CS wrote:

I want to fit a small fan to vent my battery box when the battery is
charging. I only want the fan to run during charging, so was thinking
of a relay that would only close when the voltage is above 28v or so.
Any ideas???

What's the charging source?

Switch the fan on based upon the presence of the charging source. An AC
relay for a shore powered charger, a relay off an engine signal (oil
pressure) for the alternator, a relay ahead of a blocking diode off of a
solar panel, etc.

As long as there's a power source, there's no harm in running a small
brushless fan (brushless to prevent igniting a hydrogen buildup)
continuously.

--
Paul Hovnanian
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I think you left the stove on.