Slight confusion with steam-engine

[Danny_uk5]

Hello fellow builders!

I've come across some confusion and I need someone to help me.

My confusion lies with steam-engine.org. Let's take one of my builds - We have a dual clapton, wrapped 5 times which gives us a ohms reading of 0.263.

The ohms law page says we should have:

13.31 amps @ 46.58w

But! The battery drain page says -

15.97 amps @ 67.07w

Am I correct in assuming that the battery holds a form of resistance and thus making up a 20.29 watt difference? Or is there something else that I don't know about?

 

[bwh79]

67 watts is right for a battery at full charge of 4.2v. The 46 watts is at 3.5v after the battery has drained a bit.

4.2 * 4.2 / .263 = 67.07...
3.5 * 3.5 / .263 = 46.57...

 

[Danny_uk5]

Great! And thanks for the reply.

After putting in 4.2 volts into the ohms law page I get the correct reading. This is what you get for wearing sunglasses and not your glasses!

 

[nyiddle]

67 watts is for a battery at full charge of 4.2v. The 46 watts is at 3.5v after the battery has drained a bit.

This.

The battery drain and ohm calculator are looking at two different things. The battery drain calc (by default) assumes you're working with a single 18650 battery at full charge (4.2V) and looks at what the amperage will be at this nominal charge, as well as at it's lowest (3.5V). Using the resistance and the specifications for the battery, it can estimate how long your battery should last.

If you're just looking at finding the amp requirements of your build, all you need is the ohm calculator. Assuming you're using a single 18650 mech mod, 4.2V would be at full charge. If you're not going over the amp limit at full charge, you won't be over the amp limit at any point (because the outputted amperage goes down as the voltage goes down).

 

[Danny_uk5]

Thank you both for your replies. That makes sense. Handy knowledge is handy!