DS3231 RTC + LION 2032 + MAXIM1551

Hi

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.

When the unit is not used for several months, does it also get cold.

I.e. I presume it's not used in the winter.

This may affect battery performance.

This is a bit of a off the wall suggestion, but there articles that I've read which describe how to trickle charge normal button cells, with ultra low currents. They may be worth investigating.

As you application is ultra low current, I'm not sure whether you'd need a dedicated charger ic for the battery, often you can get away with charging from the 3.3v line via a high value resistor e.g. 47k

But I realise that these are not orthodox methods, however if your unit isn't designed to last forever you can often cut down on the design complexity and cost

rogerClark: When the unit is not used for several months, does it also get cold.

I.e. I presume it's not used in the winter. This may affect battery performance.

I don't think this is a big issue, because most of my customers store their car inside. But thanks for pointing it out.

This is a bit of a off the wall suggestion, but there articles that I've read which describe how to trickle charge normal button cells, with ultra low currents. They may be worth investigating.

As you application is ultra low current, I'm not sure whether you'd need a dedicated charger ic for the battery, often you can get away with charging from the 3.3v line via a high value resistor e.g. 47k

But I realise that these are not orthodox methods, however if your unit isn't designed to last forever you can often cut down on the design complexity and cost

The reason I started investigating this IC is that (in contrast with other charging IC's) this one doesn't need any other peripheral components, therefore very easy to integrate into the circuit. If using a charger IC like this improves battery health and lifetime, it's price of $1,20 is a non-issue. But if you say just a high value resistor will suffice, maybe i should give that a chance.

Thanks for you help so far.