You might send a note to these folks to see if the have a voltage controlled
relay kit that has an adjustable voltage setpoint. I did a search on their
site and could not find one but they have a kit that switches at 12 volts,
QK156.
www.qkits.com/

Laurier Gendron's Handy Dandy Little Circuits webpage has a circuit for a
voltage detector/relay. Needs a 9V battery for power so that might not be
convenient. http://members.shaw.ca/roma/one.html

"Steve (another one)" wrote in message
...
Dear folks

I have a blower motor venting 3 battery compartments on a 24V system.
The motor is said to draw 2.5A. I want this to run whenever the
batteries are charging, they may be charged from engine, genny or
shorepower. It seems to me that the simplest solution would be a voltage
sensitive switch to activate the motor whenever the system voltage rises
above a threshold of around 27V.

I can't seem to find a suitable switch. A battery combining relay could
be used I am sure, but I can't find a small one for 24V systems and it
seems crazy to waste money and space on a massively over-rated combiner.

Such systems must exist, eg to prevent fridges killing batteries but I
can't seem to find a supplier.

Does anyone have any suggestions ?

Thanks

Steve