We had an earlier success tearing down a Dell laptop battery pack, where the six salvaged cells still have 70% of original capacity after ten years of service. However, that was from a laptop that could still boot and run from its battery pack. This XPS M1330 battery pack is in far worse shape. How much worse, we were about to find out.
The first critical detail was realizing the battery pack was not the original Dell battery pack. It is an aftermarket type of unknown manufacture. The earlier battery pack tear down yielded Samsung cells, we’re probably not going to get anything nearly as nice this time around.
Once the case was cracked open the suspicion was confirmed: These appear to be generic 18650-sized lithium cells with no manufacturer branding. The nine cells of the battery pack were divided into three modules in series, each module had three cells wired in parallel. The module in the worst shape exhibited severe corrosion and had no voltage across their terminals.
The other two modules were in slightly better shape, but they have self-discharged down to approximately 1 volt DC, well under the recommended voltage range. A web search found some details on what happens to overly discharged lithium cells. In short: the chemistry inside the cell starts dissolving itself. If recharged, the dissolved metals may reform in inconvenient ways. Trying to use these cells has three potential outcomes:
- Best case: The metals dissolved into the electrolyte will hamper chemical reaction, resulting in reduced capacity.
- Medium case: The dissolved metals will reform in a way that damages the cell, causing it to fail as an open-circuit. (As if no battery was present.)
- Worst case: The dissolved metals will reform in a way that damages the cell, but causing it to fail as a closed circuit. Short-circuiting the internals will release a lot of energy very quickly, resulting in high-pressure venting and/or fire.
The corroded cells that have discharged down to zero volts have the highest risk and will be discarded. The remaining cells will be slowly (and carefully) charged back up to gauge their behavior.