Battery connected backwards to robot
 

So in the rush to get ready for SVR this week, I wired the leads on one of our batteries backwards. I didn't notice and no one else noticed, so when we plugged it in apparently our breaker made a clicking noise and may have tripped at some point. Regardless, it was left plugged in backwards and on for roughly 10 seconds.

The bad part - the damage. Apparently our VRM is shot, our PCM may be nonfunctional, and our PDP only supplies power to certain rails. Is this a problem that someone else has ran into and had to fix? Or are we just out of luck?

Cheers,
Devin Ardeshna (FRC Team 8)

Re: Battery connected backwards to robot
 

Sounds like you need to start replacing components. Unfortunately, those components aren't really designed to handle reverse voltage.

Re: Battery connected backwards to robot
 

Interestingly, none of these are the failures I would expect.

The VRM is reverse polarity protected. The PCM is not as explicit in the documentation, but I thought it was as well.

The PDP outputs are just copper, to damage it like that you have described, you would have had to fry the traces off the board. Are you sure it isn't the devices attached to those rails that are damaged?

Your motor controllers are the most likely to be damaged by a reverse polarity event.

Re: Battery connected backwards to robot
 

Devin,
To check the output of the PDP disconnect all your loads and try a known good device connected to each output. As pointed out above, there is nothing in the distro that can go bad. However, there is a lot of electronics in the PDP that will be affected by reverse polarity. My suggestion (I know you don't want to hear this) is recycle everything. If it isn't bad now it will fail when you need it the most. The clicking you heard is the resettable breakers.

Re: Battery connected backwards to robot
 

Are you sure your PDP is shot? Talons/motor controllers burn out super easily; I would check those before replacing the PDP, as it's a pretty nice piece of equipment that is well protected. I wouldn't be surprised if your VRM and PCM needed to be scrapped though; sorry about that.

Re: Battery connected backwards to robot
 

I'm wondering how common this mistake is, and if other teams have implemented a process change as a result.

Re: Battery connected backwards to robot
 

Hello! have you checked the 20A and 10A fuse on the PDP? we had a similar issue and it was a bad fuse.

Re: Battery connected backwards to robot
 

You could also test the outputs with a multimeter.

In addition to those, I would recommend checking all of the fuses on the robot.

Re: Battery connected backwards to robot
 

I know it's prohibitively difficult to put reverse voltage protection on the PDP input. But what about adding an inexpensive (~$2) 70dB buzzer with a diode so that it screams when you apply power backwards? That might give the user a chance to open the breaker quickly enough to avoid frying everything.

Re: Battery connected backwards to robot
 

Ever since we fried a D-link in my first year as a Mentor, all the dlink/camera connections are tested for polarity and voltage before first use.

Similarly, a few years ago, our team acquired a Battery Beak to test batteries before use. It has the added feature of testing for polarity. When a battery is first wired, we use the Battery Beak to test for correct polarity.

http://www.andymark.com/product-p/am-0995.htm

Re: Battery connected backwards to robot
 

Ideally, you put it on the Main Breaker, so it screams before you close the breaker and complete the power connection.

Re: Battery connected backwards to robot
 

That only protects against a backwards-wired battery. I'm suggesting something built in to the PDP, which would detect any polarity swaps from the battery through the SB-50 connectors all the way to the PDP input itself. I imagine it could be added to the "smart module" with the CAN connectors.

Re: Battery connected backwards to robot
 

I deal with a lot of robots each year (about 240 this year after this weekend (though probably only 160 unique teams), and I have not seen this on the battery before that I can recall. That obviously doesn't mean it never happens, but I would say it is not that frequent.
I do/did see frequently where leads get mixed up on motor controllers for the inputs. This was much more prevalent with the old victors/talons than the new ones. This would burn out certain controllers. Most often this occurred during a rushed add or wiring job. One team had burned up the current and spare before having me take a look at their robot.

Re: Battery connected backwards to robot
 

It is pretty easy to make an SB50 connector with an LED and resistor in it. Attached to the battery correctly wired and it will light.

Re: Battery connected backwards to robot
 

I think most of he vulnerable items would fry at nearly the speed of light; realistically too quickly for a reaction to help.

A big honkin' diode across the inputs - reverse biased normally - would act like a short in a RP situation and perhaps trip the main breaker while dropping the voltage to a less harmful level. Needs to handle several hundred Amps for many milliseconds.

Re: Battery connected backwards to robot
 

An interesting cost/benefit analysis... assuming that this worked, adding $2 to the cost of every PDP would cost more than replacing the occasional one that is hooked up backwards.

