Author Topic: 21700 batteries  (Read 386 times)

Online Fulk

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21700 batteries
« on: November 01, 2020, 10:08:11 am »
I believe that 21700 batteries are the same size as 18650s. Is that right? If so, are they interchangeable? i.e. can I replace a 3500-mW-hr 18650 with a 5000-mW-hr 21700 and get 10/7 times more run-time?

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21700 batteries
« on: November 01, 2020, 10:08:11 am »

Offline royfellows

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Re: 21700 batteries
« Reply #1 on: November 01, 2020, 10:58:55 am »
No, the number gives size if you remove the end zero.
Thus, 18650 are 18mm X 65mm
20700 are 20mm X 70mm
21700 are 21mm X 70mm

Big deal about the larger sizes is capacity and max safe discharge rate
Capacity tends to be inversely proportional to discharge rate. Example, highest capacity 18650 (quality manufacture not dubious) at this time is 3500 mAh , or 3.5 ampere hours, which has a safe discharge rate of 10 amps.

20700 are rated at 4250 mAh, but average tested capacity is more like 4000 mAh, however they support a discharge of up to 15 amps.
21700 is something I have not yet used or looked into, but a quick search reveals Nitecore, which is probably a rebrand of a quality OEM, at 5000 mAh and a safe discharge rate of 5 amps.


1/Quality manufacture, typically Sanyo or Panasonic against "BangFire" or whatever.
2/Earlier Sanyo 3500 mAh cells were rated at 8 amps
3/Chemistry denoted in type legend. example, latest 18650 is NCR 18650GA. "NCR" = Nickel Cobalt Rechargeable.

This probably more than answers your question, typical me, sorry.
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Offline Stuart France

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Re: 21700 batteries
« Reply #2 on: November 01, 2020, 11:20:46 am »
As Roy says, the 5-digit battery code specifies its dimensions in mm (diameter x length) with a redundant zero on the end for some unknown reason.  Bigger number sounds more impressive?

21x70 is 45% bigger in volume than 18x65 is, assuming the steel wall is very thin.

If the genuine 'best capacity' 18650 cell is 3.5AH and then a replacement with 4AH in a 21700 package seems a waste of time and money as that's an increase in energy content of only 15% for the volume being bloated by 45%, and no doubt the cost too.

If the 21700 has 5AH capacity then that's an increase of 42% which better fits the volumetric maths.

The downside is that neither battery is going to fit into a container designed for the other one.  It will be either too fat/thin and too long/short.  So incompatible.

It is also only a matter of time before someone discovers you can fill a 21700 with less chemicals than other manufacturers do but sell it for the same price as the full-fat version, so caveat emptor.  This applies to any battery of course including "D" size alkaline/rechargeable batteries which contain only a "C" amount of chemicals and, I suppose, 18650 too.
« Last Edit: November 01, 2020, 11:43:03 am by Stuart France »

Online Fulk

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Re: 21700 batteries
« Reply #3 on: November 01, 2020, 02:19:42 pm »
Thanks for your help, Roy & Stuart.

Offline wellyjen

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Re: 21700 batteries
« Reply #4 on: November 01, 2020, 03:10:01 pm »
The redundant zero at the end does have a meaning. The first two digits are diameter in mm, the subsequent digits are its length in tenths of mm. It comes in to its own for coin cells, like CR2032's. 2032 = 20mm diameter by 3.2mm thick.
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