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I have a system running at 3.3v requiring 70mA at most times and 220mA at peaks (roughly 20% of the time). I'd like to find a way to power this from coin cell batteries or other small portable source. Battery life needs to be about 1h but size should be as small as possible (which is why something like AA's won't work)

I see a couple ways of doing this:

  • Put several 4 button cell alkalines in series and use an LDO voltage regulator to get the voltage down to 3.3v
  • Use ~3 alkaline button cells in parallel and boost the voltage using TPS61200 (datasheet: http://www.ti.com/lit/ds/slvs577c/slvs577c.pdf). Schematic similar to this: http://www.sparkfun.com/datasheets/Prototyping/LiPower-v11.pdf
  • Same as the previous one but with the batteries in series. The TPS61200 will regulate down to 3.3v when the batteries are full and boost to 3.3v in the end
  • Same as above but with a single cell LiPo battery. I'd like to stick to easy to find batteries though. Rechargeability is not important.
  • Use 3 AAA batteries in series with a voltage regulator (size is a bit of a concern..)

In terms of efficiency does it matter if i put 3 alkaline batteries in series or parallel (TPS61200 can work down to 0.3v with a 0.6v startup voltage)?

So far i tried using a CR2032 lithium battery with NCP1450A (http://www.onsemi.com/pub_link/Collateral/NCP1450-D.PDF) to boost it to 3.3v but the output voltage stays around 1.4v. It may be that something else is wrong in my circuit but from what i understand the battery can't provide enough current anyway (4mA continuous, 15mA peaks) so there's no point continuing that path?

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  • \$\begingroup\$ What's your question? It seems like you have already listed a bunch of possibilities. \$\endgroup\$ – Chris Laplante Feb 27 '13 at 15:50
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Coin cells will not work well with your needs. While it varies by brand and size, most coin cells have low capacity, and an internal resistance that limits current output. A CR2032 has a nominal 250ma capacity (and that varies based on current load btw), and an internal resistance (Equivalent Series Resistance or ESR) of 18 to 30ohms, again based on current load). Depending on the speed of the current draw, you are going to get voltage drops, and quickly drain the coin cell. If you can get that much current out of it at a given time. 70mA is pushing it even for high quality CR2032, and 220mA is improbable, frankly, impossible. That is why your CR2032 boosted to 3.3v isn't working. The current draw of the regulator is causing the cell's voltage to drop.

That is, without having multiple in parallel (or using high capacity caps). That's the second part you asked. Batteries in series are a sum of the voltages, with the same amount of current. Batteries in parallel are a sum of the current capacities, with the same voltage.

As for your options, one or two AAA batteries with a boost converter would suit you better. It gives you relatively high capacity while still keeping space to a minimal. The TPS61200 you mention would work, if you can deal with a tiny (3x3mm leadless package. The TPS6107x family would do the same, same minimal components, in a sot6 package. You only need 1 battery for either of these. An AA gives 2400mAh nominal, an AAA gives 1200mAh nominal.

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  • \$\begingroup\$ I think i'll have to go the AAA route as you and others suggested. Not ideal in terms of size but i guess we can't change physics or chemistry. Say i have two AAA batteries: would it be better to put them in series to have a higher starting voltage or put them in parallel for higher capacity? Both approaches should work fine for the TI chips (thanks for the tip on TPS6107X!), just wondering what is better for efficiency \$\endgroup\$ – Antti Feb 28 '13 at 8:09
  • \$\begingroup\$ Just looked at the graph in the datasheet. Max Output Current vs Input Voltage. Seems for 3.3v out, max current is 425ma when input voltage is 3.0v. For your needs, 220ma (Lets round to 250ma), you need at least 1.9v. At 1.9~3v, you are getting 87~95% efficiency at 50~100ma draw, which is your "normal" draw (Varies based on voltage and current). On the other end, the 6120x (0 adjust or 1 3.3 preset) only needs 1v in for 300ma max draw (more at more voltage) but you would get probably 65/70% efficiency at 1.5v (So in parallel would help). Its a bit guessing as 220ma/1.5v are not standard points \$\endgroup\$ – Passerby Feb 28 '13 at 9:08
  • \$\begingroup\$ The NCP1450A (version 33T1 since it's a fixed regulator) that you have should work, with either one or two AAA batteries, at the current you need, if you have the correct transistor, but I can't understand it's datasheet as much, and have no experience with regulators without internal switches. Efficiency isn't listed for the 3.3 version, but the 3.0v version seems to show 77~85% efficiency with anything from 0.9~2.5v (higher is better) at the current you need. The 3.3v should have slightly better efficiency. \$\endgroup\$ – Passerby Feb 28 '13 at 9:27
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Have a look at CR2 batteries. They have most of the characteristics you want, especially if you can run directly from 3V, and are used in cameras so you can buy them from supermarkets or Amazon. They are however a bit pricey and non-rechargeable.

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Use 2 AAA cells with a boost convertor - if size matters, no need for 3 of them. That TPS61200 looks as if it'll do nicely.

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For such current requirements, I would suggest LiSOCl batteries. They provide 3.6V and have the required pulse capability. A 2/3AA ER type will have 1.7Ah capacity and able to provide enough current on pulse. They are a bit harder to find than CR types (Lithium manganese) and a bit more expansive. Some brands are much more expansive, but also much more reliable.

Also, if you decide upon CR type (Coin cells) use a well known brand name, even if it means paying few more cents. TI did a comparison between branded and no name types and the differences are quite large.

Since your constant current is quite large (70mA), a regulator is recommended.

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