How to use two new cells for a one-cell gadget?

[Fallingwater]

I just received a cheap chinese knockoff smartphone (loving it already ). It came with two 1300mah LiIon cells.
I can use them alternatively, charging one while using the other one in the phone and replacing them when needed.
Or I can discharge one to 40% of its capacity and keep it in storage while I use the other one in the phone. When that one dies in two or three years I take the first one out of storage and use that.

What would you do?

Come to think of it: what voltage (open circuit or under load?) does a LiIon have when it's at 40%?

Also: I heard about storing cells in the freezer when not using them for a long time to slow down the chemical breakdown and prolong their life, but it was in an article about NiMH. Would a LiIon benefit from this too? Or would its chemistry get damaged by the freezing temperature?

Thanks.

 

[Valolammas]

I have heard, that li-ions degrade in storage almost as much as in use, so there's no point in storing them. After a couple of years, your spare one will have aged as well, even if you have never used it. So I'd say use them both.

Don't know about the 40% voltage or freezing.

 

[Fallingwater]

No, LiIon degradation is considerably lower if stored at a charge of 40%.

 

[VidPro]

i would use them both, knowing that both will be dead in eventuality
or , store one discharged some, and cooler.

which brings up , you should check to see if the curcuit on them would discharge them all by itself. if that occurs, then you could come back to it in a year, and its just fully dead.
SOME protections and "info" type curcuits have a very tiny drain, some dont have hardly any. so dont assume that you can park it, ignore it for a year, and it will be at 30% charge, When you stored it at 40-50% charge. Check to see how it acts, then ignore it.

i think that would be the biggest issue with storing a "Pack" like a cell phone or pda type battery, if it does or does not drain due to the curcuit.

also you might cycle it once every 3-6months just to check its condition, and exercize it. then store it low again.
a simple method for doing that might be to:
refridgerate it sealed from dampness,
take it out bring it back to room temps,
Recharge it
then switch it with the other one, After you had used the other one to about 1/2.
store the one you were using, low and cool, and use the one you stored for a bit.
that way both get stored (properly), both get used, and both get cycled.

 

[Fallingwater]

Hmm. Good points.

Now I just need to know how to figure out 40% level. What's the voltage, and should I measure it with some current draw or open circuit?

 

[legtu]

a li-ion @ 40% is around 3.8v, unloaded.

you can do a forum search if you need more info.

 

[DM51]

Legtu's figures are correct.

However if I were you, I would take advantage of having 2 batteries, and use both alternately. They will last well if you recharge them little and often - don't let them run down to below ~40% on the readout (if your phone has a batt level indicator, which most do.)

The batteries will probably last longer than the phone, lol. You are bound to be looking to replace it long before these batteries die.

 

[Daniel_sk]

At room temperature, with full charge, you are going to loose 20% of the capacity each year. But you are just going to loose 4%/year if you store it at 40% charged (room temperature).
It's mainly the heat that destroys a battery, at 60°C and 100% charge - there is a 40% permanent loss of capacity in 3 months. For example some notebooks get quite hot and this affects the battery capacity..

Picture and information taken from wikipedia:

 

[Fallingwater]

The batteries will probably last longer than the phone, lol. You are bound to be looking to replace it long before these batteries die.

I wouldn't be so sure. I kept my last phone for almost four years. I very rarely replace stuff if I don't have a need for something new.

I hear what you say, but I think I'll discharge the second cell to 40% and stick it in the freezer in a plastic bag. If that graph is to be believed it'll lose about 2% capacity every year... so when I'll need it, in two to three years, it'll have rather more than 90% of its original capacity left.

And if the phone breaks before then, I have a LiIon cell to play with

 

[DM51]

I really would NOT advise freezing it. Refrigerator temperature (~4degC) will be fine. Put it inside a Ziploc, then maybe that inside another one, then that in a plastic box with a big label on it, otherwise in 3 yrs time you'll have forgotten what it is lol.

 

[Daniel_sk]

Another quote from wikipedia:
"According to one book,lithium-ion batteries should not be frozen (most lithium-ion battery electrolytes freeze at approximately −40 °C; this is much colder than the lowest temperature reached by household freezers, however). "

 

[legtu]

i prefer 'storing' a spare batt for later usage as compared to maintaining and switching between two separate ones. less hassle and 'wear' on the gadget.

no need to freeze, low temperatures is enough. aside from storing them in an air-tight container, i also include a couple of silica gel packets along with the batt to minimize/eliminate moisture.

 

[Fallingwater]

Hmm.
To freeze, or not to freeze: that is the question.

Freezing would probably slow down the chemical breakdown of the electrolyte even more, but could it damage the cell?
I like wikipedia a lot, but it's not always 100% accurate. Has anyone got any experiences about storing LiIon cells in the freezer?

Thanks.

 

[Daniel_sk]

I don't have experiemence with that, I'd say - store it arround 0°C-5°C, there isn't much difference in storing below this point.

Here is a quote about storing batteries from batteryuniversity.com:
"Keep batteries in a cool and dry storage area. Refrigeration is recommended but freezers should be avoided. When refrigerated, the battery should be placed in a plastic bag to protect against condensation"

Also some other source do not recommend freezers. Refrigerators are OK. I would go the safer route...

 

[Daniel_sk]

This is also a good article about prolonging the life of li-on batteries:
http://www.batteryuniversity.com/parttwo-34.htm

 

[Fallingwater]

Ok then, no freezing. It'll just go in the fridge.
Thanks

 

[VidPro]

Ok then, no freezing. It'll just go in the fridge.
Thanks

my thing about freezing is, if you can keep it at a balanced freezing temperature , then its fine, but freezers fluxuate in temps quite a bit, which expands and contracts stuff.
expanding and contracting cant be good for electrolyte rolls, a refridgerator can be more balanced in temps 40to32F, and doesnt go all extreeme.

also some people had OTHER issues with freezing related to curcuits attached to, mabey the electrolyte survived, but moisture (enemy) and expansion contaction, caused curcuit issues.