4P10S Pack - w/o Welded Tabs

[Geekineer]

Hi all,

I thought I'd share - and hope to get some feedback on a pack I'm building.
Nothing against welding nickel tabs on 18650 cells - they work reliably for me as well.
But, I wanted to experiment with connecting cells - no welding or soldering.

Here's the design.
See attached photo.
1. Groups of (4) 18650 cells are connected in parallel using a double sided PCB. (I'm using Samsung 35Es).
Each board connects two 4 cell groups of the
in series. Plus on one group to Minus on the other.
(9) boards are required for a 10S pack. I'm using plain FR4 material with
1oz copper on each side. Boards are cut on a fine tooth carbide chop saw, and
'drilled' out on my small CNC mill.
2, Each of the 8 cell contact holes in the board are filled with a brass eyelet, crimped, then soldered
to make good electrical and mechanical connection from one side of the board to the other. (no plated
through boards required.)
The head side of the eyelets provides a raised bump to make contact with the center of each cell on either the
positive or negatives ends. I've tinned the head of the eyelets to prevent oxidation.
3. Each group of 4 cells is held in a controlled square pattern - matching the PCB eyelet pattern, by inserting
two 3D printed spacers in the center gap. See the grey pieces in the photo. This locks together the 2X2 array when tightly
taped.
4. The PCB, cell, spacer, assembly is clamped together with 8-32 all thread rods and nuts. TWO NYLON SHOULDER WASHERS
are used, to insulated the 8-32 rod from the PCB connections. Deflection of the PCB and tension in the approx. 3" rod provide
the 'spring' to assure good 4 cell conctact (so far so good).

5. ADVANTAGES: Cell groups or individual cells easily replaced. Connections from each 4 cell group to BMS is a simple solder
connection. And pack power connections are equally simple soldered cables (12ga stranded). All materials are readily available and low cost. (you could get a mail order house to make the PCB but this method doesn't need etching, solder mask, silkscreen, or plated through holes. )
6. DISADVANTAGES: temperature, corrosion, stability long term TBD.....

Please weigh in with any thoughts - I might try copper rivets with the next one instead of brass eyelets.

 

[IdleMind]

Good idea and execution. Will you be adding a balance harness. I suppose you could do 6p by inserting an extra threaded rod. You could use 2-3/4 inch long 8/32 screws instead of cutting threaded rods to length. I'm going to keep watching to see how the whole pack turns out.

 

[Geekineer]

Thanks for your thoughts - Yes you could do the 6P as you suggest - I'd really like to look into getting 26650 cells and just be 'done with it". I will be adding 10 soldered connections for balance charger.

Or another version is to do round copper discs over just 4 cells, long 4 point 'star' spacer down the center core - then thread 8-32 into each end of the insulating spacer. Now you have threaded stud on each end of a '4 pack' to interconnect with wires/ring terminals....clean but probably more work than what I've got.....

 

[Hillhater]

Very neat and simple idea.
Is there a reason you want to ensure electrical contact on the outside of the boards ?

.....Each of the 8 cell contact holes in the board are filled with a brass eyelet, crimped, then soldered
to make good electrical and mechanical connection from one side of the board to the other. (no plated
through boards required.)

Why not use a single sided conductor board for electrical security ?
Also, i have previously seen a similar system using nylon threaded rods instead to ensure insulation.

 

[Geekineer]

Thanks for the reply - double sided board used to help distribute the load - if you use 2 oz. copper then single sided should be OK. But I'm pull about 20 amps and I needed the extra conductivity. Only had 1 oz stock on hand

I like the steel threaded rod better since nylon tends to 'cold flow' over time - I'm counting on some yield of the steel rod as part of the end cap tension - also better if things get 'hot'.