Both need protection (internal or external) from excessive discharge (or excessive charge, and short circuits etc)
That is where our new library to check battery level goes in, we could dream to have an internal mysensor security check of a LiPo battery ! they do so in 'copters

I may pass on that. I'm not used to software glitches having quite so much potential impact, and I don't need to save every gram. But to each their own!

Both need protection (internal or external) from excessive discharge (or excessive charge, and short circuits etc)

That is where our new library to check battery level goes in, we could dream to have an internal mysensor security check of a LiPo battery ! they do so in 'copters

Li-ion. (lithium ion) @korttoma pictured one of these early one, and another was linked a few message above. Typically cylindrical in 18650 or 14550 size tho others exist; the size matters since they fit in holders like AA. Some have protection circuits in little disc shaped PCBs at the end, some do not. Lots of fake "name brands" from chinese forums. Flashlight afictionados seems to know a lot about them.

Li-Po (lithium polymer) These are usually rectangular and flat or flattish, and have wires to some little connector. Many sizes (since all you need is the right connector and enough space). Sometimes packed with multiple in parallel and/or in series for higher current or voltage. The rc model community seem to be the experts on these.

Both have similar voltages, and similar chemistries, and similar energy density per volume. LiPo is a bit lighter so it has better energy densidty per weight, important for flying things. LiPo is also useful where rectangular fits better. Both need protection (internal or external) from excessive discharge (or excessive charge, and short circuits etc). You can get by with draining a NiMH or Lead Acid or NiCd battery to zero (tho it's not good for any of them), but don't do that with either Lithium type!

I have Li-Ion in my flashlight, and LiPo in my micro-quadcopter.

As far as I can tell, some of the messages above may be saying "LiPo" when referring to Li-ion or vice versa. So I wanted to get us on the same page.

Thanks. The RC community has worked with batteries for a long time. Guess they're experts ;)

http://hobbyking.com/hobbyking/store/uh_listCategoriesAndProducts.asp?idCategory=86&LiPoConfig=1&sortlist=&CatSortOrder=desc

The MCP1702 also has good specs on quiescent current (tho not as good), and accepts higher input voltage.

I don't recall all the places I've read about these, but they are used in some low power devices.

From Adafruit website I've extracted two LiPo batteries reference, maybe we could start this way ? would there be fakes of some reference ?
PKCELL LP803860
18650 3.7v 2200mAh samsung version

Is there a componant including it ? I would have gone for an AMS1117 asproposed in the store ?

2x1.5V AA batteries and use step up converter to get from 3.0v -> 5.0v
4x1.5V AA batteries and use step down converter to get from 6.0v -> 5v

Let me try again on this one. The answer depends on what else you need to power (assuming that you feed Vcc without using the onboard regulator).. Like do you have any other 5V devices (eg: sensors, actuators), and/or any 3.3v devices (eg: sensors, radios).

If you need regulated 3.3v,and you want to sleep to save battery, I'm becoming attracted to the idea of 3c 1.5v alkaline and MCP1700 linear regulator. When your duty cycle is low, the efficiency of the regulator under load for short periods becomes less important to battery life than the quiescent current of the regulator (and the sleep mode of the processor and other devices).

As others have said, if all of your devices can handle the voltage range, using 2 or 3 batteries without a regulator becomes another option. The ATMega328P and other AVRs are remarkably tolerant of voltages (with appropriate brownout fuses if necessary), so it's more about the other devices.

If you power the 5V APMs via the VCC pins, I would at least test the sleep mode power drain and be sure the unused backward-fed onboard regulator isn't drawing too much current and remove it if so.

From what I've read, there are a lot of cheap rebuilt ones out there. Some may just substantially exaggerate the capacity but do work OK, some won't last long. And of course, they are more dangerous than most of our components. You can't tell what you are getting from the outside and many vendors take advantage of that so there are a lot of fakes out there.

A good source for info is the flashlight lover sites like http://candlepowerforums.com

http://docs.spark.io/shields/#battery-shield

Moustache shaped LiPo charger. Hmm.. ;)

The lipo-rider looks good, but there's mostly the older version on ebay. They seem to have switched JST-connector-version between their rider-revisions.

It is still hard to recommend LiPo batteries from ebay without doing some tests on them. I wouldn't use any without protection as they might explode if you use/charge them wrong.

http://www.cooking-hacks.com/lipo-rider-pro
http://www.cooking-hacks.com/shop/arduino/converters/5v-regulator-module-for-arduino
http://www.cooking-hacks.com/7-4v-lipo-2200mah-battery-arduino-power-jack
they have the same at spark.io (hardware cum software like mysensors, same radio different core) :
http://docs.spark.io/shields/#battery-shield

The battery shield is a LiPo battery charger and voltage regulator combined into one. You can use it to power your Core with any 3.7V LiPo battery and charge it at the same time via the USB port. The shield is built around Microchip's MCP73871 battery charge management controller and TI's TPS61200 boost converter for up converting 3.7V to 5.0V.

There are plenty of 3.7V lipo batteries on ebay.com ...

I'll try to add things following mysearches and experiments

http://en.wikipedia.org/wiki/Leaf_wetness

Probably just a calibration/quality thing. The technique/principle is probably the same.

For the Lipo, no idea yet (not supercaps...), I was hoping this thread would propose something interresting...

For the leaf wetness I'll try with the rain sensor but... I need some autonomous energy system for it will sleep outside...

@epierre said:

leaf wetness sensor

The only leaf wetness sensors I can find is super expensive. Like this:
http://www.ebay.com/itm/Brand-New-DAVIS-LEAF-WETNESS-SENSOR-/331222331582?pt=LH_DefaultDomain_0&hash=item4d1e63b0be

Can you (or someone else) recommend any lipo battery seller on ebay? I haven't got any buying experience just yet.

I'm searching for a leaf wetness sensor... not a rain sensor, hard to find as it seems...