Power Systems / Beginner Questions

Hello. I had a few beginner questions I would like to clear up. Here they are:

  1. I’d like to use the “RP5 Tracked Chassis Gray” that is available here as a chassis for my project. Can I use 6 of these batteries to power it? pololu.com/catalog/product/1003
    If yes, how is it that your able to power 7.2V motors using 1.2V batteries? (I would assume the battery holder wires them in series, thus multiplying the voltage by 6?)

  2. Would I be able to power two servo’s off of this supply as well? (I’ll most likely be using one of the motor controllers and one of the servo controllers available here to control the two motors/servos)

  3. What would be the simplest way to get an output of 5V, 2.5A on my robot? I was thinking to try and use another 7.2V battery (which would power all sensors/electronics, not the motors/servos) with a 5V switching voltage regulator, but sadly couldn’t find one available on the site here (for shipping, I’m trying to keep things from the same company)

  4. What would be an expected number of uses I could get out of the aforementioned batteries? (just a rough estimate)

  5. I have a pet-ridden house, meaning balls of dog/cat hair are not an uncommon sight, and I have a feeling that my robot may be subject to getting hair stuck around places that may restrict movement. If anyone has had an experience like this before, how easily is it to clean off the axles of the motors and wheels on the “RP5 Tracked Chassis Gray” chassis?

Thank you to anyone who is able to answer.

Hello Evan,

  1. Yes, you can use 6 rechargeable AA batteries to power it. The included battery holder is wired in series.

  2. Servos tend to like to be powered from 4.8-6 volts and tend to be sensitive about higher voltages (they will break). Please make sure to check what voltage your servos can handle before applying voltages to them! I would not recommend using 6 AA’s to power servos because the nominal charge is already out of spec for most servos, and, when fully charged, the battery voltage can be even higher than that. If you use the right 5 V regulator, it should be possible to power the servos and the motors with 6 AA’s.

  3. Unfortunately, we don’t have a 5V switching regulator, or even a 5V linear regulator for sale. We are planning, at some point, to make a carrier for the linear voltage regulator that we use on the Orangutan SVP, but it is not high on our priority list at the moment.

  4. We have not scientifically tested the recharge cycles/lifetime of our batteries. My casual observation is that they work pretty well for 20 recharges (I just haven’t actually tried more, so it’s probably okay after 20, and I don’t really know how much the capacity was affected by the recharges.).

  5. The chassis package includes a small hex-wrench that you can use to undo the set screws that hold each of the axles in place. After that, you can simply pull the wheel/axle assembly off and clean it.

- Ryan

Thanks for the answers. This may be an obvious question but I’d just like to be sure; for question 2, could I use a “Micro Maestro 6-channel USB Servo Controller” to power the servo’s off of a 7.2V battery, and like-wise use a “Pololu TReX Jr Dual Motor Controller DMC02” to power the two motors located on the “RP5 Tracked Chassis” off a 7.2V battery? I would hope that both the boards would regulate the voltage down to one suitable for servos/the motors they are driving?

This is just a quick curiosity question but, I’m wondering why the rechargeable battery packs (such as the ones found in RC cars/boats) tend to have a ~100 recharge usage when the single AA batteries have much lower than that? I seemed to get the impression that the packs are simply made up of the smaller AA or similar cells.

The regulators on the Micro Maestro and the TReX Jr. are just used to power the microcontrollers on the boards. On the Micro Maestro, you can connect the 7.2 V to VIN, powering the logic part of the Maestro, but I would not recommend connecting it to PWR. The TReX Jr and most motor controllers/drivers pass the voltage supplied to them directly to the motors. The motors will most definitely see 7.2 V from the TReX Jr’s motor drivers.

If you are planning to buy a Micro Maestro and a TReX Jr, you might instead consider just getting one Orangutan SVP-324. The SVP-324 might solve some of the power issues you are encountering. You can power the SVP-324 on 7.2V and the voltage regulators can most likely supply enough to power your two servos (check the servo datasheets for stall currents).

