How to power a fan only when solar is charging.

[TURNKEY]

I want to run an exhaust fan from my battery compartment to vent out hydrogen during charging cycles but I don't want it to run 24/7 as I do now. I tried powering a computer fan off my panels before the charge controller but the charge controller allows power back to the panels when they aren't charging, like at night.

The reasons I want to do this are:
-battery compartment is not that air tight and will be hard to make it so-I don't want the off gasses getting into the trailer.
-it is just a small draw..but watts are watts
-I like to do tiny tinker jobs like this one (I just installed thermostatically controlled fans in my fridge chimney)

One way would be a separate tiny panel just for the fan-expensive. Or some kind of voltage controlled switch (panels run at +17 volts when charging).

I'm running 200 watts on the roof and another 100 watts on an umbilical. Batteries are 2x225 amp golf cart bats. We almost exclusively dry camp. We do not have an inverter and run the genny once a month or less.

Any input or ideas would be welcome.

[TURNKEY]

Sure a switch would work......if I got up at dawn. That ain't gonna happen.

[CincyGus]

Can't you just put an inline toggle switch for the fan that you turn off at night and on in the morning?

[MEXICOWANDERER]

Un huh

[red31]

the 56mA load operates 0.1v less than Voc!

[MEXICOWANDERER]

And the little red light diagnoses "better" than a digital panel meter?

[red31]

What I did do is learn, the little led light of course is not operating at open circuit by definition. But its resistance has it operating at near my panel's Voc.

[MEXICOWANDERER]

And that helps ventilate a battery compartment? What you did would have no effect whatsoever on a direct panel driven fan. A 48 volt fan can be driven at 8 volts the result being a very slow fan to match 0.0 charging amps. It really isn't important unless open circuit panel voltage exceeds the voltage rating of the fan. And if fan speed is too low a higher ma fan can be used. An overwhelming majority of NORMAL charging by-product is pure hydrogen that wants to fly straight up at high speed. If PWM is coupled to an excessively noisy fan the fan speed can be discerned when pulses vary.

[red31]

No more open circuit voltage for me as I added a load to the panels, a 1.2w led landscape light at the controller input! Now when the controller is in PWM, the off voltage will not be Voc but some other to be measured voltage.

[MEXICOWANDERER]

If voltage series goes above 36 for MPPT get a 48 volt fan. Most "12" volt panels operate near 20-22 volts open circuit. Fan voltage maximum IS TO PROTECT THE FAN WHEN CHARGING STOPS IN FULL SUN.

Yes this works.
And it's simple.
And inexpensive.

[MrWizard]

on my previous RV, a DP, i put a fan in the top of the fridge roof vent

wired it directly to the solar panel wires "at the roof"
the wires went down thru the fridge compartment thru the floor and into the basement storage where the solar controller is at
(yes with either PWM or MPPT controller these were 12v panels with an operating voltage of 17.x)

that fan ran directly from solar power, the batteries and controller and solar were always all connected, unless i was working on the batteries for some reason

its that simple, just keep the voltage in range,

Diodes should NOT be needed, BUT if used, the go in the FAN WIRES not the panel output wires

"oh God" is an exclamation of exasperation
and i think it fits quite well in this chain of posts

[BFL13]

red31 wrote:
OP states "It does not discharge at night as I run a Trimetric meter and it shows no loss. "

If the power came from the battery it should show on the Trimetric as AH loss. It depends on the amount of amps if it will show. 500a shunt vs 100a shunt and all that.

The RV will show neg amps all night from the various "parasitic" or "standing" draws (pick your terminology so that guy doesn't jump in on that! 🙂 ) so you would not notice a muffin fan's amongst all that.

[red31]

OP states "It does not discharge at night as I run a Trimetric meter and it shows no loss. "

[BFL13]

red31 wrote:
trimetric shows no power to fan and it keeps spinning?

Is the neg wire for the fan on the Tri's shunt?

I tired a 12v , 12w fan too. No spin on the array terminals but ran ok on the battery or load terminals of all three controllers above.

The muffin fan spins on the battery terminals of all three, but as above for the array ones, didn't try the load terminals for the little fan.

Are you saying your landscape lights are a load with no battery? That is a no-no with some solar controllers (must have battery), but perhaps not the ones made for lighting?

[red31]

BFL13 , at night my load terminal come on when panel voltage drops below 8v. I've set it to 6v in the past (to delay the start of my landscape lights). In short I have not measured my input voltage from the panel at night but expect them to be below 8v for the load terminal to activate. 8v would be plenty to operate a low wattage 12v fan slowly.