These batteries are today best energy source for powerful LED headlamps and there is no reason to use any other technology. They are used by NASA in sattelites, consumer electronics and lately also widely in electromobiles for their clear advantages - high capacity, low weight and high reliability.
Li-ion cell have got highest capacity/weight or volume ratio. They are industry proven and safe. Cell are light and keep large volume of energy. They are manufactured only as rechargeables, so they dont have so large impact on enviroment like primary (non-rechargeable) batteries. Their lifetime is 500-1000 full cycles and will serve you many years without any problems. When dealed properly, you wont likely encounter any loss in their capacity over time. They are very reliable, if you charge them, they will always provide the energy needed.
Mostly they are cylindrical with charging voltage of 4.2 volts. There are few different sizes, but the most common size is battery 18650, cylindrical cell with 18mm diameter and 65mm length. Recently electromobiles started to use rather size 21700 because they made large battery from them easily.
All standard li-ion is charged withCC/CV algorithm to the final voltage 4.2V. Nominal voltage (average voltage) is 3.6V (resp. 3.7V). You can discharge li-ion battery to 2.5V or 2.8V (based on specification). 2.8V is safe value with some margin. Lower voltages than 2.5V would permanently damage the cell. Internal resistance of the battery is between 20 and 80 miliohms, based on manufacturer, brand and whether it is large capacity cell (resistance is higher) or high current cell (lowest internal resistance).
Currently highest possible capacity is 3450mAh with stored energy of 12.4Wh. With higher currents the capacity drops a little. Dont trust any amazing chinese 18650 cells that advertises more capacity, its not real. They are all fake!
Typical maximum continuous discharge current is 3-10A, high current batteries e.g. in powertools allow up to 30 amps of continous current.
Battery will give you 500-1000 full charging cycles until the usable capacity drops to 70-80% of orginal value. Life of the li-ion can be prolonged (slower battery ageing) with slow charging current (e.g. C/5), long-term storaging in partly charged state (3.93V = 70%) and use above 0°C. More information about prolonging lifetime can be found here: www.luciferlights.net/li-ion-ageing
Li-ion works even below freeze -e.g. in -20°C - but with such use you can permanently and irrevocably damage the cell. More about use in cold temperatures here: www.luciferlights.net/li-ion-cold
State of charge of a battery can be measured with voltmetr or multimeter. Measured voltage will exactly tell state of charge. Every cells discharge curve can be little different but table above this paragraph will generally tell state of charge of any li-ion battery.
Li-ion technology has got many advantages over NiMh or NiCd batteries. Their lifetime is much higher, count in years or even one decade. Their internal resistance is low so they can supply large currents fine. Their capacity under heavy load is high and doesnt lower much. You dont have to format batteries before first use, their capacity is well defined from the first use. You can charge them from any state of charge. They are also more user friendly, simply work from the beginning and do not require any regular capacity check or recovery formatting.
Li-ion has got very low self-discharge, they can lay on a shelf for months and their state of charge doesnt change so much.
Li-ion baterry (size 18650, 21700) offers today highest capacity/weight and capacity/volume ratio. NiMh batteries are much worse than li-ion. With the same stored energy, NiMh weights 2-3 times more.
If we have to find some disadvantages, we can mention firstly the fact, you cannot buy them everywhere in the world. Probably also your current bay cahrger wouldnt charge them as it was not designed to charge more chemistries. Last disadvantage is they store large energy and can be dangerous and catch fire when short-circuited or heavily mechanically damaged. However no normal use can cause such dangerous scenario. When you use quality products (cells, chargers) such risk is almost non existent and you can be sure its totally safe.
Customers often ask us they need to use headlamp with AA or AAA batteries because they will be few weeks of the grid. Classic AA or AAA batteries you can buy everywhere.
In fact, when we count how many weeks, days, or hours in a day they need to shine with a headlamp, we mostly find that just one additional batterypack is needed to cover all their needs. Our headlamps are very efficient, the lowest VERY LOW mode with 25 lumens has got 180h runtime, so its 15-18 full nights! One 8.4V li-ion batterypack store same energy as 6-8 AA batteries. With one additional batterypack which is also good as backup you have 360 hours of runtime, that means 15 full days or 30 full nights. That should be enough for everyone.
Protected cells have got electronics circuit to protect battery against over-discharge, overcharge and short-circuit. All these protected batteries are made of unprotected cells by adding protection circuit. Protected batteries are therfore little longer (2-4mm) and have got slightly large diameter (0.5 - 1mm). Protected cells can be safely used, charged or discharged in external bay charger by every user. There are many manufacturers of these protected cells (Panasonic, Samsung, Sony are not amongst them!). Quality of protected battery depends on quality of the unprotected cell itself and quality of protection circuit. Every protection circuit places some additional resistance to the circuit, generally from 5-100 miliohms. High resistance cause energy losses druing use so its good to have low resistance protection circuit able to handle large currents.
Unprotected li-ion batteries are often spotwelded in parallel or series to make up large batterypack, like we do it in our lucifer 8.4V and 12.6V batterypacks. There is just one protection circuit monitoring the whole batterypack. Such batterypack must be charged through its cabel and you cannot easily replace or dissassembly individual cells from it. Its common scheme for powerful LED headlamps to put more batteries in series as with higher currents the losses are lower. There is larger voltage but lower current flowing through cabeling and with Ohms law you know, that losses are I2R and strictly depend on current flowing. The lower the current, the lower losses.
Its quite common practice of chinese manufacturers to exageratte its capacity. Please understand that the very best batteries come only from large global established companies Panasonic (Sanyo), Samsung, LG a Sony. Only these 4 manufacturers can make unprotected li-ion cells with the worldclass capacity. Current record capacity (March 2020) are:
These or very similar parameters are achieved with 18650 batteries Samsung INR18650-35E, Panasonic NCR18650B, Panasonic NCR18650GA, Sony US18650VC7 and LG MJ1 18650. Really no other manufacturer made so good cells!
Batteries 21700 and 20700, used in electrovehicles Tesla even today do not give better parameters than best 18650 cells. They are at best equal in terms of weight/capacity or weight/volume ratio. Best of them are LG 21700 M50 5000mAh and Samsung INR21700-50E 5000mAh.
Li-ion technologywas invented and developed in 80th a 90th of 20th century. In the year 1994 there were 18650 cells with high cost and capacity just 1100mAh. From year 2000 there was massive develompent of li-ion and regular increase in capacity every year. There established standard of 18650 cell (18mm diameter, 65mm legth). Battery got to all notebooks. In recent years the development slowed down, maybe even stopped with 3400-3500mAh capacity mark. Its hard to get now over this value. All manufacturers struggle with it and are few years searching for new better chemistries.
Just a decade ago, li-ion got widely to all consumer battery-operated electronics. Its in powertools, lawn movers, chain saws etc. Thanks to high capacity they got also to all electrovehicles (electrocars, e-scooters, e-bikes atp.). In LED headlamps they are used normally for years but compared to electrovehicles its just small portion.
Currently there are certain physical obstacles for futher increase of capacity. Many researchers and companies are now working on a new generation of li-ion cells. Very promising seems to be solid state li-ions, cell without liquide electrolyte and batteries with graphene electrodes. None of these new batteries were made publicly available and we dont have any information about their progress. But its real they would come in next 3-5 years. Researchers estimate that the capacity could be up to 30% higher compared to todays best performers. I would be careful with such a figure but some two decimal increase is definately possible.
https://batteryuniversity.com/learn/ - large source of information about li-ion
https://lygte-info.dk/info/indexBatteriesAndChargers%20UK.html - accurate discharge graphs and measurements of all curent li-ion cells