DS1302 RAM
If you try to charge a non-rechargeable battery like the lithium cell (not the litium ion used in mobile phones and cameras), you will not only damage them but also can cause fire or explosion hazards.
Initially when I designed some circuits with DS1302 using ni-cd chargeable batteries and then shifted to lithium cells, I did not want to change the firmware, so I used a 1N4148 diode in series with the Lithium cell. At the low current that DS1302 draws, the diode only cause a very small voltage drop and the DS1302 works fine.