WARNING: Use common sense. These work well, but by building this charger you assume all risk.
Notes about the Black Box Charger
The Black Box charger is a Dewalt charger (Model DW9116) modified to charge 7.2-volt NiCd hobby batteries. (Note: I only use the Black Boxes to charge 7.2-volt hobby packs because they use the same size NiCd cells as Dewalt NiCd packs.) The Black Box is still able to charge Dewalt 7.2- to 18-volt NiCd battery packs.
Warning: Don't try and charge both a hobby battery pack and a Dewalt pack at the same time.
The Dewalt charger was developed for the construction industry, which is more cost conscious than many hobby enthusiasts. Therefore an ideal battery charger must be simple to operate and manage batteries efficiently to maximize performance. To fullfill the demand for quality chargers, Dewalt devised a method to ensure their chargers give battery packs a full charge each time with a feature called "Automatic Tune Up Mode." This feature activates when a battery pack is left on the charger. Once the battery pack reaches its full charge, the charger balances the battery pack by making sure that each individual cell in the battery pack is fully charged without overcharging any of the other battery cells.
This feature gets the most out of a battery pack since each individual cell has a slightly different charge/discharge rate from its neighbors. Over time, one or more cells in the pack will reach full charge before the rest of the pack and stop a normal charger. The older the battery pack the more pronounced this charge difference can become. This can artificially make the pack lose charge capacity, but it can be restored if all of the cells receive a full charge.
To demonstrate how a Dewalt charger with Tune Up Mode can increase the performance of a battery pack (especially if its an older battery) an experimental discharge appartus was built. Results of the comparison between the Dewalt chager, a brain peak charger and a crank charger can be found here
Building a Hobby battery charger
Step One: Buy a Dwalt Charger
Although the best reason for using a Black Box is the safe simple reliability it offers, perhaps the second best reason is its price. A Hobbico crank charger costs about $25.00 and a Brain charger costs from $30.00 to $50.00. A Black Box charger can be made for about $16.00 when purchased in bulk (five or more at a time).
The key to this incredibly cheap price is that the Dewalt DW9116 charger, as advanced as it is, has become outdated because Dewalt recently changed from NiCd batteries to Lithium ION batteries as their top end battery. The DW9116 cannot charge the new Lithium ION batteries, so it can be found at clearance prices on eBay.
Step Two: Convert the DW9116 into a Black Box Charger
To turn a DW9116 charger into a Black Box, the first task is to enlarge the opening in the top of the charger so a 7.2-volt NiCd battery can rest vertically in the charger.
This job can be done with a variety of tools (a Dremal works great!). However, if you have access to one, the best (and most awesome) way I found to quickly and accurately enlarge the top of the charger is to use a milling machine. Note: A drill press on low gear with a 2-axis vice also works.
The diameter of a NiCd cell in a 7.2-volt battery is slightly less than 1-inch, so a 1-inch flat-end-mill makes the perfect slot size. However you can use a 1/2-inch flat-end-mill or even a 1/4-inch flat-end-mill and make multiple passes.
The slot should begin 1/16-inch away from touching the left side of the rectangular pocket to just touching the right side of the rectangular pocket (This area is highlighted in green below). The depth of the cut will be about 1/32-inch above the two contact tabs a normal Dewalt battery pack connects to (shown in red below).
The best way I found to machine the slot is to plunge the flat-end-mill down onto the contact tabs with the mill turned off. Then retract the mill bit about 1/32-inch. Next, lock the depth control to keep the end-mill from moving.
With the mill at low RPM (about 500), slowly move the charger over until the mill just touches the back side of the rectangular pocket.
When finished the charger should look like the slot shown below.
Next, pull the top part of the charger off. The top is held on using five screws, four of which are on the bottom of the charger and one of which is on the top of the charger next to the contact tabs. Keep the screws somewhere safe; you will need them later.
The screws that hold on the top half of the charger are T-15 star screws with a security point that prevents someone from using a standard T-15 screw driver to remove the screws.
If you do not have a T-15 screw driver for taking off the security star screws, Husky makes a combination driver set that is sold for about $15.00 at Home Depot.
Once the top is pulled off...
... locate the two contact tabs that a Dewalt battery pack would connect to
Take a roll of 25-gauge speaker wire and cut a 7-inch length.
Strip about a 1/2-inch of the insulation off each end of the wire.
Tin each end of the wire by coating it with solder.
Solder one end of the speaker wire to the contacts, making sure the polarity is correct. The red wire should go to the tab Y2 (RED) and the black wire should go to the Y1 (YELLOW) tab. Make sure and solder the wire to the bottom of the tabs so that the top of the charger will not snag on the extra wires.
For a little cosmetic work, take a Styrofoam coffee cup and a roll of black duct tape.
With a box knife, cut the coffee cup in half. Cut the bottom semi-circle of the cup off.
Put the coffee cup half in the charger. Rest a marker on the top of the "tower" and mark the coffee cup as show in the picture below. Cut on the line, discarding the top.
When finished, the section of the coffee cut left should look and form to the charger as shown in the picture below.
Once the section of coffee cup is cut to size, wrap it with black duct tape.
Place the section of coffee cup back in the top of the charger and then wrap another piece of black duct tape around the section of coffee cup to secure it to the top of the charger (see photos below).
Use a 1/4-inch drill bit and drill a hole in the side of the charger top for the wire to go through, as shown in the picture below.
Put the top of the charger back on the base, threading the speaker wire through the 1/4-inch hole that was drilled in the top of the cover. Screw the cover back on, using the original screws to do so.
Solder power-pole connector inserts (or other type of batter connector) on the end of each speaker wire. Next, attach the power-pole connector, putting the red connector with the red speaker wire and the black connector with the black speaker wire.
If using power pole connectors, I recommend putting a drop of Elmer's white glue on the connector pin to make it does not come out: pull the connector pin half way in, put the drop of glue on, then slide the connector pin back into its hole.
You should now have a working charger than can charge 7.2-volt NiCd hobby packs and all Dewalt NiCd battery packs.