Panama wrote:

I have a 40 amp battery charger that has a 5 position switch to select
the battery charging voltage. Works great. Allows steps from about
13.2 to 17 volts. Prob done by a multi-tap transformer?

Trouble is - AC voltages in Mexico can be as high as 128-130 VAC and
other places can be as low as 105 VAC. The charger reacts by changing
the battery charging voltage up or down - a little but enough to vary
the DC amps that get to the batteries. I'm not going to replace the
charger.

I'm looking for a cheap variac/scr/triac something gizmo that I can
wire into the AC line to the charger and make the effective voltage
(RMS value?) go up or down by +/- 10% or so. The AC in is about max 9
amps. It could even be a big wired wound variable resistor I suppose.
Then I can set the AC line voltage to what my genset puts out and use
the charger at it's max 40 amps all the time.

Any ideas or products. Preferably cheap and never needs replacement
like the 27 years old charger.

(No I'm really not going to replace the charger.)

The specified/allowed 'Voltage variation' plus/minus varies (pun
intended!) a little from country to country and region to region.
E.g. North America, Europe, etc.
In some places it may be plus and minus ten (10) per cent. In
another area it may be, say, minus 6% to plus 10%. If in Mexico
the voltage is occasionally up to 128/130 on a supply that is
supposed to be 117/120 volts; that's around 9% to 11% high. i.e.
acceptable? If you are monitoring it anyway why worry? Is in
danger of burning out. i so tap it down to the next lower
ampere/voltage output setting?
Or am I missing something?