I recently upgraded my camper to 280ah raw cell lithium batteries with an Overkill BMS. Everything is working great except for when I start my microwave, the BMS reboots, dropping the load in the process which resets everything in the camper including my Ali SOC shunt battery status. When I look at the BMS app afterwards there are no errors. I have measured the max current with a Fluke clamp on meter and it records 91 amps but I am guessing there is quick large inrush current that is not being captured. I calibrated the BMS per the Overkill instructions and I tried increasing the discharge over current protection from 130A to 140A, but the problem persists. If I drop the microwave power to 50%, I can prevent the reset, but this is obviously not a great answer. To solve this problem, I could replace the BMS, but I really like the features and configurability. Another option would be to wire the inverter directly to the battery and bypass the BMS but then I will not capture the true SOC on my Ali shunt. My third option is to install a BMS bypass relay that would be controlled by a push button next to the microwave to get past the initial current surge. Think of it like a clothes dryer start button. This would maintain my SOC accuracy and keep the BMS protections once the button is released. The question is, what is the impact of doing this on the BMS? The BMS is essentially a solid-state switch so theoretically this should work, and it did in my testing but I have no idea what the long term impact would be. I would appreciate any suggestions. I have attached a wiring diagram and screen shots of my BMS settings.
Overkill BMS reset on inverter start
- Thread starter aclosson
- Start date
Bob, that is a good point but I did a full re-wire as part of this this project and I have verified good voltages at all points including the input to the inverter. Voltage drop under high load is very small which makes me think that this is an inrush issue.
If it's an inrush issue I don't get why your shunt reading is also reset. Or did you connect the shunt's power to the output of the BMS? I would suggest connecting that directly to the cells (Along with a small fuse off course)
That would prevent the shunt to reset / loose it's readings if the BMS shutdown for whatever reason.
I think the inrush current is the main issue. Having a separate relay with a switch... I won't like that, too much hassle to get the microwave running.
Do you have a remote on your inverter to switch it on? If the inverter has a manual switch (not the touch-buttons, but a normal on-off) I'd replace this switch with a small relay and connect the inverter directly to the cells, bypassing the BMS (Only keep the shunt in place - and a fuse)
The relay can be powered by a small switch, which is connected to the BMS.
All power will pass the shunt (thus is registered by the monitor), but in case there is an issue with the cells, the BMS will shutdown. BMS shutdown -> power to the small relay is also gone -> inverter shuts down, even while not connected to the BMS. The bms will only feed the 'on-off' and not the main current.
You should be able to pull full rating of the inverter with this setup, without overloading the BMS.
As stated by others, just make sure using proper wire gauge and decent cable lugs with proper crimps
That would prevent the shunt to reset / loose it's readings if the BMS shutdown for whatever reason.
I think the inrush current is the main issue. Having a separate relay with a switch... I won't like that, too much hassle to get the microwave running.
Do you have a remote on your inverter to switch it on? If the inverter has a manual switch (not the touch-buttons, but a normal on-off) I'd replace this switch with a small relay and connect the inverter directly to the cells, bypassing the BMS (Only keep the shunt in place - and a fuse)
The relay can be powered by a small switch, which is connected to the BMS.
All power will pass the shunt (thus is registered by the monitor), but in case there is an issue with the cells, the BMS will shutdown. BMS shutdown -> power to the small relay is also gone -> inverter shuts down, even while not connected to the BMS. The bms will only feed the 'on-off' and not the main current.
You should be able to pull full rating of the inverter with this setup, without overloading the BMS.
As stated by others, just make sure using proper wire gauge and decent cable lugs with proper crimps
DJ:
Do you see any problem having the shunt between the battery and BMS? This was something that I was thinking was not a good idea based on the small voltage drop across the shunt which would potentially impact the BMS reading of the last cell. Any and all input is welcome.
Thanks,
Art
Do you see any problem having the shunt between the battery and BMS? This was something that I was thinking was not a good idea based on the small voltage drop across the shunt which would potentially impact the BMS reading of the last cell. Any and all input is welcome.
Thanks,
Art
The BMS came with (4) 10awg cables for each side factory crimped. One side of the BMS runs to the battery negative and the other to the shunt. The inverter positive cable runs directly to the battery with a 2awg factory crimp and the inverter negative runs to the shunt, also with a 2awg cable with a factory crimp. My theory is that my Fluke is not able to capture the inrush current before the reset occurs. Connecting the inverter directly to the battery does work but without capturing the power usage with the SOC shunt. If I can wire the shunt between the battery and BMS without causing problems then that will be the answer, its just not how Overkill recommends at least in their documentation. I sent a support request last week but so far there has been no response from them.
You are running an inverter that is driving a microwave with 2AWG wire? If you have that same cable feeding your shunt, that is a major problem right there.
A 120A BMS is not going to work for powering a microwave. Part of the problem is you have so much voltage drop in your cables, the current is going up just from that. In addition, the surge current of the microwave is WAY more than 120A.
A 120A BMS is not going to work for powering a microwave. Part of the problem is you have so much voltage drop in your cables, the current is going up just from that. In addition, the surge current of the microwave is WAY more than 120A.
The inverter is drawing 91 amps continuous when set at 100% power. The BMS is rated at 120A and I just checked and there are (4) 8awg (which is theoretically good for 180 amps) not 10awg wires as I incorrectly stated earlier. I think we all agree that the microwave it too large a load for this BMS and I knew I was cutting it close when I ordered it. So now the question is to how best bypass the BMS and still keep the shunt and state of charge metering. My next test will be to try putting the shunt between the battery and BMS and bypass the BMS for the inverter wiring. The Prosine inverter has a programable low voltage shut off and it will easily accommodate a relay to disable the inverter if the BMS turns off the load, I just don't know how the BMS will react to the shunt.
That's basically 2 8 AWG wires but that's not the issue. You have undersized cable going to your inverter and the surge is what's killing you.
BTW, just because a cable may survive with a certain amount of current, doesn't mean it's sized correctly. It's the voltage drop in your cable runs that determine sizing
How long is the round trip between your battery and the inverter?
How did you determine 91A for your inverter 100% load?
I will be building my batteries. Similar sized batteries(270AH) and BMS as you but two in parallel so I won't have your issue. I will be running 4 or 6AWG off the BMS instead of those 8 AWG wires. I have the 6mm posts on my BMS.
I currently have 2 100AH Battleborn driving a 2000W inverter and I run a 1500W microwave so my current draw is around 135A.
I currently have 2 100AH Battleborn driving a 2000W inverter and I run a 1500W microwave so my current draw is around 135A.
Thanks for help, I will do some experiments with jumper cables to see if this makes a difference. My microwave is a small one in my camper which likely is why I am only measuring 91 amps. I did see a post from Overkill that paralleling BMS was not supported for what it is worth.
Using a capacitor is an interesting idea, along the same lines as a soft start on an RV AC unit. Thanks, Art
It's not the same. A soft start limits the current an AC uses at startup. A capacitor by itself does not do that.
