As discussed, individual cells in a li-ion pack need to be monitored, and should be kept matched to each other to prolong the pack’s service life. This real-life example shows what can happen if cells are not kept in balance. This is a battery pack from a cordless power tool. The user of the tool observed decreasing battery life after a number of usage cycles. The pack configuration is 6-series Li-Ion. When the pack was measured from the external terminals, it showed around 20V, which implied that the battery was in a mostly discharged, but in a normal range. However, the pack could not be used to operate the tool even after repeated attempts at charging. When opening up the pack by removing the external case, it became obvious that the cells were damaged. The blue powdery residue along the outer surface of the cells is caused by one cell leaking electrolyte after having been over-discharged repeatedly. This pack did have an internal circuit for battery management, but the circuit did not include a cell balancing function. This shows that just doing battery management at a pack level, without monitoring individual cells, can still lead to problems with a Li-Ion pack.