Sorry to hear about the damage... we hooked up our speed controllers the wrong way in 2004 and let the smoke out of one of them. It has NEVER happened again... I suspect this lesson will remain with your team for just as long!

The moderately expensive screw-ups we make in school and FRC are good learning for preventing the truly expensive screw-ups we can make later in life.
Jason

Re: Battery connected backwards to robot
 

A reverse diode across the input to the PDP would be huge and the short reverse voltage is likely to still produce damage in downstream electronics. Such a diode would need to withstand 600+amps and might allow a volt or more in reverse. The main breaker may still take a second or two to trip at that current. Considering the normal human reaction to the trip, this must survive at least two events before team members will go looking for the actual problem. Inspecting I have been lucky to find two batteries in team's pits that had been wired backwards but never used and one that was being wired backwards while I was in the pit.

Re: Battery connected backwards to robot
 

Honestly I think this might be the best solution. Train team members to check newly wired/acquired batteries with one of these before plugging in for the first time. Cost: a couple of dollars in parts.

Or buy a Battery Beak. More expensive, but handy.

Re: Battery connected backwards to robot
 

That's why I think a warning circuit at the Main Switch would be cheaper and more effective. If voltage is potentially going the wrong way through the switch, it could warn you (buzzer) before you close it and cause damage.

Re: Battery connected backwards to robot
 

Rich,
Two problems there...
1. There is only one polarity of wiring at the Main Breaker.
2. If you close the breaker, death will occur.
I would rather know before I put the battery in the robot. But you know, training will help this situation. Everything is already color coded, red battery terminal, red wire and a big "+" sign on the connector (both sides).

Re: Battery connected backwards to robot
 

Our PDP was re-wired in reverse, and we experienced the same clicking sound. Probably 3 seconds before getting to the breaker.

It was an oversight while trying to re-route wires inside a very crowded robot under the time crunch of getting the drive working (after a few mechanical failures).

Luckily, none of the components beyond the PDP were damaged. Actually the PDP itself still worked for us; however, the CAN was fried and we had to replace the PDP as the firmware version wouldn't be communicated otherwise.

At the moment it happened, we thought the damage would be much worse and I think we were lucky.

Re: Battery connected backwards to robot
 

Hi all,

gaving this some thought: one solution will minimize such disaster:

1. simplest is to use a 120A CB that has an internal electromagnetically activated separate coil.. so a diode in series activates on reverse power opens the CB in 50-100 ms.. I've seen these, never used them and currently disallowed part, but perhaps should be?? may be feature in future.. Emergency STOP signal opens 120A CB safely isolating electrical power, solving runaway flaming motor controllers &/or battery

2. Using supplied 120A CB a team could design a motor driven screw to pull/push the red off button down, activated by series diode (on sw'd side CB=auto turn EM sw deactivate, after some inertia for screw to clear top button after disabling CB) screw designed to be well clear for manual pressing, and screw able to me manually returned to "load")

thinking outside the box :-) for infrequent need in 26 years of competition.

in lieu >600A parallel huge diode for tripping >150A

parallel many high current FETs used as series switch , ~.0001 ohm = .1 milliohm turned on if polarity is proper, again not permitted in current rules)

Re: Battery connected backwards to robot
 

That's my point. Put a diode in the reverse of the expected polarity, and that circuit powers a buzzer. That circuit is always on (regardless of the state of the breaker). If reverse polarity occurs, it buzzes.

Yes, but the buzzer sounds as soon as you connect a battery with the reverse polarity. If you close the breaker while the buzzer is sounding ... Well, that's your fault.

The situation it does not solve is if you reverse wire the breaker to the PDP.

Re: Battery connected backwards to robot
 

Thanks for the advice. I wasn't around, but I think the robot was turned on again after the breaker tripped... Whoops! And as far as fixing the problem of wiring things backwards, well I agree that the best solution is to double check to avoid stupidity :(

The PDP does output power but the status lights don't seem to function... We can play around with the components a bit but we've replaced them with the spare set we kept on last year's robot for the time being.

If the VRM is reverse polarity protected, any thoughts on why it doesn't work now? And also ideas on why the PDP functions partly?

Re: Battery connected backwards to robot
 

The status lights on the PDP and VRM did not turn on after the incident. The PCM's status light was flashing green and orange. We checked both the 20 and 10 amp fuses on the PDP but neither had popped. We managed to replace the PDP and VRM with ones from an old robot and the speed controllers, radio, Roborio, etc... all ran as if nothing had happened. We did not have a spare PCM to switch in the robot was operational minus pneumatics.

