Energy source

A project log for Portable LED Flash

As modern photography is revolutionized by LEDs, I'm applying my experience in this field to enhance my hobby

Yann Guidon / YGDESYann Guidon / YGDES 04/04/2017 at 18:120 Comments

This kind of project wouldn't be viable without a suitable source of electricity. I happen to have studied Lithium batteries, a decade ago :-)

This project is a perfect fit for the stock of old batteries I had bought from a RC store, a while ago... 60€ for 6 2S1P KOKAM packs, that's a bargain :-)

As far as I know, the low price (10€ vs 87€) is because at that time, the RC world was transitioning from 10C to 20C cells, and these are "old 10C" that wouldn't be great for flying engines. But still excellent for our purpose, with low current and short strong spikes.

This battery is not heavy and the energy capacity is great : 7.4V×2.1Ah = 15.5Wh ! Give or take 20% due to aging (but not use because they are brand new).

The peak current is also in the desired range : 10×2A=20A !

I have 6 such packs, so I can make 6×8=48V easily. This is perfect because the LED modules require between 24 and 32V (there are 10×10 LED chips in the module). 4 packs in series will make up to 34V when fully charged (that's 8S1P...) but I have not taken the various voltage drops into consideration...

But first, let's see if they are still alive ! The last measurements for one 2S1P pack is:

Looking good !!!

So one 8S1P pack can drive short pulses of high current in the projected 4×100W modules, while also maintaining some decent light output. The pack could sustain short pulses of 20A, or about 5A/module if the 4 are wired in parallel.

The 100w modules are made of 10×10 LEDs, 100W/100=1W/LED so we estimate that each LED is designed for 300mA. 0.3A×10=3A for 100W. 4 modules in parallel will draw 4×3=12A@400W (that's a lot).

The voltages so far :

  1. 7.21
  2. 7.34
  3. 7.13
  4. 7.21
  5. 7.14
  6. Can't measure, bad contact ?

The remaining 5 packs clearly need to be re-balanced. Fortunately, none seems to have swollen and they are all from the same batch so can be used in series. I'll parallel them (through resistors) and keep the 7.34 (healthier) for an eventual project.