I've been know to get my hands on a dead Dell laptop battery
pack or two. Since there is a lot of incomplete or wrong info on just how they tick, I thought I'd share what I've found.
First of all, the pinouts. For the past few years, Dell appears to have standardized the pinout, and this is
what I believe it is:
3. SMB clock
4. SMB data
More on these later.
The vast majority of Dell laptop battery packs are
made up of some number of 18650 lithium ion cells. These are available all over the place, and billions of them have
been made. The technology is pretty well figured out. These very same cells are even used to power the Tesla Roadster
and a few other electric vehicles. (Yeah, 6,831 cells in a pack. What a fun thing to have to manage, huh?)
The photo below shows a typical such cell. This one is
actually a pretty good one - it's made by Samsung, and from the "26A" suffix, I'd say it's a 2600mAh cell. This
cell came from a pack that had 3 groups of cells in series. Each group had 3 cells in parallel. So each parallel
group was good for 2600 * 3 milliamp-hours, or 7.8 amp-hours. The three groups in series give you 3.7v * 3, or about
11.1v. 11.1v * 7.8 Ah makes up about an 85 watt-hour pack. (For example, the Dell type CF623 pack.) There
are different takes on typical voltage per cell - Dell is apparently using 3.7v.
Anyway, a typical Dell pack might look like this one. It uses 3 groups, each having 2 parallel 18650 cells.
There are a couple of things to notice here.
pack's most positive terminal is in the lower left, just under the thumbnail. It's the red wire.
There is a thermal sensor (pair of black wires with a sensor at the end) between the first two paralleled
cells on the left. It's under the white tape.
The control circuit boards run around the
lower left corner.
The foil strip across the top is the most
negative terminal, and it's insulated all around until it gets to the upper right corner, where the connection is made to
the most negative terminal in the pack.
There are "taps"
at each end of all three of the paralleled pairs. This way, the control circuitry can monitor and balance each pair.
(Each pair is treated as a single cell)
The two white doohickeys in the
far upper right are spacers. They go between the 2nd and 3rd cells and the 4th and 5th cells.
They also snap into the top and bottom case halves.
Random battery test data