B&D Chargers-

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The design of the Black and Decker NI-CD chargers is pretty hocky. I am an EE, and my brother blew up a 15.6V battery (arced 9 out of twelve cells) when his son dropped it and the charger in a bucket of oil. I have dissasembled both the battery pack and charger, and find the design of the charger to be very marginal. Current limiting is handled by using a XFRMR that can only source C/10 (210mA), followed be a 1/2W current limiting resistor. If the NI-CD cells are charged before a true discharge (considered 1.05V per cell), the limiting resistor will drop enough voltage to keep the current draw within the scope of the XFRMR. Unfortunately, people use batteries just a bit more, and a bit more, and the next thing you know, the cells are at a low enough voltage that more than 210mA draw will need to be sourced by the XFRMR. At this point, either the primary winding of the XFRMR goes, or the current limiting resistor burns up or both. Both items are cheap and easy to replace, but for around $14, you can get a new charger. Unfortunately, B&D does not always support your voltage and package style, and you are left searching the world for whatever extra's where manufactured. If the battery pack itself is bad (reads less than .95V per cell, you can either replace the bad cell (only if you know how!!!) or have one of several rebuild houses upgrade the cells. This is usually at a cost of around $4 per cell. Note that a typical NI-CD cell is around 1.2 to 1.3VDC when charged. If you have a 15.6V pack, it has 12 cells in it. A 14.4 has 11 cells. The current limiting resistor in a B&D 15.6 charger is labelled R1, and should be around 10 ohms. The one in my office is 8 ohms, but has been severely overheated due to a bad cell. The XFRMR is labeled as to what current it can source, and at what voltage. Replacement XFRMR are easy to find and run around $5. The resistor is around $0.10. The whole design is fairly simple, and has no ability to cope with deep discharge of cells. The design also has no temperature, time or current draw feedback, and is therefore limited to a C/10 charge rate that is very marginal under ideal conditions, and has a distinct failure mode when the battery is discharged beyond the point where the voltage drop accross the current limiting resistor forces the current to increase above 210mA.