K Quist
Oct 06, 2005, 05:32 PM
I have been trying to find a cheap DIY Lipo balancer, but I didn't like what I found. They were all an "oscillating type" that connects an power-resistor in parallel with the battery for some time, and then disconnects the resistor again, all during charging.
I wanted mine to have an self-adjusting load that was only drawing the neccesary current, to stabilize the battery voltage at 4.15V. It should be like charging the cells with CC/CV, but where the lipo-cells are individually changing from CC to CV when neccesary.
My first idea was to detect the max voltage of 4.15V across the lipo-cell and then control a load in a linear way, so that the load was at all time equal to 4.15V/Icharge. At first I wanted to use a mosfet, but I fast realized that it would be better to use a bipolar transistor. This makes it possible to limit the max load current, in case of fault in control circuit.
Below you can see the 1st version of the schematic for my cheap lipo balancer (I guess the cost is about 1$). I have had it through a little lab-test to test the stability of the feedback-loop, where I only tested it with lab-supplies(I don't dare to connect it to my lipo's before I have got some feedback from you guys :D ) The feedback turned out to be stable and is working very well, it stabilizes the battery voltage at 4.16V.
Please let me know what you think about it!
Note: If you are going to build it attach the power transistor to a heatsink with a thermal resistance of less than 15K/W (if you are charging with 1A)
I wanted mine to have an self-adjusting load that was only drawing the neccesary current, to stabilize the battery voltage at 4.15V. It should be like charging the cells with CC/CV, but where the lipo-cells are individually changing from CC to CV when neccesary.
My first idea was to detect the max voltage of 4.15V across the lipo-cell and then control a load in a linear way, so that the load was at all time equal to 4.15V/Icharge. At first I wanted to use a mosfet, but I fast realized that it would be better to use a bipolar transistor. This makes it possible to limit the max load current, in case of fault in control circuit.
Below you can see the 1st version of the schematic for my cheap lipo balancer (I guess the cost is about 1$). I have had it through a little lab-test to test the stability of the feedback-loop, where I only tested it with lab-supplies(I don't dare to connect it to my lipo's before I have got some feedback from you guys :D ) The feedback turned out to be stable and is working very well, it stabilizes the battery voltage at 4.16V.
Please let me know what you think about it!
Note: If you are going to build it attach the power transistor to a heatsink with a thermal resistance of less than 15K/W (if you are charging with 1A)