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Unread 14-10-2013, 08:49
Breadbocks Breadbocks is offline
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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.
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Unread 14-10-2013, 09:59
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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.
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Unread 14-10-2013, 09:59
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Re: Running Two Batteries

We've run up to 3 batteries in parallel for parades.
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Unread 14-10-2013, 09:59
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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.
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Unread 14-10-2013, 10:18
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Re: Running Two Batteries

Quote:
Originally Posted by MrForbes View Post
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.

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Unread 14-10-2013, 10:41
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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.
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Unread 14-10-2013, 11:27
Breadbocks Breadbocks is offline
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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.
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Unread 14-10-2013, 11:32
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Re: Running Two Batteries

Quote:
Originally Posted by arizonafoxx View Post
more drive time = more heat in the motors.
You could make an improvised heat sink by wrapping them in aluminum foil...
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Unread 14-10-2013, 12:03
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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.
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Unread 14-10-2013, 12:22
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Re: Running Two Batteries

Quote:
Originally Posted by BBray_T1296 View Post
You could make an improvised heat sink by wrapping them in aluminum foil...
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.
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Unread 14-10-2013, 12:32
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Re: Running Two Batteries

Quote:
Originally Posted by Breadbocks View Post
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.
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Unread 14-10-2013, 12:34
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Re: Running Two Batteries

Quote:
Originally Posted by arizonafoxx View Post
Do you have a graph plotting layers of aluminum foil to extra minutes of drive time gained.
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
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Last edited by BBray_T1296 : 14-10-2013 at 12:35. Reason: typos
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Unread 14-10-2013, 13:21
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Re: Running Two Batteries

Quote:
Originally Posted by BBray_T1296 View Post
You could make an improvised heat sink by wrapping them in aluminum foil...
I'm guessing that would actually make them run slightly hotter.



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Unread 14-10-2013, 13:52
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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.
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Unread 14-10-2013, 13:58
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Re: Running Two Batteries

Quote:
Originally Posted by Breadbocks View Post
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.
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