I'm looking to create a power supply for a heated jacket that operates at 12V and draws a max of 6.4A (though that could double with the addition of pants, gloves, etc). I'm basically trying to recreate this device:

https://www.gerbing.com/collections...d-clothing-battery-kit?variant=44879370387747

I'm hoping to have something similar, but with batteries I can charge and replace with standard components (and perhaps with a greater capacity). I also don't need the output controller (as Gerbing offers a controller for use with motorcycles that I believe is just PWM) or charging capability (I'll just use an 18650 charger).

It seems that I can just get a battery holder and wire 3 cells in series (for an output of 9.9-12.6V) and attach a connector that can handle the amps. Is it really as simple as that? I feel like I need some kind of protection that cuts off the circuit at around 3.3V/cell. It would also be nice to view the charge state too (total voltage would probably be enough).

Is this as straightforward as I'm envisioning? Would there be any advantage to using 4 cells and a voltage converter to keep the voltage closer to the 12.6-14.2V you might find from a motorcycle's electrical system?

 

Check the 18650 data sheet for the maximum current one cell can supply safely. It will take several series packs connected in parallel to get the current you need.

 

As you suggestged you should have a circuit to limit the current both in and out of your cells and also circuitry to manage the charging current. These are often referred to as battery management systems.

 

Right on, it seems like I could find a 20A 3s BMS for pretty cheap, and then it's just a bit of soldering and figuring out how to package it all. I'm surprised there isn't an existing product for this (or maybe there is and I don't know where to look??)

Would I really have to worry about draw with three cells? It looks like it's rare for even a single cell to be rated less than 10-30A...

https://www.patreon.com/posts/updated-18650-131428775

 

Let's do a quick sanity check.

You are looking for something around 12 V and 6.4 A, so right around 75 W.

An 18650 cell has something like 6 to 9 Wh of available energy in it -- let's call it 7.5 Wh. That means that, neglecting other losses in your system, each cell could give you about six minutes of use. If you want your stuff to work for an hour, you'll need at least ten cells. Each cell is gonna weigh about 50 g, so ten cells would be a bit over a pound just for the cells. You also have to consider the volume involved, too. Are those kinds of numbers acceptable for your application?

 

That means that, neglecting other losses in your system, each cell could give you about six minutes

And it is actually worse than that. The capacity of batteries is generally specified for discharge over 20 hours. Discharging faster lowers the capacity significantly. Discharging over 6 minutes might well have only 1/2 the stated capacity, if it is even possible. There is also a max discharge rate which is often about 1 hour for ordinary batteries. You would need special high discharge rate batteries for rates higher than that.

 

And it is actually worse than that. The capacity of batteries is generally specified for discharge over 20 hours. Discharging faster lowers the capacity significantly. Discharging over 6 minutes might well have only 1/2 the stated capacity, if it is even possible. There is also a max discharge rate which is often about 1 hour for ordinary batteries. You would need special high discharge rate batteries for rates higher than that.

I was looking at the data in the link he provided in the 5 A and 10 A draw columns, so the situation should actually be somewhat better as the way to extend the time is to put more strings in parallel, thus reducing the current draw from each cell, which should let them use more of their nominal stored energy.

 

Aaah, interesting. I didn't do the math right. These numbers are somewhat sobering

My plan was to use 6 cells, and I had a thought maybe I could put another 6 on the other side, but...then the "is this worth it in life" equation starts to tilt...