INR21700-40E 3.7V 4000mAh Lithium-Ion Battery (14.80Wh) - Flat Top Cell

FREQUENTLY ASKED QUESTIONS (FAQ)

1. What does "21700" mean? It is the standard physical size code. It nominally means 21mm in diameter and 70mm in length. (Note: With wrappers, this specific cell measures roughly 22mm x 71mm).

2. Is a 21700 battery better than an 18650? Yes, in terms of total energy capacity. Because it is physically larger, it holds significantly more energy (4000mAh vs the typical 2000mAh of an 18650), making it the new standard for EVs.

3. Will this fit in my 18650 battery holder or charger? No. A 21700 cell is thicker and longer than an 18650. You must buy 21700-specific plastic spacer brackets, holders, and compatible chargers.

4. Is this a flat top or button top battery? This is a flat top battery. The positive terminal is flush and does not protrude.

5. Does this battery have a built-in protection board (BMS)? No. This is an "unprotected" raw cell. You must use an external BMS board or plug it into a device with a built-in low-voltage cutoff to prevent over-discharging it.

6. Can I solder wires directly to the top and bottom of this battery? NO. Applying the high, sustained heat of a soldering iron directly to the steel casing will damage the internal chemistry and can cause the battery to vent or explode. You must use a dedicated battery spot-welder.

7. What is the nominal voltage? The nominal (average operating) voltage is 3.7V.

8. What is the voltage when it is fully charged? A fully charged 21700 will read exactly 4.2V on a multimeter.

9. How low can I safely drain this battery? You should never let an unprotected lithium-ion cell drop below 2.8V to 3.0V. Draining it below this will cause permanent chemical damage and severely reduce its capacity.

10. What does "discharge at 0.2C" mean? "C" is the total capacity. 0.2C means 20% of 4000mAh, which is 800mA. Drawing roughly 800mA continuously is the optimal efficiency rate to extract exactly 4000mAh of runtime out of the cell.

11. Can my project draw more than 800mA? Yes. 21700 cells can handle high burst currents very well. However, running it under a heavy, continuous load means the voltage will sag faster, and you may extract slightly less total capacity than the 4000mAh rating.

12. What does INR stand for? INR indicates Lithium Nickel Manganese Cobalt Oxide chemistry. It is widely used in power tools and EVs because it is safer and more stable than older ICR chemistries.

13. Can I wire three of these together to get 11.1V? Yes. Placing three cells end-to-end (in series) gives you a 3S pack. However, you must wire them through a 3S BMS protection board to ensure they charge safely.

14. Can I wire two together to get 8000mAh? Yes. Wiring them side-by-side (positive to positive, negative to negative) creates a parallel pack, keeping the voltage at 3.7V but doubling the capacity to a massive 8000mAh.

15. Can I mix this 21700 battery with an 18650 battery in a custom pack? Absolutely not. Never mix batteries of different capacities, physical sizes, brands, or ages in the same series or parallel circuit. The weaker battery will be destroyed and could catch fire.

16. What is the white ring on the top for? The white ring is a safety insulator. It prevents the positive center terminal from short-circuiting against the outer edge of the top face, which is connected to the negative casing. Do not remove it.

17. What should I do if the plastic wrapper tears? The entire metal cylinder under the wrapper is the negative terminal. If the wrapper tears, the side of the battery can easily short-circuit against a metal enclosure. Stop using it immediately and re-wrap it using custom 21700 PVC heat shrink tubing.

18. Is this battery waterproof? No. Keep it away from water and extreme humidity.

19. Why does the picture show a pink battery, but mine is a different color? Battery manufacturers frequently change their PVC wrapper colors depending on the production batch. The color of the wrapper does not affect the performance of the cell.

20. How should I store this battery if I am not using it for months? Do not store it at 100% (4.2V) or 0% (dead). Store it in a cool, dry place at a "storage charge" of roughly 3.7V to 3.8V to significantly prolong its overall lifespan.