Plug Bug: Battery Bus Bars

Electric Bug

There needs to be a way to connect one 3.2v Thundersky battery to another to form the battery pack. I’ll have a total of 48 cells in two separate packs. Apparently some batteries come with interconnects, and others don’t. Evolve Electrics sells them separate for $4.50 each. Multiply it by 42 (the last two don’t need a connection as it will be connected via 2/0 wire) and you get $189. In addition, they may not be quite the right size that I needed, as I was planning on a crazy configuration. So, I was going to have to make some of them anyways, and decided to just make them all and save some bucks.

I found the volt 914 project that did a cool job on making them here. Thanks Ross for posting that information! Ross used 7 layers of 0.020” copper to make his interconnects. I also talked to Travis (who also made an EV bug) and he also went with the layered approach and used 5 layers of 24 gauge sheet (~.026 thickness). People use layers with a slight kink in the middle to allow them to stretch out when the batteries (possibly) swell up.

So, how thick do I need?

The cross sectional diameter of 2/0 wire is 0.03648”. Times pi that is ~0.115 in^2 (I’m not sure how Ross got 0.105 in^2 — maybe it was a typo). I like the idea of using 24 gauge copper to make the bars — but I might use 6 layers since: 0.026” (thickness) * 6 (layers) * 0.j75” (width) = 0.117 in^2 — definitely bigger than 2/0 wire. Or, I could use 1” wide and 5 layers, which might be easier (I think it would have been).

Or, I could just use solid copper bar stock and avoid cutting out a bunch of small pieces. I liked that idea the best, even though it might be “extra rigid”. The fear of using solid bar stock is that the cells will swell a bit and might tear apart if the bus bars don’t have some give in them. However, people use solid aluminum bus bars and don’t seem to have a problem with tearing the cells apart (as far as I can tell), and swelling is only a problem if you don’t compress your cells good enough, or abuse them by running them past their constraints. I hope to not do that, so I’m using solid stock. I also think my cells are quite well contained. If they do end up moving then I’ll have to readdress this.

I bought some 1/8″ by 1″ metal copper bar stock from onlinemetals.com. Two 96″ pieces and one 48″ piece for $114 (including shipping).

I also had to buy hardware — I stupidly thought these came with the cells, but instead they come with the bus bars. I bought the following from Bolt Depot:

Product #4815
Metric lock washers, Stainless A-2 (18-8), 8mm
Quantity: 1 box of 100
Price: $5.81
Subtotal: $5.81

Product #4778
Metric hex nuts, Stainless A-2 (18-8), 8mm x 1.25
Quantity: 1 box of 100
Price: $9.85
Subtotal: $9.85

Product #5024
Socket head screws, Alloy steel, 1/4-20 x 1/2
Quantity: 1 box of 100
Price: $5.89
Subtotal: $5.89

Product #5027
Socket head screws, Alloy steel, 1/4-20 x 1
Quantity: 1 box of 100
Price: $8.00
Subtotal: $8.00

Product #8749
Metric socket set screw cup point, Stainless A-2 (18-8), 8mm x 1.25 x 30mm
Quantity: 1 box of 100
Price: $30.19
Subtotal: $30.19

Another $76. So, it would have been cheaper to just buy the bus bars in the first place. Live and learn! I think my solid copper bars will conduct better than what the Thundersky ones do.

corbin

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corbin
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Ed Despard
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Ed Despard

Those aluminum ones don’t look plated. The worry with aluminum conductors is oxidation, which then makes them non conductive. It is safest to nickel plate the Al parts that will be used as bus bars in any application.

corbin
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Good point, ed! For my copper bus bars, i did put lots of noalox on the ss screws to help with any problems. copper should have less oxidation issues (i hope!)

Peter Thompson
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Peter Thompson

Hi Corbin,

I also used copper bars, as it is really easy to work with. Since the batteries are all tied together and don’t move I didn’t think it would be a problem. I’m sure the 1/4″ thick is overkill, but when dealing with this much power it’s a nice safety factor. :)

delicacy
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delicacy

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