I wasn’t saying 20 charges was a maximum but rather just anecdotal evidence that it worked for me up to that number of recharge cycles (it didn’t fail after that many, I just stopped needing those batteries). Unfortunately, we don’t have good numbers for how long they will last.

- Ryan

Hi again Evan,

I think my response to you about battery recharge cycles wasn’t really appropriate. I don’t know much about batteries at all, and my figure of 20 recharge cycles is totally low-balling it.

I spoke with someone here who knows much more about batteries than I do, and here is what I know now: generally, Ni-Mh batteries are good for hundreds of cycles. We haven’t cycled our Ni-Mh batteries to their limit, but, depending on the details of your usage, they should probably work about as well as other Ni-Mh batteries.

- Ryan

Ah, thanks again for the responses, and clearing up my confusion. Glad to hear about the batteries as well, now those sound like a perfectly good candidate for powering my robot. The problem of controlling the motors/servos remains however, since I already have an arduino micro controller and plan on using it. You wouldn’t happen to know of any other solution to powering motors/servo’s easily? I may just have to bite the bullet and create my own power regulation circuit.

Actually, I just realized that the RP5 Tracked Chassis motors run at 7.2V, and therefore would it be possible to use a "Pololu Qik 2s9v1 Dual Serial Motor Controller"
to control the motors? Does the 7.2V input need to be regulated in some manner before it can be used by the motors/motor controller? Also, just to clear this up for myself, the 3A peak means it would be able to handle the 2.4A stall current that the RP5 Tracked Chassis motors might generate? The arduino provides 5V logic power so that wouldn’t be a problem. Also, I’ll have to look into simply using the arduino’s 5V for the servos (hopefully it provides enough Ampere’s)


You can definitely power motors and motor controllers directly from 7.2 V if they are designed to do it, which the RP5 motors and qik 2s9v1 motor controller are. However, (RC) servos are different, and many won’t handle a well-charged 7.2 V battery.

The qik 2s9v1 is underpowered for the RP5 motors (the 3A peak is for a very small fraction of a second), but you might be able to get away with it if you don’t drive too aggressively and maybe stick a heat sink on the motor driver.

I don’t think there’s much reason to expect the Arduino to power a servo unless the servo is really small.

- Jan

Thank you both for guiding me through this. I think I’ve got it mostly figured out now, so hopefully this will be my final post in this thread:
So, I can use the 7.2V that the batteries will give me to power the “Pololu TReX Jr Dual Motor Controller DMC02” and the motors in the RP5. The arduino would control the TReX Jr via serial communication.
I have seen others power the smaller servos I’m using (Hitec HS-422) using only the 5V at 40mA provided by the arduino, so I guess I’ll go for that. If it fails, I’ll attempt to build my own 5V regulator circuit to get the 7.2V down to 5V and up the amperes from 40mA to the 150mA that the servo is reported needing with “no load running”. Does this look correct?

Edit: And one quick conceptual thing I want to make sure I’m understanding correctly: if I have a 2200mAh battery, I can hook up any component that draws any amount of current (with in reason for the 2200mAh) and not have to worry about the component “over-drawing” to much current or anything like that (as long as the component doesn’t exceed the max Amp draw of the battery)? Such as, using a 5000mAh High power delivery battery with a max of 40A discharge rate with a 2mA draw from a component won’t damage it, the component will only discharge the battery very slowly at 2mA?

Your setup looks mostly correct except for the servo power. Unless you are sure your servos will be under no load, you want to be able to power your servos up to (or close to) their maximum current draw at a given voltage. I found this forum thread about a the HS-422 that suggests the current draw might be 590-700 mA at its maximum.

What you say about avoiding over-drawing the battery is correct. It should always be okay to draw a very small amount of current directly from your batteries.

- Ryan