tiny li-ion cell

[free_electron]

i am looking for an smd mountable li-ion cell to run an cpu off.
i am looking for something with 1 millampHour or less capacity at 3.6 volts.
the cpu average current consumption with the current code is about 0.78 microamps.
i only need to be able to run the cpu for two or  three days. 1 milliamphour would already give me more than 1000 hours.
so even 1/2 millamphour would be fine.  it needs to be rechargeable.

i tried supercaps but i need 0.5 farad to get there ... that is already big and bulky.

the cell cant be larger than 6mm diameter and 1.2 to 1.5 millimeter thick

[splin]

Tricky. If you can tolerate a voltage lower than 3.3V the MS412FE at 4.8mm x 1.2mm might do the trick:

http://www.sii.co.jp/en/me/files/2014/11/BAC3011EJ-01C1409_web__.pdf

If its charged to 3.3V you might, if you're lucky, get your 2 or 3 days with the voltage above 3V, and a lot longer >= 2.8V. Low temperatures will reduce the voltage though and charging to 3.3V will reduce the cell's life but the datasheet is not very helpful as to how much.

At 4.8mm, would that give you enough space to squeeze in a boost converter such as the TLV61225? Its 2.4mm x 2.15 x 1.1mm and needs an inductor; they also recommend 2 10uF caps but you might get away with the battery (and perhaps a small cap) on the input and you might already have enough capacitance on the o/p side?

If you haven't got space for a separate converter, could you toggle a spare pin on your CPU to drive a charge pump with a capacitor and 2 diodes to boost the voltage? Might be a bit tricky bootstrapping the micro out of a low power sleep mode though. And don't charge the cell above, say 2.7V so the o/p voltage doesn't get too high, depending on what voltage regulator(s) you have. Or add a zener diode after the charge pump if you have room.

Alternatively, could you use a different form factor? EnFilm (now owned by ST) make a .7mAh cell with nominal voltage 3.9V. Its 25.7mm x 25.7mm, but its only .2mm thick. Unfortunately I don't think you can fold them or roll them up!. They're also horribly expensive at $38 one-off from Digikey! See:

http://www.st.com/web/catalog/sense_power/FM142/CL848/SC1107/PF250531?icmp=pf250531_pron_nb_jun2014&sc=enfilm-nb

[IanB]

the cell cant be larger than 6mm diameter and 1.2 to 1.5 millimeter thick

The construction of li-ion cells means they will either be long thin cylinders (like a swiss roll) or flat and rectangular (like a flat sandwich).

Here is an example of a small cell in the rectangular format: http://www.powerstream.com/ultra-light.htm

SMT would seem to pose an issue since the internals are heat sensitive and would not survive reflow or soldering operations.

[juanfermed]

Would this enerchip batteries from cymbet work? http://www.digikey.com/product-search/en/battery-products/batteries-rechargeable-secondary/394468?k=cymbet%20enerchip

They are around 55uAh, 3.8V and almost the dimensions you needed.