Inverter/Charger - Overcome BMS discharge disconnect

[techguy22]

I have a Sungold 3000w 12V inverter/charger that I'm getting ready to put into my RV. I'm trying to put it through the paces to make sure I have all my angles covered and I did a discharge cycle on my lifepo4 pack until the BMS shut off the discharge due to low pack voltage. (BMS is a common port for charge and discharge)

The issue I'm running into is that the inverter/charger will not turn on at all unless it sees battery voltage on the 12v terminals. So, it will not turn on and charge the batteries because the BMS isn't letting it see 12v and it won't turn on even with shore power. I hooked up my bench power supply for a second to the terminals and woke up the inverter. Then it started charging and the BMS turned back on the discharge and everything was back to normal.

I'm wondering though what would be the proper way to design this into the system in my RV to overcome this condition? I'm thinking maybe a pushbutton switch to temporarily bypass the BMS? I'm concerned about possible overload of that switch though, especially if it was pushed when no shore power was available.

Any ideas? I haven't been able to find any answers on the forum, but I'm also not even sure how to search for this.

 

[John Frum]

What bms are you using?
What is the bms low pack trigger voltage?
What is the bms low cell trigger voltage?
What is the bms low pack trigger release voltage?
What is the bms low cell trigger releae voltage?
Does your inverter/charger have a configurable low voltage disconnect?
If yes, what voltage is it set to?

Does the inverter resume operation when the cells rest and recover enough to hit the release voltage?

 

[techguy22]

What bms are you using?
What is the bms low pack trigger voltage?
What is the bms low cell trigger voltage?
What is the bms low pack trigger release voltage?
What is the bms low cell trigger releae voltage?
Does your inverter/charger have a configurable low voltage disconnect?
If yes, what voltage is it set to?

Does the inverter resume operation when the cells rest and recover enough to hit the release voltage?

I'm using a JBD 150a BMS like this one...
I have my voltages set according to the defaults in the Overkill Solar manual as a starting point. So, my pack under-voltage is 10v, low cell at 2.5v and release voltage at 3v, and pack release at 12v.

The only setting the inverter has is a dip switch to turn off the inverter at either 10v or 10.5v. This works, but the voltage it reads isn't the most accurate. I set it for 10.5 and it did disconnect and I think the pack was around 10.8v. After the inverter disconnected I ran the pack down to the battery disconnect. The inverter isn't the only load on the batteries as I'm powering all of my 12v coach loads off of it. So its entirely possible that even though the inverter shuts down before draining the battery too much, other 12v loads could.

I'm guessing an option from your post might be to reduce the under voltage cuttoff to release voltage difference so that the natural return of the cells to a higher voltage might allow the discharge to turn back on and the inverter to charge?

 

[John Frum]

I'm using a JBD 150a BMS like this one...
I have my voltages set according to the defaults in the Overkill Solar manual as a starting point. So, my pack under-voltage is 10v, low cell at 2.5v and release voltage at 3v, and pack release at 12v.

2.5 volts per cell or 10 volts at the pack level is dead empty.
Its quite surprising that your bms disconnected on low pack and not low cell.
It suggests that either you have perfectly balanced cells or bottom balanced cells or a malfunctioning bms.
For normal operation I would set the bms to disconnect at 3.0 volts per cell or 24 volts for the pack.

The only setting the inverter has is a dip switch to turn off the inverter at either 10v or 10.5v. This works, but the voltage it reads isn't the most accurate. I set it for 10.5 and it did disconnect and I think the pack was around 10.8v. After the inverter disconnected I ran the pack down to the battery disconnect. The inverter isn't the only load on the batteries as I'm powering all of my 12v coach loads off of it. So its entirely possible that even though the inverter shuts down before draining the battery too much, other 12v loads could.

I'm guessing an option from your post might be to reduce the under voltage cuttoff to release voltage difference so that the natural return of the cells to a higher voltage might allow the discharge to turn back on and the inverter to charge?

