Wednesday, March 15, 2017

An Update on My eBike Project

I mentioned in the last post that I bought 120 pieces of used 18650 cells from recycled laptop batteries to replace my exhausted 14S 8Ah LiPo cells from Hobbyking.

This post is meant to give some updates on the progress.

Commenced by sorting the cells by initial voltage that it holds.

Found 8 bins of them. The one in the box were under 1V.

Built a 1A constant current load to simplify the process of measuring internal-resistance of the cells.

Another view of the constant current load. Schematic is visible above the board. It was designed to be powered through USB 5V DC supply.

Most of the cells are around 200milliohms. All matched internal resistance cells grouped together in a pack of 12. This will make it a 12P cell.

This is the pack that I used (4S7P) while working on the rest. The blue cells are from my colleague (Zhang Yifeng), Thanks! Had to dismantled it for the new configuration (10S12P).

Built a passive dissipative cell balancer. The circuit can be found in TL431 datasheet. It was called "shunt regulator".

Back view of the lithium cell balancer.

Top view of the balancer.

Bottom view of the ballancer.

It ran so hot that I needed to use an adjustable wrench as the heatsink.

Box building time! I used the tools (drill and drill bits) that Zhang Yifeng gave to me. (Thanks again!)

I bought a black plastic box from Poundland (try to search for "battery box" on ebay, the price is significant higher). The structure is quite thin, so, I needed something to reinforce it. Luckily, I got a plywood from my landlord (Adrian Chapman - Thanks!). I cut it to size using handsaw also from Poundland.

I use the plywood to strengthen the floor of the box. The UHU glue was used as a temporary adhesive to help me drill mounting holes.

Time to 3D print some brackets. ;)

I also added some foam sheets underneath the box to help to distribute local mounting pressure. The goal was to avoid the box from cracking. 

Checking for dimension before drilling. My dad bought a rear bicycle rack for me, but, sadly, there are no mounting points on my bike, so, I had to buy a new rack. :/

All holes were drilled and brackets were successfully mounted on the box and the rack.

Testing for the rigidity of the bracket. Notice that I use dishwashing sponges as a temporary impact absorber (the battery was quite heavy). The wooden box was from Aldi's Clementines. So, it is a box within a box. ;)

All wiring is now done. I use cell level fuses because I had one experience where one cell shorts all other 7 cells and kill them all :(. I know I should be using bus bars, but, I ran out of copper wire, so, this should do it for now.

This is how it looks on the bike.

Overall, I am very pleased with how it turned to be. I think I like the 18650 more than pouch cells now. ;)

I am planning to develop a low-cost non-dissipative active cell balancer in the future.

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