First of all, I'm sorry for my poor English. This is my first question, since all my other questions were already answered in other threads.
I'm trying to build a LiPo charger circuit with non-specific parts. I've searched the local market here in Iran, but I was unable to find a LiPo charger IC. So, I've decided to build my own. There are lots of complex circuits to charge LiPo batteries, but I need a simple one.
Charging a LiPo, consists of two steps: Constant Current and Constant Voltage. I have a 150mAh battery, so I need a constant current step of less than 150mA.
So the questions:
1. Since the USB voltage is 5V, if I down it to 4.2V (the maximum voltage of the battery and the voltage of the constant voltage step), then could I omit the constant voltage step circuit? Since the constant current step is until the battery voltage be about 4.2V, and then the charging current will be dropped until it reaches about 0.01C. Because our highest available voltage is 4.2V, the constant current circuit will be unable to increase voltage due to less current consumption. Am I right?!
2. If the first question is right, may I use the simple current sources in WikiPedia (with one transistor and diode) with a less output current and ignore its dis-accuracy?
3. The maximum current a 2N3906 PNP transistor is able to conduct is about 100mA. Then if I place just this transistor between my USB power and the battery, It will decrease the voltage for 0.65V (5 - 0.65 = 4.35V) and also supply a current of 100mA for charging the battery. Will this work?
4. I have an empty 4000mAh LiPo battery. While trying the charging methods, I connected it directly to a 4.5V, 1000mA source and measured the current. It was about 120mA. Why the charging current is this low while I provided it 1000mA?
Again, I'm so sorry for my bad English. If it didn't burn your eyes yet, I will be so thankful if you guide me through this. I'm fighting this for more than 1 month. Thank you very much.