Re: Running Two Batteries
 

Al,

I've never heard of this back-feed seesaw action between 2 parallel batteries before. Could you please provide a couple of links to technical articles?

Thanks

Re: Running Two Batteries
 

It is not just GM many diesel pickups do that to this day as does just about every MD and HD truck, sometimes up to 4 batteries in parallel. In those applications there is one set of cables running to the engine and a bunch of short cables connecting all the batteries together.

This can be done quite reliably, however to do so it is always recommended that the batteries be a matched set, IE same model and age. The other key to making it work is that the batteries are always discharged and charged as a set. The fact that in most cases the vehicles do not sit unused for long periods so the inter-battery discharge is limited in duration. Once the vehicle is started the fact that they are charged together keeps the difference in voltage to a minimum. However if the vehicle is left to sit for an extended period of time the batteries will discharge at a rate higher than the normal self discharge that would occur if they were not interconnected.

Now in a robot application the batteries are likely of different age and/or have been subject to a different amount of discharge/charge cycles. They are also usually charged independently. However if you disconnect them after each use the inter-battery discharge will be minimal.

Re: Running Two Batteries
 

We had a deep cycle (that we took from a Wall-E robot designed by our mentors) on our cart that we used to power our robot in the pits & to re-charge our pneumatics while we were lined up before our matches, that seemed to work pretty well. Also, you'll want to be sure that you don't use a car battery inside, as it has chemicals. I don't know about the deep cycle, but I assume it is ok to run inside.

Re: Running Two Batteries
 

Deep cycle marine batteries are flooded lead acid, just like car batteries.

Re: Running Two Batteries
 

One of our mentors brought in a 12 volt marine battery to use in demos (mainly used last fall; a big festival in the town where our team is located happened right on the day of Kettering Kickoff, so all our robot batteries were at the competition).

Some of you older mentors (no offense) may remember our team's go-cart that we brought to competitions in the past (we still use it in parades and pep assemblies). It runs off of 4 golf cart batteries. 2 are connected in parallel, then the pairs are connected in series. The only problem we've encountered with the batteries is corrosion from old age; No electrical issues.

Re: Running Two Batteries
 

For what it's worth, if you're compressor is running 'constantly' or in on more often than not, it might be worth looking into the efficiency of your pneumatic setup before chasing additional batter capacity - as other's said before in this thread, the duty cycle of the current FRC compressor is relatively low and the older compressors aren't all that much better in the real world. (Though better in some FRC cases, IMO.)

It sounds almost like one of two things are going on; you're using a lot of air for auxiliary functions (relative to the amount being stored) / storing too little air (relative to the amount being used), or your system has a series of leaks somewhere. Of the two, leaks are probably the most simple issue to chase, especially if you've got a spray bottle filled with soap and water.

If it turns out that you're using a lot of air and/or storing too little, one of the easiest things to do is look at how much work the cylinders/actuators are actually doing. Standard FRC systems are usually run at 60psi (stored at 120) which can be (often is) overkill for the amount of work the system is doing. If you're just moving a series of mechanisms around, or something like that, you can try turning down the working pressure until you find a sweet spot. If it turns out that you can run at a pressure significantly lower than 60psi, you should notice that your stored air capacity should last longer, meaning that the compressor will run less often. The alternative is adding additional capacity, but you'll need to be conscious of how long the compressor is being run to fill that additional capacity.

Another handy alternative is to make an off board compressor setup. Basically, you'd take the compressor, control electronics, and an independent battery and then put them into some sort of self contained package. This would then allow you to charge the system without drawing from the robots primary battery, which will increase run time. This is a handy trick even if there's a compressor on the robot since you can charge the system before you start a demo and then the on board compressor will come on as needed to recharge but should never see that initial charging time (usually relatively long).

Re: Running Two Batteries
 

If your compressor is running constantly, and your programming and wiring are correct, perhaps your plumbing is faulty. I have occasionally seen robots with the main regulator connected backwards. That results in constant venting of air to keep the pressure below the set point, so the high-pressure side of the system never gets high enough to satisfy the pressure switch.

Re: Running Two Batteries
 

....or there are just some loose tubes, if they're not pressed into the fittings all the way, they will leak. Also, the tube wears out after a while, and you need to cut off about half an inch and reinsert.