Zip Zaps Monster Truck Charging Mod

A while back, Radio Shack introduced micro RC vehicles called Zip Zaps.  Not long after introducing those, they introduced a line of monster trucks that in my opinion, were way cooler than the Zip Zaps SE or even the original Zip Zaps as these monster trucks could drive for a much longer time and they can drive on carpet, tile, and most any surface you can think of.  They’re also 4WD, which is really cool!

Radio Shack Zip Zaps Monster Truck

After playing with one for a while with the stock motor, I had to have more of these, and along those lines, upgrade them w/ Radio Shack’s upgrade kits.  I also discovered some mods that were available for the monster trucks to make them not only faster, but make more efficient use of the stock battery.

The mod I’m going to detail facilitates neither of those end results.  Instead, this mod attacks the charge level of the monster truck’s battery, thus translating into longer run time, especially for older, kinda worn out batteries.

So here’s the deal with the battery contained in these little RC monster trucks (referred to as ZZMT from now on).  Under the circuit board sits a tiny NiMh (nickel-metal hydride) rechargeable battery pack.  It’s a 2.4V pack (2-cell) at 120mAh.  With this pack, depending on what motor/gears you’re running in the ZZMT and how fresh your remote batteries are, you can get a decent amount of run time for the quick charge you get from the remote.  Typically with the fastest motor and tightest gear ratio, I could get maybe 3-4 minutes of run time.  I performed the FET mod on two of my trucks for even more speed, stronger steering, and slightly extended battery life.  This was fine for a while, but as these trucks and especially the batteries aged, they don’t charge as well anymore, and I’m consistently getting short run times (like a minute) per charge.

I looked into putting a higher capacity 2.4V pack in there (people have put 3.7V LiPo’s in there, but that’s not for me), but nothing really fits easily that’s higher capacity.  After researching that a bit, it suddenly occurred to me that there’s no possible way that the 120mAh pack in the truck is getting anywhere near to fully charged in the ~45 secs it takes to charge the battery.  That last little letter there in the capacity, yeah the ‘h’, stands for hours, meaning that the number printed before mAh is how much current can supposedly be drawn from that battery for the duration of an hour.  Ok so that’s cool, under ideal circumstances, you can pull 120mA out of this pack for an hour before pronouncing it dead (to be charged again).  All that this really means is that you need to put that much power back into the battery (plus some extra for losses) to charge the battery to capacity.  Some simple math reveals that to fully charge that pack in the ZZMT, you need to charge the pack at 9.6Amps (1000mA in 1 Amp) for ~45 secs to reach full capacity.  Again, all this is ideal, but hopefully you see where I’m going with this.  Somehow 4 AAA batteries need to provide a ton of power in a short amount of time to charge the ZZMT fully, or something else is going on like the battery isn’t being fully charged….

I’ve designed battery chargers before, and the charge rate you have the charger charge a battery at is rated in ‘C’s, or multiples of the battery’s nominal capacity.  In other words, for the 120mAh battery I’m talking about, charging it at 1C means applying the charge voltage and current limiting the charge current to 120mA.  2C means 240mA…etc.  So another gut check:  To charge this 120mAh battery in 45 secs requires a charge rate of 80C.  Hmm…even the super-duper hobby NiMh packs can’t be charged at this rate.  Usually they top out around 20C or so, and the battery inside the ZZMT isn’t anything special, so charging it at like 5-10C max is probably what the battery can handle.

With all this, it was very obvious that there’s no way this battery is being charged to capacity.  And likewise, a higher capacity battery would really do nothing except appear to be better since it was new instead of old and kinda worn out.  This made me wonder if I can modify the charger to deliver more current (higher C charge rate), have a smart charger in the remote to fully charge the battery, or lengthen the charge time (good for older, worn batteries).  Upon looking at the schematics for the remote, I find that the charger is very crude and simple: A transistor turns on allowing current to flow to the battery, and a 0.22 Ohm resistor limits the current.  After a RC timer circuit hits ~45 secs, the transistor is switched off and the battery is “charged”.  This…is cheesy haha.  I didn’t really want to mess a whole lot w/ hacking a smart charger in the remote and trying to increase the current (as there’s not I can do to increase current to the battery), so I decided to change the timer circuit to allow a double-charge.  To prove that this helps, I double-charged a ZZMT and I got way longer battery life.  In fact, the more little charges you give the thing, the more run time you get (at some point you’ll hit diminishing returns, your remote batteries die, or something melts in the remote from all the current going into the 120mAh battery).

Luckily the schematics revealed a 1 meg-ohm resistor and a 22uF electrolytic cap make up the RC timer circuit.  I doubled the cap to 47uF, and voila, double-charges without having to do anything.  I could also replace the BJT w/ a FET instead and get a higher charge voltage to the ZZMT’s battery, but I just left that alone.  Oh I also added an external AA battery pack as the AAAs in the remote die quickly.  Another thing to note that after a bunch of charges, the AA’s get warm, meaning lots of current is coming out of them to charge the ZZMT.  The result of all this?  Probably roughly 10 or so minutes of run time, depending on the driving surface (lower w/ carpet).  So…easy mod to get longer run times w/o really doing much of anything except changing a capacitor!  Heat is probably an issue through the 0.22 Ohm resistor (R15), so that might need to be beefed up, but hey, I’m happy now with the run time.

