LFP cells will bloat if overcharged
The photo shows the results of an unattended charging from a regulated charger that had a failure. The cells were kept charging up to 4.8V per cell.
The result is obvious: due to the overcharge the cells start swelling and the housing expands.
On the other hand, this accident proved that LiFePO4 cells are a quite safe battery technology. In this hazardous situation, there was no significant temperature increase, no gas leak, no explosion, no fire. (With lithium polymer cells - so called „lipoly“ cells - such kind of over charge would result in an uncontrolled fire.)
The lesson taken from this accident:
- Never left batteries charged unattended.
- Always install multiple protections to have a backup protection in case of failure of the primary device.
- Operate batteries in places where there is a proper fire safety (not on a wooden table in a closed room).
- Even professionals can make a mistake or meet with a failure that can lead to an accident.
The lithium iron cells do swell by overcharging. One of the solvents used in the electrolyte has a boiling point of 77C. If you overcharge enough to reach this temperature (internally) it will gas and cause the swelling. It will also cause the cell to vent in most cases.
Hint: pressing the swelling cells using the carpenter’s holders or clamps
The example of a large-scale cell damage caused by overcharge
The damage was caused by using a high-speed charger (30A) designed for charging 16 cells with a pack of just 15 cells without proper balancing and management. The voltage of some of the cells reached high above the max voltage level. The result was the swelling of the cells and partial damage of the cells.
FAQ: What to do with a swelling cell?
First: Analyze the reason for the swelling. Is the battery used properly? Is the BMS working correctly? Note: Do not use the cell any longer, if the cell has become swollen.
Second: Remove the cell, unscrew the safety valve to allow the internal pressure to be released. (Be careful not to spil the electrolyte from the cell).
Third: Put the cell under the pressure to renew its original shape. You may need to increase the pressure gradually as the cell returns to its correct shape. Please allow for many hours or even days to pass to renew the shape.
Fourth: Charge the battery correctly and check its parameters: What is the voltage? What is the capacity? Perform several cycles to get consistent results and to see if the cell can be used again.
Finally: Avoid the same situation to happen again. Remember there is no way to repair a cell that was damaged by improper use.
The packing and mounting of the LFP cells
In order to prevent the swelling of the prismatic cells, it is recommended to mount the cells with “jigs and straps”. The swelling of the cells usually occurs when: 1) the cells are overcharged, 2) there is a repeated high speed charging and discharging, 3) there is some fault of the cells.
Check our suggestions and hints under the swelling tag.
Lithium-iron-phosphate batteries from Thunder-Sky …. as they heat up, they can swell and crack the casing if they’re not squeezed together.
Solid casing for the LFP battery packs needed: Lithium-iron-phosphate batteries from Thunder-Sky. They aren’t as sensitive to temperature, but they do need to be kept under pressure. This bracket is part of a vice I’m building for the batteries. As they heat up, they can swell and crack the casing if they’re not squeezed together. (source)
FAQ: The Cell and Battery Swelling
It is necessary to observe the recommendations of the manufacturer concerning the initial charging of the cells. (See the instruction here). If the cells are initially charged too fast or overcharged, the internal structure of the cells may be damaged and the cell may start swelling. That is why it is important to charge the cells individually during the initial charge and to monitor the cells to avoid the permanent change of the shape of the cells.
Battery assembly using a packing straps
The simple solution to assemble individual cells into the battery packs: strapping them together by the plastic staps used for packing of parcels.
"The cells that are not cylindrical tend to swell up when
ever they are not compressed, even if they are used correctly.”