Battery "Supercharging"?

When using the Vex 7.2V robot batteries, after they are charged using the Vex chargers and the charger light turns green, if the battery is plugged in again, the light will turn red and the battery can charge more, making the voltage slightly over 7.2V and causing the battery to get very warm. During competitions, we often pool all our teams batteries together to share and take and put back batteries on the fly, so fully charged batteries often get plugged in again. As a follow-up, I’ve heard of teams doing this to intentionally increase performance, using all vex batteries/chargers without modification. So is this legal?

Yes, this is legal unless you are physically modifying the batteries.

This is the charger you must use, and this is default behavior for the device. So…

Yep. If you were required to never use batteries in that state, they would very likely provide some VEX device or at least a specific procedure to test for and resolve the “errant” condition.

Battery temperature always goes up when charging. (And when discharging, but it’s less noticeable.) They have internal resistance, and you’re putting a voltage across that resistor. So some of the power is wasted as heat. Many rechargeable batteries, including NiMh chemistry, show high internal resistance both at low charge and at high charge, with a lower resistance in the middle of the charge range. What this means is the battery will heat up more when it is already at a fairly high charge level. (The heating occurs at low charge levels as well, but doesn’t last long as the charge moves into the lower resistance part of the curve fairly quickly.)

The upshot is the higher temps from the higher charge aren’t a problem if you use a good modern charger. For VEX, that’s the charger they’ve been selling for a few years set to the “safe” setting.

Batteries also have a habit of catching fire when set to “fast”

Yea being unsure whether you are holding a fully charged battery can supercharge batteries. I had a friend who’s battery had 8.6 volts when he checked it on the LCD display.

I’ve had 8 6 volt batteries from normal charge. New batteries are fantastic.

we’ve had 9, not even warm to the touch…

Bear in mind that 7.2V is the nominal voltage, not the maximum voltage. If VEX batteries are composed of sub-C batteries (like every other 7.2V battery of the same size and shape) then a maximum voltage exceeding 9V isn’t out of the question.