A protected 21700 cell usually measures about 75.5mm long and 21.5mm wide, which is a few millimetres longer than an unprotected cell of the same size. That extra length is the whole story when you are packing for a trip: get it wrong and the battery will not close into your torch or sit down in your charger slot. This guide walks you through the measurements and terminal types to confirm on a protected 21700, using the XTAR USB-C cell as the example, so you know it fits the gear you already own before you leave.
What are the dimensions of a protected 21700 battery?
For the XTAR 21700 USB-C cell, the numbers are 75.5mm long and 21.5mm in diameter. The name says 21 and 70, but a protected cell carries a small circuit board that adds length beyond the bare cell.
So do not assume 70mm. Measure the cell you own, or read the listing figures, and treat 75.5mm as your working length for this one. The 21.5mm diameter is a shade over the nominal 21mm, which matters on torches with tight tubes.
If your torch or charger was designed around a bare 21700, that extra 5.5mm is the part that decides whether it closes.
Will 75.5mm fit your torch tube?
Often yes, but only if the torch has a spring at the tail or a tube with a little spare room. A cell that is 5.5mm longer can compress the spring fully and still leave the tail cap short of closing.
On a torch with two springs, one at each end, a protected cell usually sits fine. On a torch with a solid contact and a short body, it may not. Check the tube length in your torch manual or measure the old cell.
The XTAR 21700 USB-C cell is flat top and protected, and it is listed as fitting Ledlenser, Olight, Fenix and Nitecore flashlights. Even inside that list, always check your own model's tube length; a flat top cell needs the torch to have a raised contact or a spring to reach it.
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Does the charger slot take the longer cell?
This is where most trips go wrong, because a charger that swallowed bare cells can refuse a protected pair. Look for a bay that takes cells longer than 70mm, and check the current rate as well as the length.
Both the XTAR PB2SL V2 and the KLARUS K2A accept protected and button top cells, so the electronics will not fight the protection circuit. The KLARUS K2A is listed with room for laptop-cell sized protected cells up to 78mm, which covers the 75.5mm XTAR cell. The PB2SL V2 lists 18650, 20700 and 21700 support at up to 2A per slot.
When both bays are loaded, the KLARUS K2A drops to 1A per slot; with one cell it gives up to 2A. The PB2SL V2 holds 2A per slot with a pair. If you are charging before a night shift, that difference decides how long you wait.
Which terminal type does your device expect?
Flat top is the common terminal on 21700 cells, and it is what the XTAR 21700 USB-C uses. Button top cells add a raised cap, so a device built for flat top may not close on a button top cell at all.
Check the contact in your torch: a flat metal plate needs a button top or a spring, while a sprung contact will reach a flat top cell. On the charger side, both the PB2SL V2 and the K2A list support for button top and flat top cells.
If you already own a button top 21700, do not assume the same torch will take a flat top one without checking the contact first.
What should you check before you buy?
Run through this list once and you will avoid the two common failures: a cell that will not close into the torch, and a cell that will not sit in the charger.
- The printed length, which for the XTAR 21700 USB-C is 75.5mm.
- The printed diameter, 21.5mm on the same cell.
- Whether your torch has a spring at the tail or a solid contact.
- Whether your torch contact needs a flat top or a button top.
- The longest cell your charger bay lists, such as 78mm on the KLARUS K2A.
Once those five are confirmed, the buying part is simple, and you can browse the 21700 lithium battery cells range knowing exactly what will fit.
Does the built-in USB-C port change the length question?
No, but it changes what you pack. The XTAR 21700 USB-C cell charges through a USB-C cable plugged straight into the cell, with no separate bay charger needed, so a short trip needs one cable instead of a charger.
A small LED indicator on the cell shows remaining charge, so you can see where you stand before heading out. The cell is rated for roughly 800 recharge cycles and gives a 10A maximum continuous discharge, with 5000mAh capacity and 18Wh of energy.
That 10A rating suits ordinary torch use. If your torch draws hard current, the unprotected XTAR 21700 RAW cell is rated 35 to 90A, but it has no protection circuit and needs a device that manages its own protection.
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Where the chargers and cells fit in
If you already own a bay charger and just want a spare cell, the XTAR 21700 USB-C is the straightforward pick. If you want one device that charges cells and tops up a phone, look at the chargers.
The XTAR PB2SL V2 charges two 18650 or 21700 cells at up to 2A per slot, shows status on an LCD screen, and works as a power bank with up to 22.5W output. The KLARUS K2A gives up to 2A in one slot or 1A per slot with two cells, shows live voltage and charge percentage on a digital display, and also works as a power bank. You can see the whole range on the chargers category page.
Sorting the spare cell from the main torch
Cable charging is the simplest habit if you carry one torch and one spare. It saves the bay charger for home and keeps the bag light.
For a fuller look at cells that charge through a cable, covers the range and what each one suits.
Related guide · 6 min readUSB Rechargeable Batteries: XTAR Cells That Charge by Cable →What to do next
Measure your current cell, check your torch contact and confirm the longest cell your charger lists. With those three numbers written down, the XTAR 21700 USB-C cell at 75.5mm by 21.5mm either fits your kit or it does not, and you will know before you pack the bag.
Check the XTAR 21700KSh 3,000 · In stock
Where you would use it
- Night patrol A spare protected 21700 in the bag keeps your torch running through a full night shift on the gate.
- Camping weekend Charge the cell from a USB-C cable at the tent and skip carrying a bay charger altogether.
- Long drive Keep one charged cell in the glovebox so a torch is ready when the car stops after dark.
- Power cut at home A charged 21700 keeps a torch and the XTAR PB2SL V2 power bank working during an outage.
- Fitting a new torch Measure your torch tube first so the 75.5mm protected cell closes without forcing the tail cap.
Other options worth a look:
- XTAR VC8S 8-Slot Charger | 3A USB-C Fast Charge, LCD DisplayKSh 8,000
- XTAR MC1 Plus 1-Bay USB Lithium Battery Charger for 10440-26650 CellsKSh 1,200
- XTAR 26650 Battery, 5200mAh, Protected Flat Top, 10AKSh 2,700
- XTAR AA USB-C Rechargeable Battery, 2450mAh, 4-PackKSh 3,500
- XTAR AA 3960 Rechargeable Battery, 2500mAh, 4-PackKSh 3,000
- XTAR 18650 Battery, 3500mAh, Built-in USB-C, 10A DischargeKSh 2,500
- XTAR AAA Rechargeable Battery, 1000mAh, 1.5V, 4-PackKSh 2,700
- XTAR 18650 RAW Battery, 3500mAh, Flat Top, 10A DischargeKSh 1,600
Frequently asked questions
The XTAR 21700 USB-C cell is 75.5mm long with a 21.5mm diameter. Protection circuitry adds a few millimetres over a bare cell, so do not assume 70mm.
It fits if your torch tube has the spare length and the right contact. The XTAR 21700 USB-C cell is listed as fitting Ledlenser, Olight, Fenix and Nitecore flashlights, but check your own model's tube.
Check the longest cell your charger lists. The KLARUS K2A lists protected cells up to 78mm, and both the PB2SL V2 and the K2A accept protected and button top cells.
No. The XTAR 21700 USB-C cell has a built-in USB-C port and charges through a cable plugged straight into the cell.
Both are 21700 size, but the RAW cell is unprotected and flat top, rated 35 to 90A. It is meant for equipment that manages its own battery protection.
