Triangular 3 pack of 21700 batteries connected in series - 10.8V, 5000mah, 54whr

[thejaguar]

Folks,

I am looking to DIY my own set of backup battery packs for my underwater diving light. Essentially, it's a pack of 3 21700 batteries connected in series and packed in a triangular shape. Here are a couple of images of the battery pack that I would like to DIY - View-1 and View-2

I am decent with soldering and basic circuitry, but it's been a while since I have messed around with my soldering iron. Please suggest as to how I should go about building this pack - what precautions do I need to take, etc? The dive light is a constant output 6000L light.

Thanks a lot!

 

[docw009]

What kind of fire safety concerns are there with lithium powered dive lights? I would imagine a series combo of three cells needs overcurrent and overcharge protection built into the battery, but then again, I dunno,

In the biking world, they sell cheap 8.4V lights that have nothing. I guess no one cares if they catch on fire. I do, so I've replaced mine with packs that have battery management system (BMS) safety circuits. They sell all kinds of BMS circuits for 3S batteries like these. Cheap, but they work. Do your dive lights use them?

Solder is only a last resort, as it can damage the cells. You can find cells with spot welded solder tabs, but they are overly expensive, You can buy an inexpensive $50 spot welder. These are problematic, but will work with the supplied strips for low current dive batteries. The welder's internal Li-po battery is known to wear out after a month or so, It has its own fire hazard issues. Reliable spot welding starts to cost around $150 and up,

That's it then, You can solder up three 21700 cells and hope they don't catch on fire. At a minimum, add a BMS, Then you can investigate spot welding etc, but a cheaper solution just isn;t there.

 

[thejaguar]

Thanks @docw009 for your response - appreciate it! After I made the post, I have been reading up on this and I think it's best that I buy a spare battery from the manufacturer rather than messing around with this - I will have to get a BMS, charging board, etc and package it all up to fit into the flashlight. This is likely beyond my capabilities at this time.

 

[E-HP]

Thanks @docw009 for your response - appreciate it! After I made the post, I have been reading up on this and I think it's best that I buy a spare battery from the manufacturer rather than messing around with this - I will have to get a BMS, charging board, etc and package it all up to fit into the flashlight. This is likely beyond my capabilities at this time.

If it’s only for lighting, you can use button top protected 21700 cells, and regular spring loaded cell holders. If you’re using single holders you can place them in the triangle configuration if that’s required. Spot welding is advised for ebike batteries since having any poor connections can render the pack unusable, but for something like lighting and with so few cells, cell holders would be ok.