We have been invited to participate in a parade with our robot. However, we only have four working batteries. We will need at least one hours worth of battery. Can we replace the standard FRC battery with a car battery?
You should be able to, as long as your robot can support the weight!
It will probably hold about 4 to 5 times as much energy as a single robot battery.
If you are planning on doing this on any kind of regular basis, however, you might want to consider a “deep cycle” battery rather than standard automotive battery.
Deep cycle batteries have a slightly different plate design so that they can be repeatedly discharged and recharged without damage.
Car batteries are designed for maximum current draw… short bursts of current to the starter motor and then recharged right away from the alternator.
You might find deep cycle batteries listed as RV or Marine batteries.
But if you’re only doing it once… you should be fine. And if you don’t discharge the car battery all the way, you should be better than fine.
Keep in mind, of course, that your “robot” batteries are SEALED lead acid batteries… they don’t leak when you tip them over. Car batteries like to be kept upright… so you might not want to do too much bump climbing during the parade.
Automotive batteries are rated in the 50- 55 Amp Hour class, about three times the capacity of our robot battery. I would recommend stripping off things that you won’t need in the parade like a ball handler and tower climber to lessen the weight. In most cases your battery drain will be fairly low as you will not be pushing, turning or climbing. Please be sure to fully insulate the battery terminals, these batteries can supply over one thousand amps in a dead short. Insulated battery clamps/terminals can be purchased where you buy the battery or from McMaster. I would also recommend one with an attached handle so that it is easy to carry around. If you are going to be kicking balls, be sure to check for loose hardware often along the parade route. You don’t want to be launching parts into the crowd. Good Luck with the demo.
Alternatively, you can run 2 or more of your FRC batteries in parallel to achieve the longevity you’re looking for.
This is not recommended practice. When batteries are used in parallel they need to be nearly identical in operation. (MK and Battery Institute experts agree that same age, same lot, same charge/discharge cycles must be maintained for parallel operation) Used FRC batteries have gone through enough charge/discharge cycles that they are as likely to self discharge each other as they are to increase amp hour capacity. Diodes can be used to prevent this and are available at RV stores.
Well consider me corrected then. Good information to know, thank you.