diy solar

diy solar

Replaced ruptured cell, and now my bank won't stay balanced :(

hunchan

New Member
Joined
Jun 11, 2022
Messages
4
About three years ago, I built a small solar power system for my ambulance. I used it regularly over the past couple of years to run corded power tools and charge cordless tool batteries in a workshop. The other day, I noticed my tools beginning to run out of power, which was surprising because I had never discharged the pack to the point of failure before. When I opened up the battery bank, I found that one of the cells had ruptured, so I ordered one replacement 280AH LiFePo4 cell. After receiving the new cell I first top balanced the cells by connecting them in parallel and charging them to 3.5V using a variable power supply. I then reassembled the pack and reinstalled it in the ambulance. After using my tools for about a day. The cells had already gotten out of balance. The old cells all read in the 3.5-3.6 range, but the new cell stays around 3.35. Also, now the charge controller never switches to float mode. It just continuously charges the batteries. The battery light on the controller remains solid green, whereas before it always changed to flashing green after a days worth of charging.

I should add that the original cells were these VariCore batteries purchased from AliExpress, but I replaced the ruptured cell with an EVE cell purchased from Docan Power. When I first assembled the bank three years ago, I did not top balance them before assembly. I was a total novice and I did not know that was advised. Also, the batteries are in a pretty hot environment. In the summer it is usually well over 100°F in the back of the ambulance.

Here is a list of the main components I used to build this system:
  • 4 x 100W Grape Solar panels wired in parallel
  • Renogy Wanderer Li 30A 12V PWM
  • 4 x VariCore 3.2V 280AH LiFePO4 batteries
  • Daly 4S 120A BMS
  • Generic 2000W inverter

What can I do to regain balance?
 

Attachments

  • PXL_20220620_222843329.jpg
    PXL_20220620_222843329.jpg
    159.7 KB · Views: 48
  • PXL_20220620_223035167.jpg
    PXL_20220620_223035167.jpg
    336.9 KB · Views: 47
  • PXL_20220620_223135422.jpg
    PXL_20220620_223135422.jpg
    288.5 KB · Views: 47
Last edited:
First, triple check all your connections for proper torque and connection quality. Inspect the sensing ring terminals and connections for quality.

Given the different cell brands, ages and sources, the expectation is that they will all have different capacity and balance may be difficult to attain and maintain.

3.5V is a little low to insure saturation on new cells.

Recommend you use your power supply set at 3.65V on the low cell to bring it up to saturation and also apply thereafter to each cell.

If an option, recommend you reduce absorption to 13.8V and allow for 6 hours of absorption. This should allow you to attain very high states of charge and should help facilitate passive balancing.
 
Thanks for your help. I'm charging up the new cell on its own right now. I'll let you know how things pan out.
 
Just a note about the charging.
- Set your Variable Power Supply to the correct voltage (3.550 to 3.650*) good range BEFORE attaching to the cell(s).
- Let it charge to the set point AND WAIT till the amps drop to around 2A-3A (for a 280). This fully saturates the cell (this is why 3.550-3.600) is best.
- It is actually best if you get the New Cell up to match the others, then with cells in Parallel take it to the top setpoint (ie: 3.600) and let the Lot Saturate to 2-3A charge being taken.

* 3.650 can be done BUT depending on the class of cell
 
Comments,

The Renogy PWM charger is not the best at charging lithium batteries and after 3 years may have damaged all the cells and could account for the ruptured cell. I suggest testing each cell individually.

On a practical note, the fuse and holder in the positive feed from the battery is totally unsuitable.

Mike
 
Well, its been a few days since I saturated the new cell. All of the cells are now remaining in the range of 3.54-3.56, so it seems to have worked.
 
On a practical note, the fuse and holder in the positive feed from the battery is totally unsuitable.

Could you explain what the problem is with the inline fuse. It is rated for 100 amps. I tried using a 1500W heat gun once and the fuse melted, so it seems like it is effectively protecting the system. What do you think about this one?

 
The fuse holder and fuse is for car audio. It's a fire risk. It does not have the withstand capability for a lithium system. It will have significant volt drops and thus heating at high currents.

Mike
 
Back
Top