Li-Ion cell instead of a CR2032 for a domestic weight scale - Page 1
Forum section: electronics / beginners

author: RoGeorge
author_rank: Super Contributor
timestamp: December 17, 2022, 11:06:21 am
content: My weight scale is discharging a CR2032 battery every 6-12 months in standby. Not sure if it's a bad batch of CR2032, or it's the scale's fault. Either way, I have Li-Ion batteries sitting unused, but don't have any LDO in stock. If needed, will make a discrete voltage regulator.A CR2032 can have up to 3.6V when new, while a Li-Ion can have 3 ... 4.2V.- anybody tried replacing a CR2032 with a Li-Ion, how did that went?- should I bother making a voltage stabilizer or just wire a Li-Ion cell?- maybe add a series diode or two, to drop some voltage?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 17, 2022, 11:57:19 am
content: You can get li-ion 2032's - look for lir2032

author: Psi
author_rank: Super Contributor
timestamp: December 17, 2022, 12:20:11 pm
content: Keep in mind a rechargeable LIR2032 is like 40-75mAh vs 220mAh for a CR2032So you are going to have to recharge it every few months.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 17, 2022, 12:32:17 pm
content: I think just connect the Li-Ion to the scale. I don't think the higher voltage would hurt it. If you can get LiFePO4 battery then the voltage is much closer to that of the CR2032.

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2022, 03:18:47 pm
content: Why not just connect two AA batteries in series?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 17, 2022, 04:38:39 pm
content: I modified my scale and put in a mechanical switch to turn it off when I don't need it.

author: bidrohini
author_rank: Regular Contributor
timestamp: December 18, 2022, 07:40:25 am
content: I think you'd better bring a new lot of CR2032. Choose a good brand like the original Maxell. If still it does not work, you can consider other suggestions that you got here.

author: wraper
author_rank: Unknown
timestamp: December 18, 2022, 08:08:08 am
content: Load cells consume quite a bit of current, so it's not a surprise such small battery will discharge fast, especially if scale is left ON for prolonged time.

author: wraper
author_rank: Unknown
timestamp: December 18, 2022, 08:10:42 am
content: Quote from: Doctorandus_P on December 17, 2022, 04:38:39 pmI don't think that standby consumption should be a problem, at least if scale is designed somewhat properly.EDIT: tested my kitchen scale which consumes 2mA when ON (less than I expected) and 2uA when OFF.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 18, 2022, 10:31:42 am
content: Something else to consider - off-brand coin cells can have terrible life - I had some cheapo ones that lasted about 20% as long as a decent one

author: RoGeorge
author_rank: Super Contributor
timestamp: December 18, 2022, 12:41:44 pm
content: Measured today the CR2032 battery and the currents. Battery now has 3.03V in standby, and 2.09V during weighting.Scale model is Hausberg HB6000C. When powered from a power supply, anything less than 2.8V will display LO (low battery) instead of the measured weight.During weighting, the current is less than 2mA for a period of about 15 seconds, then it enters in standby. At other voltages, the working current stays about the same:2.8V 1.800mA3.0V 1.815mA3.3V 1.833mA3.6V 1.850mAIn standby, the current fluctuates between 5-15uA. Looks like pulses, hard to measure directly with a DMM.With 3.1V series with a 370\$\Omega\$ then a 1000uF capacitor the standby voltage drop on the 370R looks like in the attachment. Area for the displayed 2.4 seconds (the 2 spikes of standby current shown) is about 5.8mVs. Divided by 370\$\Omega\$ then by 2.4s gives an average standby current of 6.53uA.At 6.5uA average standby current, a 200mAh CR2032 should last for about 3.5 years. Seems reasonable.At 2mA average weighting current and 15 seconds per weighting a CR2032 should last for 24000 weightings.The scale seems OK, so I guess it was a bad batch of CR2032.

