DIY Li ion battery charger w adjustable shut off current

Posted on Feb 5, 2014

This project started because I wanted a Li-ion charging circuit with more flexibility than afforded by other do-it-yourself circuits, but at less cost than the programmable computerized chargers. The goal from the beginning was to design, build, test and share a charger that was able to charge Li-ion cells of any size and with any number of them in parallel to a predictable and repeatable

DIY Li ion battery charger w adjustable shut off current
Click here to download the full size of the above Circuit.

state of charge without any trickle charging. Furthermore, the design needed to be as simple as possible; with a minimum parts count, reasonable cost, and ease of contruction to allow it to be built by an amateur (such as myself). On the advice of Silverfox, I contacted MrAl about this project, and he worked with me for months carefully considering many options and designs, all tested by him virtually on his computer, until we arrived at a design that adequately met the project goals. I then built and tested the charger with much help from MrAl. This is the result. This charger has a unique feature among Li-ion chargers. It has a variable shut-off current to allow the user to terminate charging at a specified current level. This allows the user to not only charge cells to the desired state of charge, but also to charge many cells of the same capacity at once in parallel. Many chargers charge cells in series, which can cause cells to become unbalanced and even overcharged. Overcharging a Li-ion cell is one of the most dangerous things you can do with one. By placing them in parallel, they are able to equalize. Note that cells with open circuit voltages differing by more than about 0. 5 Volts should NOT be connected in parallel, but should instead be charged individually. Some chargers are made to sense a certain shut off current which makes them suitable for charging cells with a certain range of capacities. Charging larger...

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