Ohm and Voltage question

[tsangan]

So I took the dive and picked up a Chi You Megan 318, this unit uses a 26650 battery.

I have yet to purchase one yet and have been looking around online to sites people on here have suggested such as rtdvapor

I have a few questions before I make that purchase.

So for example if i pick up this battery EFEST IMR 26650 4200MAH 3.7V Max Continues Discharge: 20a

It has 3.7V and a max discharge of 20 amp if I use a .5 ohm coil i would be pushing out 7.4amp so I am well within the limit and should be safe

Is my math correct there?

So in general wouldn't a lower voltage battery let me have use a lower ohm coil? as the amp discharge rate would be lower.

 

[tsangan]

Oh and if there's a battery to suggest please do, as I know that is a VERY important part of the mod

 

[Kaezziel]

No, if you use a lower ohm coil, you will need a higher discharge rate. Though with 20A discharge, you could go as low as around 0.25 ohms, though you may very well find that a bit too hot... depending on airflow and wicking capability.
For 26650, I've been using the KeepPower that I refer to in my blog post. It is good for 30A continuous discharge, but it may or may not fit your mod. It runs a little larger than a lot of the other 26650's.

As for your math, you are a little off. Your max amperage draw on a 0.5 ohm coil will be 8.4A... be sure to use 4.2V in your calculations as that is what your batteries hold at a full (fresh off the charger) charge.

 

[Baditude]

So for example if i pick up this battery EFEST IMR 26650 4200MAH 3.7V Max Continues Discharge: 20a

It has 3.7V and a max discharge of 20 amp if I use a .5 ohm coil i would be pushing out 7.4amp so I am well within the limit and should be safe

Is my math correct there?

Your math may be correct, but you didn't use the correct numbers. When figuring your coil's amp draw with the Ohm's Law calculator, you always use 4.2 volts (that's what a fully charged battery has) instead of 3.7 volts.

A 0.5 ohm coil resistance will draw 8.4 amps from the battery: Explain it for the dumb noob: Ohm's Law calculations

1.0 ohm = 4.2 amp draw
0.9 ohm = 4.6 amp draw
0.8 ohm = 5.2 amp draw
0.7 ohms = 6 amp draw
0.6 ohms = 7 amp draw
0.5 ohms = 8.4 amp draw
0.4 ohms = 10.5 amp draw
0.3 ohms = 14.0 amp draw
0.2 ohms = 21.0 amp draw
0.1 ohms = 42.0 amp draw
0.0 ohms = dead short = battery goes into thermal runaway

Personally, I've always been a little wary about Efest batteries. The company has been known to overstate their battery specifications in the past: Purple Efest Batteries Not As Advertised I've not read of any discrepancies with the purple Efest 26650 battery, so just a buyer beware.

I'd personally use a MNKE 26650, a very reputable battery manufacturer.

 

[tsangan]

where is the 4.2 volts coming from? the battery itself states 3.7 volts, is it something im overlooking here?

Time to go read up that post you linked to learn more

I'll check out both KeepPower and MNKE 26650

thanks again guys!!! I'll be back with more questions haha

 

[Baditude]

where is the 4.2 volts coming from? the battery itself states 3.7 volts, is it something im overlooking here?

A fully charged battery will have 4.2 volts total voltage capacity. As the battery discharges during use, the average output will be 3.7 volts or thereabout for most of that charge cycle. Once the battery reaches around 3.4 - 3.5 volts, it will need to be recharged.

Battery University

 

[tsangan]

Ah okay that makes sense as the average voltage.

From the looks of it, if i stick with a .4 - .5 ohm there is no problem at all, just got to make sure the coils are good and use a proper ohm meter after doing the setup to test it before using it.

 

[tsangan]

I think this is the last question I have for battery

26650 KeepPower 5200mAh IMR26650 High Discharge Flat Top

vs

Genuine MNKE IMR 26650 3500mAh 3.7v

the only difference is the mAh which is just how much power the battery has stored so the 5200mAh just lasts longer in the general sense. From what I read you don't want to fully drain the battery anyways.

 

[Baditude]

Big difference in the capacity specs there: 5200 vs 3500 mAh. Makes me wonder how KeepPower determines their specs.

You see, sometimes the mAh rating is figured for a low draining devices like a flashlight. Other times it is figured on a high drain device like a power tool or advanced personal vaporizer. A lot of that 5200 mAh capacity may not be in the useful range for a high drain device.

Just something to ponder.

 

[tsangan]

I doing my usual search around and seems like a lot of people prefer the MNKE after no more VT26650 Sony so I'll probably go with that one

I did run into a pretty intense Vamped Battery 26650 3500 mAh 65 Amp but can't seem to find any reviews on it so think I'll have to skip on that for now.

Thanks for all the help once again guys

 

[Kaezziel]

I don't know what the max discharge of the 5200mAh KeepPower battery is. The battery that I use is the one that is in my blog post which is a 4200mAh battery. It is 30A continuous and 50A pulse limit. I still don't take it below 0.2 ohms, though. That being said, I still get a full day to a day and a half on a full charge... once it gets down to 3.6, it goes back on the charger. It's been a very good battery for me, and I've had no problems or issues with it... other than the fact that it is a tiny bit longer than most other 26650s, so mod top/bottom caps do not fully engage when everything is put together in my Gladiator mod... meh... still works fine with just a little gap on both ends.

 

[Kaezziel]

Big difference in the capacity specs there: 5200 vs 3500 mAh. Makes me wonder how KeepPower determines their specs.

You see, sometimes the mAh rating is figured for a low draining devices like a flashlight. Other times it is figured on a high drain device like a power tool or advanced personal vaporizer. A lot of that 5200 mAh capacity may not be in the useful range for a high drain device.

Just something to ponder.

Thanks for picking up my slack here, Bad... I'm just getting back here after a bit of a hiatus and am still trying to get all caught up on some threads. It's great to see you back around again!!!