Hi Sane, easy calculations but long to describe the process. I describe it on Lumonite Compass, XP-L2, 600lm and 12.6Wh battery. There are two things you must estimate - efficiency of the driver and optics losses.
1) First is to find out how many W do you need to produce 600 lumens with that type of LED
When you know LED type (XM-L,XP-L2 etc) and bin (T6, V6, W2...) then you can calculate how much current and voltage you need to have to shine that amount of light (lumens). Current multiplied by voltage give you W value - needed power. Usually manufacturer specifies that voltage and current in 25° or 85° LED die temperature so rather choose 85°C (=less efficient). Also each manufacturer states some range of light output because no two LEDs are same so they fit them into ranges described with MIN/MAX light ouput. Its up to you what number you take, but we always use MIN to be sure that at least that amount of lumens is produced. Taking just MAX value doesnt make sense. In this example XM-L2 with U2 bin in 85°C produce 589lm when supplied with 1500mA and 2.83V (taken from Cree datasheet: https://assets.cree-led.com/a/ds/x/XLamp-XML2.pdf
). Power = 1.5A * 2.83V = 4.245W
. Round it to 4.32W
to have 600lm.
2) Then you must account for voltage conversion losses
Drivers can be generally 70-99% efficient. Efficiency up to 90% is rare but possible. FIY we make our drivers with 98% efficiency but It took a long time to develop and its not in the whole voltage range and currents. But take 90% as an example, so, to supply 4.32W to LEDs
you need approx 4.8W from battery
3) Then you must account for optics losses
Light from LEDs goes through TIR optics or reflector, both have optic losses. Reflectors could have higher losses. Best and large TIR optics have 90-92% efficiency. So at least another 10% of light is lost there. When you need to have 600lm
on output, LED must produce at least 660lm
. So you should make previous calculations again for 660lm to be accurate. But lets make it easier and just simply add 10% more power to account for that, so 4.8W * 1.1 = 5.28W
4) Calculate true (best estimated) runtime
You know battery Wh rating (12.6Wh
). With constant regulated light you have runtime = 12.6Wh/5.28W = 2.39h = 2h23min
. So its not constant regulated all the time. Its okay as long as the manufacturer shows you runtime graph so you know what to expect. Chinese brands often use runtime graphs with logarithmic or pseudo scale to impress customers that the step down is not so bad - like this runtime graph
You see its not complicated. Previously Cree had pct.cree.com page where you could easily dial all LED parameters but its not there anymore after Cree renamed to Wolfspeed. So its more difficult to go through datasheets to read voltages and currents from there.
Generally two LED lamp should have higher efficiency = runtime. Piko X4 in my knowledge is also using XM-L2 LEDs but higher U4 bin, thats approx 8% brighter (or lets say more efficient). What I personally do not like is their 6500K color temperature.