This is an interesting question, but one that is potentially 'hot'.
Everyone has their own views, and many disagree on just about everything to do with this subject. The ECF position is that user error is the cause of most issues but that this is inevitable and mods should take this into account. Also there might be battery failures even where everything was done correctly, as no battery can be called absolutely safe, plus we are subjecting them to a heavier demand than they are built to take - attys pull more current than some batts are designed for.
We're trying to put together a guide that most people will agree on, based on the battery safety issues. The current position is basically this:
1. A mod needs some safety features because you can't guarantee even the best battery will not develop a fault, and also sometimes people make mistakes.
2. A single-battery mod is probably safer as there are less issues. However, if everything is properly taken care of, HV mods should be be no problem.
3. Small battery mods (whether 1 or 2 battery) are not intrinsically safe and can't be, no matter what battery is used, as the duty cycle exceeds what the battery is designed to deliver. Batteries blow because there is some kind of fault somewhere, then they are over-driven. If you remove the over-driven part of the equation then there is a much bigger safety margin.
4. To decide if a battery can handle the load an atty will demand, look at its drain rate. That might be quoted as the actual figure (like 500mA) or as a proportion of C (its capacity). The drain rate has to be 1.5 amps minimum or it can't handle an atomizer safely especially when there is some sort of battery fault. 1 amp is about the lowest acceptable. Therefore if the drain is quoted as 550mA then that battery cannot be safely used in a mod, and especially with two batteries as that will increase the risk. If a battery has a quoted capacity of 650mAh and the drain is 2C, then the drain is 1300mA - maybe just enough but it's borderline. Ideally you need a batt with a drain of 1.5A / 1500mA or better. Many batteries used currently don't comply, which is a risk factor.
We're currently working on a list of suitable batteries and chargers. Till that's done your best bet is to buy a mod with the largest battery you can personally accept, and/or buy an AW IMR Li-Mn battery / batteries for it. This is ESPECIALLY important if you are using small batts like the RCR123 or 14500 format. As I understand it, nothing smaller than this will be suggested as suitable - but this is not my department.
Check with the vendor what batts will fit. The protected Li-ions are a bit longer than the unprotected ones and mods often only work with one or the other. A mod that only fits unprotected batts is fine if you can fit an AW Li-Mn batt, in fact that's possibly the best choice.
The risk is higher when a mod has no safety features and uses small batteries.
