24V 50Ah Lithium Ion Pack

       I got a bunch of 18650 cells and some BMS boards. What to do with them?

Around 200pcs of 18650 cells from NOS laptop packs. Bought some plastic holders online.

Three different BMS boards. 3S 20A, 6S 12A and 7S 21A.

I plan to do it "Tesla" style where each cell is fused for safety. I got a bunch of this precut stranded wire (bag at background)

Built a 4S5P pack for practice and a mishap with the solder lead shorted on some of the cells. Several of the fuse links popped and prevented further damage!

A working 4S5P pack. just needs a case to hold everything.

       Onto the big one! I plan to make a 7S20P cell using LG 18650 D1 cells. These are rated at 3000mAh when charged at 4.35V but I plan to use it to 4.20V only for longer life but capacity drops to 2500mAh.

Here is the pack all wired up.

The BMS is mounted using double sided tape.

This BMS has an onboard balancer so there is no need for an external one.

Closeup of the cell connections. This is done using the iron set to 450degC so that it heats the joint as fast as possible then once the joint has been made, remove the iron to minimize heat soak of the cells.

The bus bars are copper tape with adhesive backing.

Series connection via the red wires. Yes, I have tons of those and I had to use them!

Now to look for, or make a suitable case.

... And an application.
It should work well with a charge controller set to 24V lead acid.


       19 May 2017:

I had this on the backburner for so long, but I decided to try and use this on the house lighting circuits. Time to make the pack better.

Started by making a frame to hold the cells better.

The aluminum channels are bound together with heavy duty zipties.

Mounted the terminal block and BMS board onto a 6mm thick phenolic board.

High current connections will be made using these braid shield taken from coaxes. They will be insulated using shrink tubing.

This is a 7S pack and six of the groups were at 4.12 to 4.15V but one was 1.7V. There is a leaky cell somewhere so I charged it with the IMAX.

Using the FLIR, there was no cell getting warm so the self discharge was low. It was left on the side for 9 months without any charging. It did take about 53Ah to fill up so capacity should be ok.

Connections of the BMS on one side.

And the other side

This is not yet 100% complete as I plan to add covers on the sides. Not yet decided if I should use insulating cardboard or clear polycarbonate sheet on standoffs.

Thick braided connections.

I kept the middle pin unconnected incase I find a use for it in the future.

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