Hi,
I’ve built an autonomous vehicle using 4 of the “34:1 Metal Gearmotor 25Dx52L mm HP” motors.
The documentation states that these motors consume 450 mA when in free-run, so I need at least 4x450 mA for maximum speed, so basically 2A. I understand that the current will actually be higher due to the weight of the robot (6-7 lbs). The stall current is 6A. The motors are rated for 6V.
I’m currently using the “Dual VNH2SP30 Motor Driver Carrier MD03A” and am planning on upgrading to the “Pololu Qik 2s12v10 Dual Serial Motor Controller” motor controller. Either of these can handle higher currents than the motor will draw.
I started out with 4 regular AA batteries but these didn’t provide enough power to reliably turn the motors so I am now using 8 batteries. This works better but sometimes the motors don’t turn and I hear a high pitched noise.
I know that the regular AA batteries don’t provide much current and are probably not the correct choice for these motors. I am considering trying some Energizer Ultimate Lithium batteries which have a much higher discharge current, but I’m wondering if I should really be making the jump to Lithium Polymer (li-po) batteries as used in R/C cars.
However, I notice that the only batteries that Pololu sells are NiMH battery packs, so does that mean that these are the ideal choice?
I’d love to hear any recommendations on the best batteries to use to power these motors to their full potential.
Thanks!
While the bot is starting, the motors are basically stalled, so they will temporarily draw up to the full stall current.
Also, when you shift direction, or if you PWM the motor control, they may temporarily draw up to twice the stall current.
Thus, for 4x6A, you should use two controllers that each are rated for at least 2x12A, or one controller that is rated for 2x24A.
Similarly for voltage: Because of inductivity, the controllers may temporarily see a voltage that is twice that of the supply voltage.
With 6-8V motor control, you have to have controllers rated for at least 16V.
Yes, AA size batteries will not be able to drive those motors sufficiently. Lithium batteries will also be very expensive.
I would use LiPos for this bot (and I do use LiPos for my two bots.) LiPos are lightweight and high power, but have to be carefully charged and protected against physical damage to avoid burning down your house.
Also, if you forget to turn off power on a LiPo, it will destroy itself to the point where it can’t be recovered, safely or at all.
NiMH weighs more per watt-hour, but may be lower cost, and more forgiving against light abuse. They also are easier to send in the mail, as there are limitations on shipping of LiPo batteries.
Either LiPo and NiMH will probably work fine for your bot.
[quote]Also, if you forget to turn off power on a LiPo, it will destroy itself to the point where it can’t be recovered, safely or at all.[/quote]To avoid this issue, LiPo batteries should be equipped with a “battery protection PCB”, which prevents overcharging, overcurrent and overdischarge. You can get them quite cheaply ($1/cell or so) at all-battery.com/protectivepc … packs.aspx
Thanks for the information. That is very helpful.
I have ordered an NiMH battery pack and charger from Pololu and will try this out at the weekend. For now I’ll stick with a single motor controller since that is what they use on the Dagu Wild Thumper chassis with the same motors that I am using, but I’ll consider upgrading to two controllers in the future.
A single dual-channel motor controller is fine, as long as it’s rather for suitable amperes and voltage.
You mentioned a 2A rated motor controller above. My prediction is that this is about a factor 10x or so too little to be safe and effective.
I wanted to follow up on this for anyone finding this thread in the future. I did some brief testing this weekend with the 7.2v NiMH battery and my robot is moving much better now and able to perform turns (by braking the motors on one side). Here’s a short video to show this:
This is probably going to be fine for my purposes but I do have some follow on questions as there are still gaps in my knowledge around the currents involved.
I cannot find any specifications about the maximum current the battery (pololu.com/product/2225) can put out. The only clue I have is that the wires are 22 AWG and based on a table I found, this implies an absolute maximum current of 7A. I am assuming that these batteries have internal resistance such that they would not provide a current that the wires cannot handle?
Another question I have… assuming each motor attempts to draw 6A at startup (as per the documentation) and I have two motors wired in series, does this mean the total current would still be 6A rather than 12A? If I am correct about this then it sounds like it would be ideal to have one motor controller for each side, each with its own battery, like this:
7.2V battery providing max 7A -> motor controller (max 13A continuous) -> motor1 (6A max) -> motor2 (6A max)
If am correct about the motors in series drawing max 6A then the max current overall will be 6A. If I am wrong and the motors attempt to draw up to 12A then the max current overall will be limited to whatever the battery can provide (7A based on my assumptions).
I’d appreciate any feedback on my understanding of this.
I would not assume that the wires are dimensioned for the maximum short-circuit current that the battery can put out.
In general, current dimensions come down to heat management. If you draw more current for a short while, low-resistance parts like wires can live with it if you then draw less current.
Also, you are right, the internal resistance of the battery is what limits the max (short circuit) current that the battery can deliver. This may very well be enough to melt the wires in some cases. So don’t short your batteries 
If the motors are coupled in series, then two motors starting at 6A each would still draw 6A (each would get half the voltage.) This is not a good thing for motors, though, because as the brushes make and break connections and commite the internal magnets, that may interfere with the needed current in the other motor, and vice versa. Instead, motors should be coupled in parallel. This means they both see the full voltage, the make/break cycle doesn’t affect the other motor, and the current draw of 2 motors will be double that of a single motor.
