Extractor fan switch/sensor for battery cupboard.

[Rosscrumb]

Apologies if this has been covered somewhere else but I don't know the name of the piece of kit that i'm after so couldn't find anything.

I have a small cupboard built into the house but accessed from the outside with three 250ah FLA batteries with no mechanical ventilation at present. There will be a small amount of airchange because the door is not the full height of the cupboard - 100mm gap at the bottom to allow clean opening over a gravel floor. I'm concerned about hydrogen gas buildup (particularly as the butane hot water boiler is located adjacent) and would like to install an extractor fan.

If I understand correctly, hydrogen gas will only be produced when the batteries are at float voltage so the extractor will only need to be active at this time. Is this correct? If so, is there a commercially available device to sense the voltage and switch the fan on and off and if so, does anyone have a link for a product please?

I'm in spain but location does not need to be specific for a link - if I know what i'm looking for I can find one locally.

Thanks In advance.

[bob-n]

Sorry, I don't know of a fan switching device that operates on battery voltage. This would be a simple electronic circuit, with a comparator, a voltage reference that is temperature matched to float voltage, and a power transistor. Another possibility is to use the full-charge indicator in your charger to turn on the fan. But all of these ideas require electronic tinkering.

More hydrogen is generated at full or over-charge, but some hydrogen is generated below there, too.

Can you add another gap on top so that, even without a fan, there is always air circulating?

Alternately, can you get a small fan and plan to have it always running? You might find a suitable quality used 12V fan in an old computer or computer power supply. You can find various 80mm square fans that operate from less than 1 watt to 5 watts. One little fan like that may be enough. Make sure that the fan is specified as "brushless" or BLDC. With hydrogen gas around, you don't want a fan with a brush motor.

[Mike90250]

You may have to come up with a voltage sensing relay, for > 13v when charging, the vent comes on. These are made for battery venting, bushless

http://zephyrvent.com/

[Rosscrumb]

Thanks for the responses. I'll get onto the suggested reading.