Hello,
I need battery for backup RTC and any register in MCU with logn life , may be 20yers.
My solution s use rechardable baeery for example **broken link removed**
and charge it from voltage source 5V - 6V
My idea is this simple CV charger
I1 is in real charged battery, Q1 CV source and reverse diode disconnecting battery when V1 is off
The charging circuit must limit the current to 20 mA and the voltage to 4.2V
Your circuit has no current limit.
You must also make sure that the voltage is max 4.2V even when the current is zero. That is easy to test, just remove the battery. You should measure at different temperatures.
You have true, current limit is problem. becouse my circiutrs have current limit about 150mA nad max chrge current is 40mA, 20mA std.charge
Fortunately I find Cheap and small solution BL4054-42
price is resonable