How does BMS functionality work with multiple lifepo4 batteries

[Mia]

Can someone explain how the BMS functionality works with multiple lifepo4 batteries.

For instance, if I have 2 x 260 amp hr batteries, configured in parallel, each with a 125 amp BMS, does that equate to maximum discharge of 125 amp or 250 amps?

Also, should one be able to run AC appliances, such as a coffee maker, microwave or a small toaster oven (not simultaneously of course) off of a 3,000 watt inverter?

Because, so far, we don’t seem to be able to do so, and hence need to figure out if the BMS is causing the inverter to cut out, due to the maximum discharge being deficient. And if so, would a 150 amp be sufficient; or would we need a 200 amp or even 250 amp BMS in order to run such items?

And, how does one even start to troubleshoot such an issue? The inverter just seems to be cutting out but we have no clue as to what the underlying cause might be other than possibly the BMS max discharge being inadequately low.

PS - We have not experienced issues with the inverter running the 3 way fridge/freezer on AC off the inverter while in transit, nor does it have any issues charging up the e-bike batteries either when we are stopped and camping. But it’s extremely disappointing to have enough battery storage capacity, and enough charging capacity to maintain/charge/top off the battery bank, and an inverter that should be capable to run the AC appliances, only to have something apparently not correct in the current configuration of this offgrid truck camper setup….

 

[sunshine_eggo]

Can someone explain how the BMS functionality works with multiple lifepo4 batteries.

For instance, if I have 2 x 260 amp hr batteries with a 125 amp BMS on both of them, does that equate to maximum discharge of 125 amp or 250 amps?

In parallel, 250A - however, that assumes perfect current sharing between them. Something closer to 80% of the theoretical is likely.
In series 125A.

Also, should one be able to run AC appliances, such as a coffee maker, microwave or a small toaster oven (not simultaneously of course) off of a 3,000 watt inverter?

3000W/12.8/.85 = 275A

Because, so far, we don’t seem to be able to and hence need to figure out if the BMS is causing inverter to cut out, due to the maximum discharge being deficient. And if so, would a 150 amp be sufficient or would we need a 200 amp or even 250 amp BMS in order to run such items?

Of those listed, the microwave is likely the highest at 1600W consumed for a 1000W microwave. 1600W/12.8/.85 = 147A

And, how does one even start to troubleshoot such an issue? The inverter just seems to be cutting out but no clue as to what the underlying cause might be other than possibly the BMS max discharge being inadequately low.

Check that each and every connection in the circuit is properly torqued and of high quality. No shrink pinched between contacts, etc.

Here's a voltage drop calculator:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

I used 2/0 wire that's 5' one way distance between battery and inverter.

PS - We have not experienced issues with the inverter running the 3 way fridge/freezer on AC off the inverter while in transit, nor does it have any issues charging up the e-bike batteries either when we are stopped and camping. But it’s extremely disappointing to have enough storage capacity, and enough charging capacity to maintain/charge/top off the battery bank, and an inverter that should be capable to run the appliances, only to have something apparently not correct in the current configuration of this offgrid setup….

3 way = 12V, 120VAC, propane?

If so, be very cautious. A 7.6cu-ft absorption fridge can consume up to 5kWh/day - that's about 76% of your capacity consumed in 24 hours. If that fridge runs off propane, run it off propane.

 

[Mia]

In parallel, 250A - however, that assumes perfect current sharing between them. Something closer to 80% of the theoretical is likely.
In series 125A.



3000W/12.8/.85 = 275A



Of those listed, the microwave is likely the highest at 1600W consumed for a 1000W microwave. 1600W/12.8/.85 = 147A



Check that each and every connection in the circuit is properly torqued and of high quality. No shrink pinched between contacts, etc.

Here's a voltage drop calculator:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

I used 2/0 wire that's 5' one way distance between battery and inverter.



3 way = 12V, 120VAC, propane?

If so, be very cautious. A 7.6cu-ft absorption fridge can consume up to 5kWh/day - that's about 76% of your capacity consumed in 24 hours. If that fridge runs off propane, run it off propane.

My bad… apparently somewhere along the line, i must have overwrote the additional details in my initial post noting that our current setup consists of 2*260 amp hr lifepo4 with 125 aHr bms on each; 4*100 watt panels flat mounted; a 40 amp SCC; plus a 40 amp DCDC charger, and all the various wiring was more than adequate per the wiring charts.

We have the 40 amp dcdc charger and the 400 watts of solar that we are utilizing while in transit off the inverter and battery bank and we only run the fridge off the AC, inverter etc when we are traveling. Otherwise we’re running it off propane or occasionally alternating with the 12v and propane depending on the current capacity of the battery bank.

