concept for low current battery solution, 1mA, 3V3 - Page 1
Forum section: electronics / general

author: Johannsen
author_rank: Regular Contributor
timestamp: June 06, 2020, 07:08:50 am
content: Hi,I'm looking for the best concept for an average 1mA 3V3 voltage source. The device is operated outside and should last >4months, preferably ≥6 months.My components are RFM95, TSOP4838 and ATmega328P.So far those ideas for batteries came to my mind:2 batteries in series:Varta Longlife LR14 Baby (C)-battery Alkali-Mangan7600 mAh11.4W1.5VTensai TR-C4000 NiMH Baby C - LR14, Rechargeable Battery4000mAh4.8W1.2VMurata / Sony Konion US18650VTC6, unprotected3120mAh11.2W3,6V - 3,7V1 single batterie:Keeppower 26650, protected5200mAh18.7W3,6V - 3,7VConsideratios:outdoor condition, changing temperatures, ldo vs dcdc converterWhich of the idea would you prefere and why?First I thought a big 26650 and a low current consumption LDO MCP1703 would be the easiest and most reliable way. Big capacity, simple to handle but maybe high self discharge, rechargeable.Now I'm thinking also ofTwo batteries in series allow more common batterie type with low self discharge, every time a new battery when changed, disadvantage:not rechargeable

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2020, 12:02:52 pm
content: 3 alkaline D cells in series with a linear regulator will last 18 months.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 06, 2020, 03:42:54 pm
content: Quote from: Johannsen on June 06, 2020, 07:08:50 amAre you running the ATmega in low power mode? The processor can be put in low power modes. I see running off a 32kHz RTC it pulls 0.75 uA. Also lower logic bus voltages saves power. I'm not sure about this processor but many allow the processor to sleep with a clock running that can wake up the processor to do it's work and then shut down. Page 48 on the data sheet talks about sleep modes. I guess you probably already know this.Good luck.

author: Johannsen
author_rank: Regular Contributor
timestamp: June 06, 2020, 03:58:59 pm
content: D size cells are too large for my enclosure and 3 of them also to heavy.Yes it's running, in low power mode etc but the IR receiver is quite power hungry.Anyway I need for other applications also the 1mA supply.What do you think about the suggested solutions? Any experience also with the LiIon in outdoor environment?

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2020, 04:50:06 pm
content: What are the temperatures in your outdoor environment?

author: Johannsen
author_rank: Regular Contributor
timestamp: June 06, 2020, 06:24:57 pm
content: At extreme around -20°C in winter and +35/40°C in summer

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2020, 07:18:38 pm
content: Quote from: Johannsen on June 06, 2020, 06:24:57 pmThat's a pretty big range! I don't know how well your battery choices will hold up, and the more durable LiFePo4 batteries will be heavier and larger. The only obvious solution I can think of would be a 2X2 array of AA LiFeS2 primary cells (the Energizer Ultimate Lithium AA) and a low-QC, LDO regulator like the TPS7A05, unless your chips can take 3.6 volts, in which case you could skip the regulator. Is your draw actually 1 mA average, max, or what? Is it less at times? If it truly is 1 mA average and no more then 10 mA peak, what I've just described will probably last a year or more.4 AA cells don't take up much more room, if any, than a 26650 cell and have a pretty constant 1.7-1.8V over their life and your temp range. They are also very light.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 06, 2020, 07:47:14 pm
content: LiFePO4 is not "more durable". They are not readily available from high-profile manufacturers. The chemistry-level advantages of higher cycle life and possibly relatively higher charge current (in proportion to discharge current rating) are definetely out of scope here, as is the "compatible with 12V lead acid voltage range"; what remains is far inferior energy density and uncertain quality which becomes very important in long-term or high temperature operation. You would need two in series, increasing complexity and wasting 90% of the energy of the added one if regulated 3V3 is required.For ultimate lifetime and stability, I would recommend Sony or Panasonic li-ion cells over any other manufacturer. Great stuff with almost zero leakage current, you can expect self-discharge of a percent or two per year. Can't guarantee it, but I would be surprised if they didn't last for at least a decade of such use just fine, likely more. Recharge when necessary, use some sort of UVLO circuit to prevent overdischarging below say 3.0V. I have personally tested Sony VTC5 cell for self-discharge and long-term capacity fade and IIRC it was the best among the competition.Edit: Results of VTC5:Code:Stored at 23degC for 1.5 years, charged at 4.20V: self-discharge 1.8%/year, irreversible C fade 1.3%/year, DCR rise 7%/yearStored at 23degC for 1.5 years, charged at 3.60V: self-discharge 0.4%/year, irreversible C fade 0.9%/year, DCR rise 12%/yearStored at between 23-45degC for 1.5 years, charged at 4.20V: self-discharge 4.3%/year, irreversible C fade 2.7%/year, DCR rise 15%/yearStored at 4degC for 1.5 years, charged at 4.20V: self-discharge 0.4%/year, irreversible C fade 0.7%/year, DCR rise 1.5%/yearLi-ion, having 0% SoC open-circuit voltage somewhere around 3.4V, would be a perfect match with a very low drop-out 3.3V regulator, utilizing the cell fully. (With significant loads, say Idsch > C/5, it's hard to get to 0% if you need regulated 3V3 out from a single cell. For such C/4000 operation, not a problem.)This, if you are considering rechargeable.AA alkaline cells are fine for the specifications as well, and cheap, and with decent energy density nearly that of li-ion, but obviously you need at very least 3, not 2, in series. If using just 3, it's crucial here to pick the absolute lowest drop-out regulator, especially if the 3V3 line is not allowed to drop at all, because to get most of the capacity out of alkaline cells, you need to discharge down to 1V/cell. If you pick an LDO with dropout of 100mV, you get down to 1.1V/cell which is just fine with such small load.Almost all technologies can handle the temperature range given the very low current required. All low temperature does is to significantly increase the internal DC resistance of the battery, which affects efficiency, but at such ridiculously low discharge rates, this is meaningless, voltage drop over ESR is basically zero as is ESR loss. Self-discharge dominates, which should bebetterat low temperatures.40 degC is an easy requirement, I'm assuming there is some shading against direct sun at play here, otherwise this may be optimistic specification.

