Battery charge fluctuation.. Brain, Battery, or Motor Issue?

We have two 4-month old batteries. The voltmeter says the batteries are charged between 7.23 and 8.4 v. However, the visual battery charge indicator on the brain fluctuates between full and half within minutes of putting the battery in. The drive motors also start slowing down and whining after a few minutes of driving.
Replacing the battery with a fully charged one will resolve the above issue - for about 10 minutes of driving.
How can we determine if it is a brain, battery, or motor issue?

Swap battery packs to another brain. If the symptoms follow, it’s the battery.

And my guess is it’s the battery. Dropping the battery packs can shorten their lives.

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I’ve told our teams to ignore the battery charge indicator and instead go into settings and look at the % charge when the battery is in the brain. Once they started doing this many of the battery issues resolved. They had a tendency to swap batteries and put one in that showed a full charge on the indicator, but when they looked at it in settings, it was not fully charged, even though it had showed fully charged on the charger. I also sat down with a couple of teams and looked at how they were driving their robots. Continuing to hold down on controller buttons and forcing motors in stall conditions was causing batteries to drain quicker. Once they started paying attention to this they had fewer power issues.