Using CR2032 cells for my application - Page 1
Forum section: electronics / projects

author: sentry7
author_rank: Regular Contributor
timestamp: February 14, 2016, 08:55:28 pm
content: Hey guys,I'm looking into a source of power for my microcontroller/TFT display-based project; it is an artwork display. People come in range of the module and it turns on, showing artwork.Traditionally, I have been using wall adapters, but this is a portable project. I did some research and found that alkaline batteries are not good to use wherever you need a low self-discharge, but I saw that button/coin cells are good for low current draw applications.I haven't yet made any formal power consumption measurements, but this is a 3.3v project, and one of the parts is guaranteed to pull 100mA at 3.3V worst-case scenario.This product is not going to be sitting anywhere after assembly; it's going to be used regularly and the batteries will be replaced by the user. I would like at least 84 hours of continuous device operation.Are lithium coin cells right for this project? What do you guys recommend?

author: AndyC_772
author_rank: Super Contributor
timestamp: February 14, 2016, 09:01:51 pm
content: I wouldn't use a coin cell for something requiring 100mA; they're much better suited to keeping clocks running and maintaining memories... nothing more than 1mA or so.You're much better off with an AA cell and a boost converter, or a bigger Li-ion cell and a linear regulator to drop the output to 3.3V, IMHO.

author: Audioguru
author_rank: Super Contributor
timestamp: February 14, 2016, 09:26:43 pm
content: Energizer's datasheet for their CR3032 Lithium battery coin cell shows it with a 0.19mA load. Its maximum internal resistance (for a new one) is 40 ohms so its 3V output will be reduced to half (only 1.5V) when its load current is only 37.5mA. Then its voltage drops much more as it runs down.Energizer makes a 1.5V Lithium AA cell that holds its voltage up well. It can supply 100mA for 24 hours then its voltage has dropped to 1.4V. That is 3 or 4 times better than alkaline AA.They also make a 1.5V AAA lithium cell. It can supply 100mA for 7.5 hours when its voltage has dropped to 1.3V or for 12 hours when its voltage has dropped to 1.3V.

author: uncle_bob
author_rank: Unknown
timestamp: February 14, 2016, 09:28:17 pm
content: HiAssuming that 100 ma is anywhere near correct, a CR2032 *might* last you for two hours. The voltage cutoff of your system would be a significant factor in working out if that is going to be 1 hour or 1.5 hours. That's a pretty expensive battery for a 2 hour run time. I can buy a lot of AA's for the price of a 2032.If you really need 10 to 20 year shelf life, a good brand of Lithium's is essential. There are a wide range of suppliers out there. Buying a 10 (or a 5) year old battery is not what you want to do.Something like an a CR123A would at least get you over 8 hours of run time.All these cells drop off to about 2.5V pretty quick. Some 3.3V stuff dies at 3.1V. Other stuff works fine down to 1.7V. If your display is one of the 3.1V designs, forget about running straight off batteries. You will need some sort of regulator.Lots of twists and turns in a design like this....Bob

author: Fungus
author_rank: Super Contributor
timestamp: February 14, 2016, 09:48:34 pm
content: Quote from: uncle_bob on February 14, 2016, 09:28:17 pmNope. Pulling 100mA out of a CR2032 is a bad idea.The internal resistance of a CR2032 starts at about 20 ohms (http://data.energizer.com/PDFs/cr2032.pdf).If you draw 100mA you'll see 1V (the other 2V will be lost as heat, which is bad...)

author: sentry7
author_rank: Regular Contributor
timestamp: February 14, 2016, 10:13:41 pm
content: So it looks like I'm either stepping up or down on this one. What would be the end-result differences in a single AA alkaline/step-up solution vs. an 9V alkaline/step-down solution?

