Replacing lithium ion cells in an old laptop battery pack, I want to connect the new cells in parallel before removing the old ones to avoid the BMS locking (if it even would do that). The pack uses three parallel pairs of cells in series to make 10.xx volts with low and medium taps going to the BMS.
This is why it’s a good idea to disassemble lithium-ion battery packs for its cells. In most other cases, just a single cell has failed. Remember, battery packs are made of many cells that are grouped in a specific way. So, if one cell dies, it will bring down the cells that it is immediately attached to.
The repair process begins with a thorough cell inspection and testing. As battery cells are the essential components of any lithium battery pack, it is important to ensure they are in good condition before continuing with the repair. The first step is to conduct a voltage test on each individual cell.
The following steps should be followed in order to reassemble the battery pack correctly: Ensure that all components of the lithium battery pack are present, including cells, wires, terminals, and case cover. Assemble the cells into their respective terminal connections.
If a relatively new pack has only one defective cell and a replacement is located, exchanging the affected cell makes sense. With an aged battery, however, it’s best to replace all cells. Mixing new with old causes a cell mismatch that has a short life. In a well-matched battery pack all cells have similar capacities.
If you suspect that your lithium battery is failing, it's best to replace it rather than continue to use it, as a failing battery can pose a safety risk. How Much Does It Cost To Repair A Lithium Battery Pack?