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BlueSolar MPPT 75/10: Issues error #33 (over voltage) prematurely

Presently I experiment with a small island net: Two used solar modules 24 V/70 W in series and two 12 V/26 Ah lead-gel batteries in patrallel. I want to operate my ham radio station from that.

Over the summer my BlueSolar MPPT 75/10 worked very well. But recently, since it got colder, I quite often get #33 errors. These errors come while the input voltage is hardly higher than 70 V. After the charger has turned off I get an open-circuit voltage in the 77-79 V range measured by the BlueSolar MPPT 75/10. My DVM shows a few 100 mV more.

Using a scope I found positive and negative voltage spikes with some 5 V peak amplitude at the PV inputs every 30 µs. It appears that the over-voltage protection is triggered by these spikes of the voltage converter.

As a first measure I have added a 1µF/100 V capacitor to the PV input. Take that literally: That RF-capable capacitor (no electrolytic) is directly at the connector.

On VHF/UHF I do not "hear" the solar charger. But I expect quite some EMC problems when I switch to shortwave. I will add some protection circuitry to avoid this.

But the most annoying problem still remains: The #33 error appears long before the input voltage reaches the specified 75 V.

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1 Answer
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hello Allibaba,

You are operating that unit outside of its design. This is very likely to cause damage to it.

The MPPT PV input limit is not an operating voltage limit, it is the maximum possible voltage before damage. Open circuit PV voltage above this will cause damage to the unit.

The manual for that unit is very clear on this point.

Maximum open circuit PV voltage: 75V
For example:
12V battery and mono- or polycristalline panels connected to a 75V controller. Minimum number of cells in series: 36 (12V panel).
Recommended number of cells for highest controller efficiency: 72
(2x 12V panel in series or 1x 24V panel).
Maximum: 108 cells (3x 12V panel in series).
Remark: at low temperature the open circuit voltage of a 108 cell array may exceed 75V depending on local conditions and cell specifications. In that case the number of cells in series must be reduced.

Also in the specification datasheet.

Maximum PV open circuit voltage - 75V

You must reconfigure that system to reduce the PV panel voltage to within the allowed limits.

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