In the pic below, the replaced capacitor is the brown can looking thing next to R15 (upper right):

Zip Zaps Charger Mod

At some point, I may make a stand alone smart charger to fully charge the ZZMT in an hour-ish.  Then, you can just leave the ZZMT on the charger till you want to use it and get lots of run time.  And if it wears out, throw it on the remote’s charger for some quick-charge play time.  I love these little trucks!

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6 Responses to Zip Zaps Monster Truck Charging Mod

  1. fearless says:


    I want to make my own charger for a related project. What value of C do you recommend, or how much current really is being pumped into the 2.4V / 120 mAh battery pack? (Alternately, what voltage do you see across the 0.22 ohm resistor at the start of charging?)

    (I don’t have a zipzap, but a naive analysis says that the charger puts 3V through 0.22 ohms for a charge current of 13A, which seems improbable. I’m guessing current would be limited by the AA cells instead.)

    Thanks in advance.

    – ff

    • Andy says:

      Yes, you’re right in that the “charger” is trying to pump as much current as the AA cells allow into the little nimh pack in the zip zap. Your pack that you’re trying to charge may have a datasheet that has info on the recommended charge rate. Normal charge rate for nimh/nicd/lipo is 1C. Fast charge for nimh/nicd/lipo is very cell dependent, but you can get up to probably 4C or 8C for nicd/nimh. 4C is probably a good fast charge rate, but charge termination is harder to detect when the cell isn’t in perfect condition anymore (end of charge detect is usually indicated by the current going into the cell dropping past a set threshold). If you can afford the waiting time for charging, stick w/ lower C rates of charge.

      Lipo is a very different story as fast charging those can be pretty dangerous if your cell isn’t a high-C rated cell and you’re pumping loads of current into it.


  2. Dave says:

    How is the FET upgrade done? I’ve got a few ZZs (Mt and SE) and I’d like to tweak them a bit. Nobody has pictures or good info on how to do this. Also, do you have any pictures of exactly what you did to the transmitter? It’s a bit hard to follow with just a description. Thanks!

    • Andy says:


      Unfortunately I don’t remember what I did for the FET upgrade, but I did take pics of what I did and I took a pic of the charger mod and sent those to you. I’ll update this post w/ the charger mod pic.


  3. Shane Cole says:

    I also modified my monster truck, but with a li-po. I know its not your thing. lol. But hey. I was curious. I bought a couple ZZMT’S just in case I fried one. I used a 3.7 li-po 60mAH which i scored a 4 from ebay for like 5 bucks total. i ordered micro plugs. the same ones that would fit a small rc helicoptor. so i charge it off my usb from my pc or ps3. no problems yet. wish i had a cam to show you how fast they zip. Man they work great. I got all this speed from the fastest motor and gear set up, but now I would like to get a wee bit more range out of it. Do u know of any mods for the 27mhz and 49mhz controller to get more range. or by modifying the ZZMT itself…
    Thanks…Shane Cole

    • Andy says:


      From what I’ve gathered w/ the ZZMTs, the 40MHz ZZMTs have much better range than the 27MHz ZZMTs. Maybe it’s my particular transmitter or something, I dunno. Given that none of the antennas are ideal lengths, it would make sense that the 27MHz ZZMT would have crappier range than the 49MHz, but for me it’s significant, like 1/2 to 1/3 the range.

      If you’re good w/ electronics and RF, you could make a RF amplifier to boost the transmitter power for that particular frequency, or what I’ve found to help me a lot, make a better antenna for the ZZMT. The transmitter antenna is decent I think, just keep your fingers/hands from touching the antenna and you’ll get some range gains there too.

      For my ZZMT, I wound some wire around a zip tie so it would always stick up, instead of flopping all over the place as the stock antennas tend to do. That also helped. Ideally a 27MHz or even the 49MHz antenna is like 6 or something feet long. Supposedly coiling that length of wire onto sometime gives you similar performance of a full-length whip antenna, but in my findings and other people’s findings on the net, that’s not the case. I guess that’s the reason why rubber duck antennas for lower frequency walkie-talkies or radios just plain suck. The higher the frequency, the shorter the antenna length.

      I’ve been wanting to make a RF amp for the transmitters, but…meh, the range I get is useful enough for me. I drive the things around the house and that’s sufficient.

      As for the lipo, yep, they’re great and all that’s for sure =). You do run the risk of frying something as you know. I don’t wanna fry any of mine as I don’t have spares, and I didn’t feel like modding the crap outta the things either. I just wanted to drive em. I bet that lipo sure is fast though!


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