I am using a lithium ion (~100mAh) battery in my application, which is almost all the time powered up by 12 V source. The 12V gets converted to 4.2 V through SMPS, which then further powers up the 3.3V through LDO. The purpose of battery is to act as an alternate power source for the LDO, when the 4.2 V output is absent. The battery also comes with an inbuilt circuit which is described in the diagram below

enter image description here The reason for choosing 4.2V as the SMPS voltage, is that the charging voltage of the battery +circuit combination is recommended to be 4.2-4.3 Volts. Now, given the above circuit, the under voltage, over voltage protection is already present. I am looking for a simple circuit which can do following purposes 1) Control the charging by enabling / disabling the 4.2 V output fed to battery+circuit combo. 2) Switch the power input from battery, in absence of the 12V input (hence the 4.2V SMPS output). Hooking the battery all the time on 4.2V output looks to be a workable, but not-so-safe option. Also, the 3.3V logic powers up a MCU, so the logic can also be controlled by MCU especially for purpose 1.

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    \$\begingroup\$ Charging a Li-ion battery is not as simply as connecting 4.2V to the input. You need to charge it at a constant current determined by the cell capacity until the charge voltage hits 4.2 and then at a constant voltage until the current drops to a set threshold (typically 1/10th of the current used in the constant phase). There are dedicated power/battery management parts you can get to do everything you want. Use one of them rather than trying to build your own. \$\endgroup\$ – Andrew Dec 12 '16 at 14:06
  • \$\begingroup\$ @Andrew - It is not a raw cell but one with its own BMS. If you look at the circuit diagram I posted above, there is a power mangement IC -datasheet.sii-ic.com/en/battery_protection/S8261_E.pdf. I believe since it has 3V undervoltage lockout, the battery will never drain below 3V and hence will remain in constant voltage charging zone. Please correct me if I am wrong. \$\endgroup\$ – Pudi Dec 12 '16 at 14:52
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    \$\begingroup\$ @Pudi that is not a charge controller, it is a battery protection chip. That chip is responsible for your batteries not catching fire when you hook them up like this, but a charge controller is responsible for making the circuit work like you actually want. You need them both, incidentally. \$\endgroup\$ – Bryan Boettcher Dec 12 '16 at 15:28
  • \$\begingroup\$ @Pudi as insta indicated you are wrong. Any design which relies on the protection chip is fundamentally flawed. That chip is there as a fail safe to prevent fires or explosions, not to manage the battery for you. The URL for the chip is a giveaway, /battery_protection/ not battery management or battery charging. The constant voltage charging zone starts at about 60-70% charged, below that you are in constant current mode. \$\endgroup\$ – Andrew Dec 12 '16 at 15:41
  • \$\begingroup\$ @Andrew - Got your and insta's point. Do you have any recommendation for charging circuit which I can use alongwith this battery + PCB combination? \$\endgroup\$ – Pudi Jan 5 '17 at 6:02

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