Re: Battery connected backwards to robot
 

I have not opened a PDP, but I presume the main part of the PDP is a bus - all hard wired (hard wire does not care about polarity). The part of the PDP that seems fried is all the electronic monitoring parts.

If you are done with competitions, then if the robot works, the robot works. If you are going to Worlds, then swap out the PDP for a good one. They will check that the PDP CAN bus is working (version number check).

Re: Battery connected backwards to robot
 

While reverse polarity protection is usually overlooked since it's assumed that the user should never reverse the input polarity, it's good design practice to put some safeguards in place given that the components teams are required to use are expensive. Many teams are working with the single module given to them at kickoff and would rather spend their budget on travel costs rather than blowing up electronics.

A cost effective, very easy to implement solution would be to use a reversed P-Channel MOSFET in series between the battery input terminal and the PDP distribution node. Afrotechmods has a great video on the subject here. A transistor such as the SUM110P06-07L-E3 would work for the FRC PDP.

If you needed more current capacity (240A pulsed for the P-Type vs 440A pulsed for the N-Type) you could apply the same circuit, substituting a P-Type for an N-Type such as the STB120NF10T4, on the negative (ground) side of the circuit and achieve the same result. Both of these transistors have an RdsON = ~10mOhm meaning that the maximum power dissipated in the transistor under full load will be 4W @ 20A continuous.

Either one of these configurations would adequately protect not only the PDP, but everything else connected to it.

Re: Battery connected backwards to robot
 

You already have this, sort of. Each FET in each motor controller has a diode that will conduct in reverse when biased backward. However, the diodes will still allow approx. 1.2 volts in reverse bias plus wiring losses. That is sufficient to make electronics cross over to the light on the other side. (Stay away from the light, Fluffy)

If you can find the breaker you mention, send me the link, please. The 40 amp breaker we used prior to this one, did/does have a solenoid over current sense, to trip the breaker. It is also sensitive to vibration. So hit a bump with the breaker firmly affixed to robot frame and the main breaker would trip. Search for one of my breaker posts from 2000 on and I think I included an xray of the interior of that device.

Re: Battery connected backwards to robot
 

We connected a battery backwards due to the wires being reversed on the Anderson connector. The operators noticed it wasn't behaving right after being connected and turned on for a couple of minutes. The battery was hot but I don't believe there was any damage to any of the electrical components. They put in a new battery and worked fine through competition season without replacing any parts.

Re: Battery connected backwards to robot
 

Turned what?

Re: Battery connected backwards to robot
 

Yes, any devices that will die will do so within microseconds.


Diodes large enough to clamp the battery for long enough to trip the breaker will be about the size of the main breaker we are currently using now, even if schottky diodes are used (lower voltage drop and power dissipation). It may be possible to add a semiconductor fuse in series with the existing breaker. These fuses blow very quickly, allowing the use of a smaller diode. Unfortunately, semiconductor fuses are rather expensive (over $100 each).


I am not sure the current rules allow such devices. Even if they were to be integrated into the PDP and FIRST approves it, they would still take up a lot of space.

The voltage drop of the P-channel MOSFET linked would be in the range of 0.75V at 100A and about 3.2V at 240A. The N-channel MOSFET would have similar voltage drop. There are already many threads about the dangers of voltage drops leading to brown-outs and radio and RoboRio resets.

The voltage drop across the MOSFET will also lead to power loss of about 69W at 100A and 397W at 240A. The small thermal mass of these devices means that what robot builders would consider a short peak (say 10's of milliseconds to half a second) would have to be treated as a continuous current and the heatsinks required to keep the temperatures in check would be quite large. Please note that the Safe Area of Operation charts are for case temperatures of 25C indicating the data is only good for a single pulse or applications with long cooling times between the pulses (not FRC application).

The data on the datasheet is all for a device temperature of 25C. Both the voltage drop across the MOSFET and the power dissipation will increase as the device temperature increases making the situation worse.

It is possible to use larger devices or multiple devices in parallel to reduce the voltage drop but they will still take up a lot of space. Perhaps in another 5-10 years, we might have devices with voltage drop low enough to make a series protection device practical.


Overall, it is probably best for teams to learn to check their work many times before applying power. It is not a bad thing that team members learn that some things in life are just very unforgiving, like drinking and driving, treating firearms without sufficient care and connecting electrical equipment up backwards.