Let’s talk about batteries.
The battery is the single most critical part in the whole robot. The battery’s condition will affect every other system on the robot to some degree. We ought to care about it quite a bit! Our legal batteries are all AGM (absorbed glass mat) lead-acid batteries with nominal 18AH capacity and ~12V differential. AGM lead acid batteries are well characterized by industry at this point, I’m only going to be going over a few FRC-specific points.
Would you like to know more? Visit Battery University. When I played with batteries in a former life Battery University matched up well with all of the more serious reference materials I used and presents everything in a generalized and layman-friendly format.
Charge your battery with a smart charger of some sort. The smart charger will help preserve your batteries’ capacity vs a constant current or constant voltage charger. Most chargers sold by FRC vendors are smart (maybe all of them are?) Smart Charging Article
Batteries can be charged when they are pretty darn hot, 50C/122F. This temperature will surprise most people if they touch it. For most FRC teams this means a battery can come straight out of a robot after a match and onto the charger. However, there is arguable advantage in waiting for the battery to cool before charging: a cooler battery can charge more quickly and you remove the need for temperature-compensation in your charging system. Battery charging temperature.
Charge the battery upright, or close to upright, if you can. NEVER EVER EVER use a battery upside down (posts facing gravity). If there is a charging issue or the battery is overcharged the gas purge valve will release the pressure, but if battery acid is covering that port battery acid will be purged out. Sad times.
Charging at 2-4A vs 6A can prolong usable battery life to an extent. We charge at 6A and don’t notice bad effects over a season.
Many testing options exist. Too many. Lead acid testing.
We primarily use a Battery Beak from AndyMark because it is quick and repeatable. It will find dead/dying batteries and loose terminals no problem. It cannot differentiate well between decent batteries with a slightly diminished capacity and great batteries with full capacity.
To manage capacity loss we don’t use a battery for more than one competition season. After 1 year a competition battery becomes a practice battery.
Our criteria look like this:
- <1 year old
- Internal resistance <0.015ohm
- 13V+ charged voltage
- <4 years old
- Internal resistance <0.022 ohm
- 12.4V+ charged voltage
- Failing any practice criteria
- Any physical damage to the case or posts
- Any doubt about age or observed weird performance
Name or label your batteries and use them by a schedule. This spreads out cycles across all of the batteries evenly and maximizes recharge and cool-down time.
Assign someone to manage this process. It is critical to your success.
DO NOT DO
- Do not pick up a battery by the leads, it can damage the battery immediately
- Do not touch wires or clips to the SB contacts, the arcing damage will increase resistance in that connector and hurt your performance. Only a mating SB contact should touch another SB contact, and only when the main breaker has been disconnected.
- Do not the battery as anything other than a battery. Please. I know it seems like a good hammer. It isn’t.