Running Two Batteries

[Breadbocks]

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.

[Al Skierkiewicz]

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

[sanddrag]

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

[MrForbes]

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.

[Ether]

Originally Posted by MrForbes

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.

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.

[arizonafoxx]

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.

[Breadbocks]

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.

[Joe Ross]

Originally Posted by Breadbocks

I've had other 4-cim drive bases and the battery didn't drop anywhere near as much when it was running.

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

[camtunkpa]

Originally Posted by Breadbocks

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.

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

[runneals]

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.

[MrForbes]

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

[jwallace15]

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.

[Joe Ross]

Originally Posted by Breadbocks

For the record, in addition to a 4-cim drive base and a compressor constantly running,

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.

[Jim Wilks]

Originally Posted by Breadbocks

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.

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

[magnets]

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.