Running Two Batteries
 

Some of my FRC team and I are using parts from previous competitions to build a robot for outreach. We already have so much stuff on it that when we put on a battery and run it, the voltage never goes above 12, never mind 12.4 or whatever it normally hovers around.
When we were coming up with solutions for this problem the only solution we could really think of was to run a second battery on it. If we were to do that, how would we do it? Would it be 2 completely independent circuits with power distribution boards and digital sidecars and stuff, or do we need to run them together into the same power board? If the latter, should the batteries be in parallel or series?
Thanks in advance for the help.

Re: Running Two Batteries
 

Bread,
The FRC batteries are 12 volt batteries and under any light load will read right around 12 volts. They will measure higher when you take them right off the charger. Robot rules allow one and only one battery of FRC robots. The manufacturer does not recommend placing these batteries in parallel except through diode isolation. (except in special, controlled applications under strict guidelines.) Heavy loads will measure lower than 12 volts due to the voltage drop across the internal resistance of the battery.
Al

Re: Running Two Batteries
 

We've run up to 3 batteries in parallel for parades.

Re: Running Two Batteries
 

If you connect the two batteries in parallel, you get twice the current available. If you connect them in series, you get twice the voltage. For the FRC electronics, twice the voltage would be a bad thing, as the system is designed to run on 12v (nominal).

What kind of extra stuff do you have on the robot that concerns you?

Have you considered replacing the normal FRC battery with a car battery? It would have about 4 times the "reserve capacity", so you wouldn't need to charge it as often.

Re: Running Two Batteries
 

From Wikipedia, FWIW:

A deep-cycle battery is a lead-acid battery designed to be regularly deeply discharged using most of its capacity. In contrast, starter batteries (e.g. most automotive batteries) are designed to deliver short, high-current bursts for cranking the engine, thus frequently discharging only a small part of their capacity.

The structural difference between deep cycle batteries and cranking batteries is in the lead battery plates. Deep cycle battery plates have thicker active plates, with higher-density active paste material and thicker separators. Alloys used for the plates in a deep cycle battery may contain more antimony than starting batteries. The thicker battery plates resist corrosion through extended charge and discharge cycles.

Re: Running Two Batteries
 

I would look for a deep cycle marine battery. I use those when I'm wanting to use the drive train for more than a few minutes. You do have to be careful though more drive time = more heat in the motors.

Re: Running Two Batteries
 

Thanks for the input. For the record, in addition to a 4-cim drive base and a compressor constantly running, in addition to some other electronics, and the load is only going to go up once we add some other stuff that'll drain more power like a crapton of LEDs to make it pretty. I've had other 4-cim drive bases and the battery didn't drop anywhere near as much when it was running.

It seems like I'd want to either run an extra battery in parallel or a bigger deep-cycle battery. How expensive are decent deep-cycle marine batteries? Is there any reason to go with a better battery versus running a couple in parallel? I'd like to avoid spending large amounts of money if possible so being able to use old FRC batteries would be best.

Re: Running Two Batteries
 

You could make an improvised heat sink by wrapping them in aluminum foil... :D

Re: Running Two Batteries
 

I got a pair of marine batteries for the La Vida Robot ROVs for $60 each at Walmart a week ago.

Re: Running Two Batteries
 

Do you have a graph plotting layers of aluminum foil to extra minutes of drive time gained. Usually we just add a bunch of fans. Anytime we use these bigger batteries, we obviously don't care about weight , therefore the extra couple lbs for fans is acceptable.

Re: Running Two Batteries
 

Loose electrical connections will also drop more voltage and cause the battery to drain faster. Make sure all your connections are tight.

Re: Running Two Batteries
 

No, I was just making a joke.

We did use aluminum foil wrapping once to absorb radio interference when we used the RF Vex controller near the CIMs. a few layers of aluminum insulation really helped control the RF noise

Re: Running Two Batteries
 

I'm guessing that would actually make them run slightly hotter.

Re: Running Two Batteries
 

Instead of having to go out an buy a deep cycle automotive sized battery you can run two FRC batteries without running them in parallel. Use one to power the control system and a couple of your motors and another to power the other motors and the LEDs. You will need a second "main" breaker but you don't really need to have a second power distribution board. The FRC legal circuit breakers are of the "Maxi Fuse" size and you can get Maxi fuse holders, often sold be mobile audio suppliers. You would only need one digital side car to drive all of the motor controllers. The motor controllers have opto isolators in them for the signal input. This means that they do not have to share the ground system with the power supply to function, though you certainly could tie the grounds from the two batteries together w/o causing problems.

