rechargeables for A2 (I searched, I read, please be gentle...)

[novice]

I have ended up with three A2 lights (2 4-flat bodies and 1 'round' body). I also managed to snag two used A2 strion kits and am looking forward to getting the 'onion' led rings when they come out. The last thing to do to facilitate using them more often is getting a lithium rechargeable set-up. I have been shamelessly exploiting AA Nimhs whenever possible, sometimes using extenders with various other set-ups (or M*glites), and I have been a little intimidated by the known hazards of lithium rechargeables, but I really want to start using the A2 (amongst others) more capriciously, so it's time to take the plunge.
I know that this issue has been raised many times before, but I am struggling with it for the first time. I have spent a number of hours reading hundreds of posts over the last few days.
I really, really, want to use 'protected' rechargeable cells (don't want to have to deal with a DMM for now). Runtime is less of a priority. It seems the challenge is finding protected cells that fit an A2 body.
This is what I came up with:
http://www.batteryjunction.com/rc390reliba.html

It's a Tenergy '3-3.2 volt' (-3.6v) RCR123a 900 mAh 'yellow label' cell. It has PCB protection. Its external diameter is 16.5 mm, or approximately 0.649 606 299 21 inches. On one of the many A2 posts, someone listed the internal diameter of their '4-flats' A2 as being 0.653 inches, and the round-bodies are supposed to be a little bigger. This cell's height is 34mm, or 1/2mm shorter than the dimensions that I have seen listed for one brand of primary CR123a, so it's not too long.

About the only thing that concerns me is the Battery Junction page warning against using these in incan SureFires, although it looks like some people on cpf are using cells that can be as high as 4.2v fresh off the charger, saying that any 2xCR123A SF led light can take as high as 9 volts(!)
[ http://www.candlepowerforums.com/vb/showthread.php?t=205723 , post# 3] , but the BJ product page specifically warns that SF voltage regulation takes ~12 milliseconds to kick in. Is that going to be a problem with lithium rechargeables?
I've only seen one post mentioning this cell as their choice in an A2, and I'm wondering why.
To recap, here are my main questions:
#1 – Will this Tenergy 'yellow label' work safely in my A2?
#2 – If this cell is appropriate, what is the best possible 'smart charger' for this type of cell (not including 'hobby chargers')? I get the impression that the highly-regarded Pila IBC will NOT work for this type of cell. If this cell is appropriate, and there are no clear-cut top choices amongst others more experienced than I, I would probably go with the somewhat inexpensive 'smart charger' listed at the bottom of the BJ product page for this cell.
#3 – Can I safely use this same cell in my E2L, E2DL, and U2?
#4 – What sort of decrease in brightness might I expect using 2 of these with a SF P91 lamp?
Thank you very much for reading this, and sharing your expertise.

 

[KiwiMark]

To recap, here are my main questions:
#1 – Will this Tenergy 'yellow label' work safely in my A2?

My guess is yes - I'm sure others with more experience with these will post what they know.

#2 – If this cell is appropriate, what is the best possible 'smart charger' for this type of cell (not including 'hobby chargers')? I get the impression that the highly-regarded Pila IBC will NOT work for this type of cell. If this cell is appropriate, and there are no clear-cut top choices amongst others more experienced than I, I would probably go with the somewhat inexpensive 'smart charger' listed at the bottom of the BJ product page for this cell.

You cannot use standard Li-ion chargers or hobby chargers - you MUST use chargers designed to work with these cells with their voltage reduction circuitry.

What I use with my A2 are these:
http://www.cpfmarketplace.com/mp/showthread.php?t=125353
They are higher than 3V but they aren't as bad as 4.2V, they charge to a maximum voltage of 3.6V and have a nominal voltage of 3.2V. With a minimum voltage of 2V you should find the incan light goes out (regulation circuit cuts in off when the voltage drops too low) before the cells are drained too low.

