CR2032 powering LED lasts way longer than expected - Page 1
Forum section: electronics / general

author: dj_segfault
author_rank: Newbie
timestamp: February 02, 2021, 07:03:07 am
content: The specs I see for CR2032 button cells is say the capacity is about 235-280mA. I tried doing a test with an LED (no resistor, as I heard CR2032s have enough internal resistance to do that safely) that draws 20mA, so I expected it to last something like 10-12 hours. It actually lasted about 70 hours before getting noticeably dimmer, which is fantastic, but unexpected.The battery is rated 3V, and the LED I tried is rated 2V-2.2V, but as far as I know that should not have lead to increased runtime, but maybe increased brightness and maybe reduced lifetime of the LED.Am I missing something obvious?Thanks.

author: sleemanj
author_rank: Super Contributor
timestamp: February 02, 2021, 07:43:46 am
content: Quote from: dj_segfaultErr... no, that's not how that works.The current for your LED without a current limiting resistor, is by Ohm's law( Vbatt - Vfwd ) / Rbattwhere Vbatt is the battery voltage presented, Rbatt is the internal resistance, and Vfwd is the forward voltage of the LED. The problem therein being that all three are variable and dependant on each other. The Vbatt is dependant on the amoutn of current drawn and state of charge, Rbatt is influenced by the state of charge primarily, and Vfwd is influenced by the amount of current flowing through the LED.

author: Psi
author_rank: Super Contributor
timestamp: February 02, 2021, 08:02:09 am
content: CR2032 cannot handle high currents for very long.The initial current may have been 20mA but very soon after (like 1minute) the internal resistance of the cell increased and the voltage dropped a lot.The lower voltage meant the LED was now drawing much less current, so it lasted much longer than expected.Then, as the CR2032 got flatter and flatter the current kept reducing more and more as the voltage falls which gave you longer and longer runtime.in short, CR2032 are a pain in the ass, they don't behave like you expect due to their crap internal resistance and chemistry.

author: dj_segfault
author_rank: Newbie
timestamp: February 02, 2021, 08:53:43 am
content: Sounds like there's no real way to calculate it without doing a time series simulation, but I at least understand why my math didn't make sense. Thanks for that.About CR2032s being weird: Is that something about the CR2032 specifically, or are all button batteries problematic?

author: Psi
author_rank: Super Contributor
timestamp: February 02, 2021, 11:04:49 am
content: Quote from: dj_segfault on February 02, 2021, 08:53:43 amAll lithium manganese dioxide button cells have this issue.I really have no idea why we don't have a modern replacement for the CR2032 using better chemistry. It's like 30 years old now.You can get LI2032 which is a rechargeable version but it's capacity is only 50mA so not all that useful if you need capacity.It does have MUCH higher discharge current though, max 70mA for LI2032/LIR2032 instead of max 0.2mA for CR2032

author: tunk
author_rank: Super Contributor
timestamp: February 02, 2021, 11:16:22 am
content: Here's another, running one LED from two CR2032s in series:https://translate.google.com/translate?sl=auto&tl=en&u=https%3A%2F%2Fmysku.ru

author: dj_segfault
author_rank: Newbie
timestamp: February 02, 2021, 06:21:04 pm
content: For context, this is for a 3D printed light-up necklace pendant (I need to run off one more test before I publish it), so I'm looking for a long runtime in a small easy to buy battery, and the 2032 lasted 70 hours which is awesome.I could switch to a LiPo pack, but even a LiPo twice as big would probably not last half as long.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2021, 07:14:52 pm
content: With modern LEDs one often does not need 20 mA. Often 1 mA is enough and thus is closer to what the CR2032 can deliver. So add a series resistor and this allready makes things a little more predictable (e.g. the temperature would not effect the current so much). With battery operation it often pays off to look for effivient LEDs.

author: Psi
author_rank: Super Contributor
timestamp: February 03, 2021, 10:42:30 am
content: There's a specific type of LED, I forget the chemistry maybe some else will remember. but it's super super efficient.Usually they are orange or yellow, sometimes green.When I was design some gaming event dogtags with LED illumination from a CR2032 I went through ~50 leds from digikey looking for the best one for lumens per watt.Using this type of LED will boost your runtime even more (assuming you're not already using this type).Here's one running at 2.3mA and even 95uA !!

author: CJay
author_rank: Super Contributor
timestamp: February 03, 2021, 01:41:18 pm
content: There was a very long running thread (years) on another forum (now defunct but perhaps archived) about LEDs and button cells.Long and short of it was that, despite the cells being discharged way below the Vf of the LED,s the LEDs would produce enough light to be visible in darkness at some low number of microamps.The working theory was that in daylight the LEDs were putting enough charge back into the cells that they were able to provide the couple of uA necessary to produce enough illumination.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2021, 02:10:04 pm
content: Orange and yellow LED usually have a relatively poor effciency AFAIK yellow is the wavelenght with worst efficiency in the visible for the LEDs.Some green (more like a buish green) GaN based LEDs are really good.The current coming back from an LED when in normal daylight is not very high, so I doubt one would get significant energy back. The votlage also tends to be rather low.

author: CJay
author_rank: Super Contributor
timestamp: February 03, 2021, 02:33:36 pm
content: It does, the forum has gone now so I'm afraid I can't point anyone to it or see if there was a conclusion.

author: Psi
author_rank: Super Contributor
timestamp: February 04, 2021, 07:06:07 pm
content: Quote from: Kleinstein on February 03, 2021, 02:10:04 pmPretty sure this is the orange led i used.My main requirements were high efficiency and it needed to be low enough forward voltage to run from cr2032 all the way from full to near flat. There may have been more efficient leds but if Vf was too high they stop when the cr2032 is half flat. So are less efficient for my application.http://www.kingbrightusa.com/images/catalog/SPEC/WP710A10SET.pdfAlGaInP (on GaAs substrate)

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 04, 2021, 09:33:32 pm
content: Quote from: Psi on February 02, 2021, 11:04:49 amMy Casio fx-3600P calculator that I bought in 1981 and still use is on it's first CR2025. So why change a good thing!https://www.eevblog.com/forum/reviews/calculator-with-insane-37-year-battery-life/msg1531556/#msg1531556

author: CJay
author_rank: Super Contributor
timestamp: February 05, 2021, 11:00:33 am
content: Quote from: JohnnyMalaria on February 04, 2021, 09:33:32 pmHahaha yeah, I think I have a Radioshack branded verson of that calculator and I don't ever recall having to change the cell.I think mine may be a little older, maybe 40 years now