i was wondering, like since we have protected cells which prevent the voltage from going over 4.2v when you charge them, is there also something which stops the cells going below 2.75v when you are using them? like as far as i know all the torchlights will drain the batteries until theres no voltage left and this damages lithium cells... is this a big problem, and if it is, is there a solution?
lithium battery cutoff
- Thread starter carmatic
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interesting... i sure hope that the cells i've bought will stop themselves from undervolting... so what you would get is like the light would shine normally, and then suddenly cut off or something? my T1 has a low battery warning of some sort, i think its some kind of 'reserved' brightness level or something, but with undervoltage protected cells i dont think it will matter much...
Illum
Flashaholic
from experience AW cells have overcharge/overdischarge protection and overcurrent discharge protection, but lack an overcurrent charging protection
ScubaSnyder
Enlightened
I ahve ultrafire RCR123's and 17650's that are protected and when they get low the light just shuts off out of nowhere, somewhat of a drawback but what can you do at least it saves your cells. I have been using unprotected instead just because I need light for Mt. bike riding.
DM51
Flashaholic
As stated above, protected Li-Ion cells have over-discharge (low-voltage) protection, which prevents them from being drained to empty.
However it is unwise to rely too much on this protection. Li-Ion cells prefer to be topped up after each use - they do not need to be fully discharged before each recharging.
I have moved this thread to the 'Batteries Included' section.
However it is unwise to rely too much on this protection. Li-Ion cells prefer to be topped up after each use - they do not need to be fully discharged before each recharging.
I have moved this thread to the 'Batteries Included' section.
Illum
Flashaholic
I discharge my lithiums to the specified 40% [3.8V in a PM to AW] for storage reasons....so your saying I should store them at full charge instead?
DM51
Flashaholic
No, I'm not saying that at all. What you are doing is 100% correct. For storage of Li-Ions, 3.8V is the recommended figure. It is also perfectly OK to discharge to this figure on every cycle.
I was replying to the OP and a couple of the subsequent posts, about relying on the low-voltage protection cut-off. This occurs at a much lower voltage, when the cell is fully discharged.
These figures (below) have been posted before, but it may be useful to repeat them here for those who may not have seen them before. They are the resting voltages of Li-Ion cells with the corresponding state of charge:
4.2 volts = 100%
4.1 volts = ~90%
4.0 volts = ~80%
3.9 volts = ~60%
3.8 volts = ~40%
3.7 volts = ~20%
3.6 volts = empty for practical purposes
<3.5 volts = over-discharged
4.1 volts = ~90%
4.0 volts = ~80%
3.9 volts = ~60%
3.8 volts = ~40%
3.7 volts = ~20%
3.6 volts = empty for practical purposes
<3.5 volts = over-discharged
Resting voltage is an accurate indicator of state of charge in Li-Ions. It can be measured when the cell has been resting for ~30 minutes.
Low-voltage protection is set <3.0 volts, to allow for voltage sag while the cell is under load. Ideally, a cell will bounce back up above 3.5 volts after it has been taken off load and allowed to rest, but this depends on the load (current). A small current (light load) will cause less voltage sag, so paradoxically the cell is in more danger of over-discharging if the user are not careful.
You can see from the table above that cells should be recharged before they get as low as 3.5 volts at rest. They can be topped up at any time – this will do no harm, and it is much better than running them too low. I try not to let mine get below ~3.7 volts.
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There's an interesting paper at http://www.smartsparkenergy.com/pdf/VPPPaper.pdf with a formula to calculate state-of-charge based on LIon OCV, albeit only valid from 30% up to 100% charge. There's a bit of info on cycle life in battery packs as well and how cell balancing can affect that.
i got http://www.dealextreme.com/details.dx/sku.8683 these cells...
my fenix t1 operates from like 8.4v to 3v, but i guess realistically im going to only run it down to between 6 or 7 volts.. maybe the current drain at those voltages to mantain the same brightness levels will be low and the voltage sag will make things abit easier to manage...
VidPro
Flashlight Enthusiast
those cells are protected, (when working properly) they will cutoff at a low voltage, and will stay cutoff till you charge them again.
so i think that answers your original Q, they do have both ends of protection.
the only concideration that you need for them, beyond just using them and having fun, would be to recharge when they are low, say if you discharged them very very slowly on some very low setting, they will be lower than if you discharged them really fast.
because
a high rate of discharge, causes a battery voltage droop, that triggers the protection faster. the battery is running lower voltage then.
At a very low rate of discharge, there will not be as much battery voltage droop, so you will have a more complete discharge of the cell before you reach the low cutoff.
The short version:
When your light is on HIGH and you get a cutoff, and it stops, eventually recharge it
when your light is on LOW and you get a cutoff, charge it more immediatally, so it doesnt stay so low.
at any rate, the low cutoff will work fine (when its all working right) and you shouldnt have to worry about anything at all.
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