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T-Maxx 5 "AA" NiMH Battery Pack

I picked up a Traxxas T-Maxx TRX 2.5 nitro-fueled 1/10th scale model truck shortly before Christmas last year. Actually, it was for my girlfriend, but she decided that she might get bored with it if she had to do the maintenance, so it gets to live with me.

5 AA
battery pack from sideEven though this thing runs on 20% nitro fuel, it still needs batteries for the radio receiver and the servo motors which steer and provide throttle control. Four (4) "AA" for the receiver and another eight (8) "AA" for the transmitter. Even before I burned through my first set of alkalines, I decided that replacing twelve (12) batteries was going to become way too expensive way too quickly. So I put in an order for a large number of cheap, no-name, off-brand "AA" NiMH batteries.

At this point, this little tech article should say "I put the NiMH batteries in the receiver pack and that was that." Instead, things just got good and started.

Regular old alkaline batteries have a nominal rated voltage of 1.5v. Of course, they test at a slightly higher voltage when you first open the pack and test lower than this when you finally decide they need replaced, but 1.5v is as good a figure to use in our calculations. And calculate we shall. As I mentioned before, the receiver pack uses four "AA" cells. Or...

4 alkaline X 1.5v = 6v

NiMH rechargeable batteries, on the other hand, have a nominal rated voltage of 1.2v. Again, they will test slightly higher when freshly charged and lower when discharged, but 1.2v is a nice round number. The receiver pack uses four "AA" cells. Or...

4 NiMH X 1.2v = 4.8v

Seems a bit low, doesn't it? Will it even work? In my experience, most battery-powered devices can run on +/- 25% of their nominal voltage rating. Let's calculate that range of voltages for our receiver pack...

6v x 25% = 1.5v
6v + 1.5v = 7.5v
6v - 1.5v = 4.5v

So, our receiver pack and servos should work as low as 4.5v. Obviously, at 4.5v the servos aren't going to be as peppy as they would be with a full 6v, and the receiver might experience some reduction in radio range, but they should work, at least for a while. Luckily, NiHM batteries put out a fairly constant 1.2v over most of their discharge cycle (unlike alkalines which start out strong but then drop steadily) so we can expect 4.8v most of the time. A little on the low side, but in theory it should work.

For the record, four "AA" NiMH for 4.8v worked reasonably well in my receiver pack and I hear they work well for most people. In my 'scientific' tests, I couldn't tell the difference between fully charged NiMH and the partially discharged alkalines that I had been using. I wasn't going to go out and buy new alkalines just to test radio range or servo strength. The whole point of this article is that I don't want to buy any more alkalines, after all.

I might have been happy at this point, but I'd read somewhere that real power users didn't settle for 4.8v. A full 6v is what you want. The servos have more turning power, the receiver has more range, and it makes you more attractive to members of the opposite sex. But for a full 6v you need...

5 NiMH X 1.2v = 6v

Using NiMH, you need five to reach a nominal 6v. The catch is that the receiver battery compartment on the T-Maxx only holds four "AA" batteries. Oh, they make smaller (sub-C or so) NiMH batteries in 5-packs that will fit in the battery compartment, but they are more costly and, being smaller, generally don't hold as big a charge as "AA" NiMH batteries do.

5 AA
battery pack from topThe stock T-Maxx receiver battery compartment contains a four "AA" cell holder in a 2x2 configuration. I had ordered some six "AA" holders in a 2x3 configuration when I ordered the batteries. I wanted five "AA" holders but they're hard to find. I had vague plans of building up or cutting the top out of the stock battery compartment to make room for the larger battery holder. Once I had the materials in hand, however, it didn't seem possible. Cutting big enough holes or leaving big enough gaps to make a six "AA" holder fit would make the battery compartment as a whole much less shock resistant. With my luck, one little battery would shake loose just when I most needed to steer or brake or something.

I picked up a little aluminum project box at Radio Shack, planning to make it into a custom receiver battery box, but it was a little too large to fit in the T-Maxx, and the screws that held it together shorted the batteries. And, on second thought, the six "AA" holders didn't seem too sturdy, especially after I half melted two of them trying to solder in jumper wires in place of the sixth "AA" cell.

I had almost convinced myself to try soldering five "AA" batteries together and using nylon zip ties to fasten them to the chasis, when I decided to take one more trip into town. At Radio Shack, I found a few more project boxes that looked like they'd almost work, and a collection of "AA" battery holders of varying sizes. I almost passed them over, but I noticed a few were fully enclosed and secured with little screws. They looked almost water tight. In fact, they looked a lot like a T-Maxx battery compartment.

Naturally, the beefy versions came in two, four, and eight cell varieties. No fives, which is what I really wanted, and no sixes, which I could easily convert with a jumper wire, now that I'd had practice soldering them. I even looked for a single "AA" cell holder, figuring I could piggyback it on top of the existing battery compartment, but all those were flimsy open-sided deals. I finally settled on a two "AA" cell holder (side by side).

