7. Inside A Lithium-Ion Battery Pack

Let's take a look at a more modern Lithium-Ion battery pack from Dell. This is a battery that went bad on me. It is rated 14.8v @ 3600mAh.

While the seams on the Dell battery pack enclosure are pretty visible, they are also really, really tight seams. To give you an idea, here's the edge of the screwdriver compared to the thickness of the seams. Pretty tight:

But luckily, looking very carefully, I saw one spot near the connector that had a slightly larger opening in the seam. So I took a stab with the screwdriver and voila:

Now, by gently prying around the edge of seam all around, I spied an interesting brand on the inside:

While, this is no surprise if you've been paying attention to the news lately, it looks like the Sony Energytec factory has been producing Dell batteries for some time. What you are looking at is a Sony Lithium-Ion charging circuit, complete with a 5 LED charge status array. This is pretty sophisticated stuff compared to the Toshiba battery. If you recall, the NiMH battery had the terminals wired directly to the cells with nothing really in between. Why this level of sophistication? To repeat myself, Lithium-Ion cells are easily pissed off. If you note from the table earlier, the protective circuitry is responsible for preventing dangerous situations from occurring.

Opening the case further reveals:

Two sets of 4 green batteries. These look a lot like AA cells, but they are not. In principle they are constructed the same, but with different chemistry and sizes. Looking a bit closer:

These appear to be Sony Energytec 18650GR cells. These are very common cells used in many products and are rated 1800mAh @ 3.7v. Since the whole pack is rated for 3600mAh @ 14.8 volts. As you can see from the picture above, the pack is wired up with pairs of parallel batteries in series.

Each pair is worth 3600mAh @ 3.7V chained together like this yielding the 14.8V @ 3600mAh.

In general, batteries are constructed to have many sets of smaller cells for a couple of reasons: 1) the charge circuit can be simplified 2) the charger can top up many different sets at a lower voltage than the pack voltage 3) if any individual set of cells goes bad, the charge circuit can identify the cells and switch them off, or declare the entire pack bad. Some of the newer protective circuits can detect an impending explosion and open a vent on the battery in order to release a jet of hot gas to thwart or reduce an explosion. A jet of hot flaming gas is probably not good news for your lap, but, that's life in the fast lane.