Yes, absolutely a NC contactor would work, I didn't mention it because I believe it has a few negatives.
1) If a critical battery condition occurs the point is to disconnect the load. A NC contactor requires power to maintain a disconnected state. You are then relying on a component that has a known problem but has to function properly to keep itself off-line. Poor engineering.
Fair point, though even NC contact its can fail in a stuck closed condition, no?
Backup protection, etc is a whole ‘nuther topic.
Looking at the Heltec offering more closely, the relay they are using appears to be dual-coil design (2 pairs of control leads and a add-on rectangle) so hopefully NO power consumption is more modest.
They claim 80uA self consumption reducing to only 60uA after disconnect, but I suspect this is not including the consumption of the relay.
Also, for purposes of scale, my use case is powering 300W of load (fridges) 24/7, so I could accept as much as 30W of coil power but would be happier with a fraction of that.
In any case, you’ve confirmed ON consumption of the relay itself is perhaps the most significant drawback of the relay approach to BMS, so thanks.
2) It would require the BMS to function properly and maintain a sustained "active" state this is also poor engineering practice for fail safe devices. Passive safegurards are better. The controller PCB's I developed have on-board capacitors that will open a latching contactor for any reason the control signal is cut. Could a BMS fail in such a way that the relay control signal persisted even though the BMS otherwise was not working properly? Sure but much less likely.
Again, whole ‘nuther’ discussion (probably best for a separate thread). I am planning to eventually have a PLC monitoring the entire setup and will probably have a second relay/contactor to shut everything down.
The possibility of two NC contactors failing to open from the same failure event is risk I’m comfortable with…
3) NC contactors are less common and not always stocked. Although this is definitely true for magnetic latching contactors as well.
Good point. If I start with a NO contactor, that design can probably be switched to NC using an intermediary relay or signal inverter.
Very true, and this logic is exactly why I would actually choose the remote trip breaker option because under normal circumstances the BMS should not trip. Thus manual intervention is probably needed anyway and its not such a big concern that the system needs to automatically restart itself.
Agreed.
Absolutely, I agree. In your OP you asked for a discussion of General Pros and Cons. This is what we are doing. I wasn't advocating for one thing over another.
Understood and appreciated.
True but there is a trade off. If a system is designed to have a certain kWh rating to carry the loads for a specified period of time, parasitic losses have to be accounted for and the system has to be up-sized accordingly which is not free.
Likewise, value priced All-in-One inverters have lower up front costs to the buyer but then they also waste 2 to 3 times as much energy for stand-by and operating losses as compared to higher priced brands. As I mentioned, when you do the math wasted energy adds up fast when it’s a 24/7 issue.
I’m in an unusual position of having excess energy to waste (within limits). Either I’ll be throttling my solar power because battery is fully-charged and export limit has been reached or I can burn a bit of that power that otherwise would be getting dumped.
More worried about damaging components than I am about achieving highest possible efficiency.
But yeah, +10% to power the relay is about the most that would make sense.
I’ll reach out to Heltec to see what they can tell me about the relay they are using…