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TimD
Newbie

Joined: 23/02/2021
Location: United Kingdom
Posts: 30

Posted: 11:34pm 06 Sep 2025

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Hi,

I'm in the process of setting up a couple of small automatic bilge pumps sunk into the garden to alleviate the waterlogging we get when we have heavy rain. Both pumps are rated 12V 5A, although when I tested one out it was drawing more like 2A. I want to power these with a battery charged by a solar panel. If it's rainy then each pump could end up running for several minutes within a day. I'm looking for any advice about solar panel size, battery chemistry & capacity, and what sort of charge controller to look for. Also, any recommendation/links about brands. I'm hoping for a setup that's reliable without being too expensive...

Thanks,
Tim.

Solar Mike
Guru

Joined: 08/02/2015
Location: New Zealand
Posts: 1171

Posted: 08:47am 07 Sep 2025

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As the pumping system will only be running when its raining, any solar panel input will be minimal; so your battery size is the determining factor in how long the pumps will run before going flat. As the setup will only run on occasion when it rains, the rest of the time it will sit fully charged, this precludes any Lithium type battery as it doesn't do them any good to sit in that state. Get a lead acid type and a small PWM PV charger plus panel 20-30watts.

Batteries are expensive, suggest you go to your local car wrecker and get a large 12v lead acid used one 40-60 AH, may not be suitable for starting a car but should be ok for 5-6 amp load to run a couple of pumps.

You will need a sensor like a float switch + a 12v relay to turn the pumps on and switch off when the water level drops. Most PV chargers have a load output, use that to control the float switch power, run the pumps via the relay direct to the battery + fuse; the controller load outputs are not designed to run high surge current devices like motors. The load output will protect the battery from being discharged too much and resulting in a dead battery.

Cheers
Mike

TimD
Newbie

Joined: 23/02/2021
Location: United Kingdom
Posts: 30

Posted: 05:35pm 07 Sep 2025

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Hi Mike,

Thanks very much for your reply. These pumps actually have the float switches built in, so presumably the idea would be that the 'Load' output directly drives the relay coil, and the relay contacts control the battery supply to the pumps (which are wired in parallel). This will mean that the coil will continuously draw a few tens of mA while the battery is above the controller's threshold voltage, but I guess that's not a big deal.

Would I need a flyback diode across the relay coil, or would the charge controller be resilient enough without it?

Regards,
Tim.

Solar Mike
Guru

Joined: 08/02/2015
Location: New Zealand
Posts: 1171

Posted: 10:40pm 07 Sep 2025

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>> Would I need a flyback diode across the relay coil, or would the charge controller be resilient enough without it?

Best to put one in across the relay coil, use a small 1Amp 1N4004-7 type.

Mike

TimD
Newbie

Joined: 23/02/2021
Location: United Kingdom
Posts: 30

Posted: 10:43pm 07 Sep 2025

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Thanks Mike!

Kind regards,
Tim.

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