This should be a short and sweet Instructable and may not even require a schematic. I'm not sure how obvious this type of charge control scheme is, but I've been working on charging lithium ion batteries for many years and today I think I've found a more simple way of charging and balancing multiple cells in series. Let me know what you think!
Multicell Lithium Ion charge controllers are kind of elusive as far as understanding all the details. Lots of charge controllers sacrifice cost for capacity and even allow for cells to go bad over time. My idea was to have a charge controller on every cell in order to squeeze the maximum capacity out of every cell instead of requiring some higher 3V cut-off voltage safety margin. Unfortunately, most multi-cell balancers and charge controllers take a very conservative charge scheme to ensure battery performance and with a charge controller on every battery, such bulk multi-cell safety margins can be relaxed.
2 USB wall chargers
2 USB Type A to Micro cables
2 Connector or wires with bare leads (bare plated through holes will work too and probably safer)
2 18650 batteries
1 Multimeter with probes (I didn't have clamps probes that have a banana jack)
3 Tiny clamp probes (2 red 1 black)
2 18650 battery holders with connectors or pre-wired
2 Lithium Ion charge controller circuit boards with charge/discharge capability. I used my own which can be seen from the attached files. You can use something similar, basically any charge controller that has a battery voltage output with current limit and low cut-off voltage.
How it works:
The trick is to isolate the grounds of both charge controllers (trust me, I tried a common ground and ended up shorting a battery, DO NOT use common grounds for both charge controllers!). Using two USB wall adapters effectively turns each charge controller in to its own power supply, which can be tied in series giving each battery its own independent charge circuit and can balance the charge independently from the other. You can see in the last photos that the battery on the right is fully charged so no charge LED indicator is on, while the other battery on the left is low on power so it is charging with the green LED turned on. Moving the multimeter probes directly to the battery on the left shows that the battery is at 3.87V, which is pretty low considering the full charge is 4.2V. These battery charge controllers connected in series can be used to power devices while being charged. If one battery voltage so low that it is at the cut-off voltage while the other isn't, the voltages are still added together, but the current will be shut off. However, I still should do more tests for these different charge states just to make sure it performs as predicted.
My future goal is to try and miniaturize this circuit or even try to make it expandable to plug multiple circuits together.
Step 1: Instructions:
1. Plug in the two USB wall adapters and plug in the USB cables.
2. Plug the USB cables to the Lithium Ion charger controller circuit boards.
3. Plug the batteries to the charge controller.
4. Turn on the multimeter, set it to DC voltage, and connect the red and black clamp probes to the respective red and black multimeter probes (I didn't have clamp probes with banana jacks that could directly connect to the multimeter).
5. Connect the black clamp probe that is attached to the multimeter to the Output ground of one of the charge controllers.
6. Connect the red clamp probe that is attached to the multimeter to the Output voltage of the other charge controller.
7. Now use the last red clamp probe to connect to the open Output voltage pin of the first charge controller and connect the other end to the open Output ground pin of the second charge controller.
8. Congratulations! You should see the voltages of both batteries added up in series with the charge controllers balancing them both in real time!