Solar Panel Voltage Question

I have my two 290 Watt panels installed in parallel on my RV. They have Vpmax of 32.4V and Ipmax of 8.97 amps. They are then connected to a Victron Smartsolar Controller (100/50) and then into Lifepo4 400 AH batteries. During testing today they were bouncing around in Voltage of 24V to 32V. There was no shading on either panel other than cloud cover on/off during the test. Is that normal or is there a bad connection? The Controller was in Bulk charge mode at the time.

sailcan88 said:

There was no shading on either panel other than cloud cover on/off during the test.

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Did you notice a correlation between the clouds covering the sun and the lower voltage? I would be surprised if they were not directly related.

MisterSandals said:

Did you notice a correlation between the clouds covering the sun and the lower voltage? I would be surprised if they were not directly related.

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Cloud cover on/of affects the current and not the voltage of the panels. And the battery would have held the voltage stable (CC, bulk stage) and the controller should have held the voltage stable during absorb.

My guess is that this fluctuation happened during absorb stage due to poor PWM algorithm. Before other point out this is an MPPT controller, the absorb stage is always PWM.

The fluctuations are happening almost instantaneously. ie. in full sun or in partial cloud.

sailcan88 said:

The fluctuations are happening almost instantaneously. ie. in full sun or in partial cloud.

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the batteries are in absorb when these fluctuations happen, aren't they? If so, check if Victron has addressed this in a firmware fix.

sailcan88 said:

The Controller was in Bulk charge mode at the time.

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What is the interval of the bounce? In MPPT mode the Victron controllers periodically do a full sweep to avoid the knee points that other controllers get stuck on.

fat_old_sun said:

Before other point out this is an MPPT controller, the absorb stage is always PWM.

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Nope. It is more about the watts from the panels being able to satisfy the load. If there is still sufficient load the controller will still be MPPTing in an attempt to hold the voltage at the absorb voltage point. Only once the load can no longer absorb all the current the controller makes available will it step off MPP since it can no longer hold that point due to, well, insufficient load.

When it steps off MPP due to light loading, it is just being a buck regulator. Whilst this type of regulator can be referred to as PWM it is not PWM in the sense that most people think of when applied to SCCs where PWM implies pulling the array down to battery voltage.

I wonder if this could have something to do with an intermittently faulty internal connection that is causing one of the strings to be bypassed. That could explain the oscillation between two distinct voltages maybe.

gnubie said:

Nope. It is more about the watts from the panels being able to satisfy the load. .....Whilst this type of regulator can be referred to as PWM it is not PWM in the sense that most people think of when applied to SCCs where PWM implies pulling the array down to battery voltage.

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I agree on both points. I could/should have expressed my response better.

However, there's something else odd about the whole thing. Why did the BMS allow the voltage to go to 32V? It should have isolated the battery at about 29.5V The 2nd odd thing is that a current of less than 20 Amps can't jerk around the voltage of a partially discharged 400 Ah LFP pack. The charging rate amounts to 0.05C.

OP, are you 100% confident that the system was in bulk?

I guess I could have been a little bit clearer. The voltage swings are as reported by the Victron App from the input of the solar panels. The output to the batteries is 13.5 V at about 20 amps. I will get up on the roof to test the individual panels when the sun comes out next.

sailcan88 said:

I guess I could have been a little bit clearer. The voltage swings are as reported by the Victron App from the input of the solar panels. The output to the batteries is 13.5 V at about 20 amps. I will get up on the roof to test the individual panels when the sun comes out next.

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Then there's no issue at all! The controller is actively hunting for the sweet spot on the solar side so that it can deliver the most power to the batteries. If the panels are not very close in their individual specs, there can be multiple max power points - pseudo max power points - and the controller will have a tough time setting in one of those.