I would like to use my 18V (5Ah) lithium-ion cordless drill battery to power two DC motors for an Arduino RC car I’m building.
According to the seller of the motors the maximum voltage I should supply to the motors is 7.4V with a maximum current of 58A. See this link for full specs.
So my problem is that I need to supply a lower voltage to my motors than my battery is giving me. I don’t know anything about voltage converters, but after googling I understand that “linear voltage regulators” can be used to easily convert just about any input voltage to any output voltage. However, I’ve also read that it is not advisable to use these when the difference between input and output voltage is large, which it is in my case, and that they are inefficient and generate a lot of heat.
Could anyone give me some hints on what type of hardware I should use to best make this conversion? Preferably a relatively simple solution since I’m not that good with electronics in general. Or are there perhaps DC motor drivers for the Arduino that automatically does the DC-DC conversion for me so I don’t need a separate voltage conversion?
The simple solution is to just buy a good 2S Lipo to run the motors. That's what they're designed for.
A DC-DC converter for the sort of power you're talking about, around 1000W for 2 motors, would be large, heavy and expensive. Though you probably don't really need that much power and in reality the motors won't last long if you run them at that sort of power.
slipstick:
The simple solution is to just buy a good 2S Lipo to run the motors. That's what they're designed for.
A DC-DC converter for the sort of power you're talking about, around 1000W for 2 motors, would be large, heavy and expensive. Though you probably don't really need that much power and in reality the motors won't last long if you run them at that sort of power.
Steve
That’s too bad, I was really hoping on being able to use these batteries for all kinds of projects in the future. 1000 W sounds pretty extreme for a small RC vehicle. What if it’s enough for me to run the motors at 1/2, 1/3 or 1/4 their maximum current, would that change anything or would it still require a large, heavy and expensive solution?
What about a “buck converter”? According to wikipedia that sounds like the converter I need (steps down voltage while stepping up current).
Edit: After now having looked through the available motor shields/drivers at sparkfun’s store I realize that most shields/drivers only output about 2A. So I will only need 2A and 7.4V, not 58A.
You can't just pick random values to suit yourself. At 2A I doubt if those motors will even turn. They definitely won't drive a car. They are normally run on at least 30A max and through an ESC (Electronic Speed Control) not a motor driver. Hobbyking have lots of ESCs that would suit.
But your "buck converter" would still need to output around 50-60A or approaching 500W. Just Google "DC-DC converter 500W" and see what you find.
Perhaps we need to start again with exactly what it is that you are trying to do. If you really want to run motors from 18V batteries then it might be better to find some higher voltage motors. Those motors are basically brushed 540-size motors and they are available in all sorts of winds and therefore voltages.
slipstick:
You can't just pick random values to suit yourself. At 2A I doubt if those motors will even turn. They definitely won't drive a car. They are normally run on at least 30A max and through an ESC (Electronic Speed Control) not a motor driver. Hobbyking have lots of ESCs that would suit.
But your "buck converter" would still need to output around 50-60A or approaching 500W. Just Google "DC-DC converter 500W" and see what you find.
Perhaps we need to start again with exactly what it is that you are trying to do. If you really want to run motors from 18V batteries then it might be better to find some higher voltage motors. Those motors are basically brushed 540-size motors and they are available in all sorts of winds and therefore voltages.
Steve
Thinks a lot for the info.
That’s a shame, I specifically bought brushed motors because I’ve read that brushless motors require an ESC, making them more expensive and complicated, whilst brushed motors do not. So I was hoping I could avoid those by going with brushed motors.
But if ESC is the best way to go I guess I will have to learn how to use them..
Do you think this one will work with my motors? Rated at 45A. I’m building a medium-sized (perhaps 30 cm long, 20 cm wide) robot/crawler/tank, I don’t need it to go particularly fast at all, but I would like to have decent torque and good low speed control. Does that play in to my choice of ESC?
