Solar Panels Voltage much lower at MPPT than at panels

Hooked my panels up to my system today and tried to get several readings over the course of the day with my multimeter and MPPT.

At the multimeter I seem to get slightly over 20V on each panel (at the peak) and around 5.5a (also peak) which seems to line up with the 110w rating. At my MPPT, I'm getting only around 15-16V each. The amps seem to hold true on the MPPT.

What's the reason behind this?

Ohm's law would have me believe that there's some resistance in the circuit between the panels and the MPPT. The only thing that's in between them is the solar hookup wire: 6mm2 (10awg), 3m (10ft), 84×0,3mm tin plated copper)

Could this really be it that the wire would be causing enough resistance to result in a loss of ~20-25W?

Panel Specs:

Panels: 2x 110W Flexible SunPower Cells

Vmp = 18.5v
Voc = 22.5v
Imp = 5.94a
Isc = 6.24a

Measure the solar panels at the wire just before it enters the mppt.

Sunny Days said:

Measure the solar panels at the wire just before it enters the mppt.

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I'll give that a try tomorrow thanks!

Are you using the same multimeter for both measurements, or using what the MPPT is reporting or something?

Either the cable is damaged/grossly undersized, or you have a poor connection somewhere.
Start measuring the voltage across cables/connections.

Aaron_85 said:

Are you using the same multimeter for both measurements, or using what the MPPT is reporting or something?

Either the cable is damaged/grossly undersized, or you have a poor connection somewhere.
Start measuring the voltage across cables/connections.

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The cable is definitely not undersized. It's a 6mm2 (10awg) solar cable for 6a...

I measured it both ways: with the multimeter at the MPPT and based on the MPPT reading as well.

The MPPT and multimeter readings at the MPPT are similar. However, so are the readings at the panels and at the end of the 3m cable once I connect it to the panels (but before I connect them to the MPPT).

The funny thing is when I first plug the panels in, the voltage jumps to what the multimeter reads (20.3V) and then drops back down to 15-16V. This is on a fully sunny day where the multimeter is reading 20.3V and around 6a at the panels.

loverofpeace said:

The cable is definitely not undersized. It's a 6mm2 (10awg) solar cable for 6a...

I measured it both ways: with the multimeter at the MPPT and based on the MPPT reading as well.

The MPPT and multimeter readings at the MPPT are similar. However, so are the readings at the panels and at the end of the 3m cable once I connect it to the panels (but before I connect them to the MPPT).

The funny thing is when I first plug the panels in, the voltage jumps to what the multimeter reads (20.3V) and then drops back down to 15-16V. This is on a fully sunny day where the multimeter is reading 20.3V and around 6a at the panels.

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This is what a SCC does.
It adjusts panel voltage to battery charge levels.
If your SCC is shunting down the panels, then it is a PWM controller.

Supervstech said:

This is what a SCC does.
It adjusts panel voltage to battery charge levels.
If your SCC is shunting down the panels, then it is a PWM controller.

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Sure I understand that but I'm running a Victron 100/20 MPPT. Losses this high don't make sense as it's rated at 98% peak efficiency.

loverofpeace said:

Sure I understand that but I'm running a Victron 100/20 MPPT. Losses this high don't make sense as it's rated at 98% peak efficiency.

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Is the battery mostly charged when you're doing this test?

The last bit of charging on a lead-acid battery is a trickle charge, so the charger will charge it very slow.

JoeyB said:

Is the battery mostly charged when you're doing this test?

The last bit of charging on a lead-acid battery is a trickle charge, so the charger will charge it very slow.

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Almost completely discharged but good point. I’ll give it a try with my Lifepo4 battery on Friday when I get my box together for it.

It’s victron? Then something is wrong. Either the batteries are shot, or there is a connection issue on the panels.
I would start tracing every connection and look for corrosion or signs of insulation warping...

Supervstech said:

It’s victron? Then something is wrong. Either the batteries are shot, or there is a connection issue on the panels.
I would start tracing every connection and look for corrosion or signs of insulation warping...

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I’ll give it a try with the lifepo4 battery on Friday and report back. I think you may be right about the Lead acid one being near the end of its life. It’s been in my van for 4 years and hasn’t been driven much in that time so thatd make sense.