Difference in amps consumption between 2 parallel LifePO4-battery banks

[HaraldK]

Hi everyone!
I’m new at this forum and run into the following:
A few weeks ago I installed two Ninthcit LIFEPO4-battery banks in my sailing boat. They are connected in parallel. Each bank consists of 4 cells serial connected making it a 12v system and each bank has its own BMS.

I noticed that since a few days the amps consumed are unequally drawn from the battery banks and I can’t find the cause. For instance: if I use 78.5 Amps, one bank provides 44.1 amps and the other 34.4.

Also at lower consumption levels there is a difference. During last night one battery bank provided all 3 amps per hour and the other bank zero amps.

- I checked all the terminals to be clean, well fixed, etc.
- I switched the minus cable of both BMS’s to see if the difference is caused by one of them. The difference of each battery pack remains the same, only measured now by the other BMS. That kind of rules out that one of the BMS’s causes the problem.
- I switched the + cables from both battery packs, but made no difference to check if the internal resistance of the wires is different. Made no difference.
- the + cables as well as the - cables of both packs are of equal length, quality, age and thickness.
- It is a new battery system (few weeks) and until just a few days ago these differences were not occurring.
It seems preferable to me that both packs use the same amount of amps all the time.

My question is:
Is it usual that these differences occur?
If not, what can be causing the difference and what solution is there?

Looking forward to your replies.

Thanks!

 

[FilterGuy]

Is it usual that these differences occur?
If not, what can be causing the difference and what solution is there?

There will *always* be some differences but your example is a bit more than I would expect.

The cause could be many things, but almost everything comes down to resistance in the paths.

Inside the battery.
* In a decently manufactured battery, the most resistance will be through the BMS. If the BMS resistance of one is very different than the other, you will get a substantial difference in discharge rate.
* Of course all the wiring and connections inside the battery are opportunities for differences in resistance.

Outside the battery.
How the two batteries are wired can make a difference. I don't know your configuration but one common mistake is to put both the Positive leas to the inverter and the negative lead to the inverter on the same battery. It is better to put the Positive lead to the inverter on the positive lead of one battery and the negative lead of the inverter to the negative lead aon the other battery.

I assume you have two wires connecting the two batteries in parallel, Are these as identical as you can make them? Length, lug type, wire size, etc. Differences in resistance in these two wires can create a discharge imbalance. A bad crimp on one of them can create a huge imbalance.

You already mentioned you cleaned the connections. Did you check all of the lugs to make sure they don't have burs or bends that might prevent a good connection on the terminals?


Try swapping the two batteries, if the imbalance follows the battery, the problem is related to the battery. Otherwise, the problem is in the external wiring.

 

[HaraldK]

Both positive and negative leas of consumers/charging sources (shore power, dc dc charger behind the alternator and solar power) have a bus bar. The + and - of the inverter are connected to their respective bus bars as well.

I swapped the batteries. Most of the problem followed the battery. There was only a small difference. I will do it again in the morning ( night here now).
The strangest thing is that at some point this evening they were both drawing 2.7 amps, according to the BMS’s. So totally equal.

I will have a closer look at the lugs tomorrow as well. Many thanks for now!!

 

[HRTKD]

Put a heavy load on the system and let it run for a few minutes. Then, with your finger, touch each connection and cable. If you find a spot that is hotter than the rest, you've found your problem. If you have an infrared thermometer, that is a more objective way to look for the hot spot.

Does each battery have its own fuse?

Have you checked the voltage at every connection?

 

[RCinFLA]

Even if your connections, and cabling resistance is perfectly matched, battery matching has a large effect on paralleled batteries current sharing.

Battery matching is not just the same capacity for batteries. It also means the same overpotential cell terminal voltage slump for the same cell current.

Overpotential is the required overhead required to drive the cell lithium-ion migration for a demanded cell current. It has an exponential time decay. For an LFP cell, the decay to a level voltage equilibrium takes 1-3 minutes to settle.

It is much more significant than the mostly conductive resistance of a cell measured with a 1 kHz impedance meter, like a YR1035+ meter.

For example, a 280 AH cell may measure 0.2 milliohm when measured with a 1 kHz impedance meter. At 40 amps cell current that would yield a 0.0002 ohms x 40 amps = 8 mV of voltage drop from rested open circuit cell voltage, but if the voltage directly across the cell's terminals is measured after about 3 minutes of 40 amp load on 280 AH cell @ 25 degs C, the voltage drop from rested open circuit voltage will measure between 40mV and 70mv on a good 280 AH cell. That voltage difference on paralleled cells under load current has a large effect on the load current sharing between the cells.