And just to make sure I don’t misunderstand this ESC thing: My Arduino will communicate with the ESC which in turn will control the speed of my motor, and therefore I will need two ESCs if I want to independently control the speed and direction of my motors, correct? Or can one ESC be used to control several motors?
petters:
I would like to use my 18V (5Ah) lithium-ion cordless drill battery to power two DC motors for an Arduino RC car I’m building.
According to the seller of the motors the maximum voltage I should supply to the motors is 7.4V with a maximum current of 58A. See this link for full specs.
Just use a big motor controller and never turn the throttle fully up?
I know this is a bit tricky without some monitoring of the average voltage and of the current, but it involves
no unnecessary complication in the power circuitry, which would be very expensive at that power level.
Yes an ESC can connect to an Arduino and control speed and direction (check the ESC spec because not all ESCs have reverse). If you need independent control of the motors then they will need an ESC each. The ESC you linked to looks reasonable and does have reverse.
ESCs are controlled like servos so there is lots of library support making the programming for them relatively straightforward.
MarkT:
Just use a big motor controller and never turn the throttle fully up?
I know this is a bit tricky without some monitoring of the average voltage and of the current, but it involves
no unnecessary complication in the power circuitry, which would be very expensive at that power level.
Thanks for the suggestion. The simplicity of that solution is very tempting I must say. However, all the motor controllers I’ve seen are extremely far away from the 58*2 A rating of my motors. So I worry that If I were to buy a relatively expensive big driver and try this, I would end up frying it almost immediately since I’m not knowledgeable enough to control my voltage and current properly. Since I’m a newbie who barley understands what I’m doing I think it would be best if I could experiment with my setup without a constant risk of damaging my electronics.
slipstick:
Yes an ESC can connect to an Arduino and control speed and direction (check the ESC spec because not all ESCs have reverse). If you need independent control of the motors then they will need an ESC each. The ESC you linked to looks reasonable and does have reverse.
ESCs are controlled like servos so there is lots of library support making the programming for them relatively straightforward.
Have you decided to go with a normal 2S lipo?
Steve
Ok, sounds good! I guess I will buy myself two ESC’s then and start reading tutorials on how they work.
However, the ESC I linked is rated at 45A of continuous current, since my motors are rated at 58A, does that mean that the ESC will fry if I give the motors full throttle for more than a few seconds? Or do ESC’s normally have built in protection against that so I don’t have to warry about destroying it?
And yes, it seems 18V to 7.4V DC-DC conversion at these amps is impossible to find without going for some sort of industrial looking converter – which is expensive and unreasonable to have in a RC car. So I guess I will buy a 7.4V battery and a charger as well. Do you think these will work?
It's very unlikely that ESC will have any serious over-current protection. But motors only draw the current they need for the load on them so provided you don't do something really silly like stalling the car/tank so it can't move and still holding full throttle on for some time you should be o.k. Most ESCs can usually tolerate 2 or 3 times their rated continuous current for a few seconds.
The battery is o.k. though I'd probably go for 30C. But I'd definitely spend a bit more for a decent charger. That one will work but at only 800mA charge rate it will take around 6 hours to charge your battery.
It occurs to me that an 18v Lithium-Ion battery is almost certainly made up of 4 or 5 x 3.7v cells. The easiest way to get 7.4v might be to take it apart and just use 2 cells.
Robin2:
It occurs to me that an 18v Lithium-Ion battery is almost certainly made up of 4 or 5 x 3.7v cells. The easiest way to get 7.4v might be to take it apart and just use 2 cells.
...R
I took a dead battery apart and got 4 good 18650 lithium ion cells out of mine. Throw them on a charger and use them in projects.
Just be careful, now that they're unprotected, if you discharge them under 2v the batteries may be dead for good. Use a good charger don't over charge them. Read up on lithium ion safety. High current usage can cause heat to build up which isn't safe either...
Your battery pack has thermal, current, balancing and voltage sensing circuitry that protects the batteries. That's why they cost so much.