Oversized Inverter

[Dagoth Ur Does Solar]

Here's a question:

Let's say I have an inverter that is realistically oversized for my system- let's say a 12v 3000W inverter with a battery bank that can only do a continuous output of 200A. If I only intend to ever pull a max of, say, 1500W continuous from this inverter, is it sensible for safety reasons to use something like a re-settable 200A DC breaker to prevent someone from potentially hooking up something trying to pull a full 3kW from the inverter?

I don't particularly want to rely on the battery overcurrent protection- it's there, but my thought was that in addition to adding fuses to all components for safety, I could artificially limit the max continuous draw from the inverter... As well as not worrying about overheating my wiring (this would be using quality 4/0 wire regardless).

I know there's stuff to think about in terms of the trip curves for breakers, but is this a reasonable safety precaution, and would it actually work?

 

[Supervstech]

Since 1500 at 12V would be 125A setting the limit to 200A seems excessive.

 

[Dagoth Ur Does Solar]

Since 1500 at 12V would be 125A setting the limit to 200A seems excessive.

Sorry, I should have mentioned the numbers were hypothetical. It was more of an, in principal, "will artificially limiting the draw from the inverter with a breaker work to prevent overloading a battery bank?"

 

[MisterSandals]

It was more of an, in principal, "will artificially limiting the draw from the inverter with a breaker work to prevent overloading a battery bank?"

I wouldn't call that "artificially limiting", that is real limiting (assuming its a quality breaker).
Breakers and fuses are mostly used to protect wiring but using them to protect a battery seems very appropriate.

Presumably your BMS is sized and programmed to protect the battery too. Or is the 200A you mention the BMS limit?

 

[profesor79]

I am using a current limiter on the AC side that disconnects the circut at requested amperage for 5 seconds (so we know that we overloaded the one inverer and need to wait or switch on second unit).
This works as a breaker mounted on a din rail

 

[Dagoth Ur Does Solar]

I wouldn't call that "artificially limiting", that is real limiting (assuming its a quality breaker).
Breakers and fuses are mostly used to protect wiring but using them to protect a battery seems very appropriate.

Presumably your BMS is sized and programmed to protect the battery too. Or is the 200A you mention the BMS limit?

BMS limit is 200A max for concurrent loads, I would need to pull the spec sheet but I believe it's maybe 3x that surge for a few seconds max.

 

[MisterSandals]

What size breaker you choose will have tradeoffs between nuisance trips and how close to the 200A BMS limit you want to allow. There is not really a right and wrong in which way you go, both have their pros and cons.
If you suspect that its a cheapish BMS, i'd stay as far away from 200A as you can while putting up with nuisance trips.

If you have known loads, you should get a good idea how many watts/amps they pull from your inverter. And your inverter is maybe 85% efficient so take that into account:

1500W AC load / .85 efficiency / 12.8V = 138A

If you consider your inverters (likely?) 10V cutoff:
1500W / .85 / 10V = 176A (which would make a 200A breaker barely enough)

 

[Supervstech]

It really depends on what your goals are with the inverter.
1500W of lighting is significantly less than 1500W of motor loads.

 

[Dagoth Ur Does Solar]

It really depends on what your goals are with the inverter.
1500W of lighting is significantly less than 1500W of motor loads.

Actually, this system really would be used for compressor motors in freezers/refrigerators. So, very high spike when it needs to start up the locked compressor, but then relatively low constant current. My proof of concept with a chest freezer that only pulls around 90-100W continuous has enough surge current to trip my AIO's inverter that is supposedly capable of 2kW of surge. It immediately overloads and switches over to bypass mode when it tries to start up.

Kind of sucks, because if I understand inverter efficiency curves, this type of application (high inrush for startup, then very low constant consumption) is really bad efficiency-wise, right? My quick searching indicated that most inverters have very, very poor efficiency when they are at a low % utilization, and tend to be much more efficient when more heavily loaded.

What size breaker you choose will have tradeoffs between nuisance trips and how close to the 200A BMS limit you want to allow. There is not really a right and wrong in which way you go, both have their pros and cons.
If you suspect that its a cheapish BMS, i'd stay as far away from 200A as you can while putting up with nuisance trips.

If you have known loads, you should get a good idea how many watts/amps they pull from your inverter. And your inverter is maybe 85% efficient so take that into account:

1500W AC load / .85 efficiency / 12.8V = 138A

If you consider your inverters (likely?) 10V cutoff:
1500W / .85 / 10V = 176A (which would make a 200A breaker barely enough)

Understood- and with my current plans described above, the only time I could see a nuisance trip happening is if multiple compressors happened to fire up simultaneously and manage to overload the inverter's surge capacity (which is something I'd be curious to test).

Anyhow, I appreciate the answers from everyone. I think my plans are solid then to protect the BMS/rest of the system by simply fusing off or putting a re-settable breaker between the inverter and the rest of the system to ensure it can't overload anything else if someone were to come haphazardly plug in two space heaters.