That's a really hard question to answer. From what I remember, internal batteries actually have a more dangerous chemistry. However, external batteries are out and about more so more things tend to happen to them. The key point to remember is that batteries don't just decide to blow up for the heck of it. To stay safe:
- Don't ever carry a battery loose in your pocket. Actually, don't ever let a battery hang around loose. When not in the mod or the charger, make sure it's always in a case or skin.
- Do not use a battery that's physically damaged, and that includes damage to the wrap (the plastic that covers it). If the wrap gets nicked, it's cheap and easy to rewrap a battery. If you don't want to do that, many vape shops will do it for free.
- Don't ask too much of the battery. This usually happens in mech mods so it's not so much of an issue for you, but also stay under the 50 - 60 w I mentioned earlier. It's very unlikely a battery would blow up even if you turned the wattage up as high as it would go, but it's better not to try it.
- Get a good charger - one of the ones I mentioned earlier will be fine. A cheap charger can overcharge batteries, which causes problems.
- If something 'weird' happens - a battery really heats up, something strange happens with the charger - recycle the battery. If you're not sure if something's a problem, drop by the forum and ask before you use the battery again.
The most common causes of fires or explosions (and such events are extremely rare, despite the attention they get) are batteries carried loose in a pocket (which bump into something metal and short-circuit) or mech users who don't understand what they're doing using a coil with resistance that's too low.