I don't know if this qualifies as cheap enough but one way is to use a
variable autotransformer, a transformer with a variable tap. With
120VAC in you can output from 0130 VAC.
Here is a link to a 10amp unit all cased up and ready to plug in. If you
are handy you can get the transformer itself for a lot less.
http://www.tenma.com/070.html
BTW, 9 amps seems kinda high, I would expect a 40amp charger to draw
about half that on the line side.
The other thing to keep in mind is that these use a wiper to vary the
tap, I have no idea how long that would last on a boat.
RB
..


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.)