This may be a dumb question but I can’t seem to find the answer. I want to put multiple 18650’s in a PVC tube to power an underwater light. I currently run the light with one battery and have to change batteries way to often. I’m using Cree XML leds two of them
How do I run multiple batteries in parrellel, vertical I can put 2 side by side but want about 8 in the tube.
Thanks for any help, Chad.
Just a thought, if the tube allows 2x 18650 parallel, then it has an internal diameter of at least 36mm.
So you could maybe put a 26350 cell in sideways?
There is a group buy on at the moment for 26350’s
Would make building a parallel bank that fits really easy.
All you need to do is connect all of the positive (+) terminals together and all of the negative (-) terminals together. If you have the batteries in pairs side by side, you should have plenty of room to run a positive and negative cable the length of the tube. This will give you 8x the capacity of a single cell, but the same voltage. For example, if each cell is 3000mAh, 3.7V, this configuration will yield 24000mAh, 3.7V.
Nice clear diagram in the post above.
Of course if you do it that way, you will have to be very careful to make sure that none of those adjacent positive and negative ends can touch when the cells are pushed together in the tube, you will need some sort of spacing between them, which will then add to the overall length.
Now if you were to turn every second pair of cells upside down, you wouldn’t have that problem.
Perfect, that’s what I was thinking, 2nd diagram, I’ll hear shrink everything together and wire it all up. Thanks everyone
The first diagram is nice too. Thank you for sharing. I just don’t have anything on hand to separate the cells. I am trying to keep the complexity and weight down. I’m using 1 1/4” pvc
According to online measurements of the ID of 1 1/4 pvc is 1.38”.
2 18650’s would be a minimum of 1.417”. Not sure 2 will fit.
If you move up to 1 1/2 pvc the ID is 1.61” you might even be able to fit 4 per stack reducing the length to half with a 1/4” bigger diameter.
Lol thank you for that tidbit of info. I’m actually about to walk into the hardware store to get a roll of insulation. I’ll grab 1.5” pvc. That seems more practical. I’ll put 8 batteries in the tube and should be able to increase my run time to 6+hr
Whatever the diameter of the cell, you can pack:
— 3 in a tube that’s 2.154× the cell’s diameter,
— 4 in a tube that’s 2.414× the cell’s diameter, and
— 5 in a tube that’s 2.701× the cell’s diameter,
so just increasing the diameter a smidge can buy you a shorter tube with less-complex wiring.
Go triple, and you can pack 7 in 1 cell-height.
How is you know those multipliers?
Geometry 101. 
Actually is pretty easy to “prove”, but there are common multiplier tables online. I had one a while back when thinking of doing the exact same thing (only 1 level high, though).
Hang on…
Goggle “circle packing” (“enclosed circles” didn’t work too well). Eg Circle packing in a circle - Wikipedia .
Thanks for that . I don’t remember that from Geometry in high school but then again it was the 70s.
You didn’t say how deep you planned to take the battery pack, but if pressure or durability is a potential issue “Schedule 80” PVC pipe (usually gray rather than white) might be worth considering as it has thicker walls. You might not be able to find it in a big box store, but dedicated plumbing supply stores usually carry it. Sizes/thicknesses are all over the internets.
It’s flounder lights, I have been using them for a while I just want to upgrade the battery pack. I have schedule 40 and 20. I’m going to mess around and see what fits. I bought 1 1/4 and 1 1/2” pvc so I’ll report back when completed. I am remodeling my kitchen too so hobbies are bottom of the totom pole. Lol.