This is just one example of how to assemble a simple low drain battery good for 2-3 Amps for running electronics. There are other ways to do this and other components. It includes a BMS battery management board which prevents over-discharge, over-charge and balances the cells.
The BMS does cut off the output when the battery gets low. It also cuts off charge when the battery voltage is high. But I’m not sure it effectively balances cells in a short time when the charge voltage is slightly low. In theory, if the charge voltage is a bit low, the battery won’t fully charge, so you won’t get full capacity but it also means the cells should last longer.
The 3 18650 cells are held together in a couple of plastic holders. Because the holders make assembly quicker and easier, provide some mechanical protection to the finished battery and result in a consistent shape. It also simplifies mounting of the BMS board to ensure that there will not be any unexpected shorts within the battery.
- clip tabs from 2 3-cell plastic mounts – they are not needed, get in the way later and make a less-tidy battery.
- prepare 3 cells and glue them into the mounts up-down-up orientation – this ensures a rigid battery as cells will rotate in the mounts – construction adhesive is convenient. Leave overnight to cure.
- drill mounts for wires to pass to other end.
- cut wires for BMS and main lead.
- strip wire ends and solder tin all but one end of the main lead.
- crimp connector to the un-tinned end of the main lead and twist wires.
- add 30mm of 5mm heatshrink to main lead.
- remove metal pads from BMS board and attach wires.
- cut metal strips for cells – 2 long, 2 short.
- spot weld metal strips to cells solder tin strips for wire attachment.
- solder BMS wires to cells/strips starting at B- and working to B+.
- DON’T LET ANYTHING SHORT CIRCUIT – may damage BMS.
- install insulator – foam PVC – 50x15x3 mm – tape BMS in place.
- solder main lead to BMS.
- connect to charger to activate BMS output.
- test and charge if required.
- heatshrink covering – end-end – 100×70 mm
- heatshrink covering – around – 100×83 mm