In previous posts I’ve mentioned accumulating and testing used laptop batteries and other sources of 18650 size cells. A project came up that makes good use of 91 of these cells (the range extender for the car may or may not still happen). Many people have already done this, but the following is my method of assembling a 24v 26Ah bicycle battery specifically designed to fit in the existing case and charge using the existing charger. The bike originally used a pair of 12v 12Ah lead acid batteries.
The first step was to layout the connection scheme for the 91 cells and create a drawing and then tool paths to drill and engrave the copper clad board which will be used for the bulk of the cell connections.
Then drill and engrave the PCB material.
The layout of the PCB combined with the layout and orientation of the cells gives the desired series/parallel arrangement.
Tin the tops of the cells, take care not to heat the cells too much, others have said this can damage the cells.
I used resistor leads for the connections from cell to PCB, bend the lead as shown to make a good electrical and mechanical connection to the top of the cells.
Solder the lead in the very center of the cell.
Trim the lead, leaving roughly 1/2″ sticking up. I found that each resistor would make 4 connections. After all the cells are done, the PCB is placed overtop of the protruding leads, this takes careful alignment to make sure they all stick through, careful soldering in the previous step helps out a lot here.
Bend and solder the leads to the PCB making sure not to get too close to the engraved dividing lines.
Repeat on the other side with the appropriate PCB and as many modules are being assembled. Due to the shape/size required I had to make 4 stacks of 5 cells to the side of the main blocks, these were simply soldered together with wire.
Complete the electrical connections (don’t forget to include a fuse!)
Connect the cell log 8 for monitoring and plug in the charger.
Once the pack passes basic tests use electrical tape or other insulating material to cover the exposed electrical connections.
For this application the pack will be balanced on the first charge and simply monitored and rebalanced as required. If the pack won’t stay balanced using this method either a top balancing BMS or RC charger with balancing leads may be adapted for long term use. The existing lead acid charger charges at 3A CC-CV to 28.82v which is 4.117v per cell on a 7S13P pack, this is low enough that minor cell imbalance over time shouldn’t be an issue.
The whole system is a compact box with built in charger. The original battery was a series pair of 12v 12Ah SLA UPS style batteries, at 50% discharge that’s about 6Ah if Peukert doesn’t decrease it even more. The new battery is 25.9v nominal, giving a slightly higher average voltage during discharge and a capacity of 26Ah, though it shouldn’t be discharged 100% it can be done with slightly accelerated aging. This gives 4.3x the Ah’s but with the higher voltage during discharge it’s about 5x the usable energy in a package that weights about 2/3 as much as the original lead.