# Project: Building an underwater SCUBA dive light

Discussion in 'The Projects Forum' started by nweakland, Mar 10, 2008.

1. ### nweakland Thread Starter New Member

Mar 10, 2008
4
0
I'm hoping this group is willing to give information freely because I have very little to give back. In reading a few of the posts, I can already see that you are all way over my head. I do have the very basics of DC circuitry down.
Please read what I am planning to do and what I have to work with, and please post any criticism, both positive and negative.

I wish to build a 12v underwater dive light with a rechargeable battery pack.
Here is the commercial product that I would like to replicate.
http://www.diverite.com/products/catalog/lights/lt6400mr11-ss

Materials I plan on using:
I have the 10w HID light head and bulb
I have a large supply of 3.6v LiIon cells
I have a 20v 4.5A DC power supply for the charging

My original question was going to be "How can I make 12v out of 3.6v cells" but in reading this forum, I found that the forward voltage drop of a diode is .7v...
Can I put 4 cells (14.4v) and 3 (2.1v drop) diodes in series to get me to 12.3v??
how close does the voltage have to be to 12v without damaging my expensive HID light.
If this is possible, how can I charge the circuit with diodes in the way?
Again, I apologize for asking so much and giving so little. Any help is appreciated.
Thanks

2. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
Hello,

Welcome to the forums

Your HID light, do you have a suitable ballast for it? Can you merely apply 12V and it will work? Secondly, if this is true, can you get information about the power requirements of the assumed ballast/HID combination? A lot of times, input voltage is specified over a range, but a nominal voltage is usually expected. It would be nice to simply apply 14.4V to it and call it a day.

You can use a linear regulator that can handle at least 1.5A current. such as

http://www.national.com/ds/LM/LM1085.pdf

As batteries discharge, their voltage goes down. This can be problematic since linear regulators have a specification called a 'dropout voltage'. This is the maximum difference between the output voltage and the input voltage of the regulator. For the above chosen regulator, you need to always be above 1.5V + 12V = 13.5V. Following a typical lithium-ion discharge curve, you will approach the minimum dropout range as the battery has discharged about 90% of its life. You should mount the regulator directly on the case, which is hopefully in contact with the water. The regulator will want to dissipate about 2W of heat. In open air, that amounts to about a 125degree C temperature.

The only way around this solution is to use a drop-in 12V switching power supply module if you can find one. They would accept your complete range of battery voltages, then be able to work at a decent efficiency. This is at the cost of complexity though.

Another note, be careful with the connections from your inverter underwater. Salt-water plus a few thousand volts from the ballast equals an unhappy diving experience. If you notice fish dropping around you, you might want to look into it

Steve

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
I suggest that for your safety, you do not attempt this project.

The commercial dive light product has been tested and certified for use at depths up to 400ft.

You will have no way of testing your light, except under actual use.

Your circuit will have to be hermetically sealed to keep the water from it. This is a non-trivial task.

Your life is worth far more than this experiment.

4. ### nweakland Thread Starter New Member

Mar 10, 2008
4
0
Thanks so much for the reply, Steve.
The ballast is in the head...I simply apply 12v and the light works fine. I don't know about the range...I was hoping that was a set rule you guys could tell me. If it depends on the light, I have no way of knowing. I bought the light of a company called American Underwater Lighting, but they went out of business and I cannot find any literature.

I'm not sure what you mean by inverter connections. If you mean the charge connection, I plan to have all that contained inside the waterproof canister.

5. ### nweakland Thread Starter New Member

Mar 10, 2008
4
0
To SgtWookie:
I am glad that you are putting safety first. Thank you.
Am I making an understatement when I say there is no life-threatening danger?
I mean I have a small battery pack contained in a tube to power a light. Even in the case of a total flood and the battery is naked to the water, how bad can that be? Isn't it true that water is a poor conductor? I just see me ruining a set of batteries.

6. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
You're welcome for the reply. Typically, you can read the input requirements of the device next to where the connector goes. There may be a switching converter inside of the device, which typically is okay with a variation of input. If these are pricey, then I would refrain from experimenting with this. If it could be taken apart easily, then an experienced designer may infer if it is feasible, but even in that case, it is a gamble.

In regards to safety, you might be underestimating danger. Two things strike me as major concerns: high-voltage from ballast, and heat/explosion from batteries upon shorting. Lithium ion batteries do not like to be shorted out, they may heat and/or explode violently. Salt water is a decent conductor of electricity. Also, if there were exposed connections from the ballast, then you may be putting yourself at risk to a high voltage transient.

I don't think I need to explain that for each meter of water depth, there is x-amount of pressure increase assuming that you are a scuba-diver. You should calculate the maximum pressure that you intend to come across, then make sure that your container can withstand such pressure. You can get PVC that is rated to that sort of pressure, but making the connections to outside the tube may not be easy. I have heard of people suspending their electronics in mineral oil so that it may withstand pressure better, but I don't know how well that would work, especially if you have to recharge them a lot

Good luck!

Steve

7. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
My concern is that unless your circuit, wiring, etc. is hermetically sealed, water will eventually force it's way inside the components used and destroy them, and you'll find yourself suddenly in the dark. That might be OK if you have a backup light immediately available. It would not be OK if you were a hundred feet down and 50 feet inside a wreck, with no backup light.

8. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
Just an update, I've been looking into conductivity of salt water. It seems I was wrong about worrying about the salt water conducting. I do not think, based upon what I read, that sufficient current would be drawn fast enough to upset the li-ion batteries. Nonetheless, it would still discharge them quite fast.

Steve

9. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Like most diving equipment I note the commercial light uses O ring seals. These are not 'hermetic' but use the water pressure against itself - a very effective method actually.

But you have to be able to engineer the retaining grooves to use this method.

10. ### Audioguru New Member

Dec 20, 2007
9,411
896
BEWARE! Lithium is extremely active.
If a lithium-ion battery is over-discharged or over-charged then it becomes unstable and catches on fire with an extremely hot white flame. I saw one.
Water touching lithium causes it to become unstable and it might catch on fire.

11. ### gee_emm Active Member

Jan 16, 2008
34
0
Perhaps LEDs in a "cluster" could be used, to avoid using a ballast. Then maybe some blues could be added for better penetration underwater. I mean, just a thought, I cant even swim, so I have no idea what underwater looks like.

12. ### nweakland Thread Starter New Member

Mar 10, 2008
4
0
Thanks to each of you for your valuable input.
I'm not at all concerned about the waterproof housing...I have a couple options for that. I just need help with the circuitry.
I am considering using NiCad batteries now after the warnings of LiIon...maybe I'll just buy the "C" or "D" 1.2v cell type and use 10 of them. That puts me at 12v and I can charge it with a trickle-charger in my garage.
Anyone see any problems with that? Will a trickle charger be too fast for a battery pack that small?
Thanks again

Jul 17, 2007
22,183
1,728