Rechargeable lithium ion (Li-Ion) batteries have become very expensive in recent years. It's not a matter of greedy manufacturers – these are industry-standard cells which are used in many devices, not only torches, so there's plenty of pressure on them to cut their prices to the bone.

Li-Ions come in a variety of sizes which are reflected in their model numbers, so an 18650 cell is 18mm in diameter and 65mm long. The amount of energy they can store is limited by their size – 18650 Li-Ions seem to have hit their peak at 3,500mAh while 21700 cells currently hold up to 5,000mAh.

There are a few other variables, such as whether they are "protected", have "button" or "flat" tops, and the amount of current (amps) they can safely supply but, in general terms, they are interchangeable with other cells of the same physical size.

Unfortunately anything that's small, expensive and impossible to evaluate just by looking at it draws skellums like flies to dung. As a result there are lots of bad and fake Li-Ions around, making outrageous claims, selling at bargain prices, and potentially burning people's houses down.

No, I'm not kidding.

All Li-Ions – including those in cellphones, laptops and cameras – have the potential to burst into flame and, once burning, they are almost impossible to put out. That's why airlines place restrictions on them – as a passenger you must carry them in your hand-luggage so that, if one starts burning, the cabin crew can put it into a special box where it can burn out.

But don't get too freaked out. If this was common you'd hear of electronic gadgets bursting into flames all the time – the reason you don't is because good quality Li-Ions are pretty safe. When we import them, our suppliers have to provide the shipping company with certificates to show their Li-Ions have been tested by a recognised authority and found to be safe.

However crappy and counterfeit batteries somehow dodge these restrictions, so you really are taking a chance when you buy them. And, even if they don't burn your house down, independent tests show they don't last or deliver the capacity they claim, so you're not really saving money.

Li-Ions are most likely to burst into flame while being charged, which is why it's a good idea not to leave them plugged in when you're not at home and, if you're a belt-and-braces kind of person, consider charging all your devices in a fireplace/kaggel.

So how can you tell if a battery is fake or junk? The first indication is the specification – there's no such thing as an 18650 with a higher (genuine) capacity than 3,500mAh, so avoid batteries that claim bigger numbers.

The second thing is the asking price – I'm really struggling to import and sell good quality Chinese "unprotected" 18650s for less than R270 each, and "protected" Li-Ions for R540 each, after discount. A 5,000mAh (unprotected) 21700 is currently going for R450 after discount. 

A genuine Korean cell (e.g. Samsung) will cost much more, and a Japanese Li-Ion (Panasonic) twice as much, although the best Chinese cells are now every bit as good IMHO. If you're offered Li-Ions for less than this they are probably fake – Google for websites that show tell-tale signs of counterfeit cells.

I would LOVE to sell Li-Ions for less because it would bring down the price of all my torches, but they are expensive and tricky to import so I genuinely can't. If you'd like to buy your Li-Ions elsewhere that's fine by me – they may not last, or work at all, but it's rare for them to damage torches so I won't try to scare you into buying mine.

You wouldn't be the first customer to come back to me saying "I tried some cheaper batteries I found on [name of online store] but they didn't last" and I promise not to say I told you so.