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We built a home in a country where electricity is a rare ressource. In order to have energy available all day we bought a bunch of solar panels and several 12 volt batteries. We then used a 8000 watt inverter to power the house.

We are concerned about battery life so we thought about adding an automatic shut off to disconnect the load when the battery discharge to a critical level.

At first we wanted to add a contactor or a relay with a voltage meter to trigger the relay and disconnect the load. The problem is finding a relay that can handle a 8000 watt load on a 12 volt circuit is quite hard.

The other solutions is to use the inverter built in switch to cut off the load. The inverter came with a module that you plug in to remotly power off and power on the inverter. It would be pretty easy to hook it up to a small relay.

I have been looking online for a solution but most module I found only handles a maximum of 100 amp load, since the inverter is capable of 8000 watt I don't think 100 amp would be enough, maybe I'm wrong?

We do not want to spend over 300$ and the module cannot be too heavy since we are limited with transport.

I'm reaching out to the community for a solution.

Edit: I think the safest and easiest solution is to use a volt meter that triggers the inverter low voltage switch, using a relay of some sort, when the batteries voltage is too low. Since the load exceeds 100 amp the load should not go through the volt meter. Maybe by using a shunt, if necessary.

Does anyone know a module that can do that?

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  • \$\begingroup\$ Examine an automotive starter-relay, perhaps from a truck. \$\endgroup\$
    – analogsystemsrf
    Commented Oct 10, 2018 at 4:37
  • \$\begingroup\$ Good Inverters might be around 10W/$ so 8kW may be expensive. Given a deep discharge battery of 12V >1kWh you need 8 for 1 hr storage drawing 8kW. So the energy storage and conversion costs may be greater than the panels. \$\endgroup\$
    – D.A.S.
    Commented Oct 10, 2018 at 4:52
  • \$\begingroup\$ does the inverter remote control power down the inverter completely? .... or does the inverter still draw small amount of power from the batteries when it is shut down or in standby? \$\endgroup\$
    – jsotola
    Commented Oct 10, 2018 at 6:11
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    \$\begingroup\$ I suspect the subject must be legally (locally) regulated. You should have already spent amounts to obtain panels and related hardware, and the logical way to proceed would be to check with their manufacturers if they have solutions you need, and if these solutions are certified. If you have already done so, please share. \$\endgroup\$
    – Anonymous
    Commented Oct 10, 2018 at 7:29
  • \$\begingroup\$ Doesn't the inverter have a built-in low battery shut down? \$\endgroup\$
    – Simon B
    Commented Oct 10, 2018 at 9:28

1 Answer 1

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8000W / 12V = ~666A, which is not going to be trivial to switch. Most power devices are designed to be run at higher voltages, with lower current to avoid this exact situation.

That being said, most industrial size relays are called contactors. If you can

  1. 100% guarantee that the contactor will never be switched while under load, and
  2. assuming you are not trying to meet building codes,

you could use a three phase contactor with all phases connected in parallel to switch the DC Positive line.

The better overall approach in my opinion would be to use an inverter with a low voltage cutoff!

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  • \$\begingroup\$ Hi, FYI the link in the answer is malformed (and therefore broken), Can you please fix it to point to the intended destination? Thanks. (I'll delete this comment when I see the link is fixed.) \$\endgroup\$
    – SamGibson ♦
    Commented Oct 10, 2018 at 21:14
  • \$\begingroup\$ This is a pretty good idea. I've been doing some readings about contactors. The inverter does come with a low voltage cut off but it doesn't detect the voltage. Maybe a voltage sensor could trigger the low voltage switch to turn off the inverter output? \$\endgroup\$
    – DustyMan
    Commented Oct 11, 2018 at 0:41
  • \$\begingroup\$ That is strange, do you have a link to the inverter you are using? \$\endgroup\$
    – TuxRobotics
    Commented Oct 11, 2018 at 20:04

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