Solar output voltage dropping 50% when connected to battery bank

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larrikin asked Jun 28, '24

Solar output voltage dropping 50% when connected to battery bank

I have 2 go power flex 190w solar panels connected in series to a smart solar MPPT 100-20 controller and then to 4-6volt agm 220ah batteries in series and parallel. All was working ok until recently when the solar voltage dropped to 21 volts. While troubleshooting I disconnected the controller leads from the battery bank and the voltage from the panels returned to it's normal 45ish volts. Does anyone have any ideas what could cause this. I am about to borrow a load tester for the batteries to see if the bank is the issue.

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Answer 1 · 2024-06-28T20:14:53Z

pwfarnell answered · Jun 28 at 08:14 PM

A poor connection within the solar panel system, it could be a loose cable in the controller, a corroded MC4 connector, a damaged cable or damaged solar panel causing a high resistance. When the solar panels are producing power there will be a voltage drop over this poor connection due to the resistance, when you are drawing no power (battery disconnected) tjere is no current flow so no voltage drop, the connection is poor but is still good enough to let enough current through to read the full voltage.

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Answer 2 · 2024-06-28T23:23:16Z

larrikin answered · Jun 28 at 11:23 PM

Thanks for that information, I will do a cleaning of the terminals and see if it makes a difference. I have also reconnected the leads at the controller they are tight. Do you think that this could cause an issue with my battery bank to discharge as well? The batteries all checked out ok with a load test, no voltage drop at all, but they went from a full charge to 10.5 volts in about 2 hours last week only running the fridge (6amps) and a freezer (10amps)

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pwfarnell commented · Jun 29 at 08:12 AM

If the batteries are degraded, and it sounds like they may be the panel voltage should still be up around the normal voltage regardless if they are taking charge or not. More.info might be helpful such as battery voltage and current as well as panel voltage and current. I would also question if your install is large enough, you have 440Ah at 12V but can only charge at 20A max = 280W, have you been constantly under charging your batteries hence they have sulphated.

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