Since the inverter/charger's low voltage disconnect is too low to play nice with lifepo4 you might consider implementing something like this https://www.amazon.ca/gp/product/B07QDW2WN3
I have not actually tried it but if it does what it advertises it should work.
I would set that to disconnect at ~25 volts.
If you are interested and need help setting up the voltage monitoring relay just holler.

Did you top balance your cells?

 

[techguy22]

2.5 volts per cell or 10 volts at the pack level is dead empty.
Its quite surprising that your bms disconnected on low pack and not low cell.
It suggests that either you have perfectly balanced cells or bottom balanced cells or a malfunctioning bms.
For normal operation I would set the bms to disconnect at 3.0 volts per cell or 24 volts for the pack.

My BMS did disconnect on the low cell voltage, not the pack voltage. My inverter did shutdown before it reached this level though. I have it set now to shut down at 10.5v now and for what its worth it tends to read lower than the actual pack is so it shuts itself down before the pack ever gets near 10.5v. I believe that the pack was at around 11v which would be 2.75v per cell which should be ok. I have a 12v pack btw. I would rather have the inverter turn itself off than the bms if possible.

Since the inverter/charger's low voltage disconnect is too low to play nice with lifepo4 you might consider implementing something like this https://www.amazon.ca/gp/product/B07QDW2WN3
I have not actually tried it but if it does what it advertises it should work.
I would set that to disconnect at ~25 volts.
If you are interested and need help setting up the voltage monitoring relay just holler.

I've looked into similar things to hookup to the inverter because I was concerned about the 10.5v setting being too low also. I would have to hack into the remote control switch for the inverter to make it work. In the end I decide to try to avoid it for cost and complexity of things to go wrong in the system.

Did you top balance your cells?

Yes, I did top balance my cells. They seemed to work together pretty well. I only did one cycle on them so far though. I was hoping to get a good read on capacity but the BMS is not as accurate as I had hoped and I'll need to add a victron smart shunt or similar to the system. I do not have the cells compressed and I did notice the bulging of the cells at high SOC and I want to build a fixture to hold them snug before I cycle thru more times.

My main hope for this post though would be to figure out how to recover from low voltage BMS disconnect where I need to charge the batteries again with shore power and my inverter won't power up because it can't "see" the battery connected.

 

[John Frum]

My main hope for this post though would be to figure out how to recover from low voltage BMS disconnect where I need to charge the batteries again with shore power and my inverter won't power up because it can't "see" the battery connected.

The bms has release trigger to correspond with each disconnect trigger.
When the bms disconnects the cells should rise in voltage as they rest.
Once the bms reconnects the inverter should resume as long as the voltage is above the inverter's release trigger.

 

[techguy22]

Thanks for helping. Ok, so I'll have to do some testing and find that sweet spot. One hitch in my plan is that the manual says that the inverter won't turn back on until it sees 13vdc

From the manual:
Battery voltage recover start
After low battery voltage shut off (10V for 12V model/20V for 24V model/40V for 48V model), the inverter
is able to restore operation after the battery voltage recovers to 13Vdc/26Vdc/52Vdc (with power switch still
in the “On” position). This function helps to save the users extra labor to reactivate the inverter when the
low battery voltage returns to an acceptable range in the renewable energy systems. The built in battery
charger will automatically reactivate as soon as city/generator ac has been stable for 15 seconds.

I just had an idea from looking at the manual... what if I put a simple "wall wart" kind of AC to DC transformer in on the shore power side of the inverter input. Whenever there is shore power it would power up and it could supply 13ish volts to the terminals of the inverter? Maybe it would need some sort of current limiting resistor to keep it from blowing up, I'm not sure. But it would be a way to tell the inverter that there is shore power available and start charging. I would need someone who knows more about electronics to tell me if it would work or not. I think it might because I was able to jump start it with my 10A benchtop power supply when it happened last time.

 

[John Frum]

Maybe it would need some sort of current limiting resistor to keep it from blowing up, I'm not sure.

You are looking for a cc/cv=constant current constant voltage power supply.
The trick is getting one that outputs of 13ish volts and that should work.

"The built in battery
charger will automatically reactivate as soon as city/generator ac has been stable for 15 seconds."

IMHO that is what the inverter/charger ought to be doing.