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21700 batteries

So I have been researching battery options. I see a lot of cool ideas on the forums about battery holders a couple of you are building and other things about charging and all that GREAT INFO !!!

However the one thing I haven’t seen is any mention of the 21700 cells … they seem to have a higher Mah rating and a little cheaper to get ahold of. I guess what I am asking is other than the physical size ( obviously the 21700 is bigger) are / would there be a down size to building a pack with these cells to get a longer run time ?

Maybe no one has tried it yet?

Just keep in mind that most 18650 (and I guess 21700) are rated for lower currents. Our 18650 cells are the INR18650-30Q which can handle bursts up to 20A. We use a 4s6p configuration with allows the ROV to draw up to 120A without damaging the battery.

So I advise you to take a good look at the datasheets before moving on, and then let us know how how it goes!

William,

Thank you for your response and for the data on the batteries you use. I have looked at the data, and researched the forum. I found one other person / post talking about these 21700 batteries. And the only downfall I can see is the size difference. However the specs allow for a lot of benefit to use these … you can get them in 5000mah ( and bigger) which is a significant increase over the 3000mah in the 18650 along with more run time, they seem to be more cost effective as well. I have seen these if you build a 4s6p config the same as yours using the the 21700 that should give you well over the peak amps you currently get … correct ? is that a bad thing to have that available power ?

Any advice anyone can give me would be greatly appreciated… I am looking at building this as a sealed unit … so I wont have to open the enclosure to swap batteries , in theory you would just quick release the battery and place on another one … kinda like a battery pack for a cordless drill of sorts …

I made a battery pack out of 21700 cells. The main disadvantage is that they don’t fit efficiently into the 3 inch battery tube that comes with the ROV. They had lots of power and a nice boost in capacity, though. I think my cells were 4800 mah, and the pack had about 30 Ah in a 4s7p configuration.

I mounted them in a 4 inch pvc tube, which required some extensive modifications to fit under the BlueROV. But otherwise it was great. I went with 18650 cells for my next battery pack, though. Makes for a much more streamlined ROV.

I’m using The Samsung INR21700-50E. Very happy with quality and performance, though I run relatively low loads (a 4S2P 10 Ah giving me a full day). With a cheap battery spot welder (under $200) there’s enormous flexibility in how you can configure them is a tight space.

Yeh I was looking at making like a flat sealed pack, with custom flat sealed “box” to mount under a 3inch tube on my custom rov … that is good info to know that running a 4s2p works I was wondering how many parallels I would need . I am gonna do some research in building a pressure vessel for it.