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I am working on a project that checks a sensor every 12 hours and reports the status via cellular modem to our server ..

Power consumption is less than 1mA during sleep mode and spikes to 900mA While transmitting for 30 or 45 seconds every 12 hours .

My question is what battery do i need to use to get the longest run time with the smallest form factor ?

Battery needs to be able to handle the following temperature range -20C to 55C

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  • \$\begingroup\$ Why must it be a battery? Opportunity doesn't run on batteries, and it lasted over 10 years. \$\endgroup\$ – Aron Aug 21 '15 at 3:56
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Lithium. Cells made by the Israeli company Tadiran are popular for this sort of application.

If I did the math right, it's about 41Ah for the cellular, assuming it runs directly from 3.7V. Not much you can do about that. 1mA for the standby is very high and you should try to reduce that. That's another 43Ah. Try to reduce that to uA. The biggest I see on their website is 19Ah (max 500mA output current) so you'd need several of them.

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    \$\begingroup\$ Just looked at the processor i will be using . In power save mode it will be consuming .75uA so if i disconnect all other circuitry should be able to reduce my standby power consumption from 1mA to .75uA \$\endgroup\$ – user83711 Aug 20 '15 at 21:42
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    \$\begingroup\$ Yeah, but you've still got your (minimum) 41 Ah for the transmitter. Plus, your capacity will be reduced at -20 C. The only good news is that with this large capacity requirement, your current draw is only C/40 or thereabouts. \$\endgroup\$ – WhatRoughBeast Aug 20 '15 at 22:22
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    \$\begingroup\$ leakage of diodes and capacitors may amount to hundreds of uA, so be careful what type are used. \$\endgroup\$ – KyranF Aug 20 '15 at 22:56
  • \$\begingroup\$ As the above comments say, watch the leakage at high temperature and capacity reduction at temperature extremes. \$\endgroup\$ – Spehro Pefhany Aug 20 '15 at 23:56
  • \$\begingroup\$ (Confirmed: if the cell runs for 45 secs every 12 hours, will consume 41Ah in five years.) \$\endgroup\$ – Daniel Griscom Aug 21 '15 at 2:50
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This was going to be a comment, but I felt it ran too long, so I expanded it a bit more... some of you guys know me by now ;-)

This seems like a brilliant opportunity to include some energy harvesting methods, next to a safety backup cell to keep the proc active if all else fails.

If the measurement isn't complex you could consider buffering a couple, knowing your lithium primary coin cell will keep your core memory powered 99.999% of all possible cases, and reducing the number of busts of cell-power per year.

If for example you collect 32bytes of data every twelve hours and your proc has 2kByte, assuming you need 1kByte for other smart stuff, leaves room for 1000/32 =~ 31 measurements (I'm using the new, more pessimistic kibiByte here, but it should probably have been 1024 gives 32 measurements).

Now, if you're smart, you send them out in a bunch of 30 every 10 measurements and use your 1kByte memory as a 30 place ring buffer, always adding new stuff, but leaving every one of the last 30 measurements in memory. That gives you a lot of redundancy, while still only sending stuff 1/10th of the times you are planning now, gives 1/10th the power requirement over time. If you can then energy harvest that power back into a LithiumPolymer or LiFePO4 (more suited to sub-zero temperatures) cell, bang! problem solved. Small cell that recharges itself over time between transmissions.

For power harvesting you can look at solar, thermal or piezo or possibly many more, depending on the situation your system is in. (on land is a good candidate for a 1 or 2W solar cell, under water might be a case for hydro-powered, etc, etc).

Your battery setup could look like this:

schematic

simulate this circuit – Schematic created using CircuitLab

(I had to compact the schematic a bit to make the image turn out at least a little useful, so my apologies if some text crosses a few lines.)

If you do the maths right on the power use per x hours and minimum power gain per same x hours, you can potentially optimise the main battery down to a single 1Ah Lithium Ion cell, though I'd advise you go with a flat-pack LiFePO4, since they can also be discharged safely with pretty good capacity at temperatures below freezing. Still best to only charge them at +5 degrees Celsius or above, since it's a Lithium chemistry.

Of course, if charging is not at all possible in any way, the only solution to the battery is what Spehro suggests: Big ass Lithium battery with good quality.

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