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I am working on my mountain cabin and am wiring it with some new electricity. In the long run, I want to switch to only LiPo based batteries but for now I still want to keep my old Pb batteries as a backup. I have two new and very strong 400 W panels which I use to charge my LiPo pack and I use a separate charger for my old Pb pack since connecting panels of different power can significantly decrease the power output of the stronger panels.

What I want to achieve somehow is that I can use the peak power of the five batteries all together when I need it for example to heat water, solder, run my 3D printer etc. I know that connecting them is not a good idea because of the different discharge characteristics and voltage levels. I could just put a simple switch and use either or but of course would loose my maximum power output.

Which circuit could I design to put at the [?] that somehow combines the output of the two sides? Please note that each battery back can deliver up to more or less 100 A so anything placed where the [?] is, needs to sustain high currents.

Overview over circuit

So of course, any form of parallel or series connection is off the table because the batteries have different voltage and current specs. Would it be sufficient to run the positive wire through diodes for each pack to avoid back-flow of current to the other battery pack?


Edit: my current idea is to connect a buck converter to the LiPo battery to pull down the voltage to around 12 V before I merge it with the Pb battery. The Pb battery behaves very soft, i.e. it will have a decreasing voltage on high load. Through this, in the moment a high load is operating, both batteries can start delivering power.

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Would it be sufficient to run the positive wire through diodes for each pack to avoid back-flow of current to the other battery pack?

That, and a fuse in case the diode fails short-circuit, would be sufficient to make the connection safe. However, because the two packs have different voltage ranges and capacities, it would not share the load evenly between them. That would be adequate but not optimal for total capacity, but not for high power draw because there would be long periods where only one pack is being used, and the other is either full or empty. (And you would need separate under-voltage protection for each pack.)

A more efficient and flexible configuration would be two separate DC-AC converters (inverters) with their outputs wired in parallel, communicating to share the load. (This is a specific feature that some have.) With two inverters, each one can independently convert the voltage available from its associated battery bank to the output 120 V, and each separately configured for maximum current and minimum battery voltage.

(I can't say more because I haven't researched such systems — I just know that they do exist.)

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