
21-03-2016, 22:39
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LabVIEW aficionado and robot addict
AKA: The guy with the yellow hat
 FRC #5987 (Galaxia)
Team Role: Mentor
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Join Date: Mar 2015
Rookie Year: 2012
Location: Haifa, Israel
Posts: 571
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Re: Longer battery wires
Quote:
Originally Posted by kiettyyyy
Absolutely. I had helped a team in San Diego that had about a 6 FT run of 6 AWG going from their main breaker to the PDP. Shortening the run solved their issue.
Battery voltage can be affected by three factors, temperature, state of charge and current. The one we're really talking about here is current. A lead acid battery has internal resistance. As you move current through the battery, the voltage changes due to its internal resistance.
Lets simplify it a little bit.
When you're charging a battery with some low current, you'll see the battery voltage increase. Charging it with a larger current, you'll really see it increase.
Same thing applies to the discharge. Discharge it with a small current, you'll see it decrease; discharge it with a large current, you'll see the battery voltage really decrease.
Now apply this same theory to having long interconnects with your battery. Say you're drawing current at some C rating that's near the limits of the battery, your voltage will likely be pretty low. Combine that with the voltage drop due to large current draw over a long run of cable.
This situation is extremely prone to causing brownout conditions.
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According to Ether, 6 AWG wire loses .1 V at 260 A PER FOOT. Running an extra 6 feet would lose .6 V, or a 10.2% loss of mechanical power. This is something many teams would notice.
__________________
2017-present: Mentor FRC 5987
2017-present: CSA for FIRST in Israel
2012-2016: Member FRC 423
2013: Programmer
2014: Head Programmer, Wiring
2015: Head Programmer, Wiring
2016: Captain, Head Programmer, Wiring, Manipulator, Chassis, CAD, Business, Outreach (basically everything)

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