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One or Two Cell Li-Ion Battery Charger

2016-05-10 11:21  
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One or Two Cell Li-Ion Battery Charger
When the battery is low, the voltage at the reference pin of the TL431 will be less than 2.5 volts, causing the TL431 to switch off, increasing the transistor base voltage and charge current. Current is limited to around 300 mA by the 18 ohm resistor (2 cell setup). As the battery approaches full charge, the TL431 reference pin approaches 2.5 volts, increasing the TL431 current and reducing the transistor base voltage and charge current. Using 2 cells (8.2 volts, 1000 mAH), the current drops from 300mA to about 100mA as the charge reaches 75% of capacity in 200 minutes. Another hour is needed to bring the charge to 85% Note, the value of 4.1 rather than 4.2 was chosen for a little more margin, and less stress on the battery at full charge. From the data above, it looks like only 5% of capacity is lost. The diode prevents a reverse voltage across the e/b transistor junction in the event the power supply connections are shorted while the battery is still connected. The 220 ohm resistor was selected for about 20mA base current. The minimum transistor gain is 30, so 20mA should produce at least 600mA. The open circuit output voltage is set with a voltage divider for either 4.1 or 8.2 volts. Two jumpers are used to select the desired voltage and current limit. For example, to charge a single Li-ion cell to 4.1 volts, the current through the 10K resistor will be (4.1-2.5)/ 10K = 160 microamps. The series combination of the other 2 resistors should total 2.5 / 160 uA = 15625 ohms. A 15K in series with 620 might be used, and the 620 adjusted to compensate for the 15K being slightly more or less. I ended up 15K and 750 since the 15K was a little low. In the 2-cell (8.2 volt) case, two additional resistors are added in parallel with the 15625 (using the jumper) to increase the output voltage from 4.1 to 8.2. I ended up with 5.6K in series with 430 ohms. The 430 can be adjusted to get it just right. A second jumper is used (across the 12 ohm resistor) to maintain approximately the same charge current with either single or double cell operation. Both jumpers are installed for 8.2 volt operation and removed for 4.1 volt operation. Note: The circuit board picture shows two 5 watt 12 ohm resistors. One of the resistors is out of tolerance and is actually 17 ohms. Caution: Be careful not to set the jumpers for 8.2 volt operation while connected to a single cell (4.1 volt) battery. Use a DMM to verify the open circuit voltage is what you want before connecting the battery.