author: Johannsen
author_rank: Regular Contributor
timestamp: June 06, 2020, 09:03:51 pm
content: Yes 1mA is average and most of the time. Some peaks can occur during transmisson of around 150mA - but they are rare.I didnt want to use AA cells as I would need 3-4 cells per device and the capacity of C cells is much more and I only would need 2 of these.It can easily be around 10 devices I need that power solution for and I find it more convientient to just change 1-2 cells instead of 4.The results of the Sony VTC5 are looking great. I expected a much higher self-discharge. The only problem I have with those cells are that In my enclosure only unprotected cells would fit because of the length. If anybody besides me are handling the devices I'm a bit worried because of the unprotected cells..The 26650 I found have a pcb and as far as I saw the holders for the 26650 should fit also for the protected versions.Thanks for the explanation about the relation of temperature und resistance - I didnt' know that.

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2020, 12:24:22 am
content: I would reconsider whether to use a linear or switching regulator. A big alkaline D cell plus a micropower boost converter rated to start on a single cell more than meets your requirements. A similar configuration but using a micropower SEPIC converter rather than boost converter will work with a single cell of whatever lithium chemistry you choose.Suitable parts include the LT1307, LT1316, and LT1610.

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2020, 02:05:18 am
content: Quote from: Siwastaja on June 06, 2020, 07:47:14 pmThe trail cam guys--the only common application I can think of for long-term outdoor low-drain use-- would disagree with you. Just because you can't readily obtain them doesn't mean they aren't out there. Trail cams have a use profile very similar to the OP and the first choice for primary batteries are the ones I mentioned. For rechargeable, it's LiFePo4 or, as a second choice for smaller batteries, LiPo pouches.Your data is interesting, but for whatever reason Li-Ion cells, especially the NMC types like the Sony you mentioned, just don't do well in simple unconditioned outdoor installations, at least AFAIK. I don't know the reason, perhaps the actual temps go much higher than the weather would indicate--keep in mind I'm in southern California--or maybe it's the temperature cycling. I also don't know how they would do in the OPs application, given the very low drain. In any case, if he goes that route, wouldn't the Panasonic NCR18650B be a better choice?

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2020, 03:12:45 am
content: Quote from: Johannsen on June 06, 2020, 09:03:51 pmYou might want a good sized output filter cap depending on the length of that 150ma pulse, at least if you use the TPS7A05, because the dropout voltage will be a lot higher than at 1mA.It sounds like your main constraint is physical dimensions. Is cost a real issue? Can you fit in just one D-cell? If so, this one will likely last several years with the TPS7A05 and will always have about 300mV excess voltage, enough to cover regulator drop at 150mA.https://www.batteryjunction.com/xl-205f.html?gclid=Cj0KCQjw_ez2BRCyARIsAJfg-kvQoKvz-KSMVax129-VrGj12HfiJ-ggwpEF4TIjLzp0_tI6ACyWaKAaAnj3EALw_wcB

