Be aware of battery and charger inconsistencies!

[philso]

Originally Posted by Ether

I believe the RCM (reserve capacity in minutes) rating of automotive batteries follows an industry standard of 25 amps load until voltage drops to 10.5V.

If so, you could multiply the RCM by (25/60) to get an Ahr rating (at 25 amps down to 10.5V).

For many automotive batteries, the RCM rating is available.

Your 25/60 number is close to the 1/2 number I found for converting RCM to an approximate Ahr. Since the RCM is measured at a constant current, regardless of battery, it isn't the same as the Ahr rating though it is close.

Originally Posted by Toa Circuit

But if you're experiencing inconsistent performance based on battery voltage, You have a bad design. You will experience voltage drop throughout the match, during movement, so basically, if your performance is hinging upon a perfectly (not fully) charged battery, you really need to rethink how you've built something. (Though this late in the season... what can you do? Maybe use the battery reading in your programming to do something like an open-loop control, or you can slap on a sensor to do closed-loop control.) (This is my #1 concern with all these motor-powered catapults this year...)

The peak currents that a typical FRC drivetrain will draw during acceleration is well off the charts provided by Enersys. Since the design of these batteries were optimized for their storage capacity rather than high discharge rate, their "output resistance" will be rather high, guaranteeing that you will get severe voltage dips.

Choosing your battery charger to give you a consistent state of charge is one way of dealing with the battery variations. As Toa states, a more robust solution would involve making your robots performance insensitive to the variations in your battery's state of charge or output voltage. FIRST has already provided us with an example; the regulated 12V output of the PDB.

Learning to devise strategies that are independent of variables such as your battery voltage is part of the STEM education you should be getting from participating in this program.