Dead battery!

This past saturday we were running our robot around at our facillity and something very odd happened. We have a big open carpetted room with a cubicle w/a metal strip along the bottom. Our robot generates a considerable amount of statc electricity when we drive it because of the carpet and the wheels and when you hit themetal strip on the cubicle it discharges. Also when you hit something hard enough w/the robot the circuit breaker clicks off. So when we were driving it we hit the cubicle and and the robot discharged and turned off. OK cool what ever. So we go to turn it on and it won’t fire up, so we shut it off again. We went to disconnect the battery and when we grabed the connector we drew back in pain because it was burning hot! We let it cool and disconnected it and put the volt meter on it and it read .25 volts. The battery went from fully charged at 13.4 volt to .25. Please explain this!

Are you sure you were measuring it correctly? It seems impossible that it rapidly discharged like this. You would have to have noticed the robot slowing down and becoming weaker. It probably would have stopped running entirely before it got anywhere near .25V.

The discharge may have been caused by some sort of connection of the battery to the frame and consequently to this metal strip. A short like that may also explain the hot connector (caused by a large amount of current flowing through it caused by the short).

Hot battery, rapid discharge, throwing the breaker.

These are all the telltale signs of a short circuit my friend.

Do you have any wires which might be coming into contact with your frame? Or would you happen to be using your frame as a ground return?


I agree with Slimbo,
Use a continuity tester and check the frame and make sure it is not grounded. Also check to make sure that all the connections on the batter and in the electrical system are tight and in good condition, if there is poor contact there is increased resistance which will cause all sorts of problems.

It is my understanding that with the current breakers it is supposed to be very dificult to trip them simply by hitting something, although with the old one it was very easy. Unless you are drawing way more current then you should be, in which case it would be easier to trip the breaker.

So my advice would be to recheck ALL the connections in your electrical system, check the connectors for any signs of wear and tear on the battery, check to make sure nothing is grounded, double check the voltage on the battery, and just make sure you didn’t overlook anything.

Check your drivetrain too. If you are careening along at a pretty good clip then you hit a wall or whatever it could be that something in your drivetrain is a little bit loose and it catches and binds, when those CIMS go from full speed to nothing with power still applied they probably draw a tremendous amount of current.

Just a thought. It doesnt really explain the battery thing, but what I can say is that what you are having happen should not happen. I know, im so helpful. :smiley:

I don’t think the CIMS alone could draw THAT much though. Last saturday we we’re running our bot’ around the warehouse and we had a draw of about 3v when at full speed, and when we reversed from full to back full there was an increase of .5 or so.

BUT, I as a programmer have implemented a “rampPWMs” program which “ramps” the output to the motor. If your team’s programmers havent written a program they should. It should help a little bit. If you want to know how to do that I’ll show you what I did. :cool:

There is something missing from this picture…The hot connector and dead battery would indicate a dead short somewhere after the main breaker (since that tripped) but you have said nothing about any of the other breakers tripping. So this leaves us with a dilemma, what could be wrong?

  1. The breaker panel itself shorted. (doubtful without some other visible sign. i.e. smoke and flame.)
  2. Wiring between the main circuit breaker and breaker panel shorted. (most likely cause, might be a main breaker that is not secure and moves against the frame.)
  3. Not using the breaker panel or not using the specified 40, 30 and 20 amp breakers in series with the motor types used for this season. (Not likely, teams don’t bypass safety features like a breaker panel and specified breakers.)

You have not mentioned that you tried a new battery and all was OK so I am assuming that you are still troubleshooting before connecting a known good battery. I would disturb as little as possible, remove all the breakers from the panel, and measure the resistance at the robot end of the battery connector. It should be infinite regardless of whether the main breaker is closed or open. If it reads less than a few thousand ohms with the main breaker closed, but not open, your problem is after the breaker. Disconnect the breaker panel and test again, if the resistance reading goes high, than suspect the breaker panel.

Sorry about the lack of info. i didn’t have much time before. But the neg. (-) terminal on the battery was exposed and the battery was held in the robot by the latex tubing and it could slide. Today when i came in the battery had a charge put back on it and when i put the volt meter on it i watched the voltage rapidly drop. Also we put a good battery in saturday and it was fine. And it would be much appreciated if you could show me how to program that rampPWM you were talking about.

I’ll have to get a copy of it from my comp at the build site, and once I do I’ll show it here. :slight_smile:

by the way if you’re wires are that burning hot id advise you to disconnetct first anyways because there just MIGHT be the chance it wont cool down (and keep getting hotter till sumthing blows or melts. at worst (with rc car experiences) you might even need to cut the wire. but never cut two wires at once.

This is to be expected. Bringing the battery below 11 volts can start to cause perminant damage, and if you really did manage to get your battery down to 0.25V, the thing is toast.

Quick question regarding one of the above posts:
Were you able to charge the battery back up? If not, then you may have an internal short (SLAs depend on metal plates with acid between them. If the plates short, then you have a problem, but that doesn’t explain the hot breaker wires). In any case, it sounds like you had a HUGE amount of current go somewhere really fast. That’s downright dangerous. One thing is niggling, though: why is it that the wires did not actually melt? It seems to me that even a somewhat discharged battery, if it were to short using the breaker wires, would fry them in a heartbeat. If the voltage levels did drop that much, then it seems semi-logical that there would be scorch marks along the path the power took to whatever circuit it seems to have made. Make sure this gets solved before competiton!


I am guessing that when the robot took a hit, the negative terminal made contact with the robot frame. I expect that an inspection (with VOM) will show that the positive lead is in contact with the frame, probably at the output of the main circuit breaker. When the battery slid against the frame, a circuit was formed through the frame, battery, main breaker and associated wiring. This kind of abuse can result in the internal plates of the battery warping and the result is there is an internal short. When the battery was put on the charger, it may have taken longer than normal to show full charge (or it never achieved full charge) and would have been much warmer than other batteries while charging. You should also check that the battery connector has not so badly deformed from the original heat that the contacts are now touching. If you are sure all of the wiring on the battery is correct and there are no sneak paths for the current, then the battery is a candidate for recycling.
Check the wiring on the robot for contact with the frame, correct what you find and always insulate both battery terminals at all times.

Battery terminals were insuled with heat shrink over them but the person who did it didn’t do a very ggod job and it came off and thats how it could complete the curcuit. The possitive(+) terminal was the hottest of the 2 on the battery and the same for the one coming off the curcuit breaker. Where as the negative(-) was as cool as a bone. The wire was hot but not quite hot enough to melt the wires plastic coating. And yes the curcuit breaker will pop open if you hit it hard enough, well atleasr ours does which won’t be very good in the competition if we get hit. And as for charging it, i don’t know how they got it to charge. Because when we put it on saturday and stuck the volt meter to it the voltage dropped. But when it was charged and the volt meter was put on it, it wold rapidly drop.

Well it sounds like this battery is going to the recycler.
The circuit breaker should not open when hit. If yours does, I would recommend replacement. We had a defective breaker this season that would open with any pressure on the red button. Opening the device showed that manufacturing had misaligned the internal parts. There are no parts that can trip open this design as there had been in the breakers from several years ago. If yours trips open with shock it is defective.

Everyone keeps saying check the battery terminal to make sure it didn’t short, if it did, there would be large scorch marks on the chassis where it made contact (I’ve seen 6awg get arc-welded onto metal by shorting). Also, check your breaker block, there was a mfg warning that said stuff inside of it may come loose during shipping and you should open it up to check for loose hardware, that could be the cause of all your problems. I would also try using a current sensor (not FIRST one, the one on a multimeter) to measure how much current you are drawing while running. In 2003 we were drawing 60amps with the bot up on blocks and the current sensor helped us pinpoint the problems (loose electrical connection and one of the transmissons). Good luck!

On the multimeter with as a current sensor:
You want to watch out with this one. Most multimeters have three ports: a sensitive voltage/resistance port, a ground, and a high-amperage port. Most meters are set up with the sensitive port and ground, and if current is measured with that probe, it will fry the meter. Even the high-amperage port has a limit. On mine, it’s 10amps for 15 seconds. If you run, say, 50 or 60 amps through it, it’s toast in no time flat. Read your multimeter carefully to see what its current limit is.


Question: Our team kinda lost (As in completely drained) a battery over the course of testing. We’ve tried almost every suggested place on the site, but nothing worked.

We need replacements and spares. Where might we find FIRST competition-legal batteries?

If you read a bit farther back, you’ll notice that ahecht said that a sealed Lead Acid battery (the kind we use) will be trashed if voltage goes too low. That tallies with my own knowlege of SLAs, and I think others would agree. In short, I think that you guys are sunk. Look around for other batteries, or maybe contact FIRST.


The gell cell batteries we use can be totally discharged without damage under normal loads. A dead short is not a normal load. In the event the battery we are discussing came in contact with the frame quickly, there is likely nothing more than a scrape where the contact took place. Arcing and burn marks are indications of high resistance contacts for low voltage DC. A completely discharged battery will take several hours to recharge even using the 6 amp charger.
Don’t discount the possibility that the battery sustained mechanical damage when used in a robot that ran into a wall. You are dealing with a device that has a lot of mass, and that mass is concentrated in defined areas within the case. Lead is not nearly as strong as steel and a fracture in any connective structure (internal connections and plates) will result in battery failure.
The batteries are available from a variety of sources, including battery stores, McMaster and Digikey. Be sure that you order is the ES or EX type battery.
Most multimeters only can display and measure 10 amps or less and are internally fused for protection. The popular Fluke meters have two internal fuses, one for 300 mA and one for 10 Amp. To measure current on the robot, you need a current probe. Fluke makes a few nice ones but they are almost as expensive as the meter itself. They clamp on to wires under test. Remember that they can only measure current on one wire at a time. If you measure both the positive and negative leads of a motor at the same time with a clamp on probe, there will be no current measured.