Question storage batteries

[tiago1986]

I followed the recommendations and I am storing my xiaomi BM20 and Nokia BP-5L batteries at 3.75v-3.8v but these voltages will drop slowly, at what voltage do I need to recharge them again for storage mode and how long for voltage drop to a voltage that needs recharging?

[Martin Miranda]

I followed the recommendations and I am storing my xiaomi BM20 and Nokia BP-5L batteries at 3.75v-3.8v but these voltages will drop slowly, at what voltage do I need to recharge them again for storage mode and how long for voltage drop to a voltage that needs recharging?

https://en.wikipedia.org/wiki/Lithium-ion_battery
you can store them for 3-4 years without worrying that it would get damaged (worst case 2.5% per year)
but you should recharge it every year and check it's condition, it's still wet chemisty battery. and you need to monitor each battery.

[Martin Miranda]

a quick test with a cheap multimeter should do. 

[Gyro]

https://www.eevblog.com/forum/renewable-energy/question-storage-battery/

https://www.eevblog.com/forum/renewable-energy/temperature-and-voltage-keep-battery-lipo/

 

[tiago1986]

i have and i test multimeter i recharged these batteries a few 3 or 4 months ago to 3.75 or 3.8v and today they are with 3.74v it is necessary for me to keep checking because it can fall too much and damage the batteries

[Siwastaja]

You don't need to check, let them be.

If they self-discharge themselves to death, it's only because of them being total crap which would have died quickly anyway.

[tiago1986]

in the worst case what is the time interval for me to recharge these batteries? clone of xiaomi BM20 batteries (seller said it is original) and clone Nokia BP-5L

[Gyro]

7.35 years  days  weeks  months.

[Siwastaja]

in the worst case what is the time interval for me to recharge these batteries? clone of xiaomi BM20 batteries (seller said it is original) and clone Nokia BP-5L

Who knows? Your question does not make sense, because:

* If the cells are decent, there is no such time interval. They keep just fine. Maybe you can check the voltage every 3-4 years to be sure?
* As the cells are counterfeit crap, anything is possible, the worst case is they burst in flames any moment and burn down your house. Unlikely, but you asked for worst case.

Let them sit, and if they die on their own, let them die.

[SilverSolder]

i have and i test multimeter i recharged these batteries a few 3 or 4 months ago to 3.75 or 3.8v and today they are with 3.74v it is necessary for me to keep checking because it can fall too much and damage the batteries

Lithium-Ion batteries stay at a flat voltage for a long time, they don't start dropping until they go very low.



It doesn't hurt to check them (it uses almost no energy to check their voltage), for your peace of mind.

[tiago1986]

i really don't know the quality of these batteries they seem good some 2 or 3 or 4 months i charged them to 3.8v and today they are in the 3.73v range but i don't know when to charge and how long to recharge them again

[Siwastaja]

Lithium-Ion batteries stay at a flat voltage for a long time, they don't start dropping until they go very low.

I don't think the curve you posted is a really representative example. I have never seen such curve in any real-life cell, but something close to that might exist of course.

Consumer cells are more likely nearly not that flat; look for practical examples at https://lygte-info.dk/info/indexBatteriesAndChargers%20UK.html (Lowest current graphs are quite close to open-circuit voltage). Most are surprisingly linear between 100%-25% and then drop faster, and this is also my observation on testing ~15-20 different cells (mostly LCO and NCA).

[Siwastaja]

i really don't know the quality of these batteries they seem good some 2 or 3 or 4 months i charged them to 3.8v and today they are in the 3.73v range but i don't know when to charge and how long to recharge them again

Measure again in January 2023 and report back.

[tiago1986]

do they hold a charge for 1 year without risk of damage and me permanently losing the batteries?

[BrokenYugo]

If they don't hold some charge for a year they weren't very good to begin with.

Something everybody should learn to accept is all rechargeable batteries are inherently limited lifespan devices. Some types/chemistries destroy themselves (lose capacity, gain ESR and self discharge) faster than others, but to the best of my knowledge they all do it, perhaps with the exception of stuff like the old school flooded lead acid motorcycle batteries that give you a bottle of acid to pour in at the time of installation, but that just gives you control over when the timer is started. Attempts at rejuvenation, meticulous long term storage, etc. are mostly a waste of time. Charge them full, put them away (ideally in a steel box away from flammable materials), and forget about it for a year, recharge any that are still above the minimum safe voltage (3V I think?) and repeat.

[tiago1986]

Fully recharge 100%? above 3.8v?

[SilverSolder]

Lithium-Ion batteries stay at a flat voltage for a long time, they don't start dropping until they go very low.

I don't think the curve you posted is a really representative example. I have never seen such curve in any real-life cell, but something close to that might exist of course.

Consumer cells are more likely nearly not that flat; look for practical examples at https://lygte-info.dk/info/indexBatteriesAndChargers%20UK.html (Lowest current graphs are quite close to open-circuit voltage). Most are surprisingly linear between 100%-25% and then drop faster, and this is also my observation on testing ~15-20 different cells (mostly LCO and NCA).

I have only experimented with small li-po bag cells, and they behaved (as a rough approximation) like the curve below at very low rates of discharge.  Leaving the batteries in the fridge is as low a rate of discharge as it gets! 



