top-quark.blog
Battery Pack Rebuild
Tue, 07/05/2005, 02:25This article's not complete yet, but I thought I'd post the beginning part to satiate all n of my readers (where n<7). Also, since this is informal, I'm looking for feedback on how I should implement a "nerdiness" meter. I know a lot of my readers aren't dyed-in-blue tech folks, so it'd probably be better to sort out the hardcore tech from the more generally appreciable articles. Let me know *somehow*.
My old POS laptop (dubbed "meow") has a really dead battery. When it was new, it was great; 4 hours a charge easily. But for the longest time now, it's been relagated to desknote status. Really, it's just good enough for switching outlets when you put it into hibernation mode. It's been (repeatedly) suggested that I get rid of it (the computer), but slow and tortuous as it is to work with, it does still have its uses.
For example... Since figuring out how to get my wireless card going in Netstumbler, I've been doing limited wardriving runs. It hasn't been all that great, though, since my inverter for my car's too noisy, and the computer gets random crashes. The solution is to go to battery power. Well, there's no way in hell I'm going to shell out $80+ on a new battery, when the computer's worth maybe 2x that (on a good day). The solution, then, like many times before, is to rebuild the battery pack. This time, though, I'll document the process.
The battery itself.. belongs to my old Compaq Presario 1244 system. Technical bits, for you search engine types...
Compaq Series 2941B P/N 330986 - B21 also 330935 - 001
It's NiMH (Ni-MH, Nickel Metal Hydride) chemistry, with a voltage of 9.6V and capacity of 3.8 Ah. Typically, a pack of this size and chemistry is filled with a number of cylindrical AA-esque batteries, attached with tabs; unsurprisingly, this one is completely typical, as we'll soon see.
Disassembly wasn't as... elegant as I like. Strong adhesive and soft plastic really made it a bear to get in. What I figured out on the ends was that a heatgun or hair dryer will soften up the adhesive to the point that you can hand-part the halves. Wear gloves, though.
Liberating the bottom half, we find that the cells are uncharacteristically umarked. That's bad, since you usually use the markings to find replacements. No matter, though. With a trusty set of calipers and this site, referencing dimensions of common battery pack cell dimensions, I was able to determine that the cells are of the 4/3A variety.
It's also good to note the thermistor that's thermally coupled to some cells with thermal compound. The temperature of the cells is a vital parameter in the charging "algorithm". On rebuild, I'll be reattaching it with some thermal compound of my own.
As with anything electronic, don't get too enthusiastic in disassembly. There's a rather fragile and semi-flexy power circuit stuck to the top of the battery casing. You don't want to damage this.
I haven't done any extensive sourcing yet, but it looks like the best deal in cells is going to be at The Battery Station (famed for their CR-123As, also). The particular item of interest is the tabbed "HR-4/3AU" sale items. At $4 each, that's $36 to rebuild my pack, or half price off the $80 new packs. Incidentally, the tabbed part is very important when constructing battery packs. Soldering can potentially damage the cells, so just leave it to the manufacturer to spot weld the tabs on; once those're attached, you can solder the tabs to your heart's content, sorta.