phr (10/28/82)
The problem is that the HP Woodstock series calculators charge the batteries at about 150 mA, three times what normal nicads are designed for. You can easily pry open the battery pack with a screwdriver, replace the cells, and put the pack back together with Krazy Glue. Normal cells (what HP uses) can be had for around $1 each at hamfests or mail-order; cells designed for fast charging are slightly harder to find and more expensive. After going through several HP packs, I replaced the cells in mine with some fast-charge cells I got from Formula International (advertiser in Radio-Electronics) for $1.50 each. I've had no trouble with them in over 3 years. Overcharging nicads at more than the C/10 rate shortens their lives pretty seriously. Your normal HP cells will probably last much longer if you only charge when needed, and make sure to unplug after about 5 hours. A 20 hour recharge with the calculator turned on might be OK - I haven't tried this too much. Don't half-charge the batteries (either by plugging in when they're only half discharged, or by pulling the plug when they're half recharged) - this causes cell memory, which decreases the operating time of the calculator on a single charge. You can erase this by putting the cells through several deep discharge/charge cycles. In a pinch, you can often repair shorted cells by passing large currents through them. Get a huge computer electrolytic cap, charge it up to 10 or 20 volts, and zap it through the cell (in the charging direction). Don't try this in explosive atmospheres! The idea is to burn out the internal short, which is usually pretty thin. Don't expect the cell to behave too well afterwards - the shorts tend to come back at more and more frequent intervals. For more information: "Eveready Battery Engineering Data", Union Carbide, is a book about the size of a World Almanac. I got mine by calling them and asking, but it says something like $15.00 on the cover. Lots of cell specs for all types of batteries, charge circuits, etc.; some folklore (the above paragraphs are folklore). The fast charge AA cells are CH500, I believe.