I am wanting to install several [maybe 4] bright LED’s to be mounted on a robot crawler for a subfloor building inspection. Weight is critical, and I am wanting to know what options are available for LED’s and Batteries running on a separate circuit to the other electronics.
I was considering using the LED - 3W Aluminum PCB (5 Pack, Cool White)
I need a battery pack(s) to run the LED’s for about 40 minutes - Total weight limit about 200 grams.
Hi Simon
As the Core page will tell you. Each LED will drop 3.2 to 3.8V and require 750mA for full brightness.(≈3W)
How bright are you expecting to run these.
I think you would be best off driving these with constant current dedicated LED drivers. I think they come as 350mA or 700mA and some selectable between values.
4 LED in parallel at 700mA = 175mA each Total 700mA
4 LED in parallel at 350mA - 87.5mA each Total 350mA
2 LED in parallel at 700mA = 350mA each X 2 drivers Total 1400mA
4 LED each at 700mA (4 drivers) = Total 280 mA
Using the constant current drivers takes care of any required current limiting resistors and allows you to use a wide voltage input range.
As for the battery. Depends on how bright you want the LED. If you need full brightness the 200gm limit may be stretching it a bit. I would suggest getting the LEDs you have flagged, they are not that expensive, and do some experimenting. They are apparently very bright so your current (mA) requirements may be fairly modest.
Cheers Bob
Core have a couple of versions of drivers.
Single channel 330mA or 660mA SKU: COM 13716
3 channel 330mA or 660mA (each channel) SKU: COM 13705
All up to 36V input
Thanks for the reply Bob. I need to get almost full brightness out of them, so I was expecting to need about 2.5A to 3A total current.
I need to light a fairly large [several square metres], dark area.
Based on 3.7v LiPo batteries [2400 mAh] I thought I could hook them up with some voltage protection - although the wiring on the batteries would need to be changed as they seem to all be limited to 1A current draw, even if the batteries are rated much higher.
At 3A [max brightness] they would use 2000 mAh?
I think at 3.7V and 3A you would need to connect directly. I don’t think that is very safe. Someone with more experience with LiPo might correct me here but I wouldn’t do it. I think LiPos can fail in a fairly spectacular manner sometimes.
I would personally go for more voltage and use drivers as above. I would also experiment as well because you may be surprised how bright these can be.
As these converters are switch mode to get into the real world do all your calculations in mWhrs and allow for efficiency which I think is quoted at 85%.
Example 3W @ 3.8V = 790mA
3W @ 85% requires 3.45W input
3.45W @ 12V = 288mA
So 3W/hr @ 3.8V per LED = 790mA/hr
Power into the driver = 3.45W/hr @ 12V = 288mA/hr
3 batteries in series will be 11.1V, You may find you only need 3 LEDs. If this were the case I would go for 3 18650 batteries in series with the triple driver or if you don’t need to drive to full brightness you may be able to reduce battery size or voltage. But, as you can see when you convert to Watts to get the real figure the mA/hr figure reduces with increased voltage. The trade off here is the battery bank gets less manageable but there will be an equilibrium somewhere…
Cheers Bob
Yes, I agree, it is certainly quite spectacular and always something to be aware of (heat, over-discharge, overcharging, physical shock, piercing, thermal shock, and static discharge are just a few of the ways to set them off 
). It is also worth noting that the salts, oxides, etc that form when the lithium inside the cells burn is also quite toxic to my understanding. In uni robotics labs there’s usually a big bucket of sand that you pour over a battery that is beginning to burn and everyone evacuates, it’s really not something you want to be around.
Yes, that’d be correct, however, the issue is when the batteries have a differential voltage between them, you’ll also need to be careful regarding the maximum current that you can pass through the wiring for each battery (when you’ve got them hooked up in series or parallel you’ll want to be careful that the wiring that is attached to each battery can handle the current load on it, single core wiring can usually handle a fair bit more current, but 99% of the time I wouldn’t recommend changing the wiring to a cell, and rather just getting a higher rated battery)
@Simon115759 Thanks for posting on the forum! Welcome!
You’ll want to be quite careful which batteries you choose to go with and how you hook them up. If we’re aiming for a near constant 3A draw at max brightness, it’s almost certainly going to be larger than any of the batteries that we carry (typically, they’re designed for use in small microcontroller and microprocessor projects with a UPS hat of some kind to keep a low-draw board running for a long time).
HobbyKing is the way to go here. If you know the capacity, voltage, max discharge current, approx dimensions, and battery chemistry that you’re after, they should be able to set you up with an appropriate battery and charger and may have some more advice on how to use it too.
I agree with your original idea. LiPo batteries, or maybe LiFePO4 are the way to go here. A NiMH or SLAB may work too, but the energy density of the cells by comparison to the increase in weight is generally quite significant.
You might have to get the robot to tow a trailer with the battery on it
Just a thought.
Cheers Bob
If this was my project I would be looking at NiMH AA cells. Less dangerous than LiPo and reasonable voltage current output for size. The post by @Bryce is quite important.
Two LEDs powered from 3 cells in series rated at 2000mAH, would met your requirements.
Cells weight about 30gram each.
You would need to calculate the series resistor need. Probably 0.47 or 1 ohm at 5 Watt.
Anyway, the warning of what can happen with LiPO’s is very relevant.
Using individual cells in series I would never do, according to what I have read.
If you need higher than 3.7V buy one that is designed with a higher voltage, much safer option.
The C rating of a LiPo is very relevant too. How much current you can safely draw from the battery.
Drone & remote control car batteries are design to output a lot of current for a short time (C20 rating). The standard LiPo is around 0.5 or 1.
Cheers
Jim
And you can get our latest projects and tips straight away by following us on:
