Enabling causes voltage to plummet

Our team has been using new batteries for the robot but every time we enable it, what started as around 12.4 V drops to 10 or 9, even 8 at times. We’ve tried multiple new batteries so we don’t think that’s the problem. Nothing was changed electrically or in the code, so we’re seriously confused as to what could cause this.

Do you have a short somewhere?

Check your DS logs. Are there any PDP channels that show an abnormally high current draw?

Does this happen when the robot is enabled and motors are running or does it start as soon as it’s enabled?

Check that the roboRIO fuse is properly seated in the PDP. If there is a high resistance there and the roboRIO draws more current when enabled than disabled, that could cause the voltage drop.

Batteries:
Do you have some batteries you used in past years? If you use any of those do you have the same issue?
For the new batteries, you must have needed to connect lugs to the batteries. Are you 100% certain that these connections are tight? If you wiggle the cables, the lug terminal should not move (rotate) on the battery terminal. Make sense?

Electrical
Same check applies at the main breaker and at the PDP. Nothing should move, everything must be rock solid. Is this the case.
Suggest that you consider removing all breakers from the PDP. This should remove all electrical load from the battery (RoboRio / radio / VRM / PCM excepted)

If you change to this configuration, does you Voltage still drop?

Get a “Battery Beak” battery tester from AndyMark or directly from Cross The Road Electronics.

What kind of charger(s) are you using. The starting voltage of 12.4 V makes me think your batteries aren’t fully charged. They should be around 13.0 to 13.5 V when you first disconnect the charger. After an hour or two, with no load, they will decrease to under 13 V.

Use a DVM to measure right on the battery terminals to see what voltage you get before and after you enable. Next, measure the voltage on the lugs of the big red and black wires going into the PDP. Last, measure the voltage at one of the pairs of outputs of the PDP with a breaker installed. This will help narrow down where your problems are.

Troubleshoot by division and elimination: First remove all circuit breakers in the PDP. Reinstall them one at a time while watching the voltage. When the circuit that is drawing too much current is reconnected, you’ll know it.

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As a thought experiment, how much resistance would be needed for a roborio that draws 5 watts idle and 45 watts max to drop a battery by 3 volts?

Did you charge the new batteries? If they start at 12.4v, they are probably not fully charged.

Just to double up on this post…because this is likely your problem…

If the difference in power draw is 40 W, at 12 V the difference in current draw should be \frac{40W}{12 V} = 3.33 A. That would correlate to a resistance of \frac{3 V}{3.33 A} = 0.9 \Omega. Pretty high by FRC robot standards, but not impossible if there’s a faulty connection or a partially-blown fuse.