123 batteries and chargers

Depends on the safe voltage range and current draw involved

Lots of variables here ... and the type of situation you'll want to know is ok before something goes

If you are talking about replacing 2x cr123 in series with 6v total with about the same diameter 16mm~ you could try a lithium ion 16650, it will not have 6v but 4.2 fully charged and it does fit in many lights that are normally dedicated cr123 x2.
Keeppower makes about the highest capacity 16650 rechargeable li-ion that I'm aware of.

There is a LiFePO3 (Lithium Iron Phosphate) that charges up to 3.5v instead of the more usual 4.2,, but you need to make sure that the light can withstand 7v instead of 6v (CR123) as max input.

Timothybil said:

There is a LiFePO3 (Lithium Iron Phosphate) that charges up to 3.5v instead of the more usual 4.2,, but you need to make sure that the light can withstand 7v instead of 6v (CR123) as max input.

Click to expand...

The OP did ask about a "single" rechargeable battery to replace 2x 123 so 2x LiFePO3 would still be two batteries or if there is such a thing as a single LiFePo in that size, I'm unaware of that.

It would probably be easier to answer your battery question if you say what the light or device is that you want to use the batteries in, then see what batteries will fit in the right size and compatible voltage range, and energy density.

I ve also tried to find a solution to this in the First-Light TMax 700 and the Tomahawk 425. One x 17670 will physically work but the voltage isn't enough for the light. The old 150 lumen Tomahawks work so/so, 425 works for a bit (not long) before it goes into lockout and the TMax doesn't want to play at all.
The circuit board can handle up to 8v I believe but the light needs 2 batteries. Otherwise an 18650 may have been used to adapt it.

If your light can benefit from a single battery then 17670 is a decent choice... it was either aw or eagletac what I have.

I second Monkey's comments, both the battery recommendation and the further info request.

I 3D printed an adapter so I could use a single 14500 in place of 2 CR123's in an old Streamlight LED.....works real well for lights that run on the lower Voltage of a 14500 (compared to 2X CR123's).