question

Fabian Dumitru avatar image
Fabian Dumitru asked

How to charge with 4 solar panels a 12v and 36v battery

Hi guys,

I know this subject is present in other threads but I still couldn't figured out how to wire my setup on my boat.

I bought 4 170W solar panels with Vmp 20.21V and 8.4A.

I need to be able to charge in my boat a 300Ah 12v LifePo4 battery and a 100AH 36v LifePo4.
I was thinking at a setup like this:

===Case1===

1. Wire 2 solar panels in series to a Victron mppt that can be set up to 36V and use those 2 for charging the 36v 100Ah battery.
2. Wire 2 solar panels in parallel to charge the 12 300Ah battery.

* In this case I think I can charge both my batteries in the same time.

-=====Case 2====
Use a switch that can commute to charge either the 12v or the 36v battery
1. Commute on position 1 and Charge the 12v battery with all 4 solar panels in parallel.
2. Commute on position 2 and charge the 36v battery with all 4 solar panels in series.

ofc, i will use two separately mppts for each case.

Question1: In the first place, do you think I can achive what I want?
Question2: What setup do you recommend?
Question3: In none of my ideas are good, what can you suggest to do to be able to have 2 batteries on my boat and be able to charge them?

Thank youscreenshot-2023-09-23-095703.png

battery charging
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2 Answers
klim8skeptic avatar image
klim8skeptic answered ·

You have done yourself a disservice buying those panels.

You need 3 panels in series to have enough voltage charge your 36v battery.

Use the mppt calculator to check your design.

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Fabian Dumitru avatar image Fabian Dumitru commented ·
Can you please explain me why?

I can still return them.


I need to be able to charge and fit on my boat both a 36v battery as well as 12v battery.

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kevgermany avatar image kevgermany ♦♦ Fabian Dumitru commented ·

MPPT will not start to charge until panel voltage is battery voltage + 5. Your battery will have an actual voltage of about 42V, so you need about 48-49V from the panels to start charging. You don't state Voc, but it's unlikely that the panels will work as a pair on your battery.

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Michelle Konzack avatar image
Michelle Konzack answered ·

If you have already the panels, the best option are two chargers which support 4 modules in series (probably a 150V type, e.G. SmartSolar MPPT 150/70) and switch the solarpanels between the two chargers.

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