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: February 14, 2016, 10:31:54 pm
content: a good idea is to start by defining the targeted autonomy of your application to choose the best capacity. while most portable application now use Li-ion battery that can handle 100mA easily and with some low quiescent current voltage regulator you can get good results

author: sentry7
author_rank: Regular Contributor
timestamp: February 14, 2016, 10:48:28 pm
content: Quote from: hamdi.tn on February 14, 2016, 10:31:54 pmI had originally planned for 84hrs of continuous device operation, but now I realize that my requirements may be too high and unrealistic. The system is microcontroller/display-based, and for comparison purposes, smartphones are essentially the same thing. Even with a li-ion cell rated at 3.6v, it's unlikely that you can go through almost a week without charging your phone (even without using it at all). Depending on what's available to me, I may have to fall back on wall power.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 12:17:05 am
content: HiConsider that *if* I'm running a MCU designed for low power operation, I can fairly easily get it down below 1ua of current drain. I can wake it up every 10 seconds, run for ~ 1ms (at 20 ma) and have a net current drain of about 3ua. That's how they get days out of a cell phone.Bob

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2016, 12:26:11 am
content: Quote from: sentry7 on February 14, 2016, 08:55:28 pmThis I don't follow. Alkaline batteries have a shelf life of years. In practical terms they don't have a self-discharge problem.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 12:32:31 am
content: HiIf you have a system that needs to sit on the shelf for 20 years and then go into service.***and***If you buy top notch lithium cells***and***They are brand new.*** and***You don't store them hot.....You can get a lot better performance out of lithiums than out of other battery systems. There is a derating you should apply, but you will have something left to run your system.Bob

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 12:55:20 am
content: Quote from: IanB on February 15, 2016, 12:26:11 amHere is the source. Maybe I misinterpreted?http://www.allaboutcircuits.com/technical-articles/how-to-choose-the-best-battery-for-your-next-project/

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 01:20:27 am
content: Quote from: sentry7 on February 15, 2016, 12:55:20 amHiCarbon zinc does not last long. A couple of years on the shelf is about it. It can easily be less.Alkaline can be better than Carbon Zinc by 2X to 5X. It depends a bit on the brand.Lithium can indeed last for decades on the shelf.As always there are some disclaimers. They apply to all batteries:Storing them hot shortens their life. Storing them below room temperature makes them last longer. Some cells do not like it really cold (the deep freeze) and will have a shorter life there. Comparing batteries stored hot to those stored cold will always favor the cold battery. Buying batteries that are years old and stored who knows how is a gamble.All batteries gradually loose capacity. A capacity decrease to 50% will give a different life than 10%. Oddly enough there is some variation in what different people call "end of life voltage". Those variables will muddy up comparisons even more.If you get into rechargeable batteries it's also complex. There are more variables. The stuff above applies directly to the non-rechargable versions. It is by no means "the whole story".Bob

author: wagon
author_rank: Frequent Contributor
timestamp: February 15, 2016, 02:13:49 am
content: Consider also something like a 1/2AA lithium cell. They typically have around 3.6V terminal voltage, and last very well in storage.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 02:18:10 am
content: Quote from: wagon on February 15, 2016, 02:13:49 amHiJust don't buy the ones with no markings at all on them from some guy named Bob on eBay.Bob (the home of the ten thousand amp hour AA cell)

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2016, 02:54:55 am
content: Quote from: sentry7 on February 15, 2016, 12:55:20 amWell you haven't said what your application is and what your requirements are. Are you building something that will be inaccessible and remain untouched for years? Or are you building something that will be in regular use where the batteries can be replaced on demand?Just saying, "alkaline batteries are bad because some website said they have worse self discharge than some other batteries" is about as meaningless as it can get.Alkaline batteries obviously are not bad as such, because they are used everywhere in everything.

author: Audioguru
author_rank: Super Contributor
timestamp: February 15, 2016, 02:57:14 am
content: The All About Circuits article about batteries is many years out of date. Energizer guarantees their alkaline batteries last for 10 years shelf life.

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 02:07:34 pm
content: This product is not going to be sitting anywhere after assembly; it's going to be used regularly and the batteries will be replaced by the user. I would like at least 84 hours of continuous device operation @ 100mA minimum.

author: Kilrah
author_rank: Unknown
timestamp: February 15, 2016, 02:20:38 pm
content: Is that 100mA average for all those 84h, or 100mA peak when everything is on and active but the system will have the screen off / be in low power mode 65% of the time because 8hr work days? I now see it's only one component (I assume display) so you'll need a somewhat more detailed power budget first to do much more.If the former you'd need 8.4Ah of capacity or about 30Wh at 3.3V. An AA cell is apparently said to hold about 3.5Wh, so going with AAs you'd need about 10 cells (and the required converter to the proper voltage) to store enough energy.But what's sure is that coin cells are definitely not a good choice, going with those numbers and assuming they actually could deliver enough current you'd need about 50 of themAlso... it seems there's quite a bit of leeway between the 2 "84h of straight battery operation" and "I'll use a wall wart to power it" extremes you've been considering in the first place. Like having enough battery for a day or 2 and plugging it to the charger at night...

