Add 36V Battery Block to 48V Battery System?

CarsonGrey

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May 23, 2020
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Hi,

ChrisD gave me a hint last year how he combined a 36V Battery to the 48V Main Battery System.
The short description is using a dc stepup module for loading the 48V Battery with the 36V one.
I thought about it and wonder how to load that 36V battery again.
I don't think that connection a step up in one direction and a step down module in the other direction is the right way, at least not without switching between them.omegle xender
So another Idea was to find a bidirectional converter module, but up to now I did not find one fitting the needs of 36V to 48 Volt.
Does anyone have an idea of such an equipment that can deliver maybe 1500W?
Or any other solution that fits the needs to combine 36V and 48 Volt Batteries?

I also asked Chris via PM for a description of his system, maybe that already would fullfill the requirements, but I'm still curious about any other idea.

thanks in advance
 
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Korishan

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Jan 7, 2017
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I vaguely remember something about this. It is not recommended in any way to mix voltages like that. Not only are you causing an voltage/amperage imbalance of the strings, you would also be creating a possible over-voltage situation if the buck converter half fails for some reason in the wrong direction.
As Overmind says, put a 12V battery in series to bring the system to 48V status.

The other option is to have 2 completely independent systems. One system runs on 36V, and the other on 48V. But this requires two inverters. It would also require 2 inverters that can work together on synchronizing the outputs.

For the amount of wattage you would need to be available to you, it would probably be more cost effective to go the 2nd system route rather than buying a very beefy buck/boost converter. Those things get crazy expensive really fast when scaling up. You would also want one that is absolutely guaranteed to work, so no cheap chinesium knock-offs for sure
 

daromer

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Bi directional DC dc is the name of the thing you look for or have 2 converters you switch on and of based on condition. Fully doable but its easier just to rebuild the pack to higher voltage in one way or another.
 

ajw22

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Nov 16, 2018
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You need 2 separate BMSs, one for the 48v battery and another on the 36v battery, to prevent over(dis)charging each.

You activate 48v -> 36v converter only when the sun is shining AND the 48v cells are starting to go above 3.9v.
You activate 36v -> 48v converter only when the sun is not shining and 48v cells are starting to go under 3.7v.

There are limitless possibilities to optimize the system using weather forecasts, power usage patterns, etc.

Depends on your total system size, but you probably only need a 300W converter or so, since you start can the transfers early (3.9v, 3.7v), long before the 48v gets full/empty.

PS. I faced a similar problem. I took apart the 36v packs and added the cells to the 48v battery. Much simpler and better in the long run. Sold the 36v devices.
 
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