Homemade Bulb Discharger
Jeffrey Zweizig Jeff Zweizig Jeffrey Zweizig Jeff Zweizig

Here's my homemade bulb discharger constructed from ten #1157 automotive bulbs. These are dual filament tail light bulbs drawing a total of 2 amps per bulb resulting in a 20 amp discharger. The use of ten #1157 bulbs is a pretty standard design. A search of the Internet reveals several variations of this theme. Commercial dischargers incorporating the same design are also avialabe, but you can build your own for about half the price.

The discharger is built on a Radio Shack project box and includes a volt meter and on/off switch. The banana plugs provide an output to connect a DMM for calibrating the built-in meter. The battery lead is made of red 12 gage stranded wire. I put a piece of black shrink tube over one wire to indicate polarity. Even at 12 gage, the wires and connector get too hot to touch.

I had some 12 gage house wiring laying around, so I stripped out the conductors and used them as a frame work to support the bulbs. I arranged the bulbs in two rows of five. Each bulb has a pair of contacts on the bottom of the socket. The contacts must be oriented in parallel with the bottom wire such that both contacts can be soldered to the wire. Otherwise, it just goes together as shown. Note how each wire has two 90 degree bends before entering the box.

Once inside the box, the wires were crimped over in the opposite direction to hold the frame to the box. The four corner wires are the "negative" side of the bulbs and connect to the negative battery lead at the blue shrink tube at the upper right. The lower center wire is the "positive" side of the bulbs and connects to the positive battery lead at the red shrink tube.

Here's the inside of the box again. The blue shrink tube is the negative battery connection. A negative connection is also made with the meter, black banana plug and the switch (because it's a lighted switch). The red shrink tube is the positive battery connection which goes through the on/off switch before leaving the box. A positive connection is also made to the meter and the red banana plug.

I use the discharger to discharger my NiCad packs. Six cell packs are discharged to 5.4 volts. Seven cell packs are discharged to 6.3 volts. The discharger is more complicated than it needs to be, but I guess I was feeling a bit whimsical when I built it. It's also way bigger than it needs to be. All you really need is ten bulbs, some wire and a battery connector. Without a meter, the proper discharge voltage can be judged by the brightness of the bulbs.

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