Hi all,
I have an upcoming stage lighting project where I'll need to build a very thin battery operated LED tube. The project has some very specific size and design requirements which mean that no off the shelf products are suitable.
The tube needs to be 1,6 metres long (±10 cm, approx), as thin as possible, but with an absolute maximum diameter of 40mm, and it needs to be completely evenly illuminated with 360 degree coverage. Battery life should be approx. one hour at full power. Light output should be roughly equivalent to what one would expect from two generic 5050 «14,4W/m» LED strips. Reliable wireless control over light intensity is required. The wireless transmitter should accept DMX input, but the protocol of the wireless communication itself does not need to be DMX.
My idea is to use these satin acrylic tubes in either OD 30mm / ID 24mm or OD 40mm / ID 34mm size, and an internal tube with the battery and electronics inside of it, and these COB LED strips on the outside.
I'm planning to put some kind of small 'duino board and and NRF24 inside the tube, and make a NRF24 transmitter with DMX interface. The LED strips will be PWM dimmed with a MOSFET. This should be straightforward, but I'm a bit uncertain about the battery side of this.
I have looked for pre-made battery packs, and it seems there is a loosely defined standard for long, slim "stick" batteries intended for airsoft guns. If I go for the OD 40mm tube, I think I can just barely make one of those fit, but I'd like to try something even slimmer, so I've looked at 14430 cells.
Let's say that the power consumption is approx. 35W max, so I'd want roughtly 35Wh capacity at around 12V. If each cell is 3,7V 600mAh, I take it a 3S5P pack would be approx 11,1V 3000mAh, right? That should be close enough, or if it isn't then 3S6P should.
The cells are nominally 14,3mm in diameter. If I can make the interconnects compact enough I could maybe fit this inside a 16mm ID tube, which would be cool.
I've read a bit about making custom 18650 packs, and I take it that this would be quite similar, just smaller. There's a lot of information on this around, but most of what I've found relates to making gigantic packs and/or recycling old cells, which isn't what I'm going for.
So I have a few questions:
- First and foremost, is there something glaringly obvious that makes any of this a very bad idea?
- It seems that a "battery management system" is recommended/required/unneccesary, depending on who you ask. Would it be useful in this case? Would I need one "3S" BMS for every 3 cells, or could one board do the job for the whole 3S5P assembly? Is the BMS even necessary? If nothing else, I take it it would simplify charging?
- I'll need to use quite a lot of LED strips to get even coverage, but I'll never need (or want) to run them all at full power, for thermal and battery reasons. While I could probably just make the microcontroller never run the MOSFET above, say, 50% duty cycle, I think I'd prefer a hardware solution. I'm thinking I could use an adjustable voltage regulator and undervolt the LED strips to, say, 9V or something (to be determined by testing), then PWM dim that slightly low voltage. Is this reasonable, or should I use some kind of current limiting to make absolutely sure that excessive current won't be drawn from the battery?
4a) How problematic is it that I don't have a tab welder?
4b) A colleague has some sort of bizarre DIY contraption that I think is a kind of spot welder. It's normally used for assembling small metal sculptures. How stupid would it be to try to use that on 14430 cells? Is it likely to result in fire, death, or worse? - Normally I'd use a metal tube as the internal tube, to act as a heatsink for the LEDs. Would this be risky when the battery is inside? It could slightly increase the risk of short circuit, I guess, but is this a realistic scenario?
Oh, and by the way, this will need to be transportable by air. It seems I'm well within the battery capacity liminations set by the airlines, though.
Any advice would be appreciated
TIA
-G