It is pretty difficult to tell the state of charge of a LiIon cell from its voltage when it is in the midde of its range.

[Munyua44]

the batteries can be stored for over 3 years without getting damaged but it could be good practice if you recharge them every year to check their conditions.

[tiago1986]

charged in 3,8v or full 100%?

[SilverSolder]

Somewhere around 50% is the least stressful for the batteries.

See attached document, it seems to answer most of your questions from a practical perspective.

[Siwastaja]

I have only experimented with small li-po bag cells, and they behaved (as a rough approximation) like the curve below at very low rates of discharge.  Leaving the batteries in the fridge is as low a rate of discharge as it gets! 

Packaging (pouch, prismatic or cylindrical) is unrelated, curve shape is defined by the chemistry (and microscopical details in anode / cathode construction), for example synthetic vs. natural graphite in anode makes a difference.

In any case, leaving the cells anywhere, fridge or not, means zero discharge rate which obviously leads to constant voltage. Did you actually measure (by logging voltage vs. integral of current) the curves of said cells, or did you just get the feeling they kinda match the graph your posted? Serious question, because I have never seen cell that is nearly as flat, but differences are indeed large so one might exist. Ignoring LFP cells, every cell I have tested has been easy to map between open-circuit voltage and SoC. Of course, in presence of large load variations that becomes more difficult to do exactly.

The example is also for the 4.35V (3.8V nominal) LCO chemistry, this is quite a rarity today. The only 4.35V cell I have tested wasn't flat, but very linear like all tested 4.2V LCO cells.

[SilverSolder]

I have only experimented with small li-po bag cells, and they behaved (as a rough approximation) like the curve below at very low rates of discharge.  Leaving the batteries in the fridge is as low a rate of discharge as it gets! 

Packaging (pouch, prismatic or cylindrical) is unrelated, curve shape is defined by the chemistry (and microscopical details in anode / cathode construction), for example synthetic vs. natural graphite in anode makes a difference.

In any case, leaving the cells anywhere, fridge or not, means zero discharge rate which obviously leads to constant voltage. Did you actually measure (by logging voltage vs. integral of current) the curves of said cells, or did you just get the feeling they kinda match the graph your posted? Serious question, because I have never seen cell that is nearly as flat, but differences are indeed large so one might exist. Ignoring LFP cells, every cell I have tested has been easy to map between open-circuit voltage and SoC. Of course, in presence of large load variations that becomes more difficult to do exactly.

The example is also for the 4.35V (3.8V nominal) LCO chemistry, this is quite a rarity today. The only 4.35V cell I have tested wasn't flat, but very linear like all tested 4.2V LCO cells.

I guess I was thinking mostly about the general shape of the curve:  that there is a quite long stretch around 3.7 Volts where the voltage doesn't change as much as it does near the beginning and the end.  The curve is good enough to see the gist of that. 

I did test the cells - but only the voltage, not the current - precisely because I was trying to estimate how long they would last while self-discharging on store shelves!  I did encounter long stretches of relative flatness around 3.7V.

My conclusion at the time (several years ago now) was that they could easily stay charged more than a year in normal temperature conditions, if charged to 50% at the factory.  The problem was to isolate them well enough (FET leakage currents was a problem - not much of an issue in the fridge, although the protection circuits do eat some tiny amount of energy).

[Siwastaja]

Oh, OK. My guess is that you measured them at 3.7V for a long time only because they simply were not self-discharging, so the SoC kept constant, you were at a single point in the curve. Self-discharge of li-ion cells is often overestimated. I have measured between 0-10% of capacity per year, around 3-4%/year being typical at room temperature, and this is for a full cell. At 3.6-3.7V, self-discharge is between 0%-1% per year.

Linked is a typical example curve from lygte-info.dk:


Notice how different this is compared to the Silicon Lightworks example? You can easily see the SoC from the voltage exactly because the curve does not have a flat area. Straight line fit between 4.15V and 3.2V does good job giving 100%-5% SoC, and you can leave 5% for the birds.

But yeah, some are flatter, but Silicon Lightworks example seem pretty extreme!

[SilverSolder]

Oh, OK. My guess is that you measured them at 3.7V for a long time only because they simply were not self-discharging, so the SoC kept constant, you were at a single point in the curve. Self-discharge of li-ion cells is often overestimated. I have measured between 0-10% of capacity per year, around 3-4%/year being typical at room temperature, and this is for a full cell. At 3.6-3.7V, self-discharge is between 0%-1% per year.

Linked is a typical example curve from lygte-info.dk:


Notice how different this is compared to the Silicon Lightworks example? You can easily see the SoC from the voltage exactly because the curve does not have a flat area. Straight line fit between 4.15V and 3.2V does good job giving 100%-5% SoC, and you can leave 5% for the birds.

But yeah, some are flatter, but Silicon Lightworks example seem pretty extreme!

The curve is flat enough that estimating the State of Charge based on voltage is challenging.  Basically, the open circuit voltage of a Lithium Ion battery is not a very precise indicator of the SoC.

People have done all kinds of desperate things, up to and including Coulomb-counting to try to better keep track of the SoC. 

E.g. see this article: https://www.lithium-battery-factory.com/lithium-battery-state-of-charge/

This whole area is a minefield! 

[tiago1986]

i charge for 3,8v what self discharge per year?