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Battery Shelf. Parallel design, bus bar vs cable.

matt1309

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Aug 5, 2022
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Hi Folks.

I currently have 1 48v 304aH LiFePo4 and I'm looking to expand this to 3. However I'm struggling to plan out a clean/effective storage shelving system for them. (Before someone says, each will have overcurrent protections and disconnect per battery on shelf)

My main concern is paralleling additional 48v batteries and keeping cable lengths/resistance of each the same. I've seen many say the solution is bus bars however they seem expensive for the size I'm looking at so wanted to confirm a few queries/sizes

1. Say I get a bus bar 6mm thick and 40mm wide that implies surface area of 240mm2 correct? I'm still checking prices but if I can find 240mm2 cable for cheaper is there any disadvantage to that or is resistance the same? Say 3 or 4 x 70mm2 cable (this cable size is readily available in my area)

2. I've seem folk emphasis the importance of exact cable lengths in paralleled battery systems and my understanding is that this is to ensure that each battery provides the same energy. However how important is this if the batteries themselves are all going to have different internal resistances, ie how close should I be checking the resistances to be between each shelf?
Before considering the above I'd planned to parallel 3 48v 304ah batteries and was going to use 70mm2 cable to connect them (readily available in my area). I would be pulling no more than 150amp's across the connections however the inverter would be closer to battery 1 (ie top of the shelf) vs battery 3 at the bottom.

Plan was main bus bar on top shelf. Then run 70mm2 cable down side of shelving unit and have a bus bar per shelf where each battery connects and where a new 70mm2 cable continues down to next shelf. However as noted in this scenario battery 1 would only be 200mm of 70mm2 cable away from main bus bar but battery 3 is closer to 1000mm of 70mm2.

3. Here are the options of connecting each shelf that I'm deciding between and my concerns/questions with each one:

Super premium: big bus bars (10mm thick, 40mm wide), very very pricey, but i imagine so low resistances that I can disregard differing distances between batteries.
Premium: Get bus bars (6mm thick and 40mm wide), very pricey, similar to above just lesser.
multiple 70mm2 cable: Use multiple cables to parallel from bus bar per shelf each bus bar connecting to one battery. (3-4 70mm cables connecting each bus bar). Is it necessary to use multiple?
one 70mm2: It doesn't matter that much and the resistance differences will be negligible on 70mm2 anyway?, Is this enough/sound logic.


Any guidance will be greatly appreciated.
Thanks,
 
Please review this article which is a part of a series I'm assembling. There's a section on shelf/rack vs flat layout and other pros/cons in parallel design. Longish but valuable nevertheless.

 
Personally, I don't worry about it. They will all give and receive current. It's never going to be perfectly even. And solar loads are so low, it doesn't matter.
 
Thanks both, I'll check out that article.

@Steve_S I couldn't spot the section of shelving, is that the right link or has that portion yet to be released?

I can see the portion on elevation and temperature differences. However I was more concerned with the connections/resistance in a stacked configuration. Or are you saying in that article that those connections if reasonably sized will result in negligible resistance difference if in a chain/stacked configuration and therefore things like differing temperature are more of an impact?
 
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It might not be the best but ive made my own bus bars out of old hvac line sets. even with a full load they don't even get warm. i use them for connecting batteries together and my connection to the breaker. lots cheaper than cable or a refab buss bar.
 
I like the Victron Power Ins. 1000A rated has both positive and negative bus bars and the can be fused
 

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