My team is at an offseason event right now and we have finally fixed our robot (mostly) save one problem: the battery drains way too fast. We turn on the robot and within 30 seconds, it drops from 12 to 8 volts. When we get on the field, we brown out before finishing the match. Is there any reason this is happening or a possible fix?
How old is the battery? This sounds like a battery that has been in use for several years, or was stored improperly.
Edit: A few specific questions.
- How many batteries are you observing this on?
- When were each of these batteries purchased?
- How do you charge your batteries?
- Are you testing with a battery beak before putting the battery in to the robot?
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Additional questions:
- What mechanisms are in use on the robot?
- Are any of the systems powered when they shouldn’t be, wasting your battery power (compressor always running non-stop, LEDs on super bright mode, etc)?
- How many batteries do you have? Is this your only battery?
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After the batteries voltage drops does it come back up after a few seconds if you stop using any subsystems? It might be that a certain subsystem (for us it was our swerve) that is trying to pull to much power, we fixed this by adding in coded ampere/amperage limits into the code so the subsystem could only pull a certain amount of power. I don’t know the specifics on the code though.
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We have observed this on all six of our batteries. We always check them before putting them on and we when used the batteries on a different robot, they were fine.
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Thanks! I think this was the problem. Our intake was taking up way too much power.
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Even then, your batteries sound problematic. I’d highly suggest checking them out.
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i agree with this no single subsystem should drain your battery in less than a minute
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Current limits are always a good idea, but consider performing the frame isolation test. Also check to see if your intake is stalling against a hard stop. Check (carefully) how hot your motors and wires are getting.
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We borrowed another team’s battery and it worked better but ground intake still causing problems. Oh well, thanks for the help!
If that is really the case, you should be able to go into the logging feature of the driver station and look at the current draw from different PDP ports. For just one mechanism to be drawing down a battery or would have to be pulling some serious amperage. Probably in the neighborhood of 80 or 100 amps continuous. That seems unlikely.
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Thats the symptoms I’d expect for a bad cell. A 100 Amp automotive tested will answer that quickly!
Bad one:
Good battery
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It seems possible but pretty unlikely that they have six different batteries all with a bad cell.
My team had this problem a few weeks ago. We initially thought it was our batteries, so bought a battery tester and tested them. While we did have some fail, it did not explain why all of our batteries would drain immediately. Turns out, we were shorting our battery on the frame, causing it to drain almost immediately. I would recommend putting plastic/rubber caps on your battery terminals, and testing to make sure that you are not shorting anything.
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we had similarish problems at our first comp last year just less bad (we could get through around 3/4 of a match before browning out). all the motors were current limited so we were stumped till I noticed that the shooter motor was drawing a lot of power when spinning up because I had not set a ramp limit (not sure if that’s the correct term but its close). after not letting the motor try to go to full speed instantly we stopped having our power problems.
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I’ve written articles on ramps and on current limits.
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I would look at the Driver Station Log Viewer and check the current draw of all your mechanisms. It could be as @bovlb guessed that the intake is stalling against a hard stop. It could also be that the intake is rubbing on the ground causing the drivetrain to use more power, or interfering with another mechanism.
This topic is pretty well covered now, but just for posterity note that your battery should never be starting at 12 V.
Although we (and the manufacturers) call them “12 volt” batteries, this is a nominal value and refers to the fact that the battery can maintain a roughly 12 V output under a moderate load. The actual fully-charged zero-current voltage of a robot battery should be ~13.2 V, or 2.2 V/cell x 6 cells. With your robot powered on with a charged battery and disabled, you should see ~12.7–13.0 V reported on the driver station (ideally closer to the top of this range).
If you are seeing 12.0 V before you enable the robot, the battery is already severely drained. If this is consistent across all your batteries, they are either far too old or you need to inspect your battery chargers.
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