author: electr_peter
author_rank: Unknown
timestamp: December 18, 2022, 02:01:00 pm
content: Old batches of CR2032 can go "bad" surprisingly quick. Get new batteries from good brand if supposedly new cell goes flat relatively quick.As CR2032 produces minuscule amounts of power (compared to AA/AAA or any DC power supply), it is difficult to make a quick measurement and get idea of battery life left. New battery it is then.

author: G-son
author_rank: Regular Contributor
timestamp: December 21, 2022, 11:27:50 am
content: Quote from: Doctorandus_P on December 17, 2022, 04:38:39 pmI did the same, got tired of finding empty batteries basically every other time I needed the scale. Measured the current when turned off, it was very small and I calculated the batteries should last for many years (like five or ten, don't remember exactly) at that current, but real life suggests that wasn't telling me the entire truth. So far, the batteries have been fine since the modification some time this summer.

author: wraper
author_rank: Unknown
timestamp: December 21, 2022, 11:49:54 am
content: Quote from: G-son on December 21, 2022, 11:27:50 amThe truth likely was counterfeit batteries with a fraction of capacity and very high ESR.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 21, 2022, 01:11:09 pm
content: I've soldered directly a Li-Ion 1035mAh (10+ years old, NOS, no swollen signs, made in Japan). Charged it at first very slow, at 5mA then at 10mA for a couple of hours, then at 50mA for another about 20 hours, until the battery was floating at 4.2V and was drawing less than 3mA charging current instead of 50mA.Didn't measure the capacity after charging. Judging by the charging time and charging current, it probably has more than 500mAh. My guess is it still has 70-80% of its nominal capacity. Even at 50% it will be twice the capacity of a good CR2032, so the remaining capacity doesn't matter much.4.2V seems a little too much judging by the LCD contrast. Next time I will charge the battery at only 3.8V. There is not much charge stored at higher voltage anyway, probably less than 20%. Most of the charging time was spent between 3.5-3.8V so I guess that's the voltage where the most energy is stored.It is all now about the self-discharging rate of the battery, which I didn't measure. Unless the self discharge is horrible, it should last for about 10 years before needing a recharge.Well, probably not 10 years, but I've found a few very old Nokia with battery about half after 10 years or so of storage. Though this one now soldered to the scale was found empty. Last time it was charged 5+ years ago.

author: electricenthusiast
author_rank: Contributor
timestamp: December 21, 2022, 03:43:36 pm
content: It can be done but it only works for 12hrs continous according to my old project (replacing li- ion instead of cr1616 in my digital watch with ldo ams-1117 3.3 ic)

author: G-son
author_rank: Regular Contributor
timestamp: December 22, 2022, 02:30:09 am
content: Quote from: wraper on December 21, 2022, 11:49:54 amIt's a possibility, but I've had those scales something like 20 years, and used batteries of many different brands from many different sources through the years.Nonehave lasted anywhere near what the calculations suggest, I put fresh batteries in it, use it once, get it out a few months later and it works but warns for low battery, put it away for another couple of months and it's dead. Fresh batteries and the cycle repeats.I added the switch this summer, still on the first set of batteries (cheap noname ones), and no low battery warning so far. There's absolutely an improvement there, even if it hasn't been tested with any accuracy.

