

*Can the two tranformers be wired in parallel, ie with all the phase stuff in AC, or will this cause trouble (wrong voltage or something). Will these transformers even work 'backwards'?

*Will the rectifier decrease or increase the voltage? It needs to be atleast 13VDC to charge the battery bank.

*Will this whole system be able to run continuously, powering about 200W continuos AC from the inverters, which means the charger needs to be able to put out 400W @ 12V, and a bit more to charge the batteries, or keep them on float?

*All this calculations are without allowing for efficiency. Will this cause a problem?

And lastly, the transformers have 3 wires coming out at each end. I know that the middle one on the 12V side conects to the 12v +ve terminal. How am I supposed to connect these transfomer to the 120VAC and the alternator rectifier?

Thanks for all your help,

Stephen.

P.S. I know that there is an article on this under 'experiments', but it does not go into detail about these aspects.

[K3CZ]

The idea is sound and practical, especially if the two UPS's are indeed identical.  Only the transformers themselves are necessary, plus the alternator rectifiers(still mounted on the alternator heatsink).  The 3 leads on the low voltage side of the transformers are a center-tapped secondary, probably 16-0-16 volts for full wave rectification.  The AC (and the resulting DC voltage after rectification)voltages are already optimized for 6-cell charging.  Of course, there will be no automatic charge rate control with this simplified setup.  If the transformers are the same, they may be connected in parallel on both primary and secondary sides, and use one set of rectifiers. (observe AC polarity when paralleling secondarys to prevent fireworks.)  Otherwise, connect the units together only after the rectifiers (DC).

This setup should work FB, but watch the transformer temperatures; as you have noted, these units were not meant for continuous service.

                                             K3CZ,  PE

[finnsawyer]

Any imbalance in the transformers will cause a problem as the one putting out a higher voltage will treat the other one as essentially a short for the voltage difference resulting in it putting out more (unusable) power.  I think you're better off rectifying each individually and dealing with any voltage difference by using two series resistors to make sure each provides exactly its share of the power.