# question

## MPPT 75|15 Bluetooth data question

I am currently have 2 100 watt moncrystaline panels connected to the PV side of my Victron MPPT 75|15 charge controller. I have 2 100 ah batteries (series connected) connected to the charge controller. I have a cell modem and an embedded processor connected to the load side of the charge controller. In general, the problem that experiencing is that the battery voltage is steadily dropping and the system is turning off, but based on what I think are good calculations, the system pv/battery system are over designed for the application.

1.) Using the BT Dongle and the Victron app. In the downloadable data, is it correct that the yield represents the amount of energy that was collected from the panel and the consumption is the amount of energy utilized by the electronics connected to the load? If this is the case, then I am a bit stumped. In my analysis, I assume that min. battery voltage is the battery Voc (night time, no charging) My energy consumption is a consistently 50-70 wh. I notice that there are several instances where my daily yield is close to, but still above the consumption on which days battery Voc drops. How can the battery voltage drop if my yield is greater than the consumption?

2.) I am considering to keep the solar panels in series (24 VDC), but changing the batteries to a parallel connection (12v). My rational is that The batteries will start charging faster because the solar panels will only need to get to 17V (12+5), instead of 29V (24+5). Is this rationale reasonable, are there any downsides to it?

Thanks for any information you can provide.

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You have to factor in the round-trip efficiency. Charging a battery is not 100% efficient. Energy is lost due to the internal resistance of the battery plus energy required for the chemical processes inside the battery. You'll need a greater solar yield than consuption to "break even". Maybe 25% more yield over consumption.

Your rationale is correct. If your series panel Voltage is too close to the battery terminal Voltage then you'll suffer from later starts and even poor charge performance.
If you can change to 12V battery configuration you'll see much better results. Very few downsides.
MPPT will loose a very small ammount of efficiency due to heat, but not worth worrying about. The other benefits will be be much greater.

You only need to worry about keeping the PV Voltage below 75V, your configuration will not get near that Voltage, so you'll be fine.

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