Rememeber, that chemistry gets older too 
Usually IR increase is first to come then. Capacity may be ok.
Sadly the 26980 has less capacity and amps than the 26800 cell.
Another vote here for the QB 6800mAh or VapeCell 6200mAh 26800
QB26800 quite hard to buy nowadays…
And yes, I decided to loose money, get wisdom and spread it, I ordered this 26700 6000mAh for test purposes
Well, call me a conservative old @#$%, but I always check if the so desired light runs on “mainstream” sized and built cells and batteries. If not, that’s definitely a deal breaker for me. Especially when the manufacturer of the cell is new to me. When in doubt, check if Keeppower (also) makes them. They make cells and batteries in all different shapes and sizes and they are not a hit-and-run operation.
EDIT: yes, I hate proprietary batteries.
The best 26800 for performance (technically, 26700, I guess :P) is a Molicel P45B and a spacer.
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Which performance, runtime or brightness? 26800 6200mah battery will give you 35 amps at slightly over 6200mah. Vapcell INR26800 6200mah 25A - Vapcell batteries and Chargers Molicel P45B will give you 40amps at slightly over 3900mah. Bench Test Results: Molicel P45B - 50A 4500mAh 21700…an incredible cell | E-Cigarette Forum
Well, strongest implies highest performance (which implies highest brightness) to me. Also, it’s 45A CDR, not 40 - 40 is the P28B/P30B.
Guys, I got 2 pcs of QB26800 6800mAh, but they seem to be used, can anyone tell me at what voltage they came from shop? Those I bought were at 4.11, which is rather odd for new, uncycled cells.
Post a picture. The green ones from Convoy are used EV cells with remainder of tabs on. The yellow one marked QB I’ve gotten have all been new. Look for scuff marks from the being cycled and recharged if not probable new.
No signs on it, those are QB yellow, but both at 4.11 is strange. But no signs, when cycled in machine there will be no signs though.
Well now you’re going to have to just enjoy the extra capacity and better fitting grip.
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OK guys, I spent money, bought this Suijia 26700 6000mAh And after several months it came to me today.
Well, no good news 
IR AC is 14.8mOhm and real capacity under contiuosu 7A automatic discharge (lowering amps at the end to empty cell completely) is only 4903mAh. So like average 21700 cells like LGM50 and worse than 21700 LG M58T which has 5800mAh and little bit better IR.
So 26700 is nothing speciall, I opened dispute to make seller pay me my money back
Other thing is, that QB 26800 is nothing speciall too, it has higher capacity, but internall resistance is very silmilar (about 13.7mOhm IR AC) and it makes EA01S and MF02s not giving full power output 
So again, money spent, apart from knowledge and some capacity with 26800 - no other gains
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