Why do Lithium Ion batteries self discharge?

Question

I've seen some questions about loss of capacity due to internal resistance and oxidation of plates. I've also learned that these batteries have protection circuits.

But I'm curious about Self discharge, the phenomenon of charging a Lithium Ion battery to 100% and leaving it unused, only to return months later to find it has not preserved a full charge and requires topping up.

Is self discharge over time attributable to the chemistry of the battery? Is it the overhead of the protection circuitry? Or is it more complicated than that or some combination of the two? What is the mechanism behind self discharging Lithium ion batteries?

Answer 1

Wikipedia says: Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf-life of batteries and causes them to initially have less than a full charge when actually put to use. (see here).

It is typically caused by chemically unstable electrodes and by impurities in the electrolyte. In case of Li-Ion batteries you have minimal self-discharge, situation is much worse with Ni-Cd and Ni-MH. Some types of lithium batteries also make use of separator between the electrodes to further reduce it. These can get self discharge less than 1% a year.

Self-discharge grows with temperature, that is why you usually want to keep them in cold.

Also note that self-discharge rates vary even within single batch made with the same process. It undergoes further changes with battery cycling, charging method and as it ages. More information on this is here

Answer 2

The devil is in the details. If the lithium battery is a multi-cell black-box (e.g. NEC's nom. 13.2v batteries which have A123 cells inside), the battery includes BMS electronics. That electronics monitors the state of each cell pair in the pack. The BMS does not use zero power---and so, the battery management system that was intended to manage cells drains the battery if left unused, even new on-the-shelf and in-the-box over a period of time. Worse, if the voltage gets below a certain point, the electronics typically intentionally turns ON a high current MOSFET (or 2 or 3) to intentionally blow built-in fuses. At that point, the battery is a brick unless you can open it up, recharge the cells directly, replace the BEL25 fuse, reseal the battery and you're in the game again. It seems wrong that these battery manufacturers don't box these up with the BMS disabled but designed to automatically enable the BMS on first charge cycle.