Nuts and Volts - February 2018

measure the distance from the bottom of the battery to the bottom of the band and the width of the circumferential band. To remove the band, carefully peel the strap from the positive terminal with needle-nose pliers and slide the band off the bottom of the battery pack. Properly dispose of the spent NiCd batteries.

Assembly

To build a new pack, you will need four new C-size rechargeable batteries. Don’t waste the effort using NiCd chemistry. Most newer NiMH batteries will work with any charger designed for NiCD or NiMH, and there is no memory effect. The THS700 trickle charges at about 50 mA, so it can’t overcharge C-size cells.

Four C-size NiMH cost about $20 for 3,000 mAh capacity (Fry’s Electronics and RadioShack.) This is about 50% more mAh than OEM packs! You will also need about 8” of 1” diameter heat-shrink tubing.

Figure 3 shows the preliminary assembly of the new batteries and the final assembly in the tubing. Slide the new cells into the tubing (paying attention to polarity), and trim the tubing to extend about 1/8” beyond the ends of the assembled batteries.

While applying a slight pressure to push the cells together, shrink the tubing carefully with a heat gun. The new cells are likely fully charged, so take care not to short anything!

The old band will not fit over the new shrink tubing, so cut the band open near the point where the circumferential ring strap is welded onto the linear strap. Fit the band over the negative end of the pack and locate it based on earlier measurements.

You want the circumferential strap to center on the internal side contact once assembled. Use vinyl tape to hold the band in place, overlapping the band by 1/8” or so at either edge. Refer to the photos, or better yet take some of your own prior to disassembly!

Now comes the only tricky part; see Figure 4a for reference. With the band positioned at the bottom, the strap must be soldered to the positive end button of the top battery.