18650 Battery - Unused / Rarely used lights best practise

[southernson]

OK, my question may raise a few curious responses, so I'll give a short version for those who have an answer and don't need to know the reasons why, and a longer version for those who are intrigued.


Short Version:

I have 3 lights that all use the same battery type (18650). Only one of these lights are likely to be used (Main light). The other two lights will rarely be used (they are backups), but need to be ready to go at a moments notice.

Am I better off:

a) Top up the 'backup' light batteries every month or two, or

b) Rotating the batteries. (ie, 'top up' a cell from a backup, put it into the main light, and put the main light battery into a backup after charging after use, or?

c) If possible, use a AAA-18650 converter and use AAA batteries instead of a 18650 in the backup lights, (not sure these lights will take an adapter yet) or?

d) something else?



Longer Version

I'm a cave diver. We are required to have a primary light, as well as a minimum of 2 backup lights on all dives. My new lights are all LED's that use 18650, but my main light is push button with multiple modes, and I have 2 backup lights (different model) that are twist rotate (on/off only) that are less brighter. As such, there will always be one particular light (push button) used frequently, and the other two will be rarely used. (The backup lights).

I understand that rechargable batteries aren't designed to be charged, and then left in a light unused for any length of time, but that's exactly what is required for my backup lights. I need the backup lights to be reliable and 'ready to go', so I'm looking for options for this to work for me.

The options I have come up with are:

a) Every month or two 'top up' the backup light batteries. As I understand it from reading other posts here, li-ion's work on a cycle basis - so 'topping up' the batteries should use little of the cycle life. But then I also understand that li-ion's aren't designed to be stored for a long time unused at full charge, so I'm not sure about this.

b) Rotate the batteries. I figure the benefit for this is that each battery gets used and a good workout, plus I should know if a batter is going bad as I would find out whilst using it as my primary light and know it's time to replace it? It's a bit more of a pain to keep 'track' of the battery rotation though.

c) I've been told that AAA cartridges may fit my lights. If this is the case, I could use AAA batteries in my backup lights (after all, they're designed to be stored for much longer I believe)?

d) Is there something else I haven't considered?

To me, (c) seems to be the best approach but I don't know if this will work - so if it's not possible, should I be going (a), (b), or (d) - or is it much of a muchness and am I over thinking this. Considering that the backups could be the difference between life and death I'm wanting to make sure that I dot all my i's and cross all my t's.

Thanks in Advance...

 

[BringerOfLight]

a) Every month or two 'top up' the backup light batteries. As I understand it from reading other posts here, li-ion's work on a cycle basis - so 'topping up' the batteries should use little of the cycle life. But then I also understand that li-ion's aren't designed to be stored for a long time unused at full charge, so I'm not sure about this.

While that is the case, rotating the batteries won't really improve things. The batteries degrade from discharge cycles (but it sounds like you won't be using them enough to matter much) and just sitting around. The sitting around degradation increases (quite significantly) with charge state and temperature. Not fully charging the batteries will help (e.g. only charge to 4.0 or 4.1V) as will storing them cool (fridge).

The damage mechanism is synergistic, so fully charged + high temperature = really bad.

Liion batteries have very little self discharge and the protection circuits generally seem to have very little parasitic drain (IIRC, HKJ measured less than 5uA for the circuits he tested). So topping off should be required very, very rarely.

c) I've been told that AAA cartridges may fit my lights. If this is the case, I could use AAA batteries in my backup lights (after all, they're designed to be stored for much longer I believe)?

3 AAA Eneloops will have 1/3 the energy of one 18650. So you might as well just use 18650s that are only half charged.

\\

How much the 18650s degrade will heavily depend on the specific cell. There have been chemistries, where the battery will be toast if it's stored at full charge at 30C for a year. Other cells may handle that well and just loose a few percent capacity.

 

[SilverFox]

Hello Southernson,

Welcome to CPF.

I agree that it would be better to simply not have a fully charged cell in your backup lights.

However, you need to make an assessment of how long you need the backup light to run. This will require some testing.

Fully charge a cell and put it into your back up light. Turn it on and start a timer. Now you have an idea of total run time. By charging the cell to something like 4.0 volts you only end up with around 80% of total capacity so now you can decide if 80% of the total run time is enough.

If it is then you can store your backup lights with cells that are at roughly 80% of full capacity. This will be much easier on your cells. You do nothing but check the light from time to time to make sure it is still working. Once a year check the voltage of the battery just to make sure there are no surprises there.

The next problem is how to get your batteries from fully charged to 80%. The easiest way is to fully charge them and then use the light for 20% of the total run time.

Tom

 

[yoyoman]

Could I be missing the obvious? I don't think you go cave diving everyday or even every weekend. So if you're not going cave diving for any length of time, discharge to cell to 80% and let them rest in peace. Then, right before you go for a dive, charge them up to 100% so you have full back ups.

 

[thedoc007]

Considering that the backups could be the difference between life and death I'm wanting to make sure that I dot all my i's and cross all my t's.

Given this, I think you are making this more complicated than it needs to be. Charge them fully, recharge every three to six months as needed (probably won't need it even this often, but you should at least check). Don't worry about rotation or discharging exactly 20% before you store it/use it. And when a year or two has gone by, buy a new 18650 for your backup lights. You have less to worry about, it is very easy to manage, and $20 per year is a small price to make sure you always have fresh cells. You can still use the 18650s for other applications, but a consumable product that is life-critical should not be used for longer than that anyway.

 

[southernson]

Hi All,

What a weath of information! Thanks very much for your replies!

80% capacity should be more than enough for my needs. Backup torches don't need to be bright, and thankfully it appears as though the drivers in the lights cut down on usage power, so after 3hrs I've still got some form of light. (Enough for me to find my way through restrictions, etc).

Very handy for my purpose, but not as handy for calculating run time for 20% discharge. Easiest option I guess is just to use a multimeter.

I haven't found a charger (yet) that allows me to choose the level of voltage to charge to - or one that will discharge to a level yet, otherwise I might consider going that way.

I've got no problems replacing the batteries every year or two. Pretty low cost considering the importance of them.

Seems like the two good options I've got are either:

a) Discharge to 80%. Then either fully charge them before I dive (if I choose to), but no concern if I don't - as 80% should be enough anyway, or

b) Charge fully, and test every 6 months or so - topping up as required, and replacing every couple of years.

Nice to have different options that are all workable. The last time I had experience with rechargable batteries was back in the 80's with AA rechargables in toy RC cars and they seemed to never hold enough compared to normal batteries, lose charge too quickly and get memories too quickly. Looks like technology has improved very nicely over the years!

Thanks again!

 

[thedoc007]

Nice to have different options that are all workable. The last time I had experience with rechargable batteries was back in the 80's with AA rechargables in toy RC cars and they seemed to never hold enough compared to normal batteries, lose charge too quickly and get memories too quickly. Looks like technology has improved very nicely over the years!

Yes, I was really surprised by how much batteries have improved. My father had a bunch of (I think) NiCads when I was growing up, and they were rubbish. Wouldn't hold a charge for anything, died suddenly with no warning, DRAMATICALLY shorter runtime compared to alkalines. But the situation has definitely changed...for anything other than super low-drain, NiMH spanks any kind of alkaline. And lithium ion is in a whole different league, especially if you go for 18650s.

Let's hear it for progress!