Large draw/batteries shut down

[cdrobbins77]

Currently running 2 300ah lifepo4 batteries (chins) in parallel, tied to a 2500w inverter. Plenty of power, no complaints at all EXCEPT for 2 circumstances in which my electrical system completely shuts down (ac side AND dc side), then turns back on moments later:
1. Microwave runs for longer than 4 minutes (approx) at a time
2. Mini split AC runs for 4-5 minutes

I originally thought i should upgrade to a 3000w inverter, but now i suspect it may be that the high current draw in these scenarios is making the bms shut down the batteries. Anyone have any thoughts?

[rffffffff]

Because the dc side is shutting down too I would guess bms too.

Low voltage cutoff could do it and it could be undersized wiring, imbalanced wiring or poor connections that are heating up and increasing resistance I would think… I’d look at the specs for the batteries and see if you’re hitting the high end of the allowed current draw.

2500w at 24v is 100 amps, 200 amps at 12v. Seems like neither device would hit 2500w by itself and the time component seems to lean towards a voltage drop or connection thing.

What are the current specs for the batteries and what kind of wiring do you have coming out and interconnecting the batteries?

[cdrobbins77]

Originally Posted by rffffffff

What are the current specs for the batteries and what kind of wiring do you have coming out and interconnecting the batteries?

4/0 for my battery wiring, and lengths are balanced. Per the battery specs the max continuous discharging current is 200a

[TheHubbardBus]

Could it be that the onboard BMS is not compatible with parallel installations?
Are you aware of what the battery SOC was during these two incidents?

[cdrobbins77]

Originally Posted by TheHubbardBus

Could it be that the onboard BMS is not compatible with parallel installations?
Are you aware of what the battery SOC was during these two incidents?

Batteries are compatible with parallel installs per the instruction manual. SOC was at least 70-75% each time.

[TheHubbardBus]

And those were the only significant loads at the time?
200A continuous is per-battery?

[Rucker]

Can you list the models of your battery and inverter? And do you have an external BMS like Daly or is it built into the battery?

Agreed that with details provided it looks like the BMS is trying to regulate a high current draw. Is the BMS configurable? Can you monitor the current draw, like through a shunt, to see what it is peaking at before it shuts down?

If the current draw rises over that four minutes, that could be an indication of a weak component heating up.

[cdrobbins77]

Batteries are Chins (inexpensive chinese batteries on amazon) with built in bms. I assume they are not configurable. Other than this issue i have been really happy with them.
Inverter is Aims 2500 pure sine wave inverter/charger

There is a possibility that the 7.6 cuft fridge is kicking on causing a surge. I could unplug the fridge and experiment i suppose.

I do have a battery monitor shunt, but not sure that's any help in diagnosing

[rffffffff]

Originally Posted by cdrobbins77

Batteries are Chins (inexpensive chinese batteries on amazon) with built in bms. I assume they are not configurable. Other than this issue i have been really happy with them.
Inverter is Aims 2500 pure sine wave inverter/charger

There is a possibility that the 7.6 cuft fridge is kicking on causing a surge. I could unplug the fridge and experiment i suppose.

I do have a battery monitor shunt, but not sure that's any help in diagnosing

I completely agree that monitoring the shunt current during the run up to shutdown is a great plan, as is monitoring the fridge or unplugging it. Feeling cables for heat could be interesting too, but 4/0 will be fine, maybe just connections or inter-battery connections.

[TheHubbardBus]

Assuming there aren't connection problems / irregularities causing one battery to bear the brunt of the load, and there's nothing else drawing power besides what you've listed, and they're actually rated at 200A continous per battery, I don't see how your refrigerator's start-up draw would cause this problem unless there was something internal to the batteries/BMS not functioning as it should.

I'm thinking one (of many) possibilities could be one or more unbalanced cells triggering an undervoltage event. Anyone else see this as a likelihood?

[Rucker]

Originally Posted by TheHubbardBus

Assuming there aren't connection problems / irregularities causing one battery to bear the brunt of the load, and there's nothing else drawing power besides what you've listed, and they're actually rated at 200A continous per battery, I don't see how your refrigerator's start-up draw would cause this problem unless there was something internal to the batteries/BMS not functioning as it should.

I'm thinking one (of many) possibilities could be one or more unbalanced cells triggering an undervoltage event. Anyone else see this as a likelihood?