author: Siwastaja
author_rank: Super Contributor
timestamp: June 07, 2020, 06:48:42 am
content: Note that the "protected" cell only has the standard over/underdischarge protection chip (and power switch MOSFETs). These are components that would normally be integrated with the device PCB. In a "protected cell", these are attached to a small PCB added on the top of the cell, exceeding the mechanical standards of said cells. These are kludges that supposedly make a professional-use-only part (a li-ion cell) a "consumer" device. But the fact these li-ion cells are not supposed to be used as replaceable batteries by consumers, and manufacturers shun such use for good reasons, does not change. (Not saying "don't do it", just offering perspective.)So I would definitely not use such "protected" cell here. Why? Because, you are designing the device here, do it the standard way and just add the protection on the device PCB. Of course, this assumes knowing what you are doing.If the 150mA pulse is short, say some dozen µs, you can easily handle it with an electrolytic capacitor. If it is in multi-milliseconds range, capacitors would get too big and waste your limited space, better use more batteries instead. If you are unsure, better assume it must be handled by batteries; assumptions about capacitors fixing pulse current requirements tend to fail, start assuming they don't work and prove by calculation they do. Conveniently defining max dV=0.1V and I=0.1A, the required capacitance F is the pulse length in s, i.e. 1ms pulse could be dealt with 1000µF but say 10ms pulse width would increase the capacitor size beyond reason, unless you go for ultracapacitors.Now this 150mA constraint is surprisingly high current at -20degC. I'm sure all 18650 li-ion cells handle this no problem whatsoever, but even with them, the under-load voltage at low SoC (say <20%) may sag below, say, 3.4V. So you might lose some last 10%-20% of the SoC due to this. Probably not a big deal, but good to remember if you are tight on space.Aged AA alkaline cells could pose a real problem with this 150mA, -20degC constraint, though.Think about smoke detectors: they start beeping low battery and when you measure the battery voltage, it seems full (often reading like 9.5V). The reason for this is that the smoke detector must know it can run the beeper which requires quite some current. To verify it, it has to simulate such load occasionally (not too often, not to drain the battery on these tests) and see if the voltage drops under load. So basically it's measuring the internal resistance of the battery, and reporting that.Quote from: bdunham7 on June 07, 2020, 02:05:18 amIndeed, before we speculate any further, I would like to hear a confirmation from OP that the +40degC is indeed a well considered number. If +40degC is the weather datapoint, measured in shade, an enclosure directly in sun can easily reach, say, +70degC.Li-ion cells tend to be rated for maximum discharge temperature of +60degC. Recommended long-term storage temperatures are even lower.With such low current, the "operation" is basically storage. But, self-discharge significantly increases on any battery type at high temperatures. Alkaline cells won't last long in such conditions, either.It makes a massive difference if you can add a shade preventing direct sunlight exposure. And it does matter whether this will be installed in Miami, or Reykjavik.Further advice depends heavily on whether +40degC maximum can be actually confirmed as realistic, and the maximum worst-case pulse length of the 150mA current is also extremely important.We engineers tend to be so accustomed to looking at "worst case" numbers, that we forget here that theaverage temperatureis interesting here as well. Seeing that hot weather increases self-discharge, there is a big difference whether it's hot all the time or only one week a year.

author: Johannsen
author_rank: Regular Contributor
timestamp: June 07, 2020, 10:50:18 pm
content: Hi all,wow thanks for that profound input!That D cell configuration defintly looks interesting. As I'm somehow limited in the available space for the battery a D cell would surprisingly even fit better than two C cells.Quotejust out of curiostiy what's the advantage of that sepic converter to a boost converter? is a SEPIC converter a buck-boost? TI also had some converter in their portfolio operating from 1.5-5volts and outputting 3.3V.(I'll study the LT converter's pdf tomorrow)@bdunham7 you'r right and on the right track, indeed it's a trail cam related accesoire.QuoteYes indeed it's the pysical dimensions and if possible as few batteries as possible because it safes time to change all of these. Cost is not such a big issue here if the power or the device works reliable and for a longer period without flaws.That's why i like primary batteries but the rechargeable are more sustainable and I try to avoid waste where I can.QuoteI guess your estimation is more accurate but I could try to shade it from direct sun, most of the time it is I would say. Considering the worst case isn't bad on the other hand as well.QuoteThat's indeed a good point I wasn't aware of 100%. I have an zhiyun camera gimbal that uses 26500 batteries and as far as I know they are not protected and I need to handle them to charge them etc. When designing an own device I would be a bit worried to use a unprotected cell..I don't want to happen an explosion etc when somebody else handles it and make a short circuit by mistake etc.In my current first version of the pcb I integrated the protection on the board itself as I couldnt find any holder that fit protected 18650s.Attached is my box with 77mm x 77mm inside

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2020, 11:24:40 pm
content: Quote from: Johannsen on June 07, 2020, 10:50:18 pmIf it helps, the thionyl batteries come in C size as well as 26650, any of which would power your unit for a year or more, although your 150mA peak current would be getting closer to their limits whereas a D-size thionyl battery can put that out continuously. And their temp ratings are something like -40F to +85C.

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2020, 12:45:08 pm
content: Quote from: Johannsen on June 07, 2020, 10:50:18 pmA SEPIC converter is a type of buck and boost converter but the term "buck-boost" refers to a specific topology which uses one inductor and two power switches. A buck-boost converter could also be used in your application.Compared to a boost converter, A SEPIC converter has an extra inductor and capacitor, can operate with an input voltage higher or lower than the output, and provides DC isolation between the input and output. Any boost converter can use the SEPIC topology because the power switch is in the same location.SEPIC converters usually have both inductors on the same core but this is not strictly required.The reason you might use a SEPIC or buck-boost converter is if the battery voltage straddles the output voltage which could be the case with multiple cells in series or lithium cells, or if you need to be able to drop the output to zero volts. The simpler boost converter is more suitable if a single alkaline cell is used.