Optimal voltage for Li-ion batteries’ storage

17 posts / 0 new
Last post
xtarflashlight
xtarflashlight's picture
Offline
Last seen: 2 days 12 hours ago
Joined: 01/21/2013 - 21:25
Posts: 206
Location: Shenzhen
Optimal voltage for Li-ion batteries’ storage

Li-ion batteries should be stored in a charged state, maintain a voltage above 2.5V before they start to break down and decompose. According to the Li-ion batteries’ chemical features, as permanent capacity loss is greatest at elevated temperatures with the batteries voltage maintained at 4.2 V (fully charged), you also couldn’t maintain them at fully charged 4.2V. For maximizing storage life, ideally, it is best to top-up the batteries at 40% of its standard (4.2V) charged state, around 3.7V. The 40% charge assures a stable condition even if self-discharge takes some of the battery’s energy. Most battery manufacturers also store Li-ion batteries at 15°C (59°F) and at 40 % charge.

If your Li-ion batteries are not used for long time, don’t forget to maintain them every 2-3 months. And how to store your Li-ion batteries at optimal voltage easily? Actually, some smart chargers can help a lot. Such as the vc8 charger, there is STORE mode setting (for CH1-CH4). You can put the Li-ion batteries on the slots, choose store mode, then the charger will charge or discharge the batteries to 3.7V automatically. It’s simple to operate, help you to maximize the batteries’ lifespan and save energy easily.

Do you like this storing function on XTAR VC8 charger? Smile

JaredM
JaredM's picture
Online
Last seen: 4 min 16 sec ago
Joined: 10/31/2011 - 13:33
Posts: 2242
Location: Pittsburgh, Pennsylvania

Didn’t know the VC8 had that feature. Good to know! I’m interested in an 8 bay advanced charger, and have bought and gifted MANY MC1 chargers over the years.

samgalax
Offline
Last seen: 3 hours 45 min ago
Joined: 03/22/2012 - 18:06
Posts: 694
Location: In front of you!

I think the storage voltage is about 3.8 volt

Please SUBSCRIBE to my youtube channel (flashlights, electronic gadgets and various stuff..)

https://www.youtube.com/channel/UCCF3RcoQ3Ff05Cr4-Gg5qAw

I am a beginner. So please help me to make the channel "grow up"

Thanks!!

Henk4U2
Henk4U2's picture
Offline
Last seen: 42 min 30 sec ago
Joined: 02/13/2014 - 17:52
Posts: 4128
Location: The heart of the Netherlands (GMT+1)

That is a very sensible question, but for me it is pure theoretical.
Atm. I own quite a few lights, and all have batteries in them.
About 60 of them are LiION’s. I top them up, once in a while (½y < x < 1y).

My main concern is not longevity, but availability.
The only batteries that ever went dead on me were high self discharge NiMH’s.
Maybe I would change my mind if I went into the battery business myself.
But until then ……….

You are a flashaholic if you are forced to come out of the closet, to make room for more flashlights.

WalkIntoTheLight
Offline
Last seen: 8 months 1 week ago
Joined: 12/05/2015 - 10:26
Posts: 2231
Location: Canada

I just checked some of my cells in long-term storage. Mostly new 18650’s, that have been sitting in a drawer for around 1.5 years. 30Q’s, VTC6, GA, 35E.

I store them between 3.70v and 3.75v, usually on the lower end of that. All are sitting at 3.70v – 3.71v, with one of the 30Q’s just under 3.70v.

So, yes, they discharge very very slowly when stored at about 50% charge. I think they’d last 10 years in storage, at that rate.

I haven’t experienced high self discharge of 30Q’s, except when they are clearly damaged in some way. Losing about 0.1v per day, which indicates an internal short. These were some cells from China, so perhaps they were “bad” 30Q’s to begin with. They only lasted a couple of dozen cycles.

The only other cells I have that self-discharge enough to notice, are cheap Chinese no-name cells (which I got when I purchased something).

WalkIntoTheLight
Offline
Last seen: 8 months 1 week ago
Joined: 12/05/2015 - 10:26
Posts: 2231
Location: Canada

Yes, all my cells were new when I put them in storage.

However, that graph looks like a “dead” cell. After 700 cycles, it’s worn out and can’t hold a charge. Decreasing by 0.8v in just 3 days means there’s either an internal short, or there’s so little capacity left in the cell that any charge is just a surface charge.

IMO, that graph shows that the VTC6 has more cycles than a 30Q, not that the 30Q has higher self-discharge. At least, those particular cells with that test setup.

The spec sheet says the 30Q is rated for 250 cycles, whereas the VTC6 is rated for 500 cycles (both to 60% of original capacity). So, it’s no surprise that after 700 cycles, the 30Q is completely dead, whereas the VTC6 might still have some life left.

zoulas
zoulas's picture
Offline
Last seen: 29 min 4 sec ago
Joined: 06/01/2020 - 08:35
Posts: 1963

So how much longer does a hypothetical 18650 battery last if stored at 3.7v vs 4.2v?

d_t_a
Offline
Last seen: 16 hours 16 min ago
Joined: 08/04/2017 - 23:58
Posts: 2557
Location: Manila, Philippines
Quote:
Samsung 30Q had 93 % of initial capacity after 700 cycles (March 2020).

