4s21p 32650 build-one section reaching overvoltage well before other sections

JohnJayMack

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Joined
Dec 22, 2020
Messages
25
I just completed my 3rd battery build and have run into a problem. I think I have a bad cell and want to verify my findings and approach before I tear the battery apart.

My build is a 21p4s battery using 6000mAh 32650 cells. This should give me a 126 Ah battery by calculation. The first two batteries built this way tested out at a little over 130 Ah. After the initial build, I brought the battery up to a section voltage of 3.65 volts using a iCharger x8 to provide the charge and monitoring functions. The pack charged normally the sections seemed to balance properly with and ending top balance variation of <1mv per section.

I finished installing the BMS and then did an additional top balance using the iCharger again and monitoring both the iCharger and the BMS. All readings between the two units match up. I did my first complete discharge but through my error, I did not catch the final Ah quantity.

I recharged the battery back to a top balance monitoring the BMS and the iCharger readings. The battery appeared to balance properly. I did another complete discharge which ended showing only 120 Ah. My discharge rate was a fairly steady 8.2 amps for approximately 11 hours and a final discharge rate of approximately 6.2 amps. My load was a pure sine wave inverter powering a floor fan. The discharge was monitored through a basic power monitor looking at a current shunt.

I put the battery on the iCharger looking to bring it back up to 3.5 volts per section.

I disconnected the iCharger and the BMS. With a Fluke 117 meter, I tested the section to section voltage and compared to the BMS and iCharger reported section voltages. My readings were within 5 mv of the BMS and iCharger readings. From negative to positive, my readings were 3.337, 3.337, 3.338, and 3.492. This last section is 0.154 volts high.

I hope that someone can verify my logic in the following analysis.

1. My observed amp Ah rating is almost exactly 6000 mAh off of calculated with would represent 1 cell not charging/discharging properly. (Note, other two batteries using same cells have exceeded calculations.

2. On the first recharge, everything worked properly. On second test, charge aborts because one section reaches high voltage before other sections. The most positive section hits the high voltage first, which would indicate a weak or bad cell. BTW, the iCharger provides active balancing during charge and has always kept variation t less than 50 mv.

My conclusion is that the most positive section has developed a bad cell.

Am I analyzing this properly? And is there a simple way to identify a bad cell in a 21p section without disassembly of the section?
 

JohnJayMack

New Member
Joined
Dec 22, 2020
Messages
25
I solved my problem with some help from another site. Basically, I could not tell a temperature change during charging because my charging capabilities are limited to about 30 amps. Also, the delta v was stopping the charge early. I charged very slowly using an active balancer until I got my entire pack up to approximately 3.65 volts per section. I then did a heavy discharge of approximately 110 amp for about 45 minutes. Using a IR thermometer, I found 1 cell with an elevated temperature of approximately 8 C above its neighbors. Over the course of the 45 minute discharge I measured multiple times to eliminate false readings. This one cell always read higher than any neighbors or any other non-neighbor cells. I replaced that cell, a pain in the posterior to do, and now the pack is behaving normally.
 
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