The fridge will draw close to a hundred amps on start. My little dorm fridge draws about 80, very briefly (remember the inrush is several times the running current, and the DC side is 10x the amps of the AC side). That might be enough of a voltage sag to trigger the BMS. Unplug the fridge and see if it makes a difference.

Cell issue unlikely thought possible, and apparently not verifiable. If the batteries were quite old, more likely.

[TheHubbardBus]

Gotcha. Thanks Rucker

[john61ct]

I doubt both the inverter and the BMS are faulting at the same time.

You need to datalog the bank level voltage at the posts, as well as AC output from the inverter.

Ideally current as well.

Really, access to internal voltages at the cell/group level for imbalance issues. That needs to be a deal-breaker requirement of your battery selection process.

So many drop-ins are just crap.

Meantime, run a little quiet inverter genset and the biggest possible charger to support the bank when running those kinds of loads.

[cdrobbins77]

Originally Posted by john61ct

Meantime, run a little quiet inverter genset and the biggest possible charger to support the bank when running those kinds of loads.

And that's exactly what we currently do, and like I said --otherwise we've been very pleased with these batteries considering the savings versus battleborn. Perhaps someday i will build my own battery bank. Thanks all for the input

[Rucker]

Originally Posted by cdrobbins77

Batteries are Chins (inexpensive chinese batteries on amazon) with built in bms. I assume they are not configurable. Other than this issue i have been really happy with them.
Inverter is Aims 2500 pure sine wave inverter/charger

There is a possibility that the 7.6 cuft fridge is kicking on causing a surge. I could unplug the fridge and experiment i suppose.

I do have a battery monitor shunt, but not sure that's any help in diagnosing

I took a look at the specs in Amazon. Your BMS is definitely kicking in; most likely due to maxxing out the current or possibly due to a significant voltage sag.

Also, did you make sure that both batteries were charged up separately at the start, and left to 'soak' so you could get a very accurate voltage reading from each? If one is slightly more discharged than the other (this could be from initial setup or, perhaps, having your negative and positive feeds on the same battery and not one lead on each) you may not be able to tell one is hitting the BMS threshold. Do you have a way to monitor your voltage? Maybe one of the batteries is closer to discharged than you think? The internal BMS will turn the device off at something slightly less than 10 volts I think.

[cdrobbins77]

Originally Posted by Rucker

I took a look at the specs in Amazon. Your BMS is definitely kicking in; most likely due to maxxing out the current or possibly due to a significant voltage sag.

Do you have a way to monitor your voltage? Maybe the battery is closer to discharged than you thing? The internal BMS will turn the device off at something slightly less than 10 volts I think.

I have verified voltage on the battery monitor matches voltage at the battery terminals, they are definitely charged enough

[Rucker]

Then I'm back to thinking it's maxxing the amp draw due to your fridge.

[john61ct]

consider using a 'soft start' device

[BussBry]

I had this same issue. my problem was that
1. in trying to baby my Lithium batteries I set the V to low 3.65 was max and I had it set to 3.35 the discharge curvature on lithium is a cliff not a nice steady decline so onec it dipped low it quickly dropped to 2.8 my cut off.
2. my batteries 3.65 lithiums in series were NOT top balanced so just one cell in the series was hitting 2.8 before all the others. I top balanced them all.
3. I upgraded bus bars and sanded all connections to fix resistance between cells.
4. Top charged to 3.63 set my BMS and charger to 3.55. No more problems.

Look at your individual cells to look for inconsistency.

[cdrobbins77]

Originally Posted by BussBry

I had this same issue. my problem was that
1. in trying to baby my Lithium batteries I set the V to low 3.65 was max and I had it set to 3.35 the discharge curvature on lithium is a cliff not a nice steady decline so onec it dipped low it quickly dropped to 2.8 my cut off.
2. my batteries 3.65 lithiums in series were NOT top balanced so just one cell in the series was hitting 2.8 before all the others. I top balanced them all.
3. I upgraded bus bars and sanded all connections to fix resistance between cells.
4. Top charged to 3.63 set my BMS and charger to 3.55. No more problems.

Look at your individual cells to look for inconsistency.

Appreciate the advice. Unfortunately these are drop in 300ah batteries and i am afraid to disassemble them to get to individual cells. Not something i've ever done before... Want to come over and help? 😉