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 02:27:43 pm
content: Quote from: sentry7 on February 15, 2016, 02:07:34 pmHiOk that's a set of numbers that can be worked with:84 hours at 0.1A is 8.4 amp hours.You need 3.3V out of the regulatorYou get 90% efficiency (drop in another number if this is wrong)84 x 0.9 x 3.3 = 249.48 watt hoursSome quick data from the first hit on Google:http://www.allaboutbatteries.com/Energy-tables.htmlAAA 1.4 WhAA 2.6 WhC 9.56 WhD 20.83 WhRoughly a dozen alkaline D cells will do the job. You can do it with ~4 non-rechargable Lithium's at a much higher cost per system. Any time you look at battery numbers, you can find stuff all over the map. Simply treat this as rough data for the moment. A further deep dive would be appropriate on whatever the final choice is. You could easily find that 20% or 30% more cells are needed in the real world. You equally could find that 20% less will do the job.Bob

author: Kilrah
author_rank: Unknown
timestamp: February 15, 2016, 02:32:12 pm
content: Quote from: uncle_bob on February 15, 2016, 02:27:43 pmThere are "only" 2 mistakes in that line

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 02:38:30 pm
content: Quote from: Kilrah on February 15, 2016, 02:32:12 pmHiOk well at 2 mistakes I'm doing better than average ...84 is not the same as 8.4 so yes, the 249 is 24.9It's early and my Swiss coffee machine is barely able to stoke me with caffeine fast enough.Bob

author: Ian.M
author_rank: Super Contributor
timestamp: February 15, 2016, 02:40:35 pm
content: If you believe that link, two D cells could provide 0.33W for 126 hours. However I'm not sure I believe their figures - I've seen rather different numbers in various battery manufacturer datasheets.As a point of comparison, a single alkaline AA is good for about 3 hours to 0.8V final voltage with a constant power discharge of 330mW (i.e boosting to 3.3V, 100mA load,ignoring converter losses), which means a 28 AA battery pack would be required - patently ridiculous. An alkaline D cell would be good for about 8 times longer (24H), so a 4 D cell battery pack with an efficient buck converter could meet the runtime requirement, with enough margin for converter losses. (All data taken from Energiser's alkaline battery datasheets.)Higher energy density battery chemistries exist, but are very expensive as primary cells.

author: Kilrah
author_rank: Unknown
timestamp: February 15, 2016, 02:42:59 pm
content: Quote from: uncle_bob on February 15, 2016, 02:38:30 pmSeems so, you didn't catch the 2nd one... you got the efficiency backwards (unless you're one of those "free energy" guys)

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 02:47:08 pm
content: Sorry for being so vague, guys. I would like to take a exact current measurement with the system in normal op state to give better numbers, but the fuse in my multimeter had blown a while back (I think). The voltmeter and ohmmeter work fine, though. Would it be valid to find the load resistance and then find current?

author: Ian.M
author_rank: Super Contributor
timestamp: February 15, 2016, 03:01:55 pm
content: No. The load current will be non-linear with respect to voltage so your ohmmeter reading will be meaningless.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 03:13:10 pm
content: Quote from: Kilrah on February 15, 2016, 02:42:59 pmHiOk, it's just plain early...Bob

