Testing Connections for Resistance

[cruxarche]

I just completed building a portable battery box. Obviously it involved creating a lot of spliced and terminal connections. I would like to go back with my multi meter and test those connections under load. Is there a standard protocol for this? I assume that measuring voltage drop is better than trying to measure resistance directly while not under load?

Any suggestions appreciated.

Thanks.

 

[john61ct]

Voltage will always sag under load, internal to the cells, nothing to do with the wiring. There are specialized ESIR meters, help to identify bad/failing cells.

Test the resistance of wiring / connections etc without any battery hooked up.

Then also, separately, measure the overall voltage drop of the rig's harness, under heavy current loads between the pack posts and the load's terminals.

Most accept a 3% drop, but 1% is much better.

An infrared gun is helpful to identify hotspots.

 

[cruxarche]

I do not have any heavy loads right now. With a 3 amp load I see these results. Is this what you were referring to? 13.19v at the battery and 13.10 at the powerpole under (3 amp) load.

 

[DaveInDenver]

Voltage drop is exactly how resistance is measured by a multimeter. You sound familiar enough with concepts to know about Ohm's Law.

A DMM produces a known (generally as precise as possible) test current, which when put on an unknown resistance produces a measurable voltage across the unknown resistance allowing you to calculate the one unknown from the two knowns you have.

It's acceptably accurate to place a sizable load on a circuit and measure voltage and current.

In the case of your cable run if you can know the load consuming 3 amps is reasonably accurate then knowing there's 13.19 - 13.10 = 0.09 volts of drop would indicate that you have 0.09 V / 3 A = 0.03 Ω (or 30 mΩ) of resistance in your cable (circuit).

The key in your measurement is to know that the current in the circuit is truly 3 amps. But for the purposes here if it's a load with a given value or perhaps a light bulb of known consumption (e.g. nominally ~36 watts) probably good enough to validate a back of napkin expectation.

If you can tell us the length and size of the cable the expected resistance can be calculated easily.

BTW, from the photos it appears the circuit is battery -> battery fuse -> wire -> fuse block bus and fuse -> wire -> PowerPole. Having 30 mΩ in a circuit such as that isn't unusual.

 

[john61ct]

You can also directly measure the resistance wth your DMM across each connection / termination / segment in the circuit.

Troubleshooting when there's a poor quality crimp in the path for example.

Put a bar heater on a big inverter to pull high currents if you don't want to invest in a specialist dummy load device.

Or DIY a rig with high-wattage old-school car headlights, or use a HWS element, or. . .

 

[cruxarche]

DaveInDenver, you have correctly identified the system configuration. Thanks for helping out.

As far as wire lengths:
Positive Run:
Battery Positive to Breaker: 8awg 14"
Breaker to Fuse Block: 8awg 16"
Fuse Block to power pole: 10awg 10" (including the three way splice)

Negative Run:
Battery Negative to Shunt: 8awg 16"
Shunt to Fuse Block: 8awg 9"
Fuse Block to Powerpole: 10awg 10" (including the the three way splice)

I just remeasured the amperage and corresponding voltages:
at 2.71 amps (per the Victron Smart Shunt)
Battery Positive: 13.19v
Fuse Block: 13.15v (0.04v/2.71amps= 0.015 ohms)
Powerpole: 13.11v (0.04v drop, same as between battery terminal and Fuse Block)

 

[cruxarche]

You can also directly measure the resistance wth your DMM across each connection / termination / segment in the circuit.

Troubleshooting when there's a poor quality crimp in the path for example.

Put a bar heater on a big inverter to pull high currents if you don't want to invest in a specialist dummy load device.

Or DIY a rig with high-wattage old-school car headlights, or use a HWS element, or. . .

I have a small inverter with powerpole adapters. When I get off the phone here I will hook that up and rerun the voltage drop measurements.

 

[DaveInDenver]

Those are very short lengths so their resistance is relatively small but FWIW values. I'll also include expected voltage drop at 2.71 amps.

Positive Run:
Battery Positive to Breaker: 8awg 14"

0.76 mΩ
2.04 mV

Breaker to Fuse Block: 8awg 16"

0.86 mΩ
2.34 mV

Fuse Block to power pole: 10awg 10" (including the three way splice)

1.37 mΩ
3.72 mV

Negative Run:
Battery Negative to Shunt: 8awg 16"

0.86 mΩ
2.34 mV

Shunt to Fuse Block: 8awg 9"

0.49 mΩ
1.32 mV

Fuse Block to Powerpole: 10awg 10" (including the the three way splice)

1.37 mΩ
3.72 mV

I just remeasured the amperage and corresponding voltages:
at 2.71 amps (per the Victron Smart Shunt)
Battery Positive: 13.19v
Fuse Block: 13.15v (0.04v/2.71amps= 0.015 ohms)
Powerpole: 13.11v (0.04v drop, same as between battery terminal and Fuse Block)

Making an assumption but for example a 15 amp ATC fuse has about 4.8 mΩ of resistance, so at 2.71 amps a typical fuse drop will be approximately 13 mV. I don't know the spec for those MRBF but probably not that much different. A 30 amp PowerPole has 0.60 mΩ contact resistance, so 1.6 mV of drop there at 2.71 amps. Include crimps and other terminals.

It all adds up.

You're also going to be limited by the resolution of the meter to read voltage. It might be getting to its practical limit at single digit mΩ or mV, too. Not familiar with the specs for a Fluke 115. So the total resistance or drop you measure could have some amount of uncertainty. Point being none of this seems out of the ordinary.

 

[cruxarche]

Thanks. It is a lot of connections and crimping etc. and I just wanted to make sure I didn't make a significant error along the way. Really appreciate the math check.

 

[DaveInDenver]

I don't see whether you mentioned it, you do see the same voltage at the battery and PowerPole terminals with no load, right?

 

[cruxarche]

I don't see whether you mentioned it, you do see the same voltage at the battery and PowerPole terminals with no load, right?

Yes. Exactly the same.

 

[DaveInDenver]

Yes. Exactly the same.

I wouldn't be concerned then between that and what looks like expected values and fine workmanship.

 

[cruxarche]

I wouldn't be concerned then between that and what looks like expected values and fine workmanship.

Thanks. And thanks!

 

[DaveInDenver]

Out of curiosity what size fuse did you use for that PowerPole circuit?

 

[cruxarche]

Since those two powerpole connectors will only be used for low current applications I daisy chained them together and they share a 20 amp fuse.

In the pic of the front of the box you can see I have another set of powerpoles which serve as inputs for the DC/DC charger and solar charge controller. The SB50 on the front is for higher amperage needs. It will also serve as an input ( 26 amps) for shore charging with my noco. That particular port is not complete as you can see. I still need to patch up the hole which is a sliver too wide...