A note about power consumption

The Lamp is a portable object which communicates with the satellites in order to find important (or not) places in the world, and then shine a light.

Pretty simple. But the details are important.

The important part of the above sentence is the concept of portability. I can’t tether the lamp to mains power, or require a giant car battery.

Of all the limitations this project has, is the battery. Right now I’m specifying a 3.7v 2000mAh Lithium Ion Battery with an alternate being the 3.7V 6600mAh Lithium Ion Battery Pack, and connecting it to the system through an Adafruit PowerBoost 500 rechargeable 5V power shield. I guess I could design my own charging system, but lithium-ion batteries scare me. They pack a lot of power per cubic centimeter, but don’t play well with others, tending to melt or blow up if they are shorted, bent, crushed or punctured. So, I will have to create a cage for it so it doesn’t hurt me.

Let’s look at what the system draws without lighting the lamp at all:

Component unit
Compass board0.11mA
Compass LED20.00mA
GPS board55.00mA

So I have 2,000 mAh at worst case battery to play with, but let’s assume a 10% reduction due to recharging, storage loss, etc. This gives me 1,800 mAh to work with. I also am assuming worst-case scenario where the lamp is fully lit (for example at a center of population). In reality, most of the time all the LED’s will be lit. I’m sure there I can use calculus to find the slope and areas of illumination to derive the typical power consumption.

Item1 hr ops2hr ops3hr ops4hr ops
System power110.11220.22330.33440.44
Lamp power remaining1689.891579.781469.671359.56
WS2812 LED @ 60mA
(round down)
28 lamps26 lamps24 lamps22 lamps
Luminous Intensity @ 233 (mcd)36524 mcd6058 mcd5592 mcd5126 mcd
R20WHT-F-0160 @20mA
(round down)
84 lamps78 lamps73 lamps67 lamps
Luminous Intensity @ 600 (mcd)350400 mcd46800 mcd43800 mcd40200 mcd

Besides the total power available, the real limiting factor of the battery is the standard discharge of 500mA which can peak at 1000mA. I don’t want to go toward peaking, because see exploding, above.

I have some options: I could source alternative batteries, use multiple batteries to run different systems or parts of systems, or find a better illumination system so I can get the maximum luminance per mA.