# Does current ever flow backward from motor controllers to the PDP/battery?

As an example, if the motor is spinning forward at full speed and the throttle/duty-cycle is set to +50%, it seems like the current would flow backward toward the PDP during the ON part of the duty cycle (because the back emf exceeds the average applied voltage). Does that backflow actually occur? If not, where does that current go?

I’m particularly interested in the answers to these questions for Talon SRX, Victor SPX, Spark MAX, and Falcon 500.

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Yes, it does. This is partly why you can’t run a robot from a normal non-battery DC power supply, as they aren’t designed to sink current.

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You can power a robot on by pushing it across the ground at a sufficient speed. Also as Peter said this is a fantastic way to fry a DC power supply, but you can buffer a DC power supply with a battery being charged in the same circuit.

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It’s fun to watch unsuspecting students push our 2014 robot when we need to move it. At some speed the 4 CIMs generate enough current to power up the motor controllers. The controllers are set to brake mode, so when they hit that speed the robot just stops. The students are left with very confused expressions.

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If this current is making it back to the battery, isn’t that bad for the battery and potentially dangerous? The SLA1116 datasheet says, “Never exceed the recommended maximum charging current allowed for that specific SLA. For the SLA1116, between 2 to 5 amps is recommended, with the lower of the two being preferred. Most SLA’s do not lend themselves to fast charging…If you are charging too fast or applying too high of a charging current, you will not only shorten the life of your battery but could also create an overcharge situation which will cause the battery case to swell/deform, and potentially catch fire.”

Seems like the back current could be in the 100s of amps at times. Are we saved by the typically short duration of such events?

So, I have never done that math, but I seriously doubt that it is 100s of amps for the case where the students are pushing the robot across the floor. First of all, the CBs on the PDP work in both directions so that will provide some level of current limiting (although you could get well above 2 to 5 amps). Secondly the amount of power that you are pushing back into the battery will be something less than the power that is being used to push the robot across the floor. I doubt that the students could push the robot with anything close to a kilowatt of power. I have seen the lights on the robot light up when pushing the robot across the floor, but I don’t think there is any significant current flow due to pushing the robot.

There is a second case involved which is when you use your motors as brakes to stop the robot. I believe this also feeds current back into the battery. This could potentially be higher than the student pushing case, but I still feel like this will not be in the realm of 100s of amps. But this is also a very short timeframe. I think the batteries are more prone to damage when the backfeeding is sustained for long periods of time which can cause the battery to overheat.

Just to clarify, I was thinking of the second case. Specifically, a robot going full speed forward which the driver slows down by cutting the throttle to 50%. If it has a 6 MiniCIM drivetrain, I get that the back current through each motor is 6V/(135mohm motor resistance + 25mohm battery/wire resistance)=37.5A and the back current coming out of the motor controller is 50% of that (because it is only flowing during the ON part of the duty cycle) or 18.75A. So the total current for 6 MiniCIMs would be 112.5A. So my “100s of amps” was too high. I’d forgotten to derate by the duty cycle. Still, I’m surprised that over 100 amps isn’t problematic.

I think the case of students pushing the robot is somewhat different. If the robot is on, but the throttle is at 0% (or in the motor controller’s deadband), then my understanding is that the current just cycles through the motor controller (brake mode) or is 0 because the motor is disconnected (coast mode). If the robot is not on, no current flows through the battery because the main breaker is open. However the back emf can provide enough voltage to turn on the RoboRIO and/or motor controllers. Still, very little current flow because they don’t need much (when the the throttle is at 0%).

I would not be surprised if many dozens of amps make it back to the battery often. But when discussing charging amperage we are talking about long term amperage where you want to give the chemistry in the cells of your battery time to migrate without damage to the electrolyte and plates. If you are using a battery and back charge it followed shortly by discharging it you haven’t really forced a lot of ions around yet and not very far. Think of it like sticking your hand in a fire, swish your hand through quickly and it gets a little warm and no damage, hold your hand in the fire for 30 minutes and you have cooked hand. This is also why most batteries have a recommended charging and discharge rate as well as a maximum rate for a short time, it takes a while under poor conditions to mess up the battery.

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Here’s an example of the voltage getting above 16v during deceleration: Gyro Issue, Voltage Spike

Thanks @Joe_Ross. That’s exactly what I was wondering about. 16V-12V=4V across, say, 25mOhms of battery/cable resistance implies ~160A. I hadn’t even considered the potential implications of the voltage spike.

And thanks to @RoboChair for the reassurance that the battery can handle the current for short time periods.

Absolutely, a motor becomes a generator when it is spinning faster than the applied voltage.
However, if the controller is in “brake” mode, or is being driven with a relatively small duty cycle compared to its speed, the current mostly cycles back through the motor and is turned into heat in the coils [This contradicts some of my year+ old quotes, but I’ve done some testing since then.] Excepting an abnormally large flywheel built for the specific purpose of back-powering the battery to failure, I don’t think that this is a battery concern.

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Yeah, our T-Shirt cannon(Jaguars) will light up if you push it at a decent speed. After we discovered that we have been trying to keep it on a cart whenever we push it around.

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