How to preserve 18650 batteries

[HellasTechn]

Good day dear friends !

This is my question. I have a bunch of 18650 batteries that i riped of a laptop battery.
I use them in my flashlight.
Now i try to figure out a way to preserve them. In other words to keep them from going dead.

I have a cheap charger.

My first thought is just to charge them every other week no matter if used or not but then again i have read that this will make the batterie chemicals to go bad.

I have also read that optimum storing voltage is between 3.5V - 4V. That i also can not control.

So what poped into my mind is this:
To fully charge the batteries and then place a component acros the terminals such as an LED lamp or a resistance (say 500 Ohm) in other words something that would drain the battery really slowly and then fully charge them again say every other week. Thinking that maybe by doing this the batteries will not end up dead after short time.

I am open to all suggestions !!!

THANK YOU ALL

 

[eKretz]

18650 Li-ion batteries should take quite some time to self-discharge. Mine usually stay pretty good for months on end.

 

[(*steve*)]

The best way to maintain them is to leave them about 70% charged (from memory).

They degrade is allowed to go flat, and keeping them fully charge is also damaging.

Also room temperature is best.

Since they're going to degrade over time anyway. The other "best" thing to do is to actually use them.

 

[HellasTechn]

keep them all in use is not possible...
That would be the best i know.

That makes me think that either best solution is a resistor across the terminals to keep the battery active untill recharge or maybe Tricle charge ?

 

[(*steve*)]

No, a resistor across them will cause them to go dead flat, and then dead. And trickle charging LiPo's will kill them too.

Determine what terminal voltage you get with a 70% charge, and charge them to this (or discharge them to this) periodically and they will be under the least stress. A little googling suggests that 3.92V is used in some situations and this may be close to the magic voltage.

 

[GreenGiant]

Room temp is actually not the best, LiPO batteries hold charge at about 50-75% state of charge (charged to 100% then discharged to 50-75%) when stored around freezing, this slows the degradation process, just make sure to warm them up for a while before using them.

You can use them while they are still cold but you are better off if they have been warmed up.

 

[HellasTechn]

My initial idea was this...

The batteries are Panasonic CGR18650CA 2000mah.

So at full charge i get 4,2 Volts. according to ohm law if i connect a 500ohm resistor it will cause a current of 8,4ma. Meaning that it will take the battery 238 Hours to full discharge (Supposingly voltage will not drop) or 119 Hours to 50% discharge.

Now if i full charge the batteries and hook up the 500ohm resistor then every 119 hours charge again that sould be enough to keep them live for quite some time.
And would probably be better if i keep them in temprature around 15 degrees celcius

 

[(*steve*)]

No, fully charge them. Then connect the resistor for about 80 hours. Then disconnect it. Then store it (the battery, not the resistor).

Repeated charge/discharge like you have repeatedly suggested will accelerate it's decline.

And as pointed out by Green Giant, I mis-spoke with respect to temperature. Elevated temperatures are BAD. Room temperature or lower is better. 15 degrees should be fine.

 

[HellasTechn]

No, fully charge them. Then connect the resistor for about 80 hours. Then disconnect it. Then store it (the battery, not the resistor).

Repeated charge/discharge like you have repeatedly suggested will accelerate it's decline.

And as pointed out by Green Giant, I mis-spoke with respect to temperature. Elevated temperatures are BAD. Room temperature or lower is better. 15 degrees should be fine.

I got the point.

But eventually i will have to repeat the charge and then the resistor thing some time in the future to prevent them from self dischargeing to deth.

 

[(*steve*)]

Yes, or (preferably) determine the voltage at which it is 70% charged and charge it up to there. (and keep charging it up tho that point) As I mentioned, it may be 3.92 volts...

See here.

 

[GreenGiant]

Theoretically with new LiPO batteries if charged and stored properly they should take years to degrade to dead.
I worked in the R&D department of a battery company and they did long term storage at different State of Charges (SOC) and different temperatures, the best performing were low temp at mid to high SOC, now these tests were sometimes years (when I got there, a few cells had been in storage for 5 years and only lost 3% SOC), just sitting in storage then pulled out and tested, most of them over the course of a year would lose 0-5% SOC sometimes a little more than that. But unless there was a major issue with the cell they would never lose more than 15-20% SOC

You may need to recharge the batteries once taken out of storage but lower temperature will be best

 

[HellasTechn]

I was wondering if someone had a diagram of a 18650 charget that could be set to stop charge at around 3.8 Volts !

 

[(*steve*)]

Find a LiFePO4 charger. These typically terminate charge at 3.6 to 3.8 volts. Other than that, they are pretty much identical to standard LiPo chargers.

 

[HellasTechn]

The schematic here is simple.

http://ww1.microchip.com/downloads/en/AppNotes/01276a.pdf

But useing theese values will the charger stop at 3,8volts ?

I have read the datasheet but i cant fully understand how you can program it to stop at 3,8 volts

 

[(*steve*)]

That device has a fixed end of charge voltage of 3.6V for a single cell.

You'll need to find something where this can be adjusted, or stick with a lower voltage (with significantly reduced capacity)

I have seen chargers that claim to charge to 3.8V, but my recent searching has not uncovered any.

Whilst you could build your own, there is so much logic contained in these chips that it would be difficult to emulate it all.

A colleague of mine made a dedicated current limited voltage source for recharging LIPO's (and clearly you could change the voltage here to whatever you like). The issue is that you lose the ability to do the fancy slow start and finally shutoff after end of charge. However, if charging LiPo to 70%, it may be safe enough. An improvement might be to have a schmitt trigger with trigger limits at 3.8V and 3.7V respectively so that charge is only started if the battery voltage is less than 3.7V and stops once it hits 3.8V.

 

[HellasTechn]

I have to do alot of digging to find a curcuit like that.
One thing for sure. I dont have the knowledge to build one my self !

 

[HellasTechn]

Does anyone know of any affordable good brand of 18650's ?

I know the ultrafire but it is hard to find original ultrafire on e-bay. and the fake ones are really poor quality.

 

[HellasTechn]

Question:

Why theese 18650 are actually 4,2v while rated 3.7v ?

 

[(*steve*)]

Question:

Why theese 18650 are actually 4,2v while rated 3.7v ?

They are nominally rated as 3.6V per cell, but the terminal voltage varies between 2.75 and 4.2 volts depending on state of charge.

Look for a graph of voltage vs charge for these batteries and you'll get the idea. There's no reason you have to charge them all the way to 4.2V by the way. You can get longer service life for them if you don't charge them all the way up. Balanced against that is the reduced available capacity.

 

[shumifan50]

Maybe I have got this wrong, but I watched a presentation about Li-ion batteries and the lecturer indicated that they do not like being discharged below 50% - that is when they go bad AND when they get hot during charging. I seem to remember him saying that the is best to keep them fully charged. NiMH on the other hand must be fully discharged on each cycle to get best life.

Please don't flame if I remember wrong.