Re: Running Two Batteries
 

I'd bet that compressor is the culprit and also extremely hot after only a few minutes of running.

Re: Running Two Batteries
 

Unless you wrapped the foil very very tightly (tightly enough to rip it), it's going to trap air between the layers and act as an insulator.

What you need for cooling motors isn't a heat sink, it's a radiator. The black surface of a CIM is probably a lot more efficient at radiating IR than shiny aluminum foil, but I have no data to back up my supposition. Fins and a fan will do a much better job of cooling the case than any amount of foil wrap. But it's the temperature of the windings inside the motor that matters, not the outside of the case, and a CIM isn't particularly good at getting rid of its interior heat to begin with.

Re: Running Two Batteries
 

Sorry guys, like I was saying several posts ago,

:mad: :mad: I WAS JUST JOKING :mad: :mad:

Re: Running Two Batteries
 

We have been running two FRC batteries in parallel for years without issue on a few of our demo bots. Once nice feature is you can hot swap batteries without turning the robot off which is especially nice for LabView programmers who haven't permacoded. We have also use a deep cycle battery on a robot without issue, just make sure the bot can handle the weight of a typically larger marine battery and watch out for leaks if you're driving it.

Re: Running Two Batteries
 

Be sure to be careful of the duty cycle of the compressor. For example, the Viar 90c is only rated for a 9% duty cycle and a 1 gallon fill (approximately 3 minutes operation for every 30 minutes). If you run it too long, it will heat up enough to melt the pneumatic tubing plugged into it.

Re: Running Two Batteries
 

Have you actually calculated what these LEDs will draw? Compared to the large current demands of motors, LEDs draw comparatively small amounts.

Re: Running Two Batteries
 

LED's won't make a difference.
Something that would make a difference would be an extra CIM motor, so I'll use that for comparison.

A single CIM motor might be expected to draw about 40 amps at 12 volts during normal usage. That's 480 watts.

If you had one really bright led, LEDs are about 90 lumens per watt, so 480 watts is 43,200 lumens, which is insanely bright. A "typical" home lightbulb is about 700 lumens. 50,000 lumens is the intensity of a Nightsun, the searchlight that gets mounted to Coast Guard helicopters. If you look into one, you can go blind very quickly.

If you had a bunch of normal LED's. A high output LED uses 20 mA at 5 V or 0.1 watts. 480 watts is around 4,800 led lights, way more than you'll ever see on a FRC robot.

Re: Running Two Batteries
 

Our underwater robot has 4 three watt LEDs on it, and they aren't actually getting that much power...it's too bright to look at.

Re: Running Two Batteries
 

LEDs are beast when it comes to using little amounts of electricity and producing tremendous amounts of light.

Re: Running Two Batteries
 

Out of pure curiosity, why is the compressor running constantly? I thought they only ran when necessary.

Re: Running Two Batteries
 

One solution to the compressor issue is to get a bigger compressor. Viar has ones with a 100% duty cycle & a stainless braid hose on the outlet to deal with heat issues.

Re: Running Two Batteries
 

What do you mean by permacoding? Sounds like it is something not everybody does...

Re: Running Two Batteries
 

I think it's a word he made up, which to him means the code has been uploaded into the cRIO's flash memory.

Re: Running Two Batteries
 

I should have stated this earlier but there is a reason the manufacturers do not recommend using the batteries in parallel. While several people have done it in the past, this is not good practice. Even if the batteries are from the same lot, made on the same day, and using the same materials, there is a difference from cell to cell within the battery that will make no two batteries exactly alike. As such, it is inevitable for one battery to have a slightly higher voltage which when placed in parallel will force some current into the lower battery. As that battery voltage diminishes, the other battery will try to back feed the first. This seesaw action will continue until both batteries are fully discharged. If the batteries are closely matched and charged and the same age (charge/discharge cycles included) this back and forth will take place very slowly in most cases. However, certain variables may change that, rapidly drawing down both batteries in a destrcutive cycle. Applications that place multiple batteries in parallel, constantly monitor battery health using specific gravity tests and other procedures to prevent damage. It is possible to use Schottky diodes to isolate the batteries from each other and from the load. These are available from RV centers and are designed to parallel batteries in RV applications.

Re: Running Two Batteries
 

I wonder how GM got away with it? They ran two batteries in parallel on the diesel engine vehicles in the 1970s-80s....connected together with big fat cables. No diodes.

Huh.

Re: Running Two Batteries
 

UPS's regularly operate many sets of batteries in parallel. The batteries we use in FRC are very commonly used in these applications.

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.