Is the 18V25 right for me?

I am “reviving” a Sears/Craftsman “C3”-powered radio controlled truck (product name “RC Truck”!) which involves keeping the existing motors, body, and wheels, but replacing everything else.

Reviving a Sears C3 RC Truck (slippery slope!)

More specifically, this includes a Futaba T2PH 75MHz AM Tx/Rx/Servo kit, a BaneBots BB-3-9 3.5A cont/9A peakESC, and an 18V HF cordless drill power pack.

What I have now, after some weeks of work, runs, but it appears that I seriously underestimated the motors’ current requirements: the BaneBots 3.5A ESC I selected is sufficient to drive the motors from my 18V battery pack, but only if I’m extremely gentle on the throttle. If I start slamming the transmitter from full forward to full reverse the motors become “sluggish”: the wheels turn very slowly and the current jumps to more than 10A.

With some generous advice from the newsgroup I was able to wire up a “visual ammeter” using a pair of back-to-back LEDs and a couple of 0.47ohm5W resistors. This doesn’t give me an exact current draw, but it tells me that a 3.5A ESC is 'way underpowered. <grin!>

Intermittent sluggish performance from homebrew R/C Truck

Finding a replacement ESC at a “reasonable price” has been surprisingly difficult. Apparently this combination of specs is an unusual one:

  • Brushed motors
  • 25A+ continuous
  • 18V battery pack

The 18V25 seems to fit the bill. Do I actually need a 25A ESC? I don’t know, and I don’t know how to simulate a realistic “load” on the motors that doesn’t involve either abrading the skin off my hands or a lot of running. (This kind of testing was much easier when I had younger brothers to help out: “Hey, Bruce! Put your hand on this rapidly rotating wheel here…” <grin!>)

I do know that the motors (two, wired in parallel) together draw 1.3A at 12V unloaded (truck lying on its back), and I know that when the motors are starting they draw more than 10A for some visible fraction of a second, but apparently much less than 10A when running flat out. The motors are “380”-sized brushed DC motors labelled “STANDARD MOTOR RS380-ST/3265 – SMCD348310”; I did find a 'Web site called, but I can’t get past the home page.

So here’s what I’d like to know before I ask you all to FedEx an 18V25 eastward to Richmond: based on the (admittedly sparse) information provided, does it sound like the 18V25 will work in my application? Or is there something in my description that hints that even 25A might not be enough?

Thanks in advance for any feedback.

Frank McKenney

“We are what we repeatedly do. Excellence, then, is not
an art, but a habit.” – Aristotle

For the curious…


Thanks for the pictures! That sounds like a cool project, and I’d love to hear how it turns out.

I expect the SMC 18v25 would be sufficient for your application as I do not think your continuous current draw will come anywhere near 25 A (assuming you are not stalling the motors for extended periods of time). You might even be able to get away with using an SMC 18v15, but going with the higher-power controller is certainly the safer option. Have you considered measuring the stall current of your motors at a much lower voltage and extrapolating from there? For example, can you hold one fixed (while removing the parallel one from your circuit) while powering it at, say, 3 V, and measure the current? The stall current at 18 V will be approximately six times the stall current at 3 V.

- Ben

Hi, Ben. Thanks for the prompt response.

Wait’ll you see the Autocross “traffic cones” I’m working on. <grin!>

Let’s just say that we don’t plan on that happening.

“Safe” is good.

Hm… Just a minute… I can’t seem to find the “smiley” that, when clicked, triggers the sound of one hand smacking the forehead. It’s bound ot be over there somewhere…

The short answer is “No”, but that’s so embarrassing that I think I’ll make it “Gee! What a wonderful idea!”. <grin!>

I did, however, pick up three feet of 10AWG copper wire this afternoon as suggested in this thread:

Home-Made Current Shunts for Measuring Motor Current

which will let me use the mv range on my DMM to read Amps (another approach I didn’t think of). Hindsight is such a wonderful thing; I just wish we could have it before we need it.



“Perfection [in design] is achieved not when there is nothing
left to add, but rather when there is nothing left to take away.”
– Antoine de Saint-Exupery

Good luck with your home-made ammeter! Note that we also carry some inexpensive current sensors that output analog voltages proportional to the currents flowing through them; you can read the output with your voltmeter and compute the current.

- Ben

Here are the current figures for the truck (both motors):

   V    Load       Drain
  ---   ----      -------
  3.0   None       0.67A      Gearbox and wheels only
        Moderate   1.2-1.3A   Pressing against wheels
        Stall      3.00A      Wheels locked

  4.5   None       0.75A
        Moderate   1.3-1.5A
        Stall      3.00A

It’s hard to be exact on the “loaded” numbers, since I was pressing the wheels hard enough to slow but not stop them, and it was hard to maintain a constant pressure.

Note: After noticing the odd “stall current” reading I checked the Power Supply… my 2A PS. Sigh.

Still, I clearly need at least a (1.3x6) or 7.8A ESC, and the 25A version gives me a larger margin of error.

Thank you all for the assistance.


Mainframe: n. Largest known peripheral for personal computers.

Thanks for the update. Assuming you have an adjustable power supply, you could keep decreasing the voltage until you see the stall voltage get below 2 A, and then extrapolate from there to 18 V. However, even without those numbers, I continue to think that the SMC 18v25 will be both sufficient and the best choice for your application. In addition to giving you the most margin for error, it will also run the coolest of all our Simple Motor Controllers for a given current.

Good luck with your project, and please let us know how it turns out. Don’t hesitate to ask if you have any more questions.

- Ben