Using LiPo batteries for Moteino outdoor projects

[Lensdigital]

I'm planning to create a mailbox sensor with solar panel recharging lipo battery.  I don't have much experience with lipos so wanted to ask if anyone had any luck with using them for outdoor projects? Heat in the summer and freezing temps in winter could be an issue, but I mostly worry about heat. Don't want to set my mailbox on fire

[Felix]

I have my mailbox notifier run on a Moteino+Power Shield+Lipo. I last charged in mid march. So over 2 months so far and voltage is at 3.87V now and I expect that to last a bit more. As voltage goes down the discharge rate also lasting longer, ie the discharge is not linear at all. For instance it's been between 3.95V and 3.85V for the last couple of weeks. It goes up with heat and lowers with colder temps. Freezing temps are worse I think than hot temps. Back in Feb the battery dropped dead after 1 month of freezing cold.
Anyway I don't have a solar panel attached to it to charge. That would make it very obvious, I dont want to attract attention. I dont mind taking it inside and recharging every few months.
A solar charging circuit is more complex from what I can tell. Adafruit has a solar-lipo charger but its a QFN chip, nothing like the simple SOT23-5 MCP73831 everyone is using to charge lipos.

[Lensdigital]

Thanks Felix!  I just obtained solar charger from Adafruit as well as 1200mAh LiPo and couple of 6V 300mA solar panels, gonna see if it will work. Charge has option for temperature monitoring so I'll use that.

[Felix]

Excellent, I am eager to see what you observe, please do follow up when you have results!
It would be interesting to see if the adafruit solar charger can keep a Moteino in RX mode all the time (charging during the day, on lipo at night).
This would make that a great candidate for a mesh network router, and especially nodes that sleep.
For listening radios in RX mode it would have to be able to provide around 16mA continuously. For sleepy nodes it would need to provide on the order of micro amps.

[Lensdigital]

I've yet to start working on this project, but I heard something disturbing. It might not be even legal do add any electronic device to US mail box, although I'm not 100% sure.  One can probably get away with hidden sensor, but having noticeable solar panel on top of the mail box might be a very very bad idea  

[Felix]

What was the source?

[Lensdigital]

http://pe.usps.com/archive/html/dmmarchive0810/d041.htm

Except under 2.11, the receptacles described in 1.1 may be used only for matter bearing postage. Other than as permitted by 2.10 or 2.11, no part of a mail receptacle may be used to deliver any matter not bearing postage, including items or matter placed upon, supported by, attached to, hung from, or inserted into a mail receptacle. Any mailable matter not bearing postage and found as described above is subject to the same postage as would be paid if it were carried by mail.

I got it from discussion of this Engadget article.

[Felix]

That's just guidelines for mailable stuff. I think they will need to change those rules one they find out about my mailbox notifier and everyone starts connecting their mailbox to the internet