# Powering Motors in War Robot

I would like to give you all some premise before asking this question. I am a highschool student from India and I generally do software development, so this is one of my first hardware related projects so I am a complete noob at this stuff.

Our team had been tasked with construction of a mini war robot for a competition. We thought that the best way to go about it would be with 4 powerful motors with PWM controlled wirelessly via bluetooth using an Arduino. I was successfully able to figure out the code and created the controller.

The problem we are now facing is what would be the most efficient and low cost way to power our 4 motors and what batteries should we buy.

The motors that we have used are 200 RPM Johnson Gear DC Motors 12V with maximum load current upto 7.5A We were also considering buying these motor drivers which I Linked here https://robokits.co.in/motor-drives/dual-dc-motor-driver-20a

My Question is what battery should we use so that the robot runs with full power for at least 30-45 mins and should we use two dual channel motor drivers(one for each motor) or one motor driver(two connected in parallel with one motor driver)

(not sure if this should be here or in robotics stack exchange figured this was a more electrical engineering question)

• Have you calculated how much energy might be taken by your motors over the 45 minute period given a hard feuding scenario? Nobody here is going to estimate that for you. Jun 6, 2018 at 18:22
• @Andyaka if you can give me link or something on how to do it I would really appreciate it. Jun 6, 2018 at 18:28
• Speak to a mechanical engineer. Jun 7, 2018 at 9:02

First, you don't want to use that driver. Your motor spec of 12V/7.5 A assumes the motor is running "normally", and for a BattleBots situation you can't count on that: if your bot gets into a shoving match with another you'll need to deal with the stall torque, which is much higher. Since the controller you're looking at is only good for 5 amps continuous (and that probably assumes good ventilation, which I'll bet you're not providing), it will probably overheat and die.

Second, if you provide a 12 volt battery and get about 12 volts max to the motor, 45 minutes is 3/4 of an hour, times 7.5 amps is about 6 amp-hours. This is perfectly reasonable from a rather small lead-acid battery. Again, though you need to look at the possibility of larger load currents and more power from the battery.

So. Get the data sheet for the motor and find the stall current. Alternatively, measure the motor winding resistance and apply this to 12 volts for worst-case current. Even better, get a motor and measure your currents.

If you have gearmotors, you need to look closely at your gear ratios. 200 rpm (I assume that's output rpm) is about 3.3 rps. Assuming 20 cm diameter wheels, your top speed will be on the order of 3.3 x 60 cm/sec, or about 2 m/sec - about 1.3 mph if you don't like metric. That is a very slow vehicle.

Finally, 4 motors suggests that you really don't know what you're doing. Steering with two motors is standard. If you have 4 wheels with a motor each, you will not get good results using differential power. It just doesn't work well. You'll need a steering mechanism like a car, and making one of those which is resistant to shock and impact will be a real test.

• hey, thanks for the detailed feedback. okay so all these are recommendations from my friends. We chose a differential drive instead of a car like mechanism because of the large turning radius which just makes the bot difficult to control. Also this is a 'mini' war robot(size of robot should be under 25x25x25cm) meaning the play area would probably be the size of a bed so speed is not an issue, I am using standard 7cm diameter wheels in a metal chassis. Stall current which I assume is No-load current is 800 ma(max). What type of controller would you recommend? Jun 6, 2018 at 19:27
• @DhruvaGoyal - "Stall current which I assume is No-load current is 800 ma(max)" Well, if that's true then the motor is not 7.5A. Make up your mind. And a large turning radius does not make it hard to control, but it does make it difficult to maneuver in a small space. You're not making much sense here. Jun 6, 2018 at 21:08
• thats what it says here, It does not specifically say stall current: robokits.co.in/motors/dc-geared-motors/… Jun 6, 2018 at 21:31
• @DhruvaGoyal - Sigh. No-load current is the exact opposite of stall current, which in this case is 7.5 A. This is (worst case) more than the controller you've specified can handle. Get the controller the link suggests. And differential steering is still not going to work for squat with a 4-wheel vehicle. Get over it. Jun 6, 2018 at 22:21