Ohms law

[Dellleeds]

Hi there everybody can some one advise me pls in the ohm call ,on my 21700 battery says on the side 3.7 volts but when it’s fully charged my charger says 4.2 on the display, my question is what number do I put under volts on the ohm calc 3.7 or 4.2
Any help or advice pls

 

[Coastal Cowboy]

In a regulated mod that allows you to vary the wattage, use the number that the mod stops firing. That's usually 3.2v.

In an unregulated or mechanical device, use the full charge of 4.2v.

Given the nature of your question, you should not be using a mechanical mod until you can calculate the equations derived under Ohm's Law and you understand the critical safety protocols of using such a device.

The 3.7v value is the nominal charge, which is halfway between the 3.2v cutoff and 4.2v full charge.

 

[Baditude]

Are you using a mechanical or regulated mod?

With a mechanical mod, coil resistance is relevant so you use Ohm's Law. Explain it to the Dumb Noob: Ohm's Law Calculations for a Mechanical Mod

3.7 volts is the nominal or average voltage of a battery; 4.2 volts is the voltage of a fully charged battery. We generally use 4.2 volts in the Ohm's Law calculations.​

With a regulated mod, coil resistance is irrelevant. What is relevant is the wattage setting that you use. You now use Watt's Law.

• Calculating battery current draw for a regulated mod (device)

WATTAGE PER SINGLE BATTERY on REGULATED MOD:
(wattage doubles when using 2 batteries/triples when using 3 batteries)

Up to 45W:

Samsung 18650 30Q, 3000 mah 15 amp CDR

Sony 18650VTC6 3000mAh 15 amp CDR

Up to 60W:

LG 18650HG2 3000mah 20 amp CDR

LG 18650HE4 2500 mah 20 amp CDR

Samsung 18650-25R, 2500 mah 20 amp CDR

Sanyo UR18650NSX, 2500 mah 20 amp CDR

Sony 18650VTC5, 2600 mah 20 amp CDR

Sony 18650VTC4, 2100 mah 23 amp CDR

AW 18650 3000 mah 20 amp CDR

Up to 75W:

LG 18650 HD4 2100 mah 25 amp CDR

LG 18650 HD2 2000 mah 25 amp CDR

Samsung 18650-24S, 2500 mah 25 amp CDR

Sony 18650VTC5A, 2500 mah 25 amp CDR

Up to 90W:

LG18650HB6 1500mah 30 amp CDR

LG18650HB2 1500mAh 30 amp CDR

LG18650HB4 1500mAh 30 amp CDR

Samsung 18650-20S 2000mah 30 amp CDR

 

[bombastinator]

The way I think of it is batteries are a bit like balloons. Their pressure decreases s they lose energy

 

[Zaryk]

If you are building for a mech, it is highly recommended to follow the links that baditude supplied and learn about the safety side of a mech before using it.

To answer your question, put 4.2 in the calculator. The 3.7 is nominal voltage, which is where the battery spends most of its time when being discharged. Always build to the highest voltage you will be at to keep things on the safe side.

 

[sultan1966]

Very good info for sure.