Hi, driver/electrician from 5943 here,
I was just wondering what an unhealthy battery would look like. I use a CTR Battery Beak to test our batteries, but I really only look at the charge % and status. For us, a normal percentage is 130%, and anything under that at full charge indicates a bad battery. But the beak also gives voltage readings. What are some signs in relation to voltage that the battery is bad?
A bad battery will have a high internal resistance, something over .025 OHMs is really bad, but i wouldn’t use one over .02 either. 130% should be what every battery charges to every time, and the status should say “good”
I’ll echo this, 0.020 Ohms is on the high-end of what I would use on a competition battery. An ideal battery should run roughly 0.015 Ohms (or less, if you can find one). Granted the rate of battery drain will also be affected by how energy efficient your robot is (last year we had brownout issues, this year we use almost all NEO motors and have had zero power issues).
Additionally, I wouldn’t go by the % readout on the BatteryBeak for more than a quick short-hand reading. Ideally, you want to look at the voltage readouts, identify the lowest one, and ensure it’s over 13v (give or take, if accounting for surface charge).
We use a dry erase board to track both the internal resistance and voltage readouts of our batteries at competitions so we can ensure we always use the best ones.
Check out this report/presentation done by The Charge, 2619. They did a presentation at Detroit Worlds last year about battery health specifically in the context of FRC. They designed and built a device that simulated a typical FRC match to evaluate battery health. A battery beak is useful for spot checks, but may not give you the whole story.
Note this can be impacted by the quality of the battery cable connections. Reworking the leads to high quality crimps, 12" of 4awg wire, and lightly de-oxidized (ie sanded) terminals on some of our heavily abused practice batteries brought me a 4-8 mOhm drop in internal resistance readings this year.
My research (internet reading) shows it can also be impacted by maintenance charging practices, but I haven’t got good data on this yet (we got new hardware for maintenance charging this year but have yet to track results).
It is quite interesting to play around with the ILITE drivetrain simulator to compare the effects of various drivetrain configurations vs. the effects of things like battery internal resistance and software current limiting.
Team 4926 conducted extensive configuration modeling in the summer of ‘18 to evaluate those effects. ILITE rocks and I recommend you try out their tool!