Different rated BMSs - normal?

[Texas-Mark]

I have two identical non-smart 12V 100ah lifepo4 batteries. I had a couple of spare Daly BMSs with BT laying around so I swapped out the existing BMSs. One of the Daly BMSs is 100A and the other is 60A. I needed to do a test with a new inverter, so I put the two batteries in parallel with 2/0 cable. Both batteries were fully charged. The inverter was drawing 36 amps from the pack. However the 100A BMS was showing a 22A draw and the 60A BMS was showing 14A. Would this be due to the 100A BMS having less resistance? I would think that at this relatively low draw, they would be closer, despite the different ratings.

Side notes. Both batteries have stand alone active balancers, so there is no top cell imbalance in either. I also swapped the battery positions, not that it should really matter with 2/0 cable and such a low current draw. Of course the 60A BMS has a lower gauge wire, but again, there is not that big of a draw.

 

[timselectric]

It's not surprising.
Even if everything is exactly the same. The draw will not be.

 

[sunshine_eggo]

I have two identical non-smart 12V 100ah lifepo4 batteries. I had a couple of spare Daly BMSs with BT laying around so I swapped out the existing BMSs. One of the Daly BMSs is 100A and the other is 60A. I needed to do a test with a new inverter, so I put the two batteries in parallel with 2/0 cable. Both batteries were fully charged. The inverter was drawing 36 amps from the pack. However the 100A BMS was showing a 22A draw and the 60A BMS was showing 14A. Would this be due to the 100A BMS having less resistance? I would think that at this relatively low draw, they would be closer, despite the different ratings.

First place to check is wiring and connections. Did you parallel the batteries in accordance with best practices? All cell interconnects properly torqued, etc.? Same SoC?

Side notes. Both batteries have stand alone active balancers, so there is no top cell imbalance in either. I also swapped the battery positions, not that it should really matter with 2/0 cable and such a low current draw. Of course the 60A BMS has a lower gauge wire, but again, there is not that big of a draw.

You can still incorrectly wire with 2/0. The thinner gauge wire will have the same effect at any draw, i.e., if the cross sectional area is 50% of the larger, it will always have 2X the resistance.

Maybe also research the reports of failed DALY BMS and DOA units before you place trust in them.

 

[timselectric]

If you monitor it for a while. I'm sure that the larger draw will switch to the other battery.

 

[Texas-Mark]

First place to check is wiring and connections. Did you parallel the batteries in accordance with best practices? All cell interconnects properly torqued, etc.? Same SoC?

Yes, as I noted above.

Maybe also research the reports of failed DALY BMS and DOA units before you place trust in them.

I get that some people hate Daly, but I have never had any issues with them.

You can still incorrectly wire with 2/0. The thinner gauge wire will have the same effect at any draw, i.e., if the cross sectional area is 50% of the larger, it will always have 2X the resistance.

I'm sure the cables are fine. However, as I noted the 60A BMS has a lower gauge I/O wire, so it does not matter how large the parallel cables are as that is the weakest link. I just did not think it would make much of a difference at that low current draw. .

If you monitor it for a while. I'm sure that the larger draw will switch to the other battery.

I could see that if they were not at the same starting SOC, but after an hour they were still reading the same difference.

 

[timselectric]

I could see that if they were not at the same starting SOC, but after an hour they were still reading the same difference.

Current flow is based on voltage and resistance.
Change either one, and the flow will change.
At some point the other battery will do more.

 

[Texas-Mark]

Current flow is based on voltage and resistance.
Change either one, and the flow will change.
At some point the other battery will do more.

I get that, but if I only were to run this setup for a few hours, the two batteries will have a large SOC difference. I have not yet looked to see if charging will have the same proportional difference. I'll look at that tomorrow.

I don't plan to use these specific batteries in parallel long term. This was just set up to test a new inverter to see if it would start a 5K BTU AC (which it did). I was just surprised at the different rate from the two at a relatively low draw.

 

[sunshine_eggo]

Yes, as I noted above.

You simply stated you used 2/0 cable and the batteries were fully charged.

That's one out of three.

You can use 2/0 or even 4/0 or even heavier gauge and still do it wrong.

I'll break them out, so you don't miss them:

Are all the cell interconnects properly torqued?
Did you parallel the batteries according to best practices?

I get that some people hate Daly, but I have never had any issues with them.

I don't hate them. I find the number of reports of failed an DOA DALY a concern. One poster on this site with a high level of technical competence identified a major flaw in his as-received DOA unit.

 

[Texas-Mark]

Are all the cell interconnects properly torqued?
Did you parallel the batteries according to best practices?

Best practices? It's not rocket science to connect two batteries in parallel. I've been doing solar and batteries for decades. Just never used two different rated BMSs in parallel before. At least not two different ones that I could monitor.

I don't hate them. I find the number of reports of failed an DOA DALY a concern. One poster on this site with a high level of technical competence identified a major flaw in his as-received DOA unit.

There are reports of failed BMSs of all brands. These have been working fine, I just no longer have the packs I originally installed them in.

In any case, I threw a charger on them and see the same disproportional current while charging. So I am going to just chalk it up to the 60A unit having more resistance either due to the smaller BMS wires, or internal resistance.

 

[timselectric]

60A unit having more resistance

I agree

 

[sunshine_eggo]

Best practices? It's not rocket science to connect two batteries in parallel.

No it's not, but it's done incorrectly resulting in uneven load an absurd number of times.

I've been doing solar and batteries for decades.

Was I supposed to know this?

There are reports of failed BMSs of all brands. These have been working fine, I just no longer have the packs I originally installed them in.

DALY failures are notably overrepresented.

In any case, I threw a charger on them and see the same disproportional current while charging. So I am going to just chalk it up to the 60A unit having more resistance either due to the smaller BMS wires, or internal resistance.

cool.