How to Build an Airsoft Gun Battery Discharger
No Drain, No Gain
If you want to be serious about getting the most from your electric power system, you need a high-amp battery discharger. It’s the tool you’ll use to drain your packs before recharging them, to train them to run at race pace and to monitor their capacity. Sounds like a high-tech piece of gear, right? Nah; it’s just a bunch of light bulbs! With a little patience, it’s easy to solder a discharger of your own. Here’s how.
How Many Bulbs?
A no. 1157 taillight bulb draws about 2 amps, so you need 10 bulbs to build a 20A discharger. Why 20 amps? A motor in a competition car that is geared to run for 5 minutes draws about 20 amps, and it’s best to consistently discharge your packs at an amp rate similar to what they’ll see in action.
Serious modified racers often use a 15-bulb discharger to pull 30 amps, but this is more than what most of us need. When in doubt, go with fewer bulbs; there isn’t any point in putting extra strain on the cells. Save the abuse for the A-main!
You’ll Need:
• 10 no. 1157 automotive taillight bulbs
• 60/40 rosin-core solder
• Soldering flux
• 3 feet of 1/8-inch brass tube
• Alligator-clips or connector to match your packs
• Steel wool or 100-grit sandpaper
• 1 foot of 14-gauge wire.
• 40W (or higher) soldering iron
• “Third hand” tool
Step 1: TIN THE BULBS. “Tinning” is the application of a light coat of solder on a part to prepare it for assembly. To tin the terminals on the bottom of each bulb, dab them with flux and melt a little solder onto them.
Use just enough solder to lightly coat the terminals; more isn’t better.
Step 2: Cut and Tin The Brass Tube. For this project, you need four pieces of tube. Cut two 5-inch lengths, and two 11/2-inch pieces. A tube cutter is the best tool for the job, but a pair of diagonal cutters will also do the trick. Next, tin the ends of each tube and at 1-inch intervals on the longer pieces. Use your sandpaper to scuff the brass for a strong joint.
The tube is used as a structure to support the bulbs and as a conductor to complete the bulb circuit. Tinning the tube now will make assembly easier later.
Step 3: Solder The Bulbs To The Tube. Begin by soldering the base of a bulb to the end of one of the 5-inch tubes. The terminal on the bottom of the bulb should contact the tube. To make the job easier, tape the bulb to the bench top and use a “third hand” to hold the tube. Repeat this step until you have soldered five bulbs onto one of the longer sections of tubes. Now solder five more bulbs to the other side of the tube.
When soldering, don’t hold the tube with your bare hand; the brass conducts heat very well, and you don’t want to burn yourself. Note that the bulbs don’t touch each other; this helps prevent breakage.
Step 4: Join The Sides Of The Bulbs. Scuff, flux and tin the sides of the bulb sockets, then place the pre-tinned tube across the base of the bulbs and solder it to the sides of the bulb sockets. The bulbs are now joined in parallel: all the positive terminals are joined to each other, and the bulbs’ sockets (which are “positive”) are joined.
Joining the sides of the bulb sockets with a brass tube completes the circuit for the upper five-bulb assembly.
Step 5: Solder The Bulb Side Rails Together. Remember the two 11/2-inch tubes cut in Step 2? Use them now to join the bulb side rails. You only need to join them at one end to complete the circuit, but the discharger will be sturdier if you link both ends.
Step 6: Add The Connector or Alligator Clips. Add the connector or alligator clips. Strip and tin two 6-inch pieces of 12- or 14-gauge wire and install the connector of your choice or alligator clips to the end of the wires. If you go with clips, don’t just crimp them to the wire; be sure to solder the connection. To complete the discharger, solder one wire to the center rails and the other to the bridge between the side rails. Don’t worry about polarity; although we’ve referred to the bulbs as having “positive” terminals and “negative” sides, the bulbs will work fine no matter how they’re hooked up. That’s it; your discharger is complete.
Alligator clips are great for hard-wiring, but if you use plugs, install them instead.
Discharger
SAFETY
Remember, those bulbs are made of glass. Don’t put the discharger in your pants pocket and be careful not to drop it; the bulbs are fragile.
Warning: hot bulbs! The brightly glowing filaments make the bulbs very hot, so be careful.
Watch out for hot alligator clips. If you use alligator clips to connect your discharger, and then dump a pack that has a lot of juice left in it, the clips will be very hot because of their electrical resistance.
Top Tips
When the bulbs dim, disconnect the pack. If you let the pack dump until the bulbs go out, that doesn’t mean the discharger is “off”; it just means that the pack doesn’t have enough power to make the bulbs glow and that the pack is being over-discharged. As soon as you see the bulbs change from being fully bright to a dull glow, the pack has been discharged enough.
Use the discharger to rate your packs. Because the discharger draws a steady 20 amps, it’s the perfect tool for testing battery capacity. Fully charge your packs, and then put each one on the discharger. Use a stopwatch to time how long it takes each pack to dump.
