2 batteries same but different

[FooFighter]

I bought 2 160 ah each lifepo4 batteries with Bluetooth for my rv which came with 2 group 27 led acid in parallel. Have them in about a week and I notice that battery 1 chrg/dischrg slower then the other even though both batteries have same voltage and even the cell voltage is almost identified from one battery to the other. So if charging with 30 amps battery 1 will get about 11 while battery 2 gets 19. Same with discharge if pulling 10 amps battery 1 will about 3 while battery 2 pulls 7. Emailed support and replied with please use a busbar. If it was a wiring,voltage drop,wire resistance issue the issue would change if I swap the batteries. Put battery one were battery 2 is and vice versa wiring exactly the same but battery 2 still gets the higher amps in and out so a busbar will not help. One of the problems with this is capacity starting the same when charged and after some use battery 2 will be much lower in capacity then battery one. Am I wrong that it's a difference in the batteries. And which of the 2 is better the one that chrgs/dischrgs faster or the one that does it slower.

 

[sunshine_eggo]

See the resource in line #6 of my signature for best practices.

Have you taken both batteries to 14.4V for at least two hours?

 

[Mattb4]

All else accounted for your batteries (cables, connectors, fuses, terminals, and other resistance type things) may not have the same capacity. This does not mean that one has below rated capacity, it could mean that one battery has much better than rated capacity.

 

[FooFighter]

Which one? Switching the battery placement eliminates anything to do with wiring etc. I understand a little about internal resistance but on lifepo4 to be so much difference between batteries. I may have another week before returning is expired. So if I was gonna return 1 of the 2 which one the one that chrg/dischrg faster or the other?

 

[GXMnow]

Like @sunshine_eggo asked... Did you charge it to 14.4 and hold it there a while?

Most likely, the batteries are just at a different state of charge. This is an issue with LFP batteries. They can be more than 20% off in SoC while measuring the exact same voltage. The only way to really balance them is to bring them up into absorption. As the cells go above 3.45 volts, the fully charged cells will try to climb faster. When you have two batteries in parallel, the one closer to fully charged will have greatly reduced current draw as their voltage tries to climb and the lower SoC battery ends up taking most of the current.

The only thing against this being the case is, when discharging, the higher SoC battery should begin to push a bit more current. Since you said it is going the other way, you are likely low in the SoC curve. And then the lower SoC battery will be the one showing less current. This can also happen while charging because the lower SoC cells also have higher internal resistance. This makes it take much longer for the cells to truly balance. It took several large cycles for my NMC pack to find their balance. And the newer cells still show a little more current in both charge and discharge because they have about 5 to 10% more total capacity. The current difference tracks the capacity difference.

You could try doing a full capacity test on each battery and see how different they are. This should also bring them into balance as they both will hit full charge separately. Do you have something like a "Smart Shunt" to measure energy in/out of a batery?

 

[sunshine_eggo]

Like @sunshine_eggo asked... Did you charge it to 14.4 and hold it there a while?

Most likely, the batteries are just at a different state of charge. This is an issue with LFP batteries.

Yep!

 

[FooFighter]

No smart shunt as of yet.