# basic beginners question



## poprock (Apr 29, 2010)

My ohmmeter shows no resitance between battery string + and battery strings common earth. Good? Also showing no resistance between battery string + and chassis. Bad? Possible causes?


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## major (Apr 4, 2008)

poprock1 said:


> My ohmmeter shows no resitance between battery string + and battery strings common earth. Good? Also showing no resistance between battery string + and chassis. Bad? Possible causes?


Hey pop,

Don't use an ohmmeter on any circuit, or component in any circuit while that circuit is energized. It will not give a correct reading and may well damage the meter. And a battery is always energized, at least to some degree. Measure voltage across a known resistance and Ohm's Law to figure resistance to ground (or anywhere else) from a circuit node having a potential.

major


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## jockepocke (Nov 24, 2009)

Try disconnecting the battery(ies) and measure the resistance between battery cable and ground.


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## rwaudio (May 22, 2008)

jockepocke said:


> Try disconnecting the battery(ies) and measure the resistance between battery cable and ground.


Most inexpensive multimeters won't be able to measure anything around or below 1 ohm, and the resistance of the cables/connectors in the meter itself will sway the results. A better way to test for resistance is measure the voltage under load. IE if there is a significant voltage under load between the most positive battery + terminal and the controller positive you may want to look at the cables/connectors. The same can be done with any part of the system, just use care and safely measure these points.


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## jockepocke (Nov 24, 2009)

rwaudio said:


> Most inexpensive multimeters won't be able to measure anything around or below 1 ohm, and the resistance of the cables/connectors in the meter itself will sway the results. A better way to test for resistance is measure the voltage under load. IE if there is a significant voltage under load between the most positive battery + terminal and the controller positive you may want to look at the cables/connectors. The same can be done with any part of the system, just use care and safely measure these points.



I was talking about the resistance between battery + and chassi, which should be very high!


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## rwaudio (May 22, 2008)

jockepocke said:


> I was talking about the resistance between battery + and chassi, which should be very high!


You are correct for that measurement, an alternative is measure voltage between pack + and chassis ground, this should be 0volts, if you get anything there is a problem. A double check is pack - to chassis ground to check if pack + is shorted to chassis ground.

Measuring the voltage will expose a problem anywhere in the battery system without having to disconnect every battery and test the cables to ground.


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## poprock (Apr 29, 2010)

rwaudio said:


> You are correct for that measurement, an alternative is measure voltage between pack + and chassis ground, this should be 0volts, if you get anything there is a problem. A double check is pack - to chassis ground to check if pack + is shorted to chassis ground.
> 
> Measuring the voltage will expose a problem anywhere in the battery system without having to disconnect every battery and test the cables to ground.


 Thank you gentlemen; it appears that we have a problem. 
Chassis to: common pack earth= 14.32v
24v + = 9.8 v
36v + = 23.1v
48v + = 33.3v 
This would seem to indicate a short between battery pack(s) and chassis.
I will start the process of elimination.


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## poprock (Apr 29, 2010)

poprock1 said:


> Thank you gentlemen; it appears that we have a problem.
> Chassis to: common pack earth= 14.32v
> 24v + = 9.8 v
> 36v + = 23.1v
> ...


Additional thought: adding the 14.32 to each measurement brings it to near enough full pack reading?


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## rwaudio (May 22, 2008)

poprock1 said:


> Additional thought: adding the 14.32 to each measurement brings it to near enough full pack reading?


Do you have a 12v battery or dc/dc converter in the vehicle as well? If the + out of that battery/converter was somehow connected to the pack you would read something like that.
Also do you have any guages? Xantrex or anything that could be tied to the pack and possibly ground?


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## poprock (Apr 29, 2010)

rwaudio said:


> Do you have a 12v battery or dc/dc converter in the vehicle as well? If the + out of that battery/converter was somehow connected to the pack you would read something like that.
> Also do you have any guages? Xantrex or anything that could be tied to the pack and possibly ground?


 Was a 12v battery since removed so now only 48, 36, 24 , packs sharing a common earth. Could it be the commutator shorting?


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## rwaudio (May 22, 2008)

poprock1 said:


> Was a 12v battery since removed so now only 48, 36, 24 , packs sharing a common earth. Could it be the commutator shorting?


Do you have any pictures of the setup?
Unless the controller is on the motors com shouldn't play any part in the equation.

Also define "pack earth" how do you have the cells wired together?


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## poprock (Apr 29, 2010)

will have to draw diagram, get back to you tomorrow. Meanwhile will follow thru on suggestions. thanks.


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## Snakub (Sep 8, 2008)

Do you have flooded lead acid batteries? This could be part of the main problem each battery should have its own contained plastic tray or tub because when you charge them the acid bubbles over the side of the battery creating a conductive path down the battery's case to anything metal you have them resting on i.e. your chassis.


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## poprock (Apr 29, 2010)

yes; they are old fla on metal frames and similar to a normal car battery I agree that this is what is happening. When I had all batteries isolated in an insulated container there was no problem , but this has arisen since repositioning the fla's.into a metal frame which is a direct path to the chassis earth. I will revert to isolated and well insulated carriers. Using old junkyard fla's the best result so far was only 10k's with 40k top speed. Since rearranging in the new carriers. it is now 3k's and 30k so something has gone wrong.Live & learn. thank you.


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## poprock (Apr 29, 2010)

Snakub said:


> Do you have flooded lead acid batteries? This could be part of the main problem each battery should have its own contained plastic tray or tub because when you charge them the acid bubbles over the side of the battery creating a conductive path down the battery's case to anything metal you have them resting on i.e. your chassis.


Found the problem. Using old batteries one had been plastic welded to repair a bolt hole. No acid was visible but a reading of 7.5v could be had by linking + to the repaired casing. It was at the bottom and touching the angle iron frame. Will do as suggested and completely isolate batteries in plastic cases. Thanks.


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