As cells age, from many charge/discharge cycles, their overpotential voltage slump increases for same cell current. Over a cell's useful life, it will increase 3x to 5x. This is why you will have more current sharing imbalance when an older battery is paralleled with a younger battery.

 

[HaraldK]

Put a heavy load on the system and let it run for a few minutes. Then, with your finger, touch each connection and cable. If you find a spot that is hotter than the rest, you've found your problem. If you have an infrared thermometer, that is a more objective way to look for the hot spot.

Does each battery have its own fuse?

Have you checked the voltage at every connection?

I put 80 amps on the system for a few minutes and checked every cell terminal and lugs. There wasn’t any significant raise in temperature in either one of them. Unfortunately I don’t have an infrared thermometer to check more precise. I could try to find one here in the marina and do that later.

Don’t have a fuse on each battery pack (yet).

Voltage of each individual cell is the same.

 

[HaraldK]

I put 80 amps on the system for a few minutes and checked every cell terminal and lugs. There wasn’t any significant raise in temperature in either one of them. Unfortunately I don’t have an infrared thermometer to check more precise. I could try to find one here in the marina and do that later.

Don’t have a fuse on each battery pack (yet).

Voltage of each individual cell is the same.

There will *always* be some differences but your example is a bit more than I would expect.

The cause could be many things, but almost everything comes down to resistance in the paths.

Inside the battery.
* In a decently manufactured battery, the most resistance will be through the BMS. If the BMS resistance of one is very different than the other, you will get a substantial difference in discharge rate.
* Of course all the wiring and connections inside the battery are opportunities for differences in resistance.

Outside the battery.
How the two batteries are wired can make a difference. I don't know your configuration but one common mistake is to put both the Positive leas to the inverter and the negative lead to the inverter on the same battery. It is better to put the Positive lead to the inverter on the positive lead of one battery and the negative lead of the inverter to the negative lead aon the other battery.

I assume you have two wires connecting the two batteries in parallel, Are these as identical as you can make them? Length, lug type, wire size, etc. Differences in resistance in these two wires can create a discharge imbalance. A bad crimp on one of them can create a huge imbalance.

You already mentioned you cleaned the connections. Did you check all of the lugs to make sure they don't have burs or bends that might prevent a good connection on the terminals?


Try swapping the two batteries, if the imbalance follows the battery, the problem is related to the battery. Otherwise, the problem is in the external wiring.

There will *always* be some differences but your example is a bit more than I would expect.

The cause could be many things, but almost everything comes down to resistance in the paths.

Inside the battery.
* In a decently manufactured battery, the most resistance will be through the BMS. If the BMS resistance of one is very different than the other, you will get a substantial difference in discharge rate.
* Of course all the wiring and connections inside the battery are opportunities for differences in resistance.

Outside the battery.
How the two batteries are wired can make a difference. I don't know your configuration but one common mistake is to put both the Positive leas to the inverter and the negative lead to the inverter on the same battery. It is better to put the Positive lead to the inverter on the positive lead of one battery and the negative lead of the inverter to the negative lead aon the other battery.

I assume you have two wires connecting the two batteries in parallel, Are these as identical as you can make them? Length, lug type, wire size, etc. Differences in resistance in these two wires can create a discharge imbalance. A bad crimp on one of them can create a huge imbalance.

You already mentioned you cleaned the connections. Did you check all of the lugs to make sure they don't have burs or bends that might prevent a good connection on the terminals?


Try swapping the two batteries, if the imbalance follows the battery, the problem is related to the battery. Otherwise, the problem is in the external wiring.

The + cables connecting the packs in parallel are the same length, diameter, quality, age, and terminals are the same. Also the resistance of the cables is the same. To make sure I swapped them and looked at the BMS’s. No difference with before the swap.
The - cables are the ones from the BMS delivered by the manufacturer. They connect together first and then go with one cable to the minus bus bar.

To check if the resistance of the BMS is the problem, I swapped both B- cables and checked the amps use. The problem remains with the battery pack 2 since the other BMS showed the lower output of battery pack 2.

I checked the lugs. They all seem to be okay. I put battery grease on each cell terminal but unfortunately that didn’t do the trick either.

The first two pictures show the difference between pack 1 and 2 before the BMS-swap.
The second two pictures show the different after the swap. The lower output follows the battery, not the BMS.

 

[HaraldK]

Each cell is 3.2V 280Ah.
Is there a way to test the internal resistance of each cell to determine which cell could be the problem?

 

[mikefitz]

Comments.
There may be calibration error in the BMS. Confirm actual current with a DC clamp meter.
The last picture shows a low battery voltage. A large cell inbalance with a delta of 0.3 with low cell voltage below 3.0, this suggests issues with this battery.
If the batteries are DIY check all connections between cells and BMS leads.

Mike