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 03:35:13 pm
content: Then approximate figures will have to do. The major parts are the MCU and display.The MCU, as mentioned before, pulls 100mA, 3.3V, at maximum clock frequency (it's always running at this frequency, so I'm assuming constant pull).The display has four backlights that sink, if I'm reading the datasheet correctly, 80mA, 3.3V, but I'm not sure if that is 80mA per LED (seems a bit much for one LED) or all together. Also, it doesn't give the current sink for the display itself without the backlights.Datasheet for TFT -https://www.adafruit.com/datasheets/MI0283QT-11%20V1.1.PDFIn total, I'm looking at 180mA load current...or more if it's 80mA per LED.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 03:48:25 pm
content: Quote from: sentry7 on February 15, 2016, 03:35:13 pmHiDown on page 7 in the TFT spec they show that they are talking about all 4 LED's in parallel with 80 ma into the set of 4.Bob

author: dom0
author_rank: Super Contributor
timestamp: February 15, 2016, 04:06:31 pm
content: Quote from: wagon on February 15, 2016, 02:13:49 amThose Lithium primary cells can supply a few MOS-RAMs fordecades. They should also be able to deliver higher currents, but higher currents ~ low battery life <=> should use a rechargeable (secondary) cell.The self-discharge of secondary Lithium cells is afaik _much_ higher than that of primary Li cells.

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 04:18:16 pm
content: Quote from: uncle_bob on February 15, 2016, 03:48:25 pmThanks for clearing that up.So final approximate load current is 180mA at 3.3V.84 hours x 180mA = 15.12 mAh

author: Ian.M
author_rank: Super Contributor
timestamp: February 15, 2016, 04:21:16 pm
content: There's a module power consumption of 220mW listed on page 4 of the datasheet. That doesn't match the backlight consumption calculated from If*Vf so must be the electronics. I calculate the total including your MCU at 822mW. Assuming a 90% efficient PSU and allowing 10% margin, that comes out at 1W, three times the load you were initially expecting. Alkaline efficiency drops off rapidly with increasing current due to the losses in the cell's internal resistance, and my best estimate is you'd be lucky to get 20h from 4 D cells.To get your required run time, you'd need 12 Alkaline D cells, at about $14 a set. You'd do better to look at rechargeable LiPO packs. An 8000mAh 4s pack would handle it with enough reserve to avoid deep discharge.

author: Kilrah
author_rank: Unknown
timestamp: February 15, 2016, 04:29:00 pm
content: Quote from: dom0 on February 15, 2016, 04:06:31 pmYep impressive, I have a training/experimentation computer on which the lithium battery stamped 10/87 is still going strongThe (lack of) consumption of the RAM is jsut as impressive as the (lack of) battery self-discharge though. These things really only need toknowthat there's something to power them to maintain their contents.

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 04:41:19 pm
content: Quote from: Ian.M on February 15, 2016, 04:21:16 pmI guess that rules out alkaline cells; I want to keep customer cost down. As far as LiPO packs are concerned, is there any compliance testing that needs to happen before I ship these?

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 04:46:13 pm
content: Quote from: Ian.M on February 15, 2016, 04:21:16 pmHiHey, back to a dozen D cells in no time flat.====Bottom line here is that the system is pulling a *lot* of power for a battery based device. The cost of the power system is indeed "non trivial". I would suggest carefully evaluating the power in each sub assembly and seeing if money spent there would make more sense than money spent on giant / expensive batteries. I would also take a look at things like the back light -- how bad is it at 1/2 power ? Is that "ok" or not? On battery powered system I have ever done, we spent a *lot* of time beating on the electronics (and firmware) to get the power down. There simply was no other way to do it that made sense cost or size wise.Bob

author: Ian.M
author_rank: Super Contributor
timestamp: February 15, 2016, 04:51:53 pm
content: While we are giving examples of ridiculously long batterry life, what about the world record?Zamboni dry pile still in operation after 175 years.https://en.wikipedia.org/wiki/Oxford_Electric_Bell@sentry7,Compliance depends on local regs, but one thing's for certain, you cant ship them Air-freight.

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 04:57:20 pm
content: Sounds like a dry pile is what I need.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 04:58:26 pm
content: Quote from: sentry7 on February 15, 2016, 04:41:19 pmHiIf you follow the news, the whole "lithium battery fire" scare is making headlines every few weeks (at least in the US). Whatever the rules are today, that hysteria is likely to make things even more complex over then next few months / years. If you will be shipping internationally, I'd bet the rules will vary by destination. Massive headache coming on ....None of this in any way relates directly to the actual hazard involved with this or that battery design. We made some wonderful NiCad's back in the day that with their "rapid charger" could and would either melt the radio or set it on fire. We fixed things and nobody outlawed NiCad batteries as a result. There are Lithium designs that are a lot more prone to nasty things than others. The same is true to a lesser extent of lithium battery suppliers.The issue will be regulations that are either written or interpreted in a broad fashion. There are an enormous number of examples of this and (unfortunately) few examples of it being done properly. Being able to dodge around what ever they write (in advance) is going to be tough.=========Is this beast going through UL or CE approval? If so that will cover a bit of this.Bob