author: oasis2020
author_rank: Newbie
timestamp: December 22, 2022, 03:38:46 am
content: Quote from: RoGeorge on December 17, 2022, 11:06:21 amYes, I do the same with my weight scale.You can use the "waste heat" of old batteries! Pretty good practice.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 22, 2022, 10:39:10 am
content: Quote from: oasis2020 on December 22, 2022, 03:38:46 amIf that is the case then simply remove the battery when you don't use it as you only use it once a couple months or so.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: December 22, 2022, 01:25:21 pm
content: Third time posting this...I have a set of digital scales that are powered from 2x CR2032 lithium coin cells. The low battery cut off voltage is 4V. These batteries would last approximately 3 months before they went flat. I then replaced the CR2032 coin cells with 18650 Li-Ion cells which last approximately 9 months. The Li-Ion cells were salvaged from a defective laptop battery pack.I have a formula for calculating the estimated battery life:time (day) = \$\ \frac{60 \times (mAh)}{(1440a + t_{min} (b - a))} \$I also have a formula for calculating the estimated milli-amp-hour capacity:capacity (mAh) = \$\ \frac{t_{day} \times (1440a+t_{min}(b-a))}{60}\$\$\ a \$ is standby current consumption (milli-amperes).\$\ b \$ is operating current consumption (milli-amperes).\$\ t_{min} \$ is time measured in minutes.\$\ t_{day} \$ is time measured in days.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 22, 2022, 02:44:25 pm
content: The Li-Ion that I've soldered instead of a CR2032 should give a standby time of 1035mAh/6.53uA = ~18+ years.

author: argentas
author_rank: Newbie
timestamp: December 22, 2022, 02:44:52 pm
content: I've been buying all of my CR2032 cells from Ikea in recent years as they seem to be good quality, have a decent shelf life, and at £1.50 / for a pack of eight in the UK are excellent value. I don't know what their pricing is like in other countries, but worth checking if you have an ikea nearby.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 22, 2022, 04:29:46 pm
content: Quote from: RoGeorge on December 22, 2022, 02:44:25 pmWhile Li-Ion does have relatively low self discharge rate but I think it won't last a year unused. Rechargeable battery for such a low power consumption device is not a good idea.

author: Psi
author_rank: Super Contributor
timestamp: December 25, 2022, 10:32:06 pm
content: Lion cell self-discharge can be around 0.5-1% per month.The rate reduces after the cell as lost the top 10%, but yeah, something to keep in mind.They do self discharge themselves and it can be quite bad on crap cells.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 17, 2024, 09:21:13 am
content: Quote from: RoGeorge on December 21, 2022, 01:11:09 pm1 year later ...Discharging log since the last recharge from 2022:2022-12-21, 4.1909V2024-02-17, 3.9641V

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2024, 09:48:57 am
content: Why not add a micropower LDO regulator? Then you can fully charge the battery and don't have to worry about the scale being damaged or malfunctioning.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 17, 2024, 10:09:18 am
content: I did fully charged it (to 4.2V) 14 months ago, and never recharged since. It worked so far without an LDO regulator.The 2 voltages from the previous post are not for top voltage during recharge, they are for eventually tracing the discharge curve, in 10-20 years from now when it'll need a recharge. Measured the voltage today only because another topic about CR2032 reminded me about this battery replacement.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 17, 2024, 10:13:57 am
content: Quote from: electr_peter on December 18, 2022, 02:01:00 pmLithium coin cells, the reputable ones, have a shelf-life of 10 years or more - either the standby current is too high or the cell was a low quality one.Li-ion cells (secondary) will self-discharge _much_ more rapidly than a primary lithium cell, in months, not a decade.

author: wraper
author_rank: Unknown
timestamp: February 17, 2024, 12:36:05 pm
content: Quote from: MarkT on February 17, 2024, 10:13:57 amLi-ion is nothing like non LSD NiMH. They'll stay charged for a few years unless something drains them.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 18, 2024, 06:40:35 am
content: Quote from: RoGeorge on February 17, 2024, 09:21:13 am2 years later ...Discharging log since the last recharge from 2022:2022-12-21, 4.1909V2024-02-17, 3.9641V2024-12-18, 3.8570V, room temp 26.9*C

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2024, 12:12:05 pm
content: Thanks for the update.I've had a similar problem with my kitchen scales, but I added an LM317L on the end of a USB cable, so I could power it off a mains powered USB charger. It worked perfectly. The scales only use 10mA peak, at which point the drop-out voltage of the LM317L is just over 1.5V, so yes it won't give the best regulation and ripple rejection, but the 5V supply is already fairly stable.https://www.onsemi.com/download/data-sheet/pdf/lm317l-d.pdf