Samsung 30Q still has 93% of initial (new) capacity after 700 (full)charge/(full)discharge cycles? That seems to be very good if true (although you mentioned they self-discharge faster)..

zoulas
zoulas's picture
Offline
Last seen: 29 min 4 sec ago
Joined: 06/01/2020 - 08:35
Posts: 1963

Am I reading this right? The difference is 1% after 200 days?

jon_slider
jon_slider's picture
Offline
Last seen: 5 hours 37 min ago
Joined: 09/08/2015 - 12:20
Posts: 5046
Location: Central North America

I store fully charged

what I call storage is actually different than what a battery seller calls storage.

my storage is: ready for use as needed, and my batteries do not sit unused for a year.

a battery/charger selling company like xtar has entirely different storage intervals

For me,
it does not make sense to store batteries in a way that Requires Charging before use.

I would have no use for a charger that discharges my batteries.

my strategy is, store fully charged,
so I can put a fully charged battery into my light.

the depleted cell, which might be at 3.6v, goes on the charger, and into my battery storage case. Ready to be used in a flashlight.

otoh
if I had a warehouse full of batteries, stored for years at a time.. well.. that would not be me..

I dont store batteries in a light for a year, I use them regularly, and recharge accordingly..

I do have a light in my car tool kit, that seldom gets used. I stored it fully charged, so it will be ready, in case I need it.

WalkIntoTheLight
Offline
Last seen: 8 months 1 week ago
Joined: 12/05/2015 - 10:26
Posts: 2231
Location: Canada

docware wrote:
Sorry for not providing enough info.
30Q spec says :
3.3 Standard charge CCCV, 1.50A, 4.20 ± 0.05 V, 150mA cut-off
3.7 Max. continuous discharge 15A(at 25℃), 60% at 250 cycle
3.8 Discharge cut-off voltage 2,5 V
7.10 Cycle life With standard charge and maximum continuous discharge.
Capacity after 250cycles, Capacity ≥ 1,800mAh (60% of the nominal capacity at 23℃)

Parameters of my cycle aging test : charge 1A to 4,1 V, 0,1 A cut-off, discharge 2,5 A to 3,3 V. So much milder conditions.
Samsung 30Q had 93 % of initial capacity after 700 cycles (March 2020).

I’m still not sure on what exactly is going on. But, it you’re seeing a cell drop hundreds of millivolts due to self-discharge over just a few days, then that cell is damaged, and is a “heater”. After 700 cycles, even if they are just partial cycles, it’s very possible the cells is just worn out.

The spec sheet for the VTC6 claims twice the number of cycles as the spec sheet for the 30Q. So, it shouldn’t be any surprise that a 30Q will wear out faster than a VTC6.

Again, I’m not sure what this has to do with self-discharge. Your worn-out 30Q is showing signs of being past the end of its life.

However, I’m unsure what you mean by it still having 93% of its capacity. It surely won’t have that after 700 cycles! If it really has plenty of capacity left, then that discharge graph is showing it has an internal short.

Helios azimuth
Offline
Last seen: 3 hours 6 min ago
Joined: 04/08/2019 - 22:23
Posts: 348
Location: Sierra Nevada Mountains

zoulas wrote:
So how much longer does a hypothetical 18650 battery last if stored at 3.7v vs 4.2v?

A study from 2017 projected capacity fade after 15 years in storage. At 77F batteries stored at 4.1v were projected to have 84% of their original capacity and batteries at 3.7v were projected at 91%.
jon_slider
jon_slider's picture
Offline
Last seen: 5 hours 37 min ago
Joined: 09/08/2015 - 12:20
Posts: 5046
Location: Central North America
Helios azimuth wrote:
A study from 2017 projected capacity fade after 15 years in storage. At 77F batteries stored at 4.1v were projected to have 84% of their original capacity and batteries at 3.7v were projected at 91%.

thanks

so, 7% less runtime capacity for the ones stored at 4.1v, ready to use without recharging.

CNCman
CNCman's picture
Online
Last seen: 13 min 29 sec ago
Joined: 04/07/2018 - 10:10
Posts: 1207
Location: Mississippi Gulf Coast

Quote:
Such as the vc8 charger, there is STORE mode setting (for CH1-CH4). You can put the Li-ion batteries on the slots, choose store mode, then the charger will charge or discharge the batteries to 3.7V automatically. It’s simple to operate, help you to maximize the batteries’ lifespan and save energy easily.

.
You just sold one ! Wink
.
phouton
Offline
Last seen: 3 hours 31 min ago
Joined: 10/23/2019 - 19:06
Posts: 515

For those curious about voltage retention in charged cells, I have an interactive chart with measured values over the course of more than a year so far: https://budgetlightforum.com/node/72772

In short, after an initial relaxation period of a few days with larger drops immediately after charging, the voltage drop asymptotically settles to a fraction of a millivolt per day, depending on the cell. In other words, it’s pretty stable.

As for the charge capacity, that isn’t immediately obvious, but since it’s related to the voltage, it is presumably similarly stable.

xtarflashlight
xtarflashlight's picture
Offline
Last seen: 2 days 12 hours ago
Joined: 01/21/2013 - 21:25
Posts: 206
Location: Shenzhen

You can try this STORE function on the vc8 charger. Cool With 8 slots, can feel free to charge more batteries at once.

xtarflashlight
xtarflashlight's picture
Offline
Last seen: 2 days 12 hours ago
Joined: 01/21/2013 - 21:25
Posts: 206
Location: Shenzhen
CNCman wrote:
Quote:
Such as the vc8 charger, there is STORE mode setting (for CH1-CH4). You can put the Li-ion batteries on the slots, choose store mode, then the charger will charge or discharge the batteries to 3.7V automatically. It’s simple to operate, help you to maximize the batteries’ lifespan and save energy easily.
. You just sold one ! Wink .

Haha, thanks for your supports! Silly Beer