This is a warning about reverse battery polarity in metal tubemods, and a discussion of related issues.
DO NOT insert APV batteries 'the wrong way round', with the negative pole inserted first and at the top of the tube
This is extremely poor practice, and should certainly never be seen as a design feature since it causes the device body to become the positive line. No electrical design ever makes a machine body the positive pole (and therefore 'live'), it is classed as a design error.
There is one practical reason and one theoretical reason. The theoretical one first: a machine casing is always negative or earth, because making it positive introduces the possibility of fault conditions that are unlikely or impossible with the casing negative, or 'earth' as it is often called (though incorrectly, if you are being fussy). This is well-illustrated by the practical reason this is never done in an APV, detailed below in 'Why reversing the battery is dangerous'.
A rechargeable battery is contained within a metal 'can' or container, and this can is negative. The entire outer surface of a battery is negative, and the positive terminal at the top is insulated. The battery can is covered by a wrapper or label, and this insulates the battery. It is not considered a vital measure to have a robust wrapper that cannot easily wear through, since any short  will be to 'earth', i.e. negative, and there are unlikely to be any serious consequences.
Even so, there are two possible consequences of wear and tear on an APV battery label:
1. If the battery can shorts to earth then the new negative path may bypass a fuse such as a hot spring. Then, if there is an over-current event such as a shorted-out adapter, the fuse will not protect against high current flow and a fire may result.
2. If the battery is a protected Li-ion cell then the protection will be bypassed, and it becomes an unprotected lithium cell.
It is better if an APV incorporates some form of fuse in the positive circuit so that a battery can short to earth does not bypass a negative-line fuse. Theoretical circuits always switch and fuse the positive line and this is why.
Why reversing the battery is dangerous
In a metal tubemod, the casing is negative, as this is standard practice for many DC devices - cars for example (although theoretically this is second-class design ). A car body is negative. It means that shorts will usually have no consequences as a negative to negative short is usually not significant, since the positive side is fused. If a pos to neg short occurs, then a fuse blows immediately.
In an APV, as we have seen, a neg to neg short introduces the possibility that fuses and protection circuits may be bypassed, and it therefore requires two faults to produce an incident - a neg to neg short through a worn label, plus a shorted-out atomizer adapter or similar.
However, if the battery is reversed, and the device's operational polarity is reversed from normal, this is a different matter. With the casing positive and 'live', a torn or worn battery label then allows a direct neg to pos short that could bypass any fuse wherever fitted, and a fire will result. The full battery discharge current will pass through the short-circuit and the battery will go into meltdown. There is an explanation of the circuit in the notes at .
This is a serious incident even with a safer-chemistry cell such as Li-Mn. In the case of a plain Li-ion cell or even a protected one (since the protection circuit is at the other end of the battery and has been bypassed), we are looking at a meltdown with flames, significant outgas and possibly an explosion, even with a single battery. With a top-quality Li-Mn cell a meltdown is assured, and possibly worse; if the cell is not of the most expensive type then there may be more risk. In fact reverse-polarizing a battery is probably the best way to create a situation where an explosive event might occur with a single battery.
NEVER reverse your battery
NEVER design a metal tube APV to work with reversed polarity and a positive machine casing
An exception to this might be for example if you have a robust internal insulator such as a thick mylar internal battery sheath that totally and absolutely prevents the battery can from shorting out, and cannot fail. But there are no good reasons to do this, in any case.
It's still extremely poor engineering practice to design a device with a positive case even if 'safety measures' are taken, and no engineer worthy of the name would do it. It's the kind of beginner's mistake that results, eventually, in them becoming experts - after a long period of time and many mistakes that cost money or injury. You can't blame beginners who weren't taught properly for making this kind of mistake; it's just that you don't want to be buying products created during their learning process...
Why do people want to insert batteries the wrong way round?
Perhaps because a battery that fails will vent first from the positive end - you can see the tiny gas vents around the positive terminal at the top. All rechargeables have these. Maybe the device being used only has gas vents at the bottom, so that the user assumes that it will be safer if the battery pos terminal is by the tubemod's gas vent.
There are so many faults in this reasoning that it is futile listing them and would take too much space. Just don't do it.
 A 'short' in electrical slang is a short-circuit / dead short - a shortened electrical path not designed for and that has no resistance in the circuit, leading to full current being drawn, and which may have unwanted consequences such as a fire, damaged components, or a blown fuse - or maybe just some sparking, if you're lucky. It's why an electrician is known as a 'sparks' in the construction trade. With lithium cells, contained in a product that is used in front of the face, the consequences can be serious and all possible measures need to be taken to avoid a fault chain leading to an explosive event. If a supplier sells a device that is predisposed to fail violently when only one fault occurs, this might be construed as negligence.
 The theoretical 'perfect' electrical circuit is an insulated 2-wire circuit. For practical or economic reasons, a machine body can be made one side of the circuit, and this side is always negative. One example is a car. The negative side is sometimes called 'earth' although strictly speaking that is incorrect as there is no real ground. In the early days of electrical design, the machine case was occasionally made positive, but that has long been avoided due to the problems it causes. You can only get away with this, if lucky, in low-voltage circuits - imagine if this was done with a mains electrical device - a good illustration of why this should not be done. The consequences of having your fridge 'live' are not pleasant; equally, there is no reason to make a low-voltage device casing live either. It introduces numerous issues even though the voltage itself is not sufficient to kill.
 Explanation of circuit: dead short with machine casing live
1. A metal tubemod with or without a hot spring fuse has a battery such as an IMR Li-Mn 18650 lithium cell inserted the wrong way round.
2. There are no electronic protections in the APV. However, this is irrelevant; just like the protection circuit on a protected Li-ion cell inserted the wrong way round, any electronic protections will be bypassed in any case, since they are at the (original) positive end of the circuit, which has been bypassed by reverse polarizing the battery. A neg to pos dead short through the battery cannister will bypass the protections.
3. After frequent charge cycles the battery label starts to wear through. Perhaps there is a sharp edge that scrapes the label; perhaps there is a high point somewhere in the tube internally; perhaps after hundreds of charges there is too much wear for the label material. It doesn't really matter why, as we know that battery labels do wear through (or get torn).
4. The battery can is negative and it shorts out on the body of the tubemod, which is 'live' or positive as the battery was inserted in reverse.
5. The device is in use, or standing unused - it doesn't matter which, since no switch operation is needed to complete this circuit: batt pos to hot spring >> hot spring to device body >> device body to battery can >> battery internal circuit to batt pos.
6. The IMR cell delivers 30 amps DC current instantly through the short-circuit, which has very low resistance.
7. The hot spring (if present) does nothing as when it collapses, the battery just follows it down, and sits directly on the tube body.
8. 30 amps continues to pass though the circuit, a fire starts at the battery can to tube body contact point and the two metal surfaces may even weld together.
9. The battery goes into meltdown.
Even a typical penlight AA cell can deliver 10 amps in a dead short. All lithium rechargeables deliver much higher currents than this, and some large models can deliver 70 amps instantly (26650 Li-FePo4). You could weld using a 2.5mm welding rod with a current as high as this.