Bad batteries

Is anyone else having issues with batteries having a flashing red light and no longer working? I’ve had three this year and VEX saying they will not replace them.

Batteries have a limited lifetime. Leaving them on chargers, deep discharging, leaving them discharged, leaving them fully charged, and general use deplete the life at various rates depending on the chemical makeup of the battery.

Li-Ion Batteries like to be stored at 50% charge, stored no higher than 68 degrees F, discharged and recharged every few months, and avoid deep cycling (fully draining the battery).

Lions can last more than 500 full charge cycles, possibly a few thousand, but that number is drastically reduced by misuse. There might be ways to recover the battery, but you could also repair them and then mark them as “NOT FOR COMPETITION”.

Here is someone else’s picture from the forum showing pretty standard Li-Ion 18650 cells. You could reasonably replace the batteries with some soldering, but that is outside of my expertise.

If one of the cells has a manufacturing defect that shortens the life, the whole unit is compromised. So having four cells means that the failure rate is about 4x as high as the 18650’s manufacturer’s failure rate (since all it takes is one).


Just wanted to note for anyone looking into this, that you CAN NOT solder lithium cells, they will overheat and be damaged, and quite possibly catch fire. You need special equipment such as a spot welder to do it even remotely safely, and it takes quite a bit of skill. I would highly recommend not attempting to repair V5 battery cells, even as an educational process, unless done under the direct supervision of someone who knows exactly what they are doing.


Agreed!!! At EP summit - took a look at battery innards - cells are not simple to swap out.

Don’t do it.


Thanks for the insight. You know more than me!


Thanks for this, helps a lot. For some reason during some games some batteries showed full bars but when they were used, they had no energy in them Maybe this was the reason? :thinking:

You can actually write some cool code for testing batteries, and I would love to see someone try. You would basically need to hook up a heckin lot of motors at once to a brain. Basically we just need to extract a lot of current from the battery at once.
The brain can access the battery temperature, voltage, current, and capacity. You would just run the motors while logging voltage and current, printing them to your console so that you can copy and paste them into a csv. Then you could multiply the voltage by the current every few seconds to find the instantaneous power, and then multiply each power bit by the time interval (lets say 5 seconds or so) and then we have the energy drain. You can drain one battery and track its total energy, capacity, voltage, and more.

You could also create variable loading conditions. Ramp up and down motor voltages in a triangle wave and see how the battery handles high and low load conditions as it discharges.

I, again, am venturing into a realm of partial knowledge, but that is only because I haven’t done this or really learned that much about batteries. I would like to hear if anyone else can recommend the value of testing batteries that are acting fishy, or what you could do with the battery sensing functions the brain has access to.


Can usually look at voltage sag under a known load. This relates to internal resistance, which typically goes hand in hand w/ cell condition/life. This covers ‘early death’ situations. Other than that, just run a discharge cycle for long term degradation.

A more realistic is 1) don’t run all the way down 2) if you do, recharge IMMEDIATELY 3) don’t leave fully charged for long periods of time 4) don’t leave on charge 5) don’t drop or treat roughly

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A battery gauge is a complex topic.

If you base it on pack volts, then it won’t read correctly under instances of depressed voltage (but full capacity/charge) such as 1) cold temps 2) sitting for long periods after charging 3) old cells 4) just coming off a high load.

Conversely, it will read falsely high under certain conditions also.

It’s possible to actually measure the electrons that have flowed “out” of the battery, this giving the most accurate ‘fuel gauge’ available. However, this method requires re-calibration periodically, and ‘resetting’ the meter can totally screw up the calibration.