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I'm wondering what type of batteries to use on a project I'm building, it's a wireless temperature data logger.

  • It spends 15 mins asleep drawing 15 microAmps of current.
  • And about 10 seconds awake drawing between 70 and 200 milliAmps of current.
  • The board is tolerant between 2.5v and 3.7v.

I've currently got it running off 2 primary Zinc Chloride Dry cell AA's as they were to hand and it's been on a month so far and it's down to 2.70v now, but I'm wondering what would give the longest life.

I don't mind if it's rechargeable or disposable batteries but I want to get the longest life span between having to change them as it won't be in a convenient place to do so.

I realise using C or D cells would give more life than AA's but this is more of a question about chemistry and self discharge rates, high current discharge rates etc. Once I have decided on the most appropriate battery chemistry I will choose the largest capacity that I can to fit the solution.

I also don't want to have to have any additional regulation circuitry to keep the complexity down and also the standby current low.

I'm concerned about using a LiPo as it's over 3.7v after initial charge plus needing cut off circuitry to prevent damage to the battery, also some NiMh & NiCd can be 1.4v after initial charge so 3 of those would 4.2v

Anyway any help appreciated.

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    \$\begingroup\$ Where is this thing installed... sounds like a good example of a device that runs off a small "solar panel". \$\endgroup\$
    – Trevor_G
    May 11, 2017 at 12:47
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    \$\begingroup\$ What target lifetime? LiFePO4 may suit. Vmax can be set slightly lower than usual if desired with small loss in capacity. A single LiFePO4 operates over about 3 to 3.6V for most of its capacity so is near ideal. Shelf life is reasonable. Capacity per size is down on LiIon or NimH. Eneloop NimH will give "some years". Good brand Alkaline will give 5 years. Note that Alkaline brand new gives up to about 1.65V so 3.3V/2. or far too much for 3. Use of an LDO regulator or buck converter makes things much easier. \$\endgroup\$
    – Russell McMahon ♦
    May 11, 2017 at 12:50
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    \$\begingroup\$ Current drain is dominated by on time. At near max Ion you need about 5 mAh/day or about 1800 mAh/year. 3 x Alkaline AA + LDO will run for about 2 years, as will top 3 x LSD NimH (Eneloop or similar). A good LiIon 18650 (eg Panasonic) at about 3000 mAh will be similar. LiFePO4 has less capacity per volume. Where is this located? Any chance of background charging? 5 mAh/day ~= 200 uA or an about 1 mW solar panel. That's about 10 mm^2 in full sunlight ! or about 10,000 mm^2 = 100mm x 100mm in 100 lux roomlight (bright but not blinding)(Full brightness LCD screen about 300 lux). . \$\endgroup\$
    – Russell McMahon ♦
    May 11, 2017 at 13:01
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    \$\begingroup\$ 10 seconds awake sounds like a lot just to measure temperature. Sure you can't make it quicker? At 1s you would have increased the run time almost tenfold. I'm guessing the wireless handshake is what's taking the most time? \$\endgroup\$
    – Dampmaskin
    May 11, 2017 at 13:37
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    \$\begingroup\$ An RTC like the PCF8563 ticks along on about 500nA. That's going to have no noticeable effect on your battery consumption. \$\endgroup\$
    – Finbarr
    May 11, 2017 at 15:48

2 Answers 2

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I want to preface this with the following: I'm by no means an expert in the field of batteries and/or their chemistry. This is just what I know/have picked up over the years.

Depending what kind of autonomy you need, you need to decide what chemistry provides the best performance.

If it is truly a very long-life product, you pretty much rule out nickel-cadmium (NiCd) and nickel-metal-hydride (NiMH). They have quite high self-discharge, and it is possible the self-discharge may deplete more of your capacity than your actual circuit does. In recent years, this has gotten a lot better with techologies like the Sanyo (now Panasonic) Eneloop type batteries. However, they are still outperformed by lithium-ion technology in most cases.

You have to also consider if you truly need rechargeable batteries. I'm not very familiar with the cost of modern (reliable) lithium-based technologies, but unless things have changed dramatically, it is very possible that it will be more expensive to use a rechargeable battery instead of replacing the battery every few months/years.

Regarding discharge rates: This depends on the technology, but also the design. However, as lithium batteries tend to have lower ESR, they (generally) perform better in this regard.

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Alkaline AA cells would be an obvious drop-in replacement for zinc chloride, and produce the same voltage.

If that's not enough, consider non-rechargeable lithium cells. These are falling out of fashion, but are still available (for instance the CR123 battery, which used to be popular for cameras). They produce 3V.

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  • \$\begingroup\$ Are they falling out of fashion or making a comeback? They seem to have been appearing in AA/AAA/PP3 sizes in the past couple of years. \$\endgroup\$
    – Andrew Morton
    May 11, 2017 at 12:57
  • \$\begingroup\$ Do you have any specs on capacity vs size? Would 2 C cell Alkalines out perform a CR123 for example? \$\endgroup\$
    – Matt
    May 11, 2017 at 13:09
  • \$\begingroup\$ Comfortably. The Energizer CR123 is rated at 1.5Ah so it wouldn't last anywhere near a year in your application. A couple of good alkaline C cells would give you 5 years for a bit less money but taking up twice as much space. \$\endgroup\$
    – Finbarr
    May 11, 2017 at 13:43

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