For full information on rechargeable batteries, see this page:
Buyers should note carefully that the only types of batteries safe to use in an APV (or 'mod': a full-feature / replaceable battery 2nd or 3rd-gen ecig) are protected rechargeable Li-ion batteries, or rechargeable Li-Mn batteries . Some Li-FePo4 batteries can be used .
USE ONLY PROTECTED OR SAFER-CHEMISTRY RECHARGEABLE BATTERIES 
DO NOT USE UNPROTECTED Li-ion BATTERIES
DO NOT USE STANDARD NON-RECHARGEABLE BATTERIES (aka primary cells, as these might be placed in a charger by mistake, subsequently exploding in the mod during use - which HAS happened)
DO NOT BUY AN APV THAT CAN ONLY USE UNPROTECTED BATTERIES - unless it can take Li-Mn or good-quality Li-FePo4 batteries and these are advised by the APV supplier. [1,2,3]
The Li-ion 3.7 volt rechargeable batteries commonly used in APVs have well-known safety issues. You must only use protected Li-ion batteries. You must treat batteries with care. Li-Mn batteries are regarded as the best option at present.
Suppliers should include VERY CLEAR warnings about unprotected batteries with their products. We advise that a printed insert is placed inside the body of the device, that tells the buyer to only use protected rechargeable batteries (or Li-Mn or Li-FePo4). Note that although the risk increases when batteries are placed in series, meltdowns have occurred with single batteries.
Li-Mn or Li-FePo4 rechargeables made by for example AW are a safer technology battery that can be used - they need no protection. They may not fit in your device, check with the maker.
Cheap Li-FePo4 batteries may not be suitable, partly because they can often be fakes.
APVs built specifically to take the Ni-Mh standard 'main street' 1.5 volt rechargeables available at local stores do not need any protection. These APVs need 3 or 4 cells in series. These cells are 1.2 volt in reality.
Li-FePo4 batteries need a special charger as they are 3 volt cells, not 3.7v.
Some Li-FePo4 cells do not have a good enough C rating to safely run an atomizer. The ratings need to be, for example, 750mAh capacity and 4C, thus giving a safe discharge current of 4 x 750 = 3000mAh or 3 amps. Use Li-FePo4 cells with a clearly-stated C rating of 4C upward (some go to 8C or 10C).
Also, the main problem with Li-FePo4 batteries now is they are extensively counterfeited. This means you may think you have Brand X Li-FePo4 cells but in fact they are fakes: reject unprotected Li-ion cells with the original wrapper stripped off and replaced with a fake cover label for a more expensive (and much safer) battery. Tenergy Li-FePo4 batteries are a major target for counterfeiters. Fake batteries are likely to be dangerous.
Some APVs have protection circuitry built-in so that PCB protection in Li-ion batteries may not be necessary. We advise that in this situation, Li-Mn cells are used if possible.
An important consideration in battery safety is simply the physical size of the battery, and its C rating or safe discharge current. These two factors are linked, as small batteries of any type cannot safely run an atomizer, because they are not capable of supplying sufficient current.
Put simply: a larger battery is a safer battery. This means that a battery of sufficient physical size for the task can be of any type, as it will have sufficient safe capacity. The smallest cell this applies to is the 18500 format.
Current information is that AW IMR Li-Mn rechargeables are the safest batteries for use in APVs.
Please note that 'safety' is relative, when discussing rechargeable cells, as they are all potentially dangerous (for different reasons). For example, safer-chemistry cells such as Li-Mn are less likely to explode; but due to their high power they can supply tens of amps in a short-circuit/dead-short situation - some safer-chemistry batteries can supply as much as 70 amps of current. ECF believes that the primary danger to be avoided if possible is that of explosion, which may be in the face of the user since faulty cells tend to fail on first high-load demand after charging. However this does not remove the problem of fires caused by high-current discharge if shorted out, and in fact it may increase the chance of this happening as safer batteries can supply more current.
What this means in practice is that APVs need gas vents, a fuse, and a master on/off switch. An APV should never be left in a pocket or purse unattended and with the master switch on. If the device has no master switch then the battery/batteries should be removed and placed in a plastic battery box. Do not place batteries loose in a pocket or purse.
Rechargeable batteries have some safety issues when charging.
Batteries should be recharged on a metal surface and should not be charged unattended. For ultimate safety, batteries should be recharged in a Li-Po charging sack, or within a cookie tin (cake tin, biscuit tin) that has gas vent holes drilled in it (any gas vented during charging is likely to be hydrogen).
It is much more likely that lithium ion cells will suffer from incidents while being recharged than while in use. Do not leave these batteries unsupervised when charging.
Do not leave batteries on charge after they are fully-charged.
Do not leave a charger plugged in - unplug it after use.
Periodically check the voltage of a charged battery with a multimeter - it should read 4.2 volts for a regular 3.6v cell ( 3.6v and 3.7v cells are the same thing, the nominal voltage of the circuit is around this value, it is just that there is no agreement whether to call them 3.6 or 3.7v).
If a battery comes off charge at 4.3v or higher then the charger is faulty and must not be used.
Do not charge 3 volt cells on a regular charger, they need a special charger (which completes at 3.6v).
Lithium ion batteries of any kind are not safe in absolute terms, they have known risks. The very high charge density and low cost come at a price: instability especially after misuse or damage. In fact you could say these cells are intrinsically unsafe, because they can meltdown or even explode under certain circumstances, while this is rare or impossible for other types.
Treat lithium ion batteries with respect. They are like a fierce guard dog: they do a good job but there is a price. Mistakes may have consequences.
Rest batteries after charging
One commonly-reported factor in almost all the incidents we hear of where batteries failed violently while in use is that they were taken directly off the charger and then used immediately, at which point they failed.
Because of this, we think it may be a good idea to rest batteries after charging them. This advice will not be found in the usual 'reference bibles' on batteries but we see more and different reports than others. Therefore we now advise:
Do not use batteries directly after charging them. Use a set you previously charged, and that have rested for several hours.
This tells you what the safe maximum output current of a battery is. It can be expressed in two ways:
- Battery capacity: 500mAh, battery rating: 4C.
- Maximum discharge capacity: 2,000mA
Both those ways of describing it mean the same thing: the battery can supply a maximum of 2.0A output (2 amps, 2,000mA, 4 x 500mA = 2,000mA - all the same thing). It is debated whether or not this is a continual discharge rating or if it refers to maximum draw over any time including short periods. Since this is likely to vary between manufacturers, although the theoretical meaning is 'continuous load', in practice it is safest to assume that the C rating applies, or at least is a guide to, intermittent loads - especially for use in front of the face.
This is around the lowest needed in a battery used to power an atomizer, which will normally take between 1.5 and 2.5 amps. A battery with for example a capacity of 550 or 750mAh and a 1C rating is obviously not a good choice for an ecig because the output required is around four or five times more than the battery is designed to supply.
MAKE SURE the C Rating for your battery is sufficient. The manufacturer must warrant it suitable for a maximum discharge current of around 2 amps (2,000mA) as a MINIMUM. Even if it is of a 'safer chemistry' type, it is not good practice to ask it to deliver 4x or 5x its designed load. This applies even more if it is going to be stacked (paired up in series).
Stacking batteries and then overdriving them by a factor of 4 or 5 above their safe marked limit IS NOT SAFE and CANNOT BE SAFE.
See also the Battery Information page:
See also the APV Safety Specification page: