Power loss problems?

Technical Electrical
1

Our team just got back from the BAE Manchester regional, and we didn’t do so good. Mainly, because we had massive power loss. Our electrical system isn’t getting enough power to the drive motors to go up the ramp. However, before shipping, it was going up the exact same ramp perfectly fine.

Our team has a small theory:

Depending on how it was shipped, and stored, could the battery have gotten frozen? It’s been pretty cold around here the last week or so, and it’s not unlikely that it could have been in an unheated area for an extended amount of time.

Were there any other teams anywhere that experienced power loss? Especially teams that are in a much warmer climate, so as to help narrow down options.

2

This is probably a stupid question but did you charge it after you took it out of the crate? They can discharge some after sitting for a while.

3

Your bot has sat idle for a while, if your using the drill transmissions they won’t be as loose as they were near the end of building. Its always good to warm up your bot in the pits on Thursday. Run the drive wheels, cylinders, arms and anything else you have. Remember lubracants settle and congeal you’ve got to spread them around.

You can easily check the voltage on the batteries. Also charge and replace them between every match.

4

Yes, all batteries were fully charged (around 13v). However, we had one battery that was not holding a charge, even without running any motors, compressor, anything, the voltage was gradually dropping (0.1v per 4-5 secs). But our other battery was working fine, and that was the one we could not go up the ramp with.

All compnents had been run many, many times and were fully functional. Our drive was working as it normally would, but it was not getting the torque it needed to get up the ramp.

Also, in case it helps, we were running our drive on chips and worm gears. We also have a drill in the rear, but that was not being used when testing up the ramp, and was often not plugged in during matches, and we were still losing power.

5

*Originally posted by sanddrag *
**This is probably a stupid question but did you charge it after you took it out of the crate? They can discharge some after sitting for a while. **

Yes Yes we did, we put them right on the charger, right out of the crate;)

*Originally posted by sevisehda *
**Your bot has sat idle for a while, if your using the drill transmissions they won’t be as loose as they were near the end of building. Its always good to warm up your bot in the pits on Thursday. Run the drive wheels, cylinders, arms and anything else you have. Remember lubracants settle and congeal you’ve got to spread them around.

You can easily check the voltage on the batteries. Also charge and replace them between every match. **

We do not use Drill transmissions, we use the Atwood motors for drive, We had been checking voltage every match, but that was our problem we think the battery had been giving us a false charge, of like 13.2 so we were like" lets get this one becasue it will make us run the best, We later tested the battery in the bot with the bot off and we watched the numbers go:
13.2
13.1
13.0
12.9
12.8

All in the space of about thirty seconds, we had checked everything else on the bot and could not figure out what was wrong with it, eventually we worked are way back to the battery and found that out, but it was fri night when we found out… and even today we didnt do so hot, ohh well there always Nats…

6

maybe you guys are tripping the 40 amp breakers its been going like that with us to.

7

It may be possible that during shipping a few wires got shaken up and you have a minor short. It may not be a large one with no load, but as soon as you bring a load on the wires, the current increases as does the current now going through that “possible” short. Just give your bot a once over, if you have a larger charger one that has the higher starting capabilities use that as a battery eliminator on the bench to see if you experience the same voltage drop and current draw.

8

*Originally posted by 18voltMilwauke *
**maybe you guys are tripping the 40 amp breakers its been going like that with us to. **

We do believe that we are tripping the breakers. However, we were not doing so before shipping, unless we were putting tremendous resistance on the robot’s forward progress. Is there some reason as to why it would be tripping at a competition and not at our production area? I know the stress on the robot is much higher during a match, but it would have to still be tripping when we were just testing going straight up the ramp.

9

Well first off I see that a few of you have misunderstood what my other teamates were trying to say, so… let me say it from an electrical point of view.

What we believe to have happened was that our crate which had our batteries in it was stored in mostlikely a warehouse which due to the recent cold temperatures could have frozen the gell cells damaging them, not that we thought they were still frozen.

Because basically the cells are made of 6 2 volt gell cell packs and we believe that if one of these cells, which are obviously hooked up in series, is damaged it will affect the whole prefromance.

The clues that give us that is that we are 1) slowly fading out and losing power throughout the match untill we get to a point where we cant move. 2) its not the breakers because the speedcontrolers are still getting power and the signal light is still on even though the pwm provides no power only the main input. 3) under no load conditions we dont have any problem meaning all our batteries are giving false charges even when charged overnight.

Now this is purely a hypothesis and we do not know if it is exactly what happened but we are going to look into the shipping that our crate underwent and report further.

10

I’m not sure because I’ve never used it before but could it possibly be due to the braking setting on the Victors?

Also, in early testing stages we had a minor short which caused breakers to trip more often and the battery to deplete much quicker. I don’t think it was like the situation you described though.

I think your best bet is that your battery got too cold. Try running it down to about 11.3 volts (no load) and charging it all the way back up again a few times. Check the voltage after charging to make sure it didn’t get a false peak.

The battery wasn’t in any way shorting against your robot or anything in the crate was it?

11

We had a very similar problem this weekend at the Buckeye regional. We had a battery that the charger indicated as charged, our volt meter read as 13.5 V, but quickly lost its charge when put under load. I actually saw the voltage readings from one practice match. They were all over the place - from 6 up to 12 volts. The computer kept on resetting and messing with our shifting and pneumatic controls. We stopped using that battery and no longer had any problems.

About a freezing battery. I have no experience with the gel batteries, but have seen a few lead acid car batteries killed by being frozen after they lost their charge. apparently the batteries have some freeze protection while charged, but when they become discharged that protection is gone and the plates can be ruined.

12

I know that we at 190 would appreciate any additional information that you could provide us that you may find out about this problem.

We had very similar problems where we would plug in our batteries and they would read 13.1, and then every 5-10 seconds or so, it would drop 0.1V with absolutely no motors running (just the light and robot controller and speed controllers). Then when taken off the robot, a voltmeter would read it as 13.1 again. We went through three batteries and they all seemed to undergo similar problems. We only stumbled across this problem early this afternoon today, so we didnt really have ample time to troubleshoot the problem fully. So any additional information anyone else may know about this peculiar problem would be greatly appreciated.

13

I’m not so confident the controller can count for volatge very accurately. We got some numbers that just seemed amazingly low under load sometimes.

If you have only the Chias driving I wouldn’t suspect them shorting at all. However, if by some stroke of bad luck you got a defective one, it could be internally shorting or arcing perhaps. Assuming that you have one motor for each side of your bot, run your robot up on blocks (wheels off ground) and listen for a different tone when running each side separately. Slowly advance the control for one side of the drive system and get a feel for when it begins rotating and the sound it makes at different points along the range. Do the same for the other side and listen if there are any major differences in the sound. Certain sounds can indicate things binding or a lot of drag in the system too which can lead to the battery draining quickly. This was a big problem (now corrected) for us this season.

14

Wow, you guys posted a lot in a short period of time. So here goes…

  1. These batteries hold up pretty well even after being subjected to low temps. As long as the gell hasn’t frozen so badly that the case is warped then I wouldn’t really worry about the cold. (The battery heats up pretty quick when in the robot driving around!)
  2. Batteries will read in 13-13.5 volt range immediately after coming off the charger but will drop after removed. The definitive test is to put the OI into battery monitor and watch the battery voltage while you drive around. (i.e. press the “select” button on the OI until the display shows something that looks like voltage, like 12.5.) This is a recommended practice whenever on the playing field and have your driver or human player monitor during matches. If the battery voltage gets low, determine what conditions causes it and avoid them. Tight turns, particularly with four wheel or tank tread designs, draw immense amounts of current. When the battery voltage falls below 8 volts for longer than a moment, the robot controller will reset. During reset you will be out of commission for at least two seconds and the rotating light will be off until the controller has reinitialized.
  3. The “chalupa” motors are very powerful when used at their design speed and torques. Unfortunately running them at speeds slower than their peak power output will stall the motors in many cases or will not give enough power to climb the hill. Teams should be very careful when comparing practice at home to official fields at competitions. An inch or two in length or height errors can vastly affect the angle of the ramp. The official FIRST structures are the only one that matter.
  4. To Team 190, even if nothing is running on the robot except the light and the controller, other systems are drawing current. The light is much lower current draw than last year but the motor and light do draw current. So do the speed controllers, fans, light sensors, and if you are tethered, the OI and operator controls as well.
  5. “Slowly fading power, light remains on and speed controllers still on but no function” This sounds like a really bad one. I would at first ask if the radio modems were showing any fault lights. If they were OK, I would check on your primary wiring, i.e. the connections between the main circuit breaker and the circuit breaker panels. If you loop power from one to another, try to isolate which parts of the robot are failing and see if that points to one panel. Check all return wiring (i.e. the wires that go back to the negative terminal of the battery.)they are as important as the positive wiring. The slow fading may be due to electronics heating up with time so check for warm components. Finally, I would take a look at the OI controller and/or power supply.
    Lastly, just as an observation and to repeat from the Robot docs
    **The motors supplied with the Kit of Parts have various power and speed capabilities. Some are quite
    powerful and draw very high current when loaded to their limits. Others have very high-speed
    capability. Some have integral transmissions that magnify their torque output. Some are equipped
    with selectable two-speed transmissions. Before choosing a motor for a particular use, be sure you
    thoroughly understand the characteristics of the motor.
    Drill motors have an internal cooling fan that is not effective at low motor speeds. Do not operate
    the Drill motors at the stall condition because they will quickly overheat and fail. **
15

95 has a problem much like this. We are using a 4 wheel 4 motor drive (2 CIMs and two drills, plus two van doors and globes being used as steering motors). After about a minute of driving, any movement will cause us to lose contact with the bot, undoubtley because the RC reset due to low voltage. We do not need to be heading up the ramp.

