Battery backups

Here's a pretty common scenario: You've got a low powered device with a RTC (real time clock) - be it internal to the processor or external chip. When your device is powered the clock uses external power - like mains or larger battery - and when not, either small lithium battery (coin cell type) or a super capacitor.

Typically this is achieved by having two separate VCC supply lines; one for external power which is used when available, and one for battery backup when main power is not available. 

Lithium batteries (CR1232 or something like that) are nice - lots of "slow" capacity in small package. Enough to run your clock about two to five years (depending on battery size and power consumption of your RTC). However, when it runs out it runs out, everything that was on chip (and I'm assuming that storage is RAM like it usually is) is gone, for good. If there were critical data on the RAM, it's gone. But then again, if your device has been unpowered for years, that data might not be relevant anymore. Or then again, you might have wanted to keep the data anyway.

Supercaps on the other hand have capacity for maybe a week or two, but at least they can be charged again and again. This is nice if your device is externally powered most of the time (like mains power), but if power does run out, same limitations as mentioned above apply. The time will be much shorter though - week or two, tops, with common supercap sizes.

But wait, what if you wanted to use simple, cheap capacitor? Something like 330u electrolytic caps are cheap these days. Well, sure, if your device is almost always externally powered, like, say, an alarm clock. It would be able to keep charge for maybe few hours, max, but at least it would tolerate short power outages. Unlike my current one, which isn't even cheapest one available but still loses all the settings even if power goes out for few minutes.

But dissing Sony designs aside, how do you choose your back up solution? That, of course, depends.

How long you want the data to be retained?
What happens when power eventually runs out?

There is no single answer, it's something that you'll need to weigh case by case.

Older Nintendo Pokemon games are an example of this. The save game data in the cartridge was stored in low-power RAM, backed up with large coin cell battery. Those batteries are now dying, and your precious Pokemon collection is about to vanish - unless you replace the battery (not an easy task, as cartridge must have that backup power supply enabled during entire operation!)

Personally I prefer actual nonvolatile storages, like EEPROM (or MRAM/FRAM these days). On paper they have limited life - quoted as 20 years or so - but in practice it is much longer. And they keep their data without being externally powered. Internal clock may stop after its power runs out but it is easy to set again - other data however may be worth a lot more. Like those Pokemons.
But apparently few pennies more for actual nonvolatile memory was too much for them...