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 05:37:10 pm
content: Quote from: uncle_bob on February 15, 2016, 04:46:13 pmThe MCU is a real beast mainly because it's a multicore parallel processor, so, there's no changing that. In order for it to do what I need it to do, it needs to be constantly running at maximum frequency, so the MCU is a constraint. The TFT is relatively small as it is, and the visibility at maximum viewing angle would just be sub-optimal at anything less than full power.The battery capability is more of a leisure than a necessity. It would just be nice if the user weren't confined to wherever there is a outlet.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 05:51:10 pm
content: Quote from: sentry7 on February 15, 2016, 05:37:10 pmHiOk, not directly related to your application (I don't know what that is):In some cases, a "people in range" sensor is used. One example is the IR sensor on your cell phone. It shuts off the display (and likely other stuff) when the phone is at your ear. Other examples are seen in other areas. The idea is the same, if nobody is there to use the display turn it off.If the battery is a "luxury", how much is it worth to the customer? Are you better off with a battery option than a "everybody gets a battery" solution. In some areas, the gizmo comes in one rack box and the battery takes up a whole other box. If they want the battery, they order it. All of the gizmo boxes have chargers etc in them. Those who don't want the hassle do not have to deal with it. Even lead acid batteries are a hassle to ship ...If you go with a rechargeable battery, consider the whole issue of recharge time (and heat). I have no idea what the turn around at the end of the 84 hours needs to be. A 24 hour charge may be fine. If a 1 hour charge to 50% is not acceptable, you are back to either disposable batteries or swappable battery packs.Lots of details.Bob

author: Ian.M
author_rank: Super Contributor
timestamp: February 15, 2016, 05:52:31 pm
content: Why not design it to run off 5V with a USB lead? A cheap USB powerbank sourced locally by your customer could easily give 24H running time, and a high capacity one could meet your 84H target.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 06:02:30 pm
content: Hi.... ok so Ian.M beat me to the punch line. Rats ...The only thing I'd add: If you have two USB connectors, you can do hot swapping of the power packs. That way you have zero turn around time on a unit that is back from it's 84 hour mission.Bob

author: sentry7
author_rank: Regular Contributor
timestamp: February 15, 2016, 06:08:22 pm
content: So a more explicit description of this is an artwork display. People come in range of the module and it turns on, showing artwork.The PIR sensor and the USB powerbank are great ideas. Doesn't make sense to leave it on when there is no one there to see it. When people leave the area, the entire unit can enter shutdown/sleep mode or just turn off altogether.If the unit is placed in a hallway or living room, it is probably going to be off more often than on. But on the other hand, if it's in the office where the person is working all the time, then the device is probably going to be on more often than off.

author: Fungus
author_rank: Super Contributor
timestamp: February 15, 2016, 06:33:03 pm
content: Quote from: sentry7 on February 15, 2016, 02:07:34 pmIt's a simple calculation: you need an absolute minimum of 8400mAh. I'd add a bit extra, just in case, so 10000mAh.(Which is quite a lot...)It's really hard to give useful advice without knowing what it is though. eg. How big? Doe sit need to fit in a pocket? What exactly is it?

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 06:36:35 pm
content: HiEven in an office, most businesses run on sort of an 8 hour day. The other part of the day there is a lot less activity. It's a pretty good bet that the net active time will be < 12 hours. I'd bet on a lot less than that in most office locations.Here at home, unless I put it in the main room, The on time would be a lot less. Here, I've got uuummm ... errrrr ... USB charge strips ... all over the place. If it's got a USB power connector on it, I know what to do.Bob

author: kolbep
author_rank: Frequent Contributor
timestamp: February 15, 2016, 06:58:04 pm
content: What about using a 12v, 7Ah Lead Acid Alarm / Gate Motor Battery (I think approx $12?), and a float chargerOr a 12v 14AH?