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I have a 4kW 220V inverter which I want to connect to 12V 50Ah battery with maximum discharge current 30A.

I calculated maximum current that inverter will take by 4kW / 12V = 333A which is much bigger than allowed for this battery. So, I want to protect battery from big current (when too powerful consumer is connected). How can I do it?

I don't plan to take more than 100-200W from that inverter, so, I want to add protection from accidentally connection of high power consumer.

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  • You probably want another forum for this one. Maybe Home Improvement?
    – RedSonja
    Aug 2, 2022 at 7:54

1 Answer 1

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What you describe is standard practice.

There's a device that does exactly what you want and it's not expensive either. It's called a "fuse". Fuses are available for 30 amperes and many other amperages. It's a one-use only device, once you have blown the fuse you will need a completely new fuse. Fuses are cheap, buy at least 10 fuses of the amperage you need.

If you are planning to blow the fuse hundreds of times, then it might be useful to consider DC circuit breakers. The trouble with these is that interrupting high-current DC such as 30 amperes ain't easy. The current will want to continue, even through the air, creating a massive arc. DC does not have a zero-crossing like AC has that would be a natural moment for the current flow to stop. Any circuit breaker must be able to extinguish this arc, in a manner that doesn't cause the expensive circuit breaker to get damaged. (A self-damaging circuit breaker would only be a very expensive single-use fuse -- or worse, it could fail to stop the current flow.)

Think about welding. Does the arc when welding stop or does it continue? That's what DC circuit breakers are dealing with.

What makes interrupting the 30 ampere current somewhat easier is the low voltage, 12 volts -- you may be even able to use an AC circuit breaker at such low voltages (although if you really do so make a few tests to ensure they work safely when short-circuited with a huge fuse like over 100 amperes in series -- ideally with a safer battery chemistry than lithium-ion like lead-acid since short-circuiting lithium-ion isn't a good idea)! For higher voltages, it would be much harder. If you can find a 12 volt 30 ampere DC circuit breaker (or any voltage rating above 12 volts is fine but the current rating has to be accurate), you can consider using it but I suspect even if you plan to blow the fuse ten times, fuses would still be less expensive than a circuit breaker. Usually smaller fuses like 30 amperes are installed in a case making swapping the fuse very simple, not even needing a screwdriver. Larger fuses like 100 amperes may be attached with screw terminals to get a connection that allows high current without too much resistive drop.

Note also that 30 amperes should use 6 mm2 wiring. You may be able to achieve 30 amperes with 4 mm2 wiring without melting the insulator, but at 12 volts and 30 amperes, the voltage drop for long 4 mm2 wires might be just a little bit too much, and if the two wires are close in a hot environment, two 4 mm2 wires could still heat just a little too much for infinite insulator lifetime.

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  • "Note also that 30 amperes should use 6 mm2 wiring. You may be able to achieve 30 amperes with 4 mm2 wiring without melting the insulator, but at 12 volts and 30 amperes, the voltage drop for long 4 mm2 wires might be just a little bit too much, and if the two wires are close in a hot environment, two 4 mm2 wires could still heat just a little too much for infinite insulator lifetime." Wow, I didn't think about it. Thank you very much.
    – Robotex
    Aug 2, 2022 at 12:23
  • Can I use automatic AC circuit breaker of 1A after the 220V invertor? Will it limit discharging current at battery by 18A? With a combination of 30A fuse on 12V side, of course.
    – Robotex
    Aug 2, 2022 at 12:25
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    220V, 1A is 220 watts, which is 18.333 amperes at 12 V. However, if you run down the battery to nearly empty, and if the efficiency of the inverter isn't 100%, it could be over 20 amperes. If you use an automatic 1A AC circuit breaker, I would still put a 30 ampere fuse in there. The AC circuit breaker should trip first (thus saving the fuse from blowing), but if for example the inverter fails in a short-circuit mode then the fuse still is there as a secondary protection.
    – juhist
    Aug 2, 2022 at 18:23
  • The last question: if battery maximum charging current is 16A, can I add 15A fuse between solar controller and battery? Should I also add 30A diode here (to protect 16A fuse from 30A current on discharge)?
    – Robotex
    Aug 3, 2022 at 15:03
  • Yes, if you use solar controller to charge it, please do add the 15 ampere fuse. Obviously make sure the controller is LiFePO4-compatible and safe. Diodes have voltage drop and 30 ampere diode would dissipate huge power. Because of the diode voltage drop, current preferentially would flow through the fuse and not the diode. I recommend attaching the load directly to battery, not via the solar controller. Then you can have 30A fuse on the load, and 15A fuse on charging -- although it's still theoretically possible if the panel is big that 15A fuse would blow because of load.
    – juhist
    Aug 3, 2022 at 20:13

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