Using a darlington and an LED as charge bypass

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Joined Feb 15, 2010
I have seen a simple device set up as a charge bypass for 3.3V LiFePo battery cells, vs. a regular BMS. Basically it looks like two small leads from the battery terminals (+) goes to right leg of a darlington TIP105 and the other terminal to the heatsink-tab, the center and left legs has an LED between it. It was described as a simple (crude?) way of bypassing the charge current once the cell hit 3.8 volts. These batteries are usually charged with a bulk charger, in my case a 72V battery bank with an appropriate sized on-board charger (I don't believe in "proprietary charging infrastructure" as a general rule for electric transportation).

This is going into an electric (commuter) motorcycle that presently has 6 AGM 55Ah batteries with 6 separate chargers. The many cells I need to equal the old pack would require a BMS, but a BMS is beyond me, so I saw this as a simple way to solve my charging issue if I go with LiFePo. I would really like to know the theory on this. It has been a long time since I got into design and my inherent curiousity (and recent years in Renewable Energy) has gotten me re-invigorated. Ideas as to how this Darlington-LED is bypassing the current? Any idea of how much will be wasted energy?


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