I run a bike light testing and review site called The Bike Light Database which started as a series of blog posts for the Bicycles Stack Exchange community blog. I've personally tested dozens of lights, and have read lots of info online about countless more. I also speak with lighting companies regularly, have visited a bike light factory in person, and have experience in manufacturing engineering (albeit in a different industry).

Simply put, most cheap imported bike lights are not safe. The batteries and chargers are the biggest safety issue, although the overall design of most of these generic lights is inherently flawed as well. I'll address the batteries first because that's the foremost issue.

The battery safety issue

Most high-powered rechargeable bike lights (and flashlights) use 18650 batteries. These are a standard size and voltage, and are interchangeable if the device is built in such a way as to allow access to the battery cells. They're found in everything from lights to electric cars to laptops. Standard 18650 batteries have a nominal voltage of 3.7 V, and a fully charged cell is about 4.2 V. Most people consider any voltage below 3.2 V to be unsafe. Capacities go up to around 3600 mAh, but are usually somewhere between 2300 and 3200 mAh. As of spring 2016, there are no commercially available over 3700 mAh, so if you see anything labeled 4000 mAh immediately dispose if it in a safe manner; it's a fake and probably dangerous.

It's important to understand that these batteries hold a lot of energy, and if released in an uncontrolled manner, will explode, burn, release explosive vapors, and yes burn your house down. Used safely, they are an awesome technology which has enabled the high-powered lights with long runtimes available today. These too-cheap-to-be-true Chinese products do not use the technology safely though.

You can get "unprotected" or "protected" 18650 cells. A protected cell has a tiny circuit board on the top which monitors the cell voltage and ensures it doesn't get too high or low. It prevents overdischarge, overcharging, and short circuits. Unprotected cells are considered safe to use in lights designed for them as long as you aren't using more than two, preferably in series. Cells should also be charged individually or in matched sets to ensure the voltage is the same for all cells in a pack. Otherwise, if one cell is already charged and another is not fully charged, the charger could over-charge the full cell causing it to fail.

The batteries sold with these cheap lights tend to be the lowest possible quality to achieve the desired price point. Even the "replacement" battery packs are often of dubious quality. For example, I took apart one such battery pack for a review of a generic Cree U2 "5000" lm light (widely marketed as the Solarstorm X2). Here is a picture of the battery pack:

Two of the cells are dummies - they aren't actually wired such that they could deliver any power. That's "OK" though because the voltages of those cells were 0.0 and 0.3 V - far below the safe cutoff of 3.2 V. The other two cells were mismatched, at 4.0 and 3.8 V. While those voltages are in the typical range, you always want your battery voltages to match. This pack was likely constructed from recycled laptop batteries, with the two disconnected cells being completely dead batteries which were added to give the pack heft and feel like it was legitimate. None of these are protected cells.

Not all cheap imported bike lights are this bad, but the problem is that you don't know how bad it'll be until you actually get it. Quality control is practically non-existent, and because hundreds of vendors sell visually identical products it's impossible to be certain if you're getting a "good one" or not. Make sure to avoid like the plague anything with "fire" in the name, like UltraFire or TrustFire. These brands are known to be widely counterfeited and often dangerous.

Design flaws

Suppose you buy a replacement battery pack which you trust – although again, I'm not sure how you would trust one without opening it to see for yourself. There are still a number of design flaws with these products:

• The specifications are almost universally exaggerated, often by 2-5x. I talk about this in more detail in this Outdoors.SE post.

• Poor voltage regulation. On these generic lights the output usually drops as the battery voltage drops. Some brand name lights do this too, but you can get brand name lights with good battery voltage regulation.

• Cheap material and poor build quality. These lights are built to be as cheap as possible. You'll see terrible hand-soldering jobs, lights that aren't even water resistant, LEDs with inadequate thermal management, plastic lenses instead of optical glass, etc.

• The optics quality are typically horrid. You get a very bright hot spot, with uneven spill. This ruins your night vision with the bright spot while not giving you enough light to the sides to see well. You can get diffusing lenses for some models, but the result is a hot bar instead of a hot spot... it's not a significant improvement.

• The mounts are weak and hard to use. These lights usually have either a wimpy rubber band mount, or an enormous, over-designed plastic clamp contraption. Neither is convenient if you ever want to remove the light quickly so it doesn't get stolen while your bike is locked up. Speaking of which...

• The disconnect cable between the battery and light is usually about a foot from the light head. This also makes it hard to pop the light off before running inside somewhere.

You get what you pay for...

...in support

Reputable lighting manufacturers have a brand to uphold, and I think that most of them do in fact care about their customers. I've met these people, and they're all passionate about biking. If you call them, a real person helps you with your problem. Good luck contacting somebody who was involved in the manufacturing of any of these generic lights. A handful of online sellers have earned a reputation for good service by immediately refunding or replacing failed lights... which is nice, until you realize that a product with hundreds of reviews saying "mine failed but they replaced it, they're so nice 5 stars!" still means hundreds of people's products failed!

...and in features

If you're on a tight budget, maybe "price" is your only feature requirement. But seriously, name brand bike lights offer many features which you won't find on cheap generic lights. Things like carefully designed optics, USB fast charging, well designed mounts, specifically designed daytime and nighttime flash patterns, side visibility, and more.

So, what are your options instead?

First of all, just to be clear, there are cheap Chinese lights which are safe to use. The problem is, these are widely counterfeited, so it's best to buy from a reputable dealer in your own country. Single-18650 lights are usually safer than multi-cell lights. Buying quality batteries is always a must. I have some recommendations on my batteries FAQ page.

I also maintain a frequently updated list of recommended bike headlights. Product recommendations are a bit outside the scope of Stack Exchange, so keep an eye on the website for recent suggestions, or come chat about it.