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Use LIR2032 Recharable Battery
- LIR2032 nominal voltage is 3,6V, minimum safe voltage 3V, charging current is 20mA, maximum charging voltage is 4,2V 10mA
- voltage divider is to protect DS1307 Vbatt input from over-voltage. In my version R6/R4 = 510k / 1.5M. Which gives you 2,3V from 3V and 3,15 from 4,2 V.
- fast fix to use CR2032 (not rechargeable) is to cut track in charging circuit (between D1 and R6). But voltage divider will be sucking 1,5 uA current from battery, so divider should be also removed (remove R4 and short R6).