I have a Dagu Wild Thumper 4WD All-Terrain Chassis, 34:1 which consumes 7.2 V and 6.6A at stall per motor so 13.2A.
To increase the capacity I would like to buy a 11.1V 7000mAh 40C LiPo Battery and this step down regulator.
From this diagram the regulator should deliver 7A as max output.
With that configuration I don’t think I will meet the 13A needed. If yes, what alternative I have to have high battery capacity without using 7.2V LiPo?
Second question, with a step down (buck) regulator is the C rating of the battery basically ignored since the step down effectively reduce the ampere?
Thanks
Hello.
The potential maximum instantaneous current draw from the Wild Thumper 4WD motors would be around 26.4A (since there are 4 of them: 4×6.6A). This condition can happen when the motors are commanded to drive full speed from rest or when the motors are all stalled. I recommend ramping up the speed (e.g. using an acceleration limit) to reduce the sudden spike in current when starting up, and in practice, it should be unlikely that all 4 motors would stall at the same time. You can still use that value as a conservative estimate though.
If you really wanted to regulate the motor voltage, you could probably get away with powering the motors on each side of the robot from a separate D24V150F7 regulator; however, please note that to do this you would need to drive the motors on each side of the robot from a separate driver, so this wouldn’t work if you are using a single two-channel motor controller like you suggested in your other thread. An easier solution would be to limit the maximum duty cycle you run the motors at. For example, since the maximum voltage of a 3S LiPo is around 12.6V, if you limit the maximum duty cycle applied to the motors to around 57%, it would be similar to driving them at 7.2V.
I am not sure what you are implying when you talk about ignoring the C rating of your battery, but if you use a regulator, it would still be powered through the battery and should still be a consideration. Power is conserved, so the power into the regulator (plus some efficiency losses) equals the power coming out of the regulator. For example, if the motors draw 4A from the regulator at 7.5V (which is 30W), the regulator would be drawing around 2.4A from the battery at 12.6V.
By the way, if you have not read the “Understanding battery capacity: Ah is not A” blog post that Patrick suggested, I also strongly recommend it.
Brandon
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Many thanks for the reply, very useful.
On my last point about amp:
- the battery (2S LiPo 11.1V 7000mAh 40C) can deliver a max of 280A.
- The 4 motors, as you stated, require a max of 26.4A
So, my assumption is that I can’t use the 7.5V, 5A Step-Down Voltage Regulator D36V50F7 since the max current is 7A and I will never reach max speed.
If the above is true, the 2 options i see are:
- use separate regulators as you suggested
- use a 2S LiPo 7V 5200mAh 50C but I won’t have additional capacity. I might put 2 of them in parallel.
P.S.
With a 12.6V battery connected to the motors with max 7V even if I limit the duty cycle am I not risking to burn the motors? I am not sure I’ve got how the wiring would work and how to test that.
I would not recommend powering those motors from the D36V50F7 regulator since it is underpowered and you will likely either trigger the thermal shutdown feature or damage the regulator (it will not actively limit the current to the motors and reduce the speed like it sounds like you are describing).
The recommended voltage for those motors is 7.2V, but in general you can run them at higher voltages to get more speed and torque, but at the cost of a shortened lifetime. However, limiting the duty cycle to the motors essentially runs them at that reduced voltage (partially due to the motor’s inductance), so I would not expect it to harm the motor. I am not sure what you mean when you talk about how the wiring would work, but to limit the duty cycle, you would just not command your motor driver to drive them at full speed (so nothing in your wiring would change).
Brandon
