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hlwerschner avatar image

MPPT 75 / 15 shows higher battery voltage

I have installed a MPPT 75/15 with 2 solar panels attached (in series 24V) and for battery charging I added 2 * 12V AGM batteries in series = 24V. In addition I have a Phoenix inverter 24/375 connected to the batteries to power my garage door motor. The installation went fine and the door opening was OK also for 2 days. This morning I did control the door mechanism but everything was dead. The inverter shows a slow blinking red light and green led blinking fast (2 times). This should indicate a low input voltage alarm at some point in time. I checked the battery and it shows 21.8 V (for both in series) which is low. The solar panel delivered voltage is OK, all cables are properly set and the MPPT battery connectors show also 21.8V and displayed steady green led -> float condition ?! When I call up the connect app on my iPad, the battery state is reported as 27.8 V instead of the actual voltage on the connector? When I disconnect the battery, the app continues to show that higher voltage. Can it be that the app does not reflect the physical battery voltage but displays the MPPT charager output that it "wants" to deliver to the battery? So, how do I see the actual battery state with that app and how can I resolve the problem with the inverter showing "low input" after every power off/on test. The data sheet says that low voltage is around 18.4V which is lower than the current battery voltage. Are the batteries dead and what could have been the cause when there should have been a shutdown of the inverter when it might be gone below 18.4V? I currently have no direct VE.direct connect to the inverter to "look into its settings". Btw: There are no other power connections to the MPPT directly. Its only the inverter for the door (chamberlain MV700 which should use around 130W for the opening/closing time). I have tested the door opening at least 15-20 times without problems and the MPPT green light always stayed on.

Battery Protect
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2 Answers
hlwerschner avatar image
hlwerschner answered ·

Hi, yes the fuse was blown. I have replaced it and have to wait until sunrise for any changes / charging. But beyond that I am quite confused about this fuse and the possible reasons for it been blown. As I said, there is nothing connected to the charges load exit, I have 2 * 100W solar panels which will never (?) input a current of 20A into the charger, therefore the maximum battery loading current can only be something like 200W / 30V ~~> 6 A. The Phoenix inverter is directly connected to the battery with own cables (for max 375 W output at 230V) and thats it. So the question is: what causes a current of > 20A to blow that fuse before any protection in the charger / inverter steps in? I am quite confused and I have studied all docs / datasheets here and can not find explanations?

I will wait until tommorrow morning to see if the charging of the battery is resumed, after insertion of the fuse, the app correctly displayed the actula battery voltage = 21.8 V. Perhaps the charger is not dead (as mentioned in a different thread here in the forum). Also there is hope that the inverter will drop its alarm if the battery goes over 22.2 V ...

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I'm not quite sure what causes the fuse to blow but it seems to be a very common problem because there are many questions about that in the community.

Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

Check the fuse at the bottom of the MPPT next to the connection terminals.

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