Li-Ion cell instead of a CR2032 for a domestic weight scale

[RoGeorge]

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?

[mikeselectricstuff]

You can get li-ion 2032's - look for lir2032

[Psi]

Keep in mind a rechargeable LIR2032 is like 40-75mAh vs 220mAh for a CR2032

So you are going to have to recharge it every few months.

[BeBuLamar]

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.

[tooki]

Why not just connect two AA batteries in series?

[Doctorandus_P]

I modified my scale and put in a mechanical switch to turn it off when I don't need it.

[bidrohini]

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. 

[wraper]

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.

[wraper]

I modified my scale and put in a mechanical switch to turn it off when I don't need it.

I 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.

[mikeselectricstuff]

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

[RoGeorge]

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   1.800mA
3.0V   1.815mA
3.3V   1.833mA
3.6V   1.850mA

In 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.

[electr_peter]

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.

[G-son]

I modified my scale and put in a mechanical switch to turn it off when I don't need it.

I 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.

[wraper]

I modified my scale and put in a mechanical switch to turn it off when I don't need it.

I 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.

The truth likely was counterfeit batteries with a fraction of capacity and very high ESR.

[RoGeorge]

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.

[electricenthusiast]

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)

[G-son]

I modified my scale and put in a mechanical switch to turn it off when I don't need it.

I 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.

The truth likely was counterfeit batteries with a fraction of capacity and very high ESR.

It'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. None have 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.

[oasis2020]

……
- maybe add a series diode or two, to drop some voltage?

Yes, I do the same with my weight scale.
You can use the "waste heat" of old batteries! Pretty good practice.

[BeBuLamar]

……
- maybe add a series diode or two, to drop some voltage?

Yes, I do the same with my weight scale.
You can use the "waste heat" of old batteries! Pretty good practice.

If 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.

[timenutgoblin]

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.

[RoGeorge]

The Li-Ion that I've soldered instead of a CR2032 should give a standby time of 1035mAh/6.53uA = ~18+ years. 

[argentas]

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.

[BeBuLamar]

The Li-Ion that I've soldered instead of a CR2032 should give a standby time of 1035mAh/6.53uA = ~18+ years. 

While 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.

[Psi]

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.

[RoGeorge]

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.
...
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. 

1 year later ...

Discharging log since the last recharge from 2022:
  2022-12-21,  4.1909V
  2024-02-17,  3.9641V

[Zero999]

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.

[RoGeorge]

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.

[MarkT]

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.

Lithium 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.

[wraper]

Li-ion cells (secondary) will self-discharge _much_ more rapidly than a primary lithium cell, in months, not a decade.

Li-ion is nothing like non LSD NiMH. They'll stay charged for a few years unless something drains them.