Wednesday, January 7, 2009

Not So Stupid Battery Tricks

As previously blogged, my Sony WinXP laptop ("sonyXPress" ... Get it?) carked it. Seems I'm not the only one to experience this sudden death effect. One possible cause was a battery failure. My Sony is more than 4 yo and battery life typically is around 2--3 yrs. Also, there can be embedded fail-safe circuits that would cause the laptop not to boot; even without the battery installed. But without proper documentation, you don't really know, and taking it to a PC repair shop will typically run you b/w $70--$100 to find out.

The Sony VAIO uses a PCGA-BP2V Lithium-ion battery. How do you tell if it has charge? You use a multimeter. One slight problem. There is a connector which mates with the motherboard and it has 9 pins arranged as: 2 + 6 + 1; the 6 being slightly smaller than the other 3 and no labels or markings to indicate what they are. Great! That's 36 combinations to find the correct terminal pair that gives me the voltage drop, if there is one. They could all be zero. I've replaced whole Li-ion packs before but never taken the time to examine how they are constructed. This seemed like a good opportunity to rectify that situation.

Carefully prizing open the plastic casing, I found 6 individual cells inside. On the outside of the casing, there's a power rating for this pack: 48.84 Wh = 11.1 V × 4,400 mAh, which are interesting numbers. If the nominal voltage is magically 3.7 volts/cell, then 11.1/3.7 = 3 Li-ion cells connected in series. If the nominal current is 2200 mAh/cell, then 4400/2200 = 2 groups of 3 cells in parallel for a total of 6 cells (as observed) and it also explains the small 6-pin portion of the external connector.

Since the battery pack can cost anywhere from $50--$150 to replace, it might be worth a DIY approach. Here's a vid showing how to do that. It's absolutely vital to draw a correct schematic of the parallel and series wiring, before unsoldering all the cells. If that's too much for you, try putting the entire battery pack in the freezer overnight to recover charge. Finally, here are some general tips for prolonging the life of Li-based batteries (dated 2003--2005). In my case, the problem turned out not to be the battery alone, so I just had to resign myself to buying that Macbook. Damn! ;-)

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