Construction was relatively straightforward, which means I sweated and cussed for hours and afterwards it seemed obvious what I should have done. First step was to grind off the raised "Traxxas" logo on the top of the stock receiver battery compartment. I needed a flat surface for gluing. Then I decided that I wasn't going to use glue, but I needed a flat surface anyway.

I carefully drilled two fairly large holes in the bottom of the two "AA" cell holder, near either end. I matched these up with holes I drilled into the top of the stock receiver battery compartment and ran a large nylon zip tie through them. This turned out to be very secure. I drilled a couple of smaller holes for wires.

I fired up the soldering iron and very carefully unsoldered one of the battery leads from the two "AA" cell holder. I moved the wire to the other end of the holder, so that the two battery leads were now looking at only one "AA" cell instead of two. All that practice soldering jumper wires paid off; I didn't melt anything this time.

I cut the wires going from the on/off switch to the old four "AA" battery holder and made some new connections:

  1. Positive (red) side of switch to red wire from old four "AA" holder
  2. Black wire from old four "AA" holder to red wire from new two "AA" holder
  3. Black wire from new two "AA" holder to negative (black) side of switch
The point was to add the new two "AA" holder to the old four "AA" holder in series so that I'd have the equivalent of a six "AA" holder, jumpered to act like five "AA" holder.

5 AA
battery pack from frontAt this point, I decided to do something about a charging jack. Otherwise, I'd have to unscrew the battery compartments and remove all the batteries every time I wanted to charge them, or even just check their charge levels. I had ordered some 9V battery connectors, since the six "AA" holders I'd ordered had 9V battery terminals on them. I had planned to just snap one of those on and wire to it. I didn't end up using those holders, but figured I could put the 9V connectors to good use. I figured I'd just wire one in parallel to my new five "AA" battery pack. I could wire another to a connector that fit my battery charger. By snapping the two 9V connectors together, I'd have a good electrical path from charger to battery pack.

I ran the 9V connector through a hole in the two "AA" holder down into the stock battery compartment, so the connector was outside and near the top where I could get to it easily. I ended up soldering the 9V connector to the unused poles of my on/off switch. It then became an on/charge switch. When "on" it connects the batteries to the receiver pack. When "off" it connects the batteries to the charging connector. I figured this would prevent me from burning out my receiver pack if I accidentally tried to charge the batteries when the switch was on.

With all the soldering done, I tightened the zip tie and carefully folded all the wires into the battery boxes. I added my five "AA" NiMH batteries (mostly charged) and hit the switch. I was surprised that it all went back together so easily. I was even more surprised when everything worked the first time I turned it on. Plenty of power to those servos now!

I wanted to top off the battery pack, so I snapped another 9V battery connector on the first, just to doublecheck the polarity before I soldered on a connector to fit my battery charger. Then I tried to remove it. And tried. And tried. I ended up damaging both 9V connectors getting them apart again. Apparently, these things aren't designed to snap onto each other. Crap. Now I'd have to take the whole thing apart and solder in some other type of connector for charging.

Then I had a brainstorm. I clipped the 9V connector off, then soldered a connector that fit my charger directly to the wires. I'd considered soldering said connector to the on/off switch, but its wires were too big to route through the compartment. Now that I had the smaller wires that had been hooked to the 9V connectors to solder to, it was easy. A little tape and heat shrink, and it was done. The finishing touch was another nylon zip tie to hold the charging connector in place.

It worked just fine when I charged the batteries. The battery compartment got warm to the touch, as NiMH batteries do tend to get hot when charging, but not so warm as to be alarming.

If I had to do it over again, I'd change a few things, or maybe I'll make changes later. Without the second battery in the new "AA" holder, I have room for a recessed charging jack. I think my charging connector is out of the way and not likely to get shorted or damaged, but only time will tell. A recessed charging jack, maybe with a little rubber plug for when it's not being used, would be more robust. And I could get rid of the external nylon zip tie, which would make it look a little more professional.

I'd also like to wire in a battery meter of some kind, so I can tell as the T-Maxx drives by if my receiver battery pack is getting low. Currently, I have to break out the old voltmeter and switch the on/off switch to "off" to check voltage.

Finally, I have vague plans of someday of building a one-button or even remote start, instead of having to lug around the Easy Start 2 and plug it in every time I need to start the T-Maxx. The starter motor uses a 7.2v NiCd battery, but I don't see why it couldn't use six "AA" NiMH. If I could find a cheap voltage converter which could turn the 7.2v into 6v (called a BEC or Battery Eliminator Circuit, I think), I could use the same six "AA" NiMH battery pack to run both the receiver pack and the starter motor.

I hope you've enjoyed this little article. I apologize for not having any "before" or "during" pictures. I'd been fiddling with this project for weeks and still wasn't convinced I'd actually go through with it until I'd drilled that first hole. Then things just sort of happened and next thing I knew, I was finished!

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This page last updated on May 06, 2005 by Troy H. Cheek