Replacing PB Battery Cells

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I replaced the exhausted cells of my original PB 145 battery with new NiMH cells, with several advantages: the original NiCd cells were rated at 1300 mAh ( or 1.3 Amp/hour ), while today's NiMH cells of the same physical size are capable of 2000 mAh output. It's a matter of carving the sealed case open (I cut along the seam around the original label), extracting the old cells, and replacing them with tabbed cells of the same size, making sure to maintain polarity, and to include the thermal shutoff sensors in the circuit. It's easier than it sounds, though my longtime soldering experience helped.

Creating a "dummy" battery to act as a feed for a large external battery pack is a very workable idea. Just remove the cells, solder two heavy wires to the contacts, and run them out the end of the pack. You'll have to drill an oblong hole in the sliding panel on the battery (pry it off to do this), to allow the wire to feed out regardless of the position of the panel. If you want to get fancy, you could mount a jack where the wires pass through the battery case. I added a jack to my old full-size camcorder for this purpose, and can run it from either a hip pouch ( which contains a 4.2 A/hr set of NiMH cells ), or from my car lighter socket. Choose a durable jack and plug with good contact area; I find that a 1/4" phono plug is reliable (good old WW II bomber technology), or a standard RCA style plug and chassis-mounting jack. Either of these will allow strong current, better than most standard power-jack designs.

Using an external battery to feed through the internal one poses some problems. For one, the ext battery will only charge the internal to the point where they are both of equal potential, and no further. Running heavy current through the weaker cells in the internal battery will damage them. Diodes must be used to prevent current draining back, preferably in both batteries. It's much better to use a dummy battery with an external pack, and charge your internal either later, or on a separate charger.

You can use remote control car battery packs easily for this purpose. I tested my PB 145, and found that it drew about 2.7 Amps at 7.5 Volts DC on startup, and then the draw dropped to around 1.3 Amps (and only 1.1 Amps when the drive spun down). So I know I need at least a 3 Amp pack providing 7.5 Volts. I can connect as many as I need in parallel to achieve this; just tape them together, connect all the red leads together, and all the black leads together, and I'm all set. I could build a light case with the same footprint as the PB, made of ply ends with doorskin top and bottom, a switch and appropriate output/charging jack. Or use a hip pouch like I did for the camcorder. I don't know the specific power requirements of the various PB models, but you can get a pretty good idea from the switching power supply (power adaptor) label, where the output is listed.

The thing to remember is: you CANNOT damage your PB by hooking it to a battery pack which supplies MORE than your Powerbook draws. If you hook it up to a car battery (assuming a 12 Volt requirement for the PB), which can output say, 500 A/hr, your PB will only draw what it would have anyway, say 2.5 A/hr - 3.5 A/hr. A large battery cannot "force" more current through your Powerbook. Voltage is another matter. My PB 145 works in a range between 6-8 volts, and that's that. Over 8 volts, I cook it, and under 6 volts, the PB circuitry puts it to sleep. There is only a minimum requirement of about 3 Amps current, and of course, no upper limit on current. With a 6 volt car battery, I could probably run it for a couple of days, but who wants to carry a lead-acid car battery around?

I apologize for the very long post, but as with my earlier soldering post, I have a feeling that many of you have ideas about this, but are just fuzzy enough on the numbers to chicken out! I can assure you, what I describe above works, and I've built these with no problems. I've also built car power adaptors, if you guys can tolerate another long post!!! ( You can't just plug a PB into a cigarette lighter socket, so please don't! ) If anyone wants info on that, or more detail on this, I'd be happy to help out. Again, sorry for the long post.

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