I'm currently developing a clock based on an atmega328 and a ds3231 rtc that should be used in a car. I have the entire clock working on a 3v coin cell. I will be selling a few of these clocks this fall. It ain't anything like mass production, i sell maybe 5 of these. The battery is only used to backup the time. The rest of the clock is powered through a 5v regulator circuit. According to my calculations the battery i'm using (a cr1225) will need replacement every two years or so.
I want to update my design to include a rechargeable battery to avoid users having to open up the dashboard to change the battery, so i came up with the following: Use a LIR2032 or similar rechargeable battery, and add a max1551 smd charger ic to keep the battery charged with minimum components. The battery would then be recharged when the car is driven. I am looking for a standby time of about 4 months (during winter when the car is not used).
Here's the datasheet to the max1551 for reference: datasheet.
I am fully aware that a non-rechargeable is a better option, but then the battery needs to be changed every 8 years. The target car is an oldtimer that will probably last for much longer. I am merely investigating the option to add a charger logic to my board, and learn some new skills on the way.
So it boils down to this: is the combination of the max1551 with the LION 2032 a good solution for this problem? Some people pointed out to me that just having a CR2032 battery would be sufficient.