I love my XPS13; screen is a little small, but generally it’s a very usable and very portable laptop.
However, over the last few months, occasionally it’s said ‘power adapter not recognized’ and refused to charge, and very lately, not charging above 58%.
I’m not one to just ‘go buy a new one’ if the old one is not quite right, so I decided to have a look at the battery to see if anything was obviously wrong.
The battery is a shaped plastic housing containing 6 of what look like fairly standard LiPo cells. Â The cells are arranged in two groups of three- i.e. three cells in parallel, in series with a second three in parallel. Â Total 7.4 volts nominal.
After peeling back the plastic covering the cells, I found that one set of three cells was at a solid 4.2v (fully charged), and the other set at 3.8 (quite discharged).  This kind of tells us where the ‘58%’ was coming from – the cells are charged in series, and when one set hits 100%, no more current can flow, so the the laptop stops charging, and the overall charge is detected at 58%….
I have an RC lipo charger which can safely charge individual lipos, and lipos can be charged in parallel, so I charged the ‘low’ side of the battery.  The charger reported putting in ~2800 mAh, and now both sides of the battery show 4.2v.
Replacing the battery in the laptop, I expected it to report 100% – but it still reported 58%.  I ran diagnostics from bios, and a couple of reboots, and nothing seemed to change, but whilst writing this with charger plugged in, all of a sudden it is reporting ‘Fully Charged 100%’, so maybe it just needed a little time to realize.
How the battery got so imbalanced is of concern.  It could  be that there is one bad cell (probably in the ‘high’ side, so in effect only two cells active?), and that effectively leads to a fast imbalance during charge/discharge cycles – the side with ‘2’ cells will charge to full 4.2v faster, and discharge faster that the side with 3 good cells.  You would have thought that Dell would have an advanced charging scheme which avoided imbalance….
If I’m lucky, the imbalance is just something which happens over time, and re-balancing will give me good battery life for another couple of years.  If I’m unlucky, then there’s a bad cell which in theory I could replace.  If I’m really unlucky, the bad cell could be about to burst into Lipo flames…
Update:  after using for a day; discharged to 5%, recharged back to 100%.  Can’t say I got an extreme time from the battery, but it’s not tuned to battery saver; just glad it did not only recharge to 58% again :).
Notes on battery dis-assembly:
The battery itself is a plastic tray with a ‘sticky back plastic’ cover.  The plastic is stuck directly to the lipos, so use caution not to damage the batteries as you peel it back.
But in actuality (depending upon what I find after a week of use), you only need  to expose the power board (near the wires), only needing to peel back an inch or so.  The battery connections to the power board are obvious – two pairs, one for the set of batteries on the left, and one for the set of batteries on the right.  These are easily accessible, and suitable for attaching the external charger.
UPDATE: after a couple of months (where the laptop has basically been on mains power the whole time), the battery is now saying 75% most of the time, so I guess it’s got imbalanced again, which probably indicates that I have one cell down.  Although I’m sure I could replace a single cell, it’s my work laptop and I can’t afford any downtime, so I’ll probably buy a new complete battery, and re-purpose the remaining good cells to powering a robot :).
btsimonh 