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progbes asked

Voltage not matching and MPPT cycling

Setup: MPPT 100/30 (420 watts solar), Multiplus 2000 inverter\charger, Smart BMV-712. Batteries are 600Ah AGM. All components and batteries are new. MPPT is set in position 2 for AGMs.

The BMV shunt is installed at the electrical panel (farthest from battery). The MPPT is wired to the battery side of the battery switch, and closer to the battery.


Question 1: Should the voltage measured on the solar controller be the same or close to that measured by the BMV? I am seeing a difference of .5v - 1v (MPPT is higher) when the controller is in sun. In the dark they differ by .02 volts.

Question 2: When the Inverter is on it seems like the MPPT is fluctuating and turning on\off - i.e. The wattage goes up to 200W then down to 0 in a matter of seconds. The BMV is reading 100% SOC, the current draw at 200W is negative, and the MPPT is saying Absorption. Then it almost immediately cycles down to almost zero watts, the current draw at the BMV goes positive, and the cycle repeats. Is this on\off normal? If I turn the Inverter off it seems like the MPPT stabilizes and goes into Float mode, though the voltage between the BMV and MPPT still differ. Does this sound normal as the current draw is almost nil once the Inverter is turned off?

note: I didn't see this MPPT cycling with my old inverter (Prosine 2000) that was recently replaced with the Multiplus.


MPPT Controllers
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Alexandra avatar image Alexandra ♦ commented ·
Cabke size? It sounds like a high resistance connection. So small cables or loose terminal.

If the inverter and the mppt are not set to the same charge voltages, weird things also happen.


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2 Answers
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progbes answered ·

10AWG to and from the MPPT with connection about 6 ft away.

The marina electrician who is doing the install measured voltages at battery switch and at panel and both matched what the BMV and MPPT were saying. Does it make sense that when the MPPT is in Bulk\Absorption\Float the battery voltage should read higher because it is charging, similar to when the Charger is getting shore power? should BMV voltage match MPPT voltage at all times?

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progbes answered ·

We found the problem. The cheezie 30A breaker that I purchased was hot to the touch, so resistance was in play. We replaced with a 40A marine grade breaker and all is good. Thanks for the advice!

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