All my batteries keep dying extremely fast

So, my robots drive keeps dying by the time I drive to the other side of the field. I have the 4 motors hooked up to the power expander and have tested about ten batteries and the problem is still present. What could this be? The cortex, motors, battery, motor controller, or power expander?

Could be the motors is it one side, one particular motor.
Could be the cortex if you use a lot of motors.

Sounds like you’re tripping the PTC in the power expander

Make sure the motors aren’t under any extreme load, and that your drive is friction less.

Can you confirm that the battery is dying? Take a multimeter and check the voltage before use and after it “dies”. Could you post the data from that test here?

is it the batteries are dying? or is the robot dying. like @Ashwin Gupta said, check the voltage. if they aren’t out of power, than it isn’t the batteries. could be the PTC in the power expander, or possibly friction on the motors.

do you mean the motors are stalling?

How heavy is your bot? Is it steel or aluminum? What gearing is your drive?

Friction. Verify all the wheel can move (Relatively) freely. If you have motors with screws removed and held together by zipties (if not, do it), remove the motors and verify the wheels can turn spin completely freely.

@Nathan Webb Are only the drive motors affected or is the whole robot “dying”?

If your batteries, after the drive dies, are still showing good voltage (you can check by placing a multimeter’s prongs to the contacts on the battery connector), then it seems like your motors may be “dying” because you’re somehow tripping the motors’ PTCs. If you don’t know what that is, it’s a special fuse inside the motor that, when it gets too hot because of excessive current, it’ll trip and generate a lot of electrical resistance, preventing the motor from running until it cools down. It gets too hot if the motors are working against an excessive load, such as a heavy robot (which can happen because of a large all-steel robot), excessive sudden reversing, etc. The PTC fuse is present in the power expander and cortex as well, but the cortex PTCs are much harder to trip than that others.

When your robot “dies” the way you describe, quickly feel the motors and see if they feel warm. If they do, it’s most likely the PTC. The PTC fuse is there for safety reasons, so it is something that you can’t “fix” directly. Rather, you have to design your robot around keeping the PTCs happy. If you’re using an all-steel robot, next time (or next season, for that matter), attempt to use only aluminum parts for your robot (or as much as available, if not enough). Check the wheels for friction as well like nenik said. There are other ways to try to lighten the robot’s load.

I will try to check the voltage of the batteries, but i do not believe it is the PTC being tripped inside the motors because they are not overheating. My wheels move freely, and my robot weighs about 13 pounds and is made out of aluminium. Is there an actual PTC inside of the power expander itself that could be tripped?

Yes – all motors are connected to 2 PTCs. Each motor has its own PTC, and a PTC each is dedicated to Cortex motor ports 1-5, Cortex motor ports 6-10, and Power Expander ports.

Try putting battery caps on them. It helps a lot for the batteries to hold a charge.

What are battery caps?

And something that “helps a lot for the batteries to hold a charge” sounds illegal.

Basically, you take the connector piece of the battery and keep it plugged into the battery, while they are not in use. This strategy is completely legal.
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You plug the battery into itself? How does that work?

How does plugging the battery into something help keep the charge longer? Not plugging it into anything should make it last longer since it has no opportunity to transfer charge to something else

Either @Not12G is mistaken, or playfully trolling. Charge doesn’t “leak” out of the end of an unplugged battery. Batteries will lose charge over time, but this is an internal chemical process. It has nothing to do with uncovered battery terminals.

@Not12G has the right idea. Actually the best way to store batteries and maintain charge is to take some 14-18 gauge wire, and run it across the positive and negative terminals on the clip like so.

(jk, dont do this, and don’t do battery caps, sounds like a great way to accidently short your batteries)