Battery Limits 101

[BusterBear]

Hey all, I'm doing some reading regarding sub-ohms, amp limits, protection, etc.

I found some online calculators, looking into what kind of coils i can safely build and paired with which batteries.

http : // www. rapidtables. com/calc/electric/watt-volt-amp-calculator.htm

So according to this.. if i type in 3.7 into volts, 10 into amps it calculates 0.37 ohms.

Meaning i could build a 0.37 ohm coil on my sony us18650v3?

Just wondering if the calculations are correct more than anything, i have no interest in sub ohm that low nor pushing my battery to its limit.

 

[scote]

I'd put 4.2v into the calculator, thats what a fully charged battery is as long as its not too old.

Sent from my EPAD using Tapatalk

 

[Ryedan]

You've using the calculator correctly.

As scote said, always use the maximum voltage your battery will be at with a fresh charge which is pretty close to 4.2V. Battery voltage will decrease as it discharges and if you play with the numbers in the calculator a bit you'll see that amps drawn will decrease when that happens.

Now, coils can short in any atomizer and much more often when using stainless steel mesh as opposed to cotton or silica. These shorts will decrease the resistance of the coil and draw more power from the battery. I would not be comfortable using more than 70% of the battery's available amperage at 4.2V. I actually use much less in my setups, but that's just the way I like it.

You may have found it already in your research, but just in case you didn't, there's a lot of information on sub-ohm safety in this thread.

Hope this helps and don't hesitate to ask if you have more questions

 

[BardicDruid]

Never use full charge voltage, the reason being it's only there for about half an hour, it will be around 3.7V for 80% of it's charge capacity, for a batt that has 2000mAh, that's usually around 8 hours. That's why the manufacturers list them as 3.7V and not 4.2V.

 

[Ryedan]

Never use full charge voltage, the reason being it's only there for about half an hour, it will be around 3.7V for 80% of it's charge capacity, for a batt that has 2000mAh, that's usually around 8 hours. That's why the manufacturers list them as 3.7V and not 4.2V.

The problem with this is if you calculate the lowest resistance coil you can use for a battery at 3.7V and set up that coil, you will be drawing too many amps when it's at 4.2V.

 

[Switched]

Hey all, I'm doing some reading regarding sub-ohms, amp limits, protection, etc.

I found some online calculators, looking into what kind of coils i can safely build and paired with which batteries.

http : // www. rapidtables. com/calc/electric/watt-volt-amp-calculator.htm

So according to this.. if i type in 3.7 into volts, 10 into amps it calculates 0.37 ohms.

Meaning i could build a 0.37 ohm coil on my sony us18650v3?

Just wondering if the calculations are correct more than anything, i have no interest in sub ohm that low nor pushing my battery to its limit.

Your calculations are correct. OTOH you never introduced a safety factor in your equation. I would use 20% Therefore...

3.7 x 8 = 0.496Ohms

In all honesty folks are going nuts wrt how far they can push things these days, and IMHO there is absolutely no need to do so. As your battery ages so does its capacity to deliver the current demanded from it.

In layman's terms...

What is internal resistance in a battery and where does it come from?
Lithium-ion batteries lose capacity through cell oxidation (cholesterol), a process that occurs naturally during normal battery use and aging. As the battery ages, internal resistance (cholesterol) increases as well. This increase in cholesterol (internal resistance) leads to plaque build up and when sufficient build up occurs. This results in a plugged artery/battery or, a battery that can no longer deliver current sufficiently to meet the workload. Not dissimilar to someone having a heart condition having difficulty going up a flight of stairs.




Does internal resistance decrease the amount of usable energy in a battery?
Yes it does. It reduces the battery's overall charge capacity. Three imaginary sections of a battery consisting of available energy, empty zone and rock content. With use and age, the rock content grows.

I have heard a term “stressing a battery”. What, if any, effect does stressing a battery have?
The short answer is rapid ageing due the increase build up of cholesterol (or internal resistance), in this particular case caused by excessive work demands. These excessive work demands increase the creation of internal resistance (cholesterol) exponentially, leading to an increased plaque build up (the rock zone).

Is stressing a battery dangerous?
Yes, emphatically so.

What could be the results of a stressed/over-stressed battery?
In milder cases, the build up of plaque results in a plugged artery and possible heart failure, or in our case the battery will no longer hold a charge or deliver current. In severe cases clogged arteries can lead to aneurysms and death. In our application, should the protection circuit fail, and we have a stressed battery, which translates to an over-current condition, the battery may vent or even explode.

 

[BardicDruid]

The problem with this is if you calculate the lowest resistance coil you can use for a battery at 3.7V and set up that coil, you will be drawing too many amps when it's at 4.2V.

And why do you think it's always recommended to not go below .5 ohms, and are you going to be swapping the batt every 30 to 45 minutes to keep it at 4.2V????

 

[BusterBear]

Thanks folks!