The cybercards are putting together a T-Shirt cannon for share with our high school booster club. We have looked at many teams cannons and seen that power is an issue.
We would like to put two FRC batteries in parallel for our system to increase the time the cannon is operational. The only issue with wiring this is the main electronics savvy student graduation.
Could someone give me description on how to wire two 12v FRC batteries in parallel?
Much appreciated,
Brett
The best way to put two used robot batteries in parallel is…not to do it in the first place.
If you have two new and well-matched AGM batteries, and take care to always keep them discharged and charged together, you can get away with it and not expect any problems. But since you’ll probably be running them down pretty far in typical operation, any difference in their capacity will show up and tend to drain one to the point where it might not recover.
If you can live with the slight voltage drop, you can put a high-current diode in series with each battery before tying them together. This will prevent the potentially harmful situation of one battery stressing out trying to keep the other charged.
My preference would be not to use otherwise valuable FRC batteries at all, and instead use a motorcycle or deep-cycle marine battery. The only real drawback I can think of would be the need to keep from tipping the battery on its side.
I agree with Alan.
We are going to use two batteries on our t-shirt shooter. One will run the drive, barrel angle motor and pneumatic solenoids and and and the other just the two compressors. They will not be hooked to the same circuits. We many even increase it to three batteries (one for each compressor) if the drain is to high.
just put two batteries in the robot (make secure mountings for both of them right next to each other), and when the first battery runs low, unplug it and plug in the other one. Pretty easy and no worries for the EE types.
Get one of these:
http://di1.shopping.com/images1/pi/38/33/9d/44169866-149x149-0-0_Perko+Perko+Main+Battery+Switches+Alternator+Field.jpg
Attach a small pneumatic piston to it. When your battery gets low, push a switch that fires off the solenoid that switches the battery.
We did this on our t-shirt shooter and it has been working great as far as I know.
http://photos-c.ak.fbcdn.net/photos-ak-sf2p/v116/83/59/1289760161/n1289760161_30022679_4392.jpg
You could try having two batteries, one powering the controller and all the drive motors, and have another powering the compressor/whatever else you have operating your cannon.
Basically there are two separate systems, but the controller from one system can control the other via PWMs.
I agree with him, the only thing that you would have to watch is ground. A common tie point for all battery ground would be sure to not have an issue.
The best way, in my opinion, would be to break it up at stated above, but maybe use more, one for each item could seem sufficient.
I also agree that you could use a deep cycle marine battery with good results.
I guess we built our robot cannon in some magical energy conserving way, because we just run one battery at a time, and it’s not a problem.
What we are planning to do for our t-shirt shooter is install a maxi block, like used back in 2008 and some years prior, backwards to connect the three batteries together. We will put fuses in to prevent any battery failure.
If I understand you correctly there is nothing in between the individual batteries to prevent ‘failure’ (like the diode alan suggested). Do you me failure like thermal runaway? Failure like a dead battery (possible)? I’m not sure what you are preventing here, fuses are for overcurrent protection.
Generally though putting them in parallel is a relatively safe bet with sealed lead acid batteries so I doubt there would be any failure to begin (faster aging if anything). Don’t combine a old battery with a newer battery in parallel. Make sure the batteries are from the same manufacturer (preferably new). Practice what Alan said about charge and discharge.
SLA batteries are commonly paralleled without any ill-effects in large industrial uninterruptable power supplies. Sometimes you’ll see entire pallet-racks full of them. In these systems, I don’t believe there are typically any diodes or switching. However, all the batteries start off new, and are all always charged and discharged together, for their entire lives, and aren’t discharged often.
Brett et al,
This question comes up every year. I have asked the manufacturer and the Battery Institute for their recommendation in the past. Neither recommends that robot batteries be wired in parallel. As pointed out, if the certain batteries are new and always used in parallel (both charge and discharge) they may be wired this way. Large UPS power supplies and large DC current loads (like WWII submarines) use batteries in this fashion but monitor the specific gravity of the electrolyte constantly to insure that one battery will not discharge the remaining batteries.
I think the large UPS systems have something that most don’t…they’re being charged all the time, except when they’re under heavy load, so there is little opportunity for one battery to run down the others.
And I think the “real life” answer to the original question here is that it should be fine to parallel the batteries we use on a robot, if they are disconnected from each other when the robot is turned off. If you could figure out a reliable way to do this, I doubt you’d have any problems. But I still don’t see the need for running them in parallel on a T shirt cannon 
I think Alan hit it on the head. Just run your robot with a single Marine battery or a car battery. That way you’d have a more than enough power with none of these multi-battery issues. Why put that much stress on your FRC batteries that have lots of other uses.
Thank you for your insight - we think we’ll go with the option put forth by Scott, Pikat and fsgond, using one battery for drive and one for the compressors.