Hey all

So I have 3 x TheSunPays 4.8kw LiFePO4 batteries.

2 of them were purchased in February 2020, the 3rd was purchased about 2 months ago.

I am having a problem of uneven discharge, the new battery discharges faster and ends on 17% SOC while the other 2 will end at 40% SOC.
The voltages across all 3 are the same (including cells).

I contacted TheSunPays and they say the new battery will discharge faster due to the lower internal resistance because its newer, which makes perfect sense.
Is there anyway around this to get the batteries to discharge at a similar rate as we are loosing about 22A of capacity due to this and the new battery would in turn degrade faster.

 

All 3 batteries BMS are connected together and the positive from the inverter is connected to the top battery with the negative at the bottom battery.

 

27 minutes ago, 87 Dream said:

Hey Bl4d3 

What inverter are you using ?

The off garage on youtube has some very interesting viewing. I would suggest you get an hour or two & watch this guys videos on the subject. Extremely useful fundamentals he plays around with & he will give the explanation much better than I could. But in basic words. Voltage can tell you nothing about the state of charge of a lithium battery. I would suggest that you charge each battery to full capacity, using a long period for complete absorption of each battery pack. Once they have all undergone the above then can you connect them together in the way that you want.

To charge a lithium battery you need to understand the concepts of Bulk, Float & Absorption stages & what each means & what each tries to achieve.

https://youtu.be/chaTAH4SG3w

This guy is really awesome at making the basics understood without using complicated words.

87

Thanks for the recommendation, will most definitely watch this evening.

I have 2 x TheSunPays 4kw Axpert MKS II Clones.

So I'm charging the batteries up now with a bulk charge voltage of 53.2v and a float of 52v and will let them sit for 2 days for the absorption to full run.

I previously did this over the weekend and let them sit over the weekend to try balance out and straight after that there was that uneven drain.

 

I noticed on another post that someone had a similar issue but with Pylontechs : 

 

I would have thought that after all most 2 months it would have evened out though.

Hey 87

We fully charged the batteries and kept them fully charged, connected to each other for 2 days.
Sadly it did not solve the problem.
 

From 100 - 75% the batteries discharge evenly, but as soon as it goes below 75%, the new takes over 50% of the current draw while each of the other 2 take 25% each.

TheSunPays is monitoring the batteries remotely though and are talking with their factory as well.

All wiring is the same length and same thickness.

One thing I did find now is that the new battery has a different firmware version than the older 2.

That might be the issue, I have sent the details onto the manufacturer 

Got feedback from TheSunPays, they're going to swap the battery out due to the firmware mismatching.

 

I must say though, the service I've received from them has been superb.

Is this battery the same as the Sacredsun? 

22 hours ago, skinnyD said:

Is this battery the same as the Sacredsun? 

It appears to be different

Received the replacement battery yesterday, fully charged the old batteries and the new battery separately. Let them sit connected to each other for a while to balance out and put it back into service.

Seems to have resolved the problem. They are discharging evenly and even when 1 goes slightly our of balance, the BMS kicks in and draws more from the other packs so that they can become balanced again.

System is working perfectly so far.
Batteries all most carried us the whole night as well