Our current configuration is running the 30 amp RV shore power which is plugged into a 30 amp outlet which was wired directly to the inverter using proper size of Romex wire.

That said, everything theoretically then sounds like it should then be fine and should be more than capable for running the appliances off the current inverter and battery bank, for the nominal amount of time they would even be in use, which is what we presumed would be the case.

So, I guess we will need to start by checking the connections and proper torque as you have kindly noted.

I have also been trying to figure out how to test the amount of amps and watts being used with a killawatt meter reader on it…but if we put it on at the shore power connection it wouldn’t be accurate, to the best of my understanding, as it would be charging off shore power which would throw off the readings then as well... And if we put it on the inverter we’re not catching the inverter draw… So maybe I need some coffee and food, but I’m just not understanding how I can capture the inverter as well as the RV draws concurrently….

 

[sunshine_eggo]

My bad… apparently somewhere along the line, i must have overwrote the additional details in my initial post noting that our current setup consists of 2*260 amp hr lifepo4 with 125 aHr bms on each; 4*100 watt panels flat mounted; a 40 amp SCC; plus a 40 amp DCDC charger, and all the various wiring was more than adequate per the wiring charts.

Gotcha.

We have the 40 amp dcdc charger and the 400 watts of solar that we are utilizing while in transit off the inverter and battery bank and we only run the fridge off the AC, inverter etc when we are traveling. Otherwise we’re running it off propane or occasionally alternating with the 12v and propane depending on the current capacity of the battery bank.

Like.

Our current configuration is running the 30 amp RV shore power which is plugged into a 30 amp outlet which was wired directly to the inverter using proper size of Romex wire.

Are you saying you can't run the microwave while connected to shore power?

That said, everything theoretically then sounds like it should then be fine and should be more than capable for running the appliances off the current inverter and battery bank, for the nominal amount of time they would even be in use, which is what we presumed would be the case.

Info:
Wire gauge that's connecting batteries together:
Wire gauge between battery and inverter:
One way cable length between battery and inverter:
Confirm that the main battery connections are across the bank, i.e., main (+) on battery one and main (-) on battery two:

So, I guess we will need to start by checking the connections and proper torque as you have kindly noted.

Poor connections cause the majority of problems.

I have also been trying to figure out how to test the amount of amps and watts being used with a killawatt meter reader on it…but if we put it on at the shore power connection it wouldn’t be accurate, to the best of my understanding, as it would be charging off shore power which would throw off the readings then as well... And if we put it on the inverter we’re not catching the inverter draw… So maybe I need some coffee and food, but I’m just not understanding how I can capture the inverter as well as the RV draws concurrently….

An inverter/charger passes through the shore AC to loads and uses surplus for charging. An inverter/charger can do both, but not at the same time.

 

[Mia]

@sunshine_eggo ,

No, all the AC electrical appliances operate off shore power, and also off the built in generator, as would be expected.
It’s just an issue running the appliances off the inverter.

It is a regular (non dual combo charging) inverter.
We didn’t opt for the combo inverter/charger as we already had a factory converter charger, which then also charges when plugged into shore power; and when not on shore power, it will also charge off the solar charge controller as well as the dcdc charger.

I don’t have the wiring details/info offhand, but we were at either proper min noted, as well as some was over min for what was noted by the requirements in the references and wiring charts.

 

[sunshine_eggo]

@sunshine_eggo ,
I don’t have the wiring details/info offhand, but we were at either proper min noted, as well as some was over min for what was noted by the requirements in the references and wiring charts.

These claims require that the initial computations are correct. Without the requested information, it's impossible to help you. The inability to power a load from an inverter arises almost exclusively from (mostly in order of likelihood):

1. Improperly torqued connections, poor crimps, insulation/shrink wrap trapped between contact surfaces, etc.
2. Undersize wiring.
3. Improper bank configuration (how connections are made to the battery matters).
4. Excessive surge load the inverter can't handle (not typically an issue with a microwave, coffee maker or toaster oven).
5. Load too high for inverter.
6. Inverter fault/failure.

 

[Mia]

These claims require that the initial computations are correct. Without the requested information, it's impossible to help you. The inability to power a load from an inverter arises almost exclusively from (mostly in order of likelihood):

1. Improperly torqued connections, poor crimps, insulation/shrink wrap trapped between contact surfaces, etc.
2. Undersize wiring.
3. Improper bank configuration (how connections are made to the battery matters).
4. Excessive surge load the inverter can't handle (not typically an issue with a microwave, coffee maker or toaster oven).
5. Load too high for inverter.
6. Inverter fault/failure.

No problem! Totally understandable!