At first, I suspected a short to the frame (why don’t inspectors check for this?). I am now 99% sure this is not the case. Nothing on the multimeter and every connection is tight and insulated, no damaged wires.

We to use worm gears. We did some current draw tests on our motors and found they were drawing around 10 amps, no load. Wow.

That is really high, under load that number will climb even higher. But, we’ve never tripped the main breaker or the 40 amp breakers, the battery voltage always drops below 8 volts and the controller resets after a minute of hard driving, probably before the breakers heat up. With no load on the battery, it will settle at around 11-12 volts.

Doesn’t that seem wrong? That is a very large voltage drop in so short a time, even with the high current draws. I had always thought that to lower the voltage that far with out tripping the beakers would require a short to the frame or a bad battery. Should the voltage stay lower when it is under no load?

I’m puzzled. On one hand, I know for a fact our motors are drawing way to much power. On the other, I am having trouble trusting these batteries. I guarantee our crates dropped well below zero (in the north east anyways). But, few teams have this problem that I know of, and they all seem to be running on worm gears.

I hope that by getting those gears loosened up we can cut our current draw. We believe that attaching a capacitor across the power terminals of the RC helped prevent resets (when voltage dropped to low the capacitor would drop power into the RC to compensate). Yes, we checked with the inspectors and this was legal. I would suggest that teams having problems with brownouts try this. In any case its a stopgap and doesn’t fix the real problem.

Perhaps the batteries are devolping mild internal shorts as they heat up? Could it be that when they heat up, some plates will warp and short to other plates and then when cool lose contact and appear normal? I don’t know nearly enough about these batteries, that just popped into my mind.

I just wish I knew for sure what is causing this. It’s probably a combination of things. I highly suspect its just binding gears, but…

-Andy A.

16

Check for binding and drag in your drivetrain. You can tell a lot just by the sound of it running up on blocks under no load. Also, you can only have one Van Door motor.

17

To repeat sanddrag, check for binding, worm gears are ineficent with out it.

Also, when my hands typed ‘vandoor motors’, my mind typed ‘window motors’. Long story as to why I confuse the two so often, but it has to do with lazy programers who won’t change their programs to reflect new motor selection.

So yes, steering is through two globe motors and two window motors. I don’t suspect them as the problem, they never stall and run for less then a second at a time.

Perhaps the problem lays with those 8 little muffin fans on the victors constanly running, you never know…

-Andy A.

18

*Originally posted by Andy A. *
**95 has a problem much like this. We are using a 4 wheel 4 motor drive (2 CIMs and two drills, plus two van doors and globes being used as steering motors)…
At first, I suspected a short to the frame (why don’t inspectors check for this?).
We to use worm gears. We did some current draw tests on our motors and found they were drawing around 10 amps, no load. Wow.
With no load on the battery, it will settle at around 11-12 volts… Doesn’t that seem wrong? That is a very large voltage drop in so short a time…
We believe that attaching a capacitor across the power terminals of the RC helped prevent resets (when voltage dropped to low the capacitor would drop power into the RC to compensate). Yes, we checked with the inspectors and this was legal.
-Andy A. **

Andy this is a lot to answer so here goes…
We have been calling for inspectors to check for frame shorts, but to no avail yet. This isn’t a serious issue but can lead to problems in certain cases.
10 amps on your motors no load isn’t bad, it’s what they are drawing in competition especially on turns that is important. Many designs will max out motor current at 80-100 amps per motor while driving which may not trip any of the 40 amp breakers and not the 120 amp breaker if the current is short term. However, with the weight of the robot on the wheels there may be significant friction to run motor current up to 50 or 60 amps per motor which will not trip any breakers but will be drawing 240 amps out of the battery.
If you look at the data sheet on the battery, you will see that terminal voltage will fall after drawing current. The amp hour rating of the battery is determined by how long it takes for the voltage to fall to 8 volts. What you describe is normal for high current draws.
Adding the capacitor and passing inspection really surprises me. In past years the capacitor was strictly illegal. We believe that Innovation First responded to this problem and put a larger capacitor inside the robot controller as part of their improvements on this year’s controller. A better fix is to make the leads between the battery and the controller the shortest on the robot and to not connect controller power downstream from motor supplies. The high current causes a voltage drop in the wiring. (Simple Ohm’s Law V=I*R)
How to fix it at this point? You might have to scale the software back a bit. I would rather be slow and play all two minutes than to go fast and break fifteen seconds into the match.