how to build a bump box

Radio Control Car Action, Apr 2004 by Gonzalez, George M

Get the holeshot every time by George M. Gonzalez

One of the oldest tricks in electric racing is to give the battery pack one last bump-charge and keep the batteries connected to the charger until the last possible moment. Unfortunately, most race directors don't allow drivers or mechanics to drag a bunch of extension cords over the track to power their chargers so they can give their batteries one last blast of juice.

The solution is to build a "bump box," which is a portable, battery-operated charger that can blast your car's batteries while it's staged on the grid. The extra jolt increases voltage for a super-fast start off the line. Sound like the hot setup for this Sunday? Here's how to build and use a bump box of your own.

you'll need

* DC-compatible charger

* Two 6-cell stick packs or 10 to 12 loose sub-C cells.

* 12- or 14-gauge wire

* Soldering iron and solder

* Double-sided tape

* Mounting platform (optional)

selecting a charger

You will need a DC-powered, peak-detecting battery charger that operates on a 12 to 14V power supply. Be sure to choose a charger that is easy to operate because you won't have time to surf through a bunch of menus and charge parameters when the race is going to start in 30 seconds or less. A charger with a simple dial that adjusts the amp rate and start button is preferred. Most racers bump-charge their battery packs at a high amp rate (8 to 10 amps) to heat up the cells quickly, so a charger with a high charge rate is preferred.

1 MOUNTING PLATFORM

You'll need a ¼-inch-thick (or slightly thicker) sheet of plastic, plywood or particleboard to use as a platform to mount the charger and power supply batteries. Some racers just tape the batteries directly to the charger, but that looks hack. You want the mounting platform just large enough to mount the charger and battery packs because the bump box should be as light as possible. I used the particleboard back from an old clipboard; it was easy to cut and cheap. Trial-fit the charger and batteries on the platform so you'll know where everything has to go.

2 WIRE THE CELLS IN SERIES

Place the two battery packs side by side. To connect two battery packs in series, all you need is a short piece of 12- or 14-gauge wire. Solder one end of the wire to the negative tab on the first battery pack and the other end of the wire to the positive tab on the second battery pack. Solder a battery bar on the positive and negative end cells so you can attach the charger's alligator clips. I used some old 2400mAh Ni-Cd packs; the 12 cells will deliver 14.4 volts. If you use loose cells, you can get away with just 10. They will deliver 12 volts; that's the minimum for DC-powered chargers.

3 MOUNT THE CHARGER

Use double-sided tape or Shoe-Goo to secure the charger to the platform. Install it as shown in the photo so the charger's leads exit through the front of the platform, and the power supply's leads exit from the rear. I used a KO Propo BX-213 charger because it's compact, lightweight and easy to operate.

4 MOUNT THE BATTERY PACKS

Use double-sided tape or Shoe-Goo to install the batteries on the platform. If you use Shoe-Goo, set the bump box aside for approximately one hour while the glue sets.

5 BUNDLE THE WIRES

Use zip-ties to bundle the excess wire. Leave the main charge leads long so they can reach the batteries with the body installed.

6 CHARGE THE POWER-SUPPLY'S BATTERY PACKS

If your charger has AC capability (or if you have a separate power supply that plugs into a wall socket), use the bump-box charger to charge the power-supply's battery packs. The packs can only power the charger for about a minute, so make sure that they're fully charged before "bumping."

HOW TO USE YOUR BUMP BOX

Using a bump box is easy, but follow these safety precautions so you don't damage your vehicle's electronics or over-charge the battery pack. Make sure your pitman follows these steps while you stand ready on the drivers' stand.

* Turn off the ESC before you connect the battery charger. Connecting a charger with the ESC turned on can damage the car's electronics.

* Connect the charger's alligator clips to the battery pack's negative and positive battery bars and start the charge process. Most racers set the amp rate between 8 to 10 amps to yield quick results.

* Have your pitman monitor the cells so he can disconnect the charger if the batteries start to get overly hot-anything more than "toasty warm" is too hot. When the "10 seconds to the start of the race" announcement is made, your pit guy should disconnect the charger, switch the ESC back on and get off the track in a hurry.