Wednesday, June 23, 2010

Dell KD476 battery disassembly

I just got a replacement battery for my Dell Inspiron 6400 laptop. The price was so good that I couldn't justify an attempt to replace the cells in my old battery. However, I was still curious about what's inside.



The case consists of two halves: the bottom, which serves as part of the bottom of the laptop, and the top, which slides over the bottom and covers the rest of the battery. The printing on the top is actually a sticker. It may be removed to see the cells, but it does not provide useful access to the battery. In order to gain access, the two halves of the case need to be separated. This is difficult because the bottom slides over the top with considerable overlap, and that overlap is glued together. I decided to carefully pry apart the overlap. I started prying the edges of the bottom away from the top (sideways) using a blade. Once more space was available, I pried apart the overlap using a screwdriver. This did cause a few cracks in the sides of the base and some damage to the overlapping part of the top, but the result is acceptable.


Here you can see the internals of the battery. The cells are connected in parallel in groups of three, and then the three groups are connected in series. At this point, it's possible to measure the voltage of individual groups of cells. The right two groups measure 4.09V and 4.08V, while the left group measures -0.92V. Those measurements imply the right two groups are fully charged and the left group is bad.

The cells are labelled LGDB118650. The number and size make me think they are 18650 cells, just like in many other laptops. At this point, I could replace cells or maybe even build a 6 cell battery out of this 9 cell battery. However, I don't know if the protection circuit needs to be reset using special software.

The protection circuit consists of two circuit boards wrapping around a corner. One board, which I shall call the power board, contains the connector, protection switch MOSFETs and fuses. It is glued to the case with some hard silicone behind the connector. The silicone can be pried away from the case to free the board.



The other board, which I shall call the control board, contains a lot more components. It also has the 5 LED charge state bar graph along one side.



Among other things, the control board has a bq29311 protection IC and a bq20857 IC which seems to be the controller. The latter may be a problem when replacing cells.

3 comments:

Beui said...

I encountered the same failure issue with the one of the outer set of batteries (2 in my case because of a 6 cell) while the remaining paired packs
((2cells@4.1v)x3) were at proper voltage.
In your case convering from a 9 cell to a 6 cell is a plausible solution, however I don't think a 3 cell pack would work in my case.

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La-Tattva said...

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