Low Sub-Ohm Battery Question

[skimmer02]

Ok, I don't sub-ohm and don't plan to start. I understand Ohm's Law and test my coils before use. The lowest I currently run is 1.25 ohm. But I am interested in all aspects of battery safety, and I know someone can answer this for me. A battery has an "internal resistance". There are those that get into the super low ohm area, I see .2 and below talked about. Now, I understand that meters can be inaccurate, possibly up to +/- .2. So, some of the risk is because you can actually get below the internal resistance of the battery? Aside from the normal considerations of exceeding battery amp limits and battery stress, how does the internal resistance of the battery come into play?

 

[K_Tech]

Aside from the normal considerations of exceeding battery amp limits and battery stress, how does the internal resistance of the battery come into play?

Internal resistance can be directly correlated to a battery's performance, but aside from that, if you are staying within the manufacturer's stated limits, it's not something that needs to be taken into account when building a coil.

Edited to add:

I don't want to dismiss internal resistance, but you'll find (generally) that cells rated for higher discharge rates have lower internal resistance, generally MUCH lower than a coil you could build safely (staying UNDER the battery's stated maximum continuous discharge rate).

 

[Baditude]

Aside from the normal considerations of exceeding battery amp limits and battery stress, how does the internal resistance of the battery come into play?

This is the part of a battery's specifications which I know the least about. However, a battery which has a higher internal resistance will be less tolerable to stress and overheating than a battery with lower internal resistance. High capacity batteries (higher mAh) tend to have a high internal resistance, while high drain batteries (IMR and IMR/hybrids) tend to have low internal resistance.

This is one of the major differences between ICR battery chemistry and IMR battery chemistry. ICR batteries require protection circuits built-in because without them they can vent flames and possibly explode. IMR batteries are inherently a safer chemistry, are more tolerant to heat and stress, and therefore do not require the same protection circuits.

Lithium ion batteries have what's called a PTC or Positive Temperature Coefficient circuit. This is built into the battery just above the positive terminal and is present regardless of any "protected" labels. The PTC is designed to raise the resistance of the battery as the temperature of the battery rises.

The theory behind PTC is that as the resistance goes up, the current draw from the battery goes down - Ohm's Law. This helps prevent most accidents from becoming catastrophic, but it should not be relied on. The PTC is designed to be unobtrusive and you can still over-draw the battery if you aren't paying attention. The PTC circuits can also fail if exposed to static electricity or from a faulty charger. When a PTC fails, it often fails in a position which allows you to continue using the battery without fault.

The tech engineers at Provape touched on the topic of internal resistance in the following thread:

Question for the tech engineers at Provape

 

[skimmer02]

Hey, thank you for the response and link. I never plan to get anywhere close to a purposefully dangerous build, but am interested in continuing to educate myself about batteries and battery safety. Like you, I understand least this internal resistance part of it, but am looking to learn a little more.

 

[Tbev]

When you build low builds you pretty much have a limit in mind that's as low as you can go based on your batteries, you'll stay over the limit keeping in mind the variations in your meter and the tendency of a build to fluctuate a bit while breaking in. Internal res isn't really something paramount. You should always have a bit of a buffer anyways.

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