Lithium ion batteries are the most dangerous battery chemistry in widespread use. They have resulted in numerous fires, some serious, and have been subject to recalls and to various restrictions on transportation.
@Wookie has written in several threads that, while operating m/v Spree, he routinely threw lithium batteries overboard that were overheating, smoking, etc., because of the hazard posed to those aboard.
Many have engaged in informed speculation that the loss of the m/v Conception, with considerable loss of life, could have been caused by a lithium battery fire.
Lithium batteries aren't necessary for diving and could easily be entirely eliminated from the sport.
How we got here
Lithium batteries have the highest specific power capacity of any battery technology, with the common 18650 cells -- that weigh 50 grams -- capable of delivering as much as 80 watts for a few minutes. They also have the highest specific
energy capacity, typically delivering around 10 watt-hours over the course of a discharge cycle. Most underwater lights and camera equipment use 18650 cells -- either individually, in removable packs, or as an integral part of the equipment.
Li-ion batteries are used in high-end equipment not because they are necessary but because it is a competitive differentiator to have a device that is smaller and lighter, even when these differences have no effect on the convenience or usability of the device.
Nickel-metal hydride
NIckel-metal hydride (NiMH) batteries are much safer. The AA size is most common, and has almost exactly half the weight and volume of an 18650 cell. The best NiMH cells for photography (and diving), including Eneloop Pro and similar competing cells, deliver a maximum of 4.5 watts per cell, and can deliver 1.6 watt-hours per cell over the course of a fairly rapid 20 minute discharge cycle, and 2.5 watt-hours if discharged over the course of two hours.
If you do a little math, you can see that, for comparable size and weight, lithium batteries deliver 2-3 times the amount of energy and around 10 times the amount of power, compared to NiMH. Before the hazards posed by lithium batteries were well understood, this seemed like an easy tradeoff. The only real drawback was cost, and dive gear is expensive enough that the battery cost isn't much of a factor.
For products that are designed to operate for 30 minutes or more on a charge, the energy limit determines how many batteries are needed to meet the needs of a particular device and application. Nearly all diving gear falls into this category, since equipment is expected to function for the typical 50 minute duration of a dive.
NiMH as a good-enough alternative
For most diving applications, NiMH batteries are good enough.
For this discussion, keep in mind that rechargeable batteries can be combined into packs for ease of handling when more than a few cells are required.
A typical handheld dive light uses one 18650 cell and weighs about 200 grams. A well designed alternative using four AA NiMH cells would have similar performance. The weight of the extra batteries would add 50 grams. The change in bulk would be insignificant.
The largest handheld dive lights (billed as "handheld primary lights") use four 18650 cells. With careful design, NiMH batteries could still be accommodated without having to resort to a canister design.
The only SCUBA product where the energy density requirement is so compelling that lithium-ion batteries are the only practicable solution is scooters.
Other benefits of NiMH
NiMH batteries offer practical benefits insofar as there are no restrictions on transporting them. There are long-life versions available that will last ten years or more even with regular use.
Withering defenses of Li-ion
Many of the problems with lithium batteries can be traced to poor quality control at the time the batteries were manufactured. Some can be traced to poorly designed charging and utilization circuits. Some are caused by impact or other damage to the battery. They are fragile, dropping them on a concrete floor from waist height can cause latent damage.
So there are going to be people who say that if you use high quality batteries and are nice to them, everything will be OK.
And for people who believe that, I'm going to say that there are some really smart people at Samsung and Apple and other high-end electronics manufacturers that work in huge volumes, and they couldn't figure it out. Are you smarter than they are? Do you know your supply chain better than they do?
Tough love
Got lithium? Get rid of it. Don't buy any more.
Use lights that operate on AA cells and use NiMH cells in them. When shopping, reviewing, etc., insist that vendors use the safest technology. You wouldn't buy a regulator that weighs less if it were less safe, so why would you buy a dive light that weighs less but is less safe?
Got a dive boat? Don't allow lithium batteries on it. Period.
There are alternatives.
Got a dive shop? You get the picture....