The LiFePO4 cells don't have as much capacity, but I have 2 sets and don't find it to be a problem. They are also a safe chemistry, no explosion risk! This makes them OK to use even though they are not protected, they are actually safer than the non-rechargeable lithium batteries.

 

[Yavox]

I use AW's IMR16340 for my A2. This is not the stock version of A2 - it is modded with Strion kit and the stock LEDs were changed to Aviatrix ring. Both of those light sources (the strion bulb and the LEDs in Aviatrix) are known to work well with 2 x 3,7V IMRs.

I think I have read somewhere that the onion ring also works well with higher voltage, but I am not sure - you should ask the manufacturer.

The IMRs don't meet your requirement of being "protected" but I am not sure if you really need protected cells for your A2 - the incan bulb is regulated and cuts off when about 25% of the cells capacity remains so there should be no risk of depleting the cells too much by the incan itself. On the other hand, remaining 3 LEDs drain so little power and work on those 25% of fuel long enough - you should have a lot of time to recharge.

Thats my 2 cents. Maybe there are some side effects and drawbacks of using modded A2 with IMRs which are known to more experienced users than me. The only thing I know is that A2 with stock LEDs should not be used with rechargeables because the LEDs can be damaged. But I have also heard from people using their stock A2s in that configuration with nothing bad happening to them.

 

[KiwiMark]

The IMRs don't meet your requirement of being "protected" but I am not sure if you really need protected cells for your A2 - the incan bulb is regulated and cuts off when about 25% of the cells capacity remains so there should be no risk of depleting the cells too much by the incan itself. On the other hand, remaining 3 LEDs drain so little power and work on those 25% of fuel long enough - you should have a lot of time to recharge.

Whoa, whoa whoa!

The Surefire A2 runs the incan bulb from a regulated circuit that will cut off when the input voltage drops too low. I believe that typically there will still be about 25% charge left in the CR123a cells when the incan cuts out and that will be enough to run the LEDs for about 4 hours. But that would NOT mean that 3.7V Li-ion cells will still have 25% of their charge when that same cut off voltage is reached. The Surefire lights have been designed to run with lithium primary cells that have a nominal voltage of 3V, what applies to them will not apply to Li-ion cells with a nominal voltage of 3.7V.

The voltage at which lithium cells are down to only 25% of their capacity left is likely to be somewhere around dead flat on Li-ion cells or even getting low enough to damage them. I would advise being a bit cautious about assuming the Li-ion batteries will be fine down the the incan cut-out voltage.

 

[Yavox]

The Surefire A2 runs the incan bulb from a regulated circuit that will cut off when the input voltage drops too low. I believe that typically there will still be about 25% charge left in the CR123a cells when the incan cuts out and that will be enough to run the LEDs for about 4 hours. But that would NOT mean that 3.7V Li-ion cells will still have 25% of their charge when that same cut off voltage is reached. The Surefire lights have been designed to run with lithium primary cells that have a nominal voltage of 3V, what applies to them will not apply to Li-ion cells with a nominal voltage of 3.7V.

The voltage at which lithium cells are down to only 25% of their capacity left is likely to be somewhere around dead flat on Li-ion cells or even getting low enough to damage them. I would advise being a bit cautious about assuming the Li-ion batteries will be fine down the the incan cut-out voltage.

Yeah... you are right, this is something I have missed completely - reaching the cut off voltage which means 25% capacity of CR123a (which started from 3.0V) means something quite different than 25% for an RCR123 which was 3.7V initially.

Maybe someone could measure the voltage of CR123 cells just after the incan part of the A2 stopped working and we could have a look at the discarge curve of an IMR16340 and see if we are still OK or in a danger zone.

 

[KiwiMark]

Maybe someone could measure the voltage of CR123 cells just after the incan part of the A2 stopped working and we could have a look at the discarge curve of an IMR16340 and see if we are still OK or in a danger zone.

That would be good to know - I tried searching for the cut-off voltage and couldn't find anything. I'd like to know the actual number if anyone has done tests and can provide it.