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Old 05-12-2008, 10:37 PM   #1
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Default bad FET?

I am running a KOpropo VFS2 and my car is slow with any motor I put in. I checked the battery with a multimeter and gettting 7.2+V through the 6 cells. The ESC is pumping the same voltage to the motor leads at full throttle.

What is a typical symptom of a bad FET?
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Old 05-13-2008, 02:46 AM   #2
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Hm... may need more info on this one... there could be a million reasons why... Are your batteries new? Your drive train is free? Wired properly?

Under no load, the battery will read 7.2V even if it is empty.
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Old 05-13-2008, 02:53 AM   #3
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Well, I am using those questionable IB4200s. They vent like crazy when I charge them. At this point, the batt seems to be a likely culprit (I hope). Is there anyway to check if the batt is good other than reading the voltage?
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Old 05-13-2008, 03:05 AM   #4
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bad battery or high resistant battery will make ur car move slow...try use other battery .....
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Old 05-13-2008, 03:23 AM   #5
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It will be easiest to troubleshoot on the batteries if you could borrow a pack from the fastest guy at your track with the same battery layout
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Old 05-13-2008, 11:21 AM   #6
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The best way to check a battery is to load it down, say 20A or higher, and then check its voltage. These days, a descent pack will be at or above 1.2 VDC per cell. A pack can have good "open circuit" voltage (no load), but it may not be so good under load. This would indicate that the cells internal resistance has gone up.

An esc can be checked with the same type of test. You would put one lead of a voltage meter on the Negative motor wire and the other lead on the the Negative battery wire. I am assuming this is a brushed controller. You would then put the car up off the ground and hold full throttle. There should be little or no voltage reading on the meter. Even loading the wheels should not produce much voltage on the meter. At 20A with an internal resitance of 0.0003 ohms you should only see .006 volts.
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Old 05-13-2008, 12:13 PM   #7
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Thanks for the replies. I'll try Kufman's method to troubleshoot them. Pretty sure those IB4200s are messing up.
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