Battery and Charger life

We have 2 robots bought new last season, and one new one we just received in August. We are down to one working charger (one from last season and all of the batteries seem to be losing charge quicker than the had. Anyone else having issues, or suggestions?

Those batteries are either nicd or nimh. My advice applies equally to both.

Do NOT run them down to less than about 25%. I’m using some generalities here because I don’t know the voltage calibration for the vex computer.

In short, unless the cells inside the pack are exactly the same capacity, some will run down more quickly than others. These cells will be drained to zero voltage and can even be reverse-charged by current flowing from the other, still-charged, cells. This will kill them.

Over-discharge is the #1 killer of nicd/nimh cells. After that it’s overcharge, from a temperature perspective, not necessarily voltage perspective.

You could add some code to your brain module. It would sound an alarm if the voltage fell below 1.0 volts per cell, which is 6.0 volts per pack. FYI, at .9 volts per cell, or 5.4 volts per pack, the overall pack is considered COMPLETELY DISCHARGED from a battery chemistry/voltage perspective. Having an alarm would let you catch times when the pack voltage dropped temporarily (under heavy motor load) that happens too fast for you to see on the display.

Both nicd/nimh have steep discharge curves. One minute pack voltage is good, the next it’s bottomed out. I strongly recommend coding a voltage alarm in the brain.

Longer explanation:
I used to build custom packs using individual cells. Ran all cells through a battery analyzer beforehand and made packs consisting of exactly-matched cells. After a few dozen charge/discharge cycles, most cells would start to change their capacity. If it got bad enough, the packs would be torn down, cells re-matched, and packs reassembled.