V5 Battery Bad Cell?

We’re having issues with our batteries. We appear to be getting an under voltage error while the battery reports to be 3/4 charged. This is happening with the majority of our batteries, which were all bought in between December 2018 and November 2019. They were not used during the Change Up season. Four batteries failed to charge at all, and five seemed ok at first, but have been slowly decreasing in useable run time.

See video for more: V5 Battery Rip - YouTube

After running the robot for a short while, the robot turned off and the red light flashes 5 times per second (indicating low charge?). However, when we press the status button (the big one) all green lights shine indicating full charge, it then goes back to flashing red until we put it to charge. Then it shines 3/4 lights solid green, with the 4th flashing flashing green.

From the battery medic, it seems one of the cells is discharging much faster than the others.

We are running the latest version of the firmware, 1.1.0.

The brain seems to be drawing a large current (~1A) once it is unplugged from the charger. We tried with another brain, and with 0 motors and it drew 0.3A. Unplugging a motor decreased current by ~0.04A, and each solenoid ~0.02A. Not sure if this is relevant.

We are assuming it is a bad cell, as in Battery dying at high percentage - #13 by Totallynotarobot. But wanted to make sure before we start lamenting at the lack of availability of new batteries.


My assumption is that we will need to purchase new batteries. Is there any information from vex about how long it will be before batteries are available?
If lead times are due to a shortage for a specific part, would it be possible to do an RMA to swap out only the bad cell?

This is probably the relevant detail; if the batteries were left discharged for over a year, then that could definitely cause damage.

As for availability estimates, I’ll ping @Bob_Mimlitch_III

In the mean time you could try contacting VEX support to see if they are able to do an RMA for you


flashing red led means shutdown due to under voltage, in your case caused by cell 4.

state of charge is indicated by the green led, this is not connected to cell voltage and will not be correct if there is a bad cell. Your battery went into low voltage protection well before the batteries theoretical charge was depleted.

yea, with the brain connected that battery is not charging at all due to the brain pulling all current from the charger. That brain must have a fault as well.

Looks like it to me.


Thank you, do you know how to go about diagnosing the issue with the brain? Could it be an issue in a motor? Do you know what is the nominal current draw?


A brain with no motors or sensors connected would use about 0.25A as you noticed on the other one you tried. I assumed you had nothing plugged into the one under test, but obviously if motors were connected it will be higher.

The battery charger can supply 1A. If the battery is charged while connected to the brain it will charge at a rate of (current used by the brain) - 1A, so battery medic would show about -0.75A (negative because current is flowing to the battery cells rather than from them). In your video I saw charge current of pretty much 0 as all current from the charger was used to power the brain.


Thanks, the brain in the video had 15 motors plus a radio and two solenoids connected. We saw about 0.04A per motor and 0.02A per solenoid, which would account for most of the additional current draw. Are these numbers expected for idle motors?

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ok, then 1A for brain is not unreasonable, it’s probably fine, but battery will not charge when connected to the brain with that many motors.



Ok, thank you!

It looks like we’ll be waiting for new batteries then.

Do you know if the performance of the batteries we have will be affected, or just the run time?

It’s difficult to know what state the batteries you have are in, not enough data. But storing discharged for a long period (if that’s what happened) will not have done them any good.

You can try putting then through a couple of full charge/discharge cycles, it’s not impossible that the cells will recover and rebalance.

If you charge with a brain connected, disconnect all the smart ports, you should then see about 0.75A charge current in battery medic, charge should stop shortly after pack voltage reaches 14.4V, when working perfectly all cells should be at about 3.6V, but I suspect you will see some go as high as 4.1V while others are still low. If pack voltage never reaches 14.4V, charge will abort after about 3 hours.
Then discharge just using brain attached, it will take 3 hours or so with a good battery.