Add battery to Solar Attic Fan

I have a Natural Light Solar Attic Fan. The model with 30 watt panel. The fan works great while there is sun but shuts off immediately if there is an ounce of shadow or when the sun goes down. Problem is my attic still needs cooling for maybe an hour or so after dark.

I wanted to add a battery to the setup so it could power the fan for an hour or so and then recharge during the day. The manufacturer doesnt offer much on the specs but does say the fan has a "custom 36v motor".

Could I just find 3 12v house alarm batteries and wire them in series and add it to the setup? I have easy access to the +- leads as they hang down for an optional thermostat. Should I add a 36V solar charge controller? Appreciate any help. Thanks.


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Add battery to Solar Attic Fan

    Welcome to the forum.

    This is one of those projects which will probably turn out to be more trouble than it's worth.

    First up is the question of how much energy the fan will draw over time of use versus how much energy the solar panel can produce during the the hours of good sun. The lack of data on the motor is not helpful with this.

    Then you have the matter of the batteries being sized so that the panel can recharge them and still being large enough to provide the desired 'extra' run time. This is an issue because it takes more energy to recharge the batteries than you can pull back out of them. In other words you need a decided surplus of available solar to pull this off, and it's not likely to be there as he company that made the unit wasn't planning on it being there.

    Let's look at it from the panel POV: 30 Watts, typically capable of 23 Watts on average. Output current unknown, because we don't know what Voltage it is actually operating at. A "36 Volt" motor could indicate Vmp 35 from the panel. If so the current is <1 Amp, which isn't much for charging a battery.

    What's more, without a thermostat or timer to shut it off the fan would just keep running 'til the batteries are dead.

    If we had more specifications of the panel and fan motor we could come closer to an answer, but don't get your hopes up.
  • GrinninGrinnin Solar Expert Posts: 39
    Re: Add battery to Solar Attic Fan
    . . . Problem is my attic still needs cooling for maybe an hour or so after dark.

    . . .

    Could I just find 3 12v house alarm batteries . . . 36V solar charge controller?
    As Cabricoot says, this could be more trouble than it's worth, but it could also just shift the time by an hour or so. You will certainly not have the fan running as long in the morning. Is losing 2 or 3 hours in the morning worth adding an hour in the evening?

    With a small battery that can sit discharged all night (such as some lithium batteries), you could use a low-voltage disconnect. In the morning the PV would start charging but the battery would take all the energy. When the battery voltage comes up, the LVD would turn on the fan. Through the afternoon the PV would provide power mainly to the fan. When the sun sets, the battery would run the fan until the battery voltage drops below the LVD voltage and the fan would turn off. No thermostat at all.

    You'll need a LVD that has high enough hysteresis so the fan doesn't short-cycle in the morning and evening.

    If you can find the right battery and right LVD, the system could be simple, but finding the right parts may not be so simple. The LVD has to be able to protect the battery from going flat; even lithium batteries that can sit discharged don't survive well when completely discharged.

    The PV panel on the fan is probably not oversized. Does the fan turn on during overcast days or when the sun is far off-axis? If the fan runs only on the clearest days then the mfr cut it close. If the fan runs on overcast days you may have some leeway.

    So, are your a tinkerer or do you want a cut-and-dried solution?
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