Batteries die quickly

Any tips for having them not die so quick? I know leaving them on the charger all night really isn’t good for them, but our 5 hour chargers just do it like that. I also try to make sure that the robot is never left on when it shouldn’t be. I know new ones would definitely be better, but money is always an issue. Ideas?

What my team did:

We programmed a brain so that all the motor ports would continuously run the motor forwards, plugged in the motors and a battery we wanted to discharge completely and stuck it into a (nearly) soundproof box overnight and then recharging it completely before competitions. I personally think it works for reviving old batteries pretty well

You don’t EVER want to completely discharge a rechargeable battery, especially using a high-current-drain method.

This document is from DeWalt. Their portable tools use power in a similar manner to robots – variable loads ranging from low to very high in a short usage cycle (meaning minutes of use rather than steady use for hours):

Over-discharging a multi-cell battery (like the Vex batteries) can lead to a single cell failing due to reverse voltage. This will ruin the battery. This link will give you more information than you ever thought you would need on NiCad and NiMH batteries:


For what it’s worth, we are still using batteries we bought in 2006 and have used in competitions since then. None of them fail to hold a charge.

Good luck.](

So if you look in the Vex guide, you’ll find some interesting facts.

The Vex brain and the transmitter have low voltage cut offs, so if you leave them on with a battery in they will drain to the best “low” voltage. I believe the brain was something like 1.1v and the transmitter was a little above this. So using your vex brain or transmitter to drain the batteries is perfectly okay!

So they actually recommend you use the two devices to run your battery most of the way down so you can charge it up fully. This can help resolve some issues with the battery, but there is no cure for an old battery. They have a limited life span and eventually they do die.

I got myself a very nice charger (I have a lot of 9.6v, 7.2v and now 8.4v batteries) that is very flexible in both charging and discharging, one of the nice features is that it will measure the amount of current it discharges from a battery so you can tell if the capacity is anywhere near your rated capacity. It’ll also tell you how much current you put into a battery, but this isn’t an accurate way of determining capacity because you generally have to put in ~120% where as when you discharge, obviously you’ll only get what is really there.

So, all that said… you might have some options for reducing your costs. Obviously, for actual vex competitions you need to use their battery (what, 2000mah?), but for practice and other situations you can use any RC car battery you want.

I got a pair of 8.4v (they have 1 more 1.2v cell than a 7.2v stick) with a rated capacity of 4200mah. That was almost 3x more than my old 1500mah 7.2v packs. I don’t do any competition, so I don’t care about that. The extra voltage makes the motors just a little faster (~10%?) and the added capacity means I can play robots for an hour and the battery still isn’t dead.

I don’t think I’d go much higher than an 8.4v pack, the 6 AA holder with alkaline batteries would be 9v, which is why 8.4v is reasonably safe.

We have NEVER had that happen…