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wsconner avatar image
wsconner asked

75/15 mppt not controlling voltage?

I just installed the BT version of this controller with a 110watt panel. I updated the firmware. I'm charging a three battery bank of Northstar 31AGMs and set the bulk charge voltage to 14.7v and the float to 13.6v. The controller seemed to be workingas I was getting 14.5V and ~2amps (cloudy), but when it went from bulk (solid green) to absorption (solid orange), the battery were showing 15V+s (?). I would have thought the controller would have maintained voltage at 13.6v?

I took readings directly from the battery and the software app, and both had the battery voltage at 15v+. Any ideas are appreciated. thx.

MPPT Controllers
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klim8skeptic avatar image klim8skeptic ♦ commented ·

Temperature compensation?

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wsconner avatar image wsconner klim8skeptic ♦ commented ·

not sure I understand.. I am not using any temp monitoring..?

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klim8skeptic avatar image klim8skeptic ♦ wsconner commented ·

Read page 5 & 10 of the manual.

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wsconner avatar image wsconner klim8skeptic ♦ commented ·

yes and thanks for that resource. All I had was the spec sheet from Northstar. Now I understand..appreciate the help.

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Jack Peters avatar image Jack Peters commented ·

Check the battery voltage setting is 12V and not 24V.

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wsconner avatar image wsconner Jack Peters commented ·

I'll double check. When I set up the app, I made sure it was set to 12V along with the bulk and absorption rates for an AGM battery. Regaring ambient temperature, if I am understanding the concept correctly, I don't this could be the case. The 75/15 controller is mounted a few feet from the battery bank. The temp is ~50deg F. The panel however is on deck and the boat is shrinkwrapped in clear plastic (green house action), so the above deck temp was probably 70+ deg F yesterday. But again, sounds like ambient temp applies to the controller unit and the batteries.

From what I've read here..sounds like the software can be a bit buggy, so just curious if there are release notes or know defect lists anywhere?

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klim8skeptic avatar image klim8skeptic ♦ wsconner commented ·

"The 75/15 controller is mounted a few feet from the battery bank. The temp is ~50deg F. "

Note, the mppt will measure it's temp at sunrise, and apply that temp for compensation for the rest of the day.

Assuming you have not altered the temp compensation value (16mV/c), 50f would equal a 0.24v rise in battery target voltage.

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wsconner avatar image wsconner klim8skeptic ♦ commented ·

ok, thanks very much. so the 16mV value in the compensation settings is the max bulk charge voltage that accounts for temp differences? If so, if I lower that value to my batteries recommend charge voltage (14.7) I'd effectively negate the purpose of the setting?

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klim8skeptic avatar image klim8skeptic ♦ wsconner commented ·

Does this match your battery? Northstar.

Please do your own calculations.

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1 Answer
Stefanie (Victron Energy Staff) avatar image
Stefanie (Victron Energy Staff) answered ·

Hi @wsconner,

the MPPT controller has a temperature sensor built in. As far as I remember temperature compensation is set on by default at -16.2mV/°C. As soon as ambient temperature inside the housing of the MPPT drops below 25°C, temperature compensation is in effect. That might explain your higher voltage.

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wsconner avatar image wsconner commented ·

ok, thanks very much. so the 16mV value in the compensation settings is the max bulk charge voltage that accounts for temp differences? If so, if I lower that value to my batteries recommend charge voltage (14.7) I'd effectively negate the purpose of the setting?

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ wsconner commented ·

Exactly. When you set temperature compensation offset to 0mV/°C, there is no temp compensation and the feature is disabled.

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wsconner avatar image wsconner Stefanie (Victron Energy Staff) ♦♦ commented ·

Ok, makes sense now. appreciate the help. thanks.

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