Can a Variac with bridge rectifier be used as a charger?

I am wondering if a Variac like these ones: http://variac.com/staco_3PN10_20.htm could be connected to a battery through a bridge rectifier so that the Variac serves as a CV charger.

With the addition of some diodes it might be possible to have safe connections to the battery (which only allow charging but not discharging) and could be accessible outside of the battery safety container.

I'll try to make a diagram later on...is there anything fundamentally wrong with this concept?

It should work, and a variac is reasonably efficient at trading lower line voltage for higher current. Be very aware that it is not isolated from the power line and you could get a shock or worse by touching a battery terminal. An isolation transformer before the variac is best, but that adds to weight and expense.

dak664 said:

It should work, and a variac is reasonably efficient at trading lower line voltage for higher current. Be very aware that it is not isolated from the power line and you could get a shock or worse by touching a battery terminal. An isolation transformer before the variac is best, but that adds to weight and expense.

Click to expand...

Thank you, dak664!
So I take it that a Variac is not an isolation transformer.

But I don't think it matters for the applications I have in mind.

Application one is the Vectrix 102s NiMH battery, I would charge it at something like 144V DC.

The other application would be the Prius NHW10 battery packs I am restoring/assessing. They consist of two packs inside the box, 20 x 6s NiMH modules (or "Sticks") in each pack. That means 120s cells + 120s cells, or 144V nominal x 2, with a charging voltage requirement of around 168V DC (or 336V DC for the whole pack).

In other words, touching the battery terminals would be potentially lethal, with or without the Variac attached.

Does it make it more dangerous to attach the Variac than it would without it?
I might be missing something important here, please explain in more detail if you have the time.

Without the variac connected you'd have to touch the battery at two places to get a shock (or one place if you ground the battery to your frame). With the variac the hot AC line is alternately connected through the bridge to both battery terminals, so every connection in the battery chain will have voltage with respect to ground. "One hand behind the back" while disconnecting a charger lead would not keep you from getting a shock. And a single short from battery to frame, insulation rubbed off a wire or whatever, which would not show up without periodic testing for voltage between battery and frame, would make the frame hot with respect to ground. A variac gives no isolation since it has only one winding.

Thanks again!

That makes perfect sense.

I bought the bridge rectifier today, 1000V 35A rated, just to be future-proof!

Having a jarn with a knowledgeable guy in the electronics shop we came up with a solution to the problem:

Grid ---> Variac ---> 240V 500VA 1:1 isolating transformer ---> bridge rectifier ---> battery terminals.

The transformer costs around AU$200.-, the Variac (2A version) AU$170.- and the bridge rectifier AU$12.-

So for about AU$ 400.- one could build a charger capable of charging most of the batteries around these days.

Am I right assuming that the short to ground (which could happen from either positive or negative terminal of the Variac) would cause a residual current device (RCD) to open before anything lethal occurs? I have several RCD's installed in the house, and also have a portable one.

But it does not sound safe anyway, I think I'll just have to find the money to buy the isolating transformer as well.

Here is a diagram of what I hope will work as a universal battery charger. The battery shown is the Vectrix VX-1 battery.
It will need constant supervision, is not intended as the everyday charger. It is intended for use in rare events, like to revive a Prius traction battery which has dropped below the minimum required voltage, or an equalization charge after battery repairs or long storage etc.

Am I correct that the tabs which leave the safety container (they will be female plug receptors) would be safe to touch due to the diodes? And that charging will be possible through them, but no voltage measurements?

Mr. Mik said:

Grid ---> Variac ---> 240V 500VA 1:1 isolating transformer ---> bridge rectifier ---> battery terminals.

The transformer costs around AU$200.-, the Variac (2A version) AU$170.- and the bridge rectifier AU$12.-

So for about AU$ 400.- one could build a charger capable of charging most of the batteries around these days.

Am I right assuming that the short to ground (which could happen from either positive or negative terminal of the Variac) would cause a residual current device (RCD) to open before anything lethal occurs? I have several RCD's installed in the house, and also have a portable one.

Click to expand...

They are called Ground Fault Interrupters (GFI) in the US. Yes, they should trigger on leakage to ground from the variac, but not of course from the isolated part of the circuit.

Others can comment on the diode protection. I think I'd use a 2 pole relay that was energised by the charger through an extra pin on the plug, so the other pins would be completely disconnected from the battery except when actually charging. It probably would take no more power to run the relay coil than would be lost in the two series diodes.

I know several people who use a variac and bridge rectifier as a charger.

An ammeter is required. You must monitor the charging current.
A GFCI is highly recommended. These are very inexpensive. Most hair dryers have them now days. They are also available in recepticle form at most home improvement stores. Any accidental connections to ground will trip the GFCI and prevent nasty things from happening.

fechter said:

I know several people who use a variac and bridge rectifier as a charger.

An ammeter is required. You must monitor the charging current.
A GFCI is highly recommended. These are very inexpensive. Most hair dryers have them now days. They are also available in recepticle form at most home improvement stores. Any accidental connections to ground will trip the GFCI and prevent nasty things from happening.

Click to expand...

Re the ammeter, absolutely, I always thought I'd put an ammeter in, but did not put it into the diagram.
A GFCI (in Australia it's called RCD=Residual Current Device) is mandatory for newly built or bought houses and public buildings in Australia. I use a mobile RCD for opportunity charging just in case.

Thanks!