newbie question about resistance

[wascalwabbit]

So I built a battery from lifepo cells and finally connected everything to the inverter/charger. I used mostly 1/0 cables, except for a longer (30 inch) run to the inverter/charger where I used 2/0 cables. Everything looks good so, far the inverter booted up without problems and the BMS also didn't show any issues. I tested the resistance at the poles of the inverter, and the value that came up was 6 milliohms. I figure that if I have a bad connection somewhere this would show up as high resistance. Am I correct in the assumption, and is 6 milliohms an acceptable number?

thanks!
Dom

 

[ThaiTaffy]

milliohms or Megaohms? Best bet is to test from the battery terminals to the inverter terminals to test lugs/connections then get back to us.

 

[jpwhit]

Yes, you are correct in your assumption that the a bad connection would show up as a higher resistance. 6 milliohms is a very low resistance so i think you're fine.

 

[ThaiTaffy]

6 mΩ is more likely the resistance of your test leads rather than the cable, 1m of 25mm² cable should be around 700µΩ.that's why I wanted to be sure your talking about mΩ rather than MΩ

 

[time2roll]

What is the voltage drop at the highest load? Any heat?

 

[jjww]

It takes a pretty fancy ohmmeter to measure down to 6 milliohm (4 wire ) and even then prone to mistakes-
what meter did you use?

 

[wascalwabbit]

thanks for all the replies. It's milliohms, the tester I used is the RC3563.

What is the voltage drop at the highest load? Any heat?

I'm not able to put a load on it as of yet

 

[740GLE]

Was this impedance measurement when everything was connected, positive to negative including the Inverter?

Another good check is say put 1000w load then measure the voltage of the positive battery terminal to the inverter terminal. Repeat for the negative terminals.

Hopefully it’s a low voltage you see.

 

[wascalwabbit]

Was this impedance measurement when everything was connected, positive to negative including the Inverter?

yes, that's what I did

voltage drop test will be difficult since there is a wall between the battery and the inverter

 

[aliarifat794]

Loose connection or measurement error. It can occur due to both.

 

[740GLE]

yes, that's what I did

voltage drop test will be difficult since there is a wall between the battery and the inverter

Impedance of the inverter (caps) skews anything worthy of measuring. You didn’t really measure anything worthwhile.

 

[wascalwabbit]

Impedance of the inverter (caps) skews anything worthy of measuring. You didn’t really measure anything worthwhile.

OK, thanks. guess I'll just have to hunt hot spots when I'm able to put a load on it

 

[740GLE]

Another option is disconnect the main cables from both the battery and inverter (and any other electronics), at one end bolt positive to negative (dead short) then measure the resistance at the other end from positive to negative. This will give you full round trip impedance of the cables.

It won’t tell you which leg has an issue but give you a good deal of confidence you’re cabling and other connections are solid.