I’m looking at using BBB for a new project which requires RTC to be maintained when the board is powered down. The application is not Internet connected so NTP syncs are not available.
I see that board revisions from A5A and newer have the battery connection lines of the TPS65217C routed to solder pads for a connector. The TPS65217C specs say a single cell Lithium battery should be used. So far so good.
Then I read in the forums that connectiong a battery will not work without a SW modification, but I find no further info on what this SW modication should consist of??
Can someone enlighten me on this issue???
/j
You need to add a battery to the PMIC via the pins provided. The battery will be powering the PMIC and the RTC rails on the board. This would require a slightly larger battery depending on the time you want to maintain the RTC voltage.
The SW will need to turn everything off except for the RTC voltage rails in order to maintain the RTC. The SW does not currently support any deep power down mode. So that would need to be added to the kernel.
Gerald
that battery will power the whole board not just the rtc
if you want a rtc when powered down you will need to design one into
your daughter board and provide a battery
like i do in my design
Hmm… I’m not sure I understand. Let’s say we have a power failure. 5V supply is cut. That leaves no chance for the kernel to do anything. I still need RTC to be maintained by the battery. How can this involve the kernel SW?
If you want the easy path, just do as evilwulfe said, Add a chip on a board with a coin cell. When power is cut, most likely you will get a corrupted SD card or eMMC
The way I mentioned will allow the board to go into a power-down mode under the kernel. The battery if large enough to allow for an orderly shutdown to prevent the previously mentioned issue.
Gerald
just as a note i will be adding a cel phone type LI cell for power
outages for an orderly shutdown
but for the RTC a I2C rtc and a button cell is my way of keeping time
since it is time critical
OK, thanks Gerald. That leaves me much wiser. Corrupt flash memories feels a bit ugly. So I would need a large battery. I found a nice 2000mAh single cell one with wire connectors at my supplier. Should be enough for a lot of shutdowns 
Wulf Man: What OS are you running? Have you looked at the kernel changes needed for shutdown to deep sleep at power outages? Anything you can share?
I am currently running debian but my software person is free to use
anything he wants
I as Gerald, am a hardware person
Awesome! That make two of us!!!
Gerald
software guy here. Me . . . just being “the software person” would put a large lead acid battery on the whole thing, run the board off a battery, and charge the battery off the mains, or solar ( we run solar here at home for our home ). But yeah lol perhaps a bit extreme for some situations.
Probably one of the most important design inputs is the maximum length of time you need to support the RTC without mains power. If no more than half a day or so, then it is probably feasible to run the BBB off a decent LI battery. Best if you can monitor the battery level (essentially a UPS) but you wouldn't need any kernel mods for a basic system. If longer than that, then the separate RTC/button cell as already suggested is the best way to go.