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 Chief Delphi Inexplicable Current Draw?
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#31
01-10-2018, 05:01 PM
 AriMB The Philadelphian emigrant AKA: Ari Meles-Braverman FRC #5987 (Galaxia) Team Role: Mentor Join Date: Mar 2015 Rookie Year: 2012 Location: Haifa, Israel Posts: 1,774
Re: Inexplicable Current Draw?

Quote:
 Originally Posted by Jaci A high resistance limits the current flow, not the inverse. Low resistances are characteristics of a high current-draw system. V = IR I = V/R therefore, as R->inf, I->0. Likewise, as R->0, I->inf. To think of this better, the voltage drop across a small section of wire is almost 0 since its resistance is negligible (current can flow). Across an air gap, the resistance is almost infinite, so no current can flow.
Sorry, I should have said a large voltage drop instead of voltage to be clearer. The voltage drop (difference between resting 12V and loaded 6V) is much higher than expected for a current draw of 50A. Something in the system is causing a high resistance, causing a large voltage drop.
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#32
01-10-2018, 05:10 PM
 Ether systems engineer (retired) no team Join Date: Nov 2009 Rookie Year: 1969 Location: US Posts: 9,126
Re: Inexplicable Current Draw? [UPDATE]

Quote:
 Originally Posted by cbale2000 That seems a bit high for free speed, are your gearboxes properly lubricated and belts/chain properly tensioned?
For reference:

https://www.chiefdelphi.com/forums/s...3&postcount=16

Quote:
 Actually a new fully charged 12v battery will generally be closer to 14v (generally around 13.7 or so).
That's surface charge. It will go back down to the normal 12.8 or so level after a couple hours.

http://autorepair.about.com/library/.../aa101604b.htm

Quote:
 "12v" is more of a nominal voltage,
12V is too low for OC 100% charge.

#33
01-10-2018, 05:29 PM
 Mike Copioli You make it pretty We make it dance no team (Retired(3539, 217)) Team Role: Engineer Join Date: Jan 2006 Rookie Year: 2001 Location: Romeo Posts: 469
Re: Inexplicable Current Draw?

Quote:
 Originally Posted by Jaci A high resistance limits the current flow, not the inverse. Low resistances are characteristics of a high current-draw system.
Correct, so if you place a HIGH resistance (NOT a low resistance) in series with the load, in this case a motor or several motors, your voltage at the load will absolutely drop. So the original post was correct.

The power paths between the battery and loads MUST be low in resistance. A bad crimp or loose terminal INCREASE the resistance causing the voltage at the load to drop.

Think of the "BAD" connection as a resistor who's value is higher than ideal therefore causing a voltage drop across the connection.

E/I*R
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#34
01-20-2018, 12:26 PM
 Al Skierkiewicz Chief Robot Inspector AKA: Big Al WFFA 2005 FRC #0111 (WildStang) Team Role: Engineer Join Date: Jun 2001 Rookie Year: 1996 Location: Wheeling, IL Posts: 11,120
Re: Inexplicable Current Draw?

So back to the original post and some real world information.
You cannot determine a battery state of charge with a voltmeter. A dead battery will show ~12 volts with no load. However, a battery showing less than 12 volts with just a light load (robot disabled) is likely discharged.
It is normal for FRC robots to show a drop of several volts when driving or turning. This is due to the current through the internal resistance of the battery and the primary (#6 AWG) wire. All things being equal, 100 amps would drop 1.1 volts across the internal resistance of a fully charged battery.
Remember that the voltage is sensed at the PDP. Any bad connection up to the PDP will drop voltage under load.
Pull on every #6 wire, there should be no movement in the connector and no wiggle in the connectors. Nearly 100% of all electrical failures due to brownouts are related to poor crimps, loose hardware or improperly made terminations. Hardware on the battery, the PDP and the main breaker needs to be tight. I highly recommend adding a #10, external tooth lockwasher, between the battery terminal and the wire terminal to prevent the terminals from moving. Any movement in these connections will loosen the hardware in a short period of time.
If you are using screw clamp terminals that do not specifically state "for stranded wire" the screw may actually cause the strands to spread out and move up the screw threads.
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