"Voltage Sensing" Relay

[Oonrahnjay]

I would like to switch on a small cooling fan in my house battery compartment when the batteries are being charged.  Seems to me that if there is such a thing as a "voltage sensing relay" that switches on at about 13.4-13.5v (i.e. when charge is being applied to the batteries, either from the auxiliary 12v alternator or by the charger on the inverter) and then drops out when the batteries aren't being charged, then that solves that problem. 

I found a likely unit   http://www.solar-electric.com/volconswit.html but the $70 price tag ($75 - $78, with shipping?) is spendy.  Nothing in Grainger (that I can find).

Is there a lower cost (simpler) component that will do this for me?  Or any other good way to do it that I've missed?   Thanks,  BH  NC USA

[Sean]

The easiest way to do this is to get an appropriately rated thermostat and stick it to one of the batteries.  Should be less than $10.

Make sure you use an ignition-protected fan (and 'stat) if your batteries are the flooded type.

Sensing voltage and activating a relay requires electronics, which is why such parts tend to be pricey.  That said, if you are good with circuits, you can cobble something together using a voltage comparator chip (less than $1), a trimmer, and an SCR.  I'm pretty sure I still have a sleeve of quad voltage comparators lying around from my leveler project, and maybe even some trimmers, if you decide to go that route.

-Sean
http://OurOdyssey.BlogSpot.com