18650 Battery Packs.

I needed a battery for a small photovoltaic system, my first idea was the traditional lead-acid, but afther some reseach found that 18650 li-ion have some very good advantages and can be found at reasonable prices, so i decided to give it a try.

This is my first attempt to do a 18650 battery, and soon found some problems...

I already have a 2000W 12V inverter, so my system should be 12V, but li-ion cell voltage range is aproximately from 2.5V fully discharged to 4.2V fully charged, but to get long life from the cells a good range is 3V to 4V.

When you try to fit theese numbers in a 12V system you realize that they just fit very bad:
If you stack 3 cells the range is 9V  to 12V.
If you stack 4 cells the range is 12V to 16V.
Voltage range of my inverter is 10.5V to 15.5V.

A better for li-ion is 24V: stacking 7 cells you get perfect voltages, but for now i have to stick to 12V, so i decided to do something modular: packs of 10 cells in parallel that i can use as building blocks for a 12V now and reuse to build a 24V later when i get a 24V inverter and more solar panels.

Having a 3D printer i decided to print my own holders, and having some brass bars around i decided to use then as bus bars.
I finally did this design:

These packs can be connected in series easily, just stacking them and one bolt for each bus pair:

Then several of these strings can be connected to main bus bar with just 2 bolts for each string, this way is fast and easy to take out just one string and keep the rest of strings connected:

Afther some intents i found some things that could be very useful:
- Screw bus bars to plastic holders, this way there is no risk to break connections with cells:
Cells are connected to bus bars with a thin wire that acts as a fuse, protecting cells from high current in case something go wrong.
- Print positive and negative sides in different color, ideally red and black, but i already had white and black so i used theese, the idea is differentiate poles to avoid connecting some pack in reverse.
- Doing a slot to "embed" fuse wires into bus bars:
Pack are stacked one over the next, so bus bars are touching and the surface needs to be flat for the packs to fit perfectly.

Note That positive busbar connection hole is threaded and negative is not:

 This way is possible do a connection just with one M3 bolt;

As fuse wires i used 1/4W resistor legs, i tested a few kind of wires, copper is too conductive so in order to get a fuse that blows below 10A it must be very thin and fragile.
This resistor legs blow up at around 7A and are still tick enought to be strong.
In adition i had a few hundred resistor that came with leds but i will problably never use, so is the perfect option for me.

Here is the battery almost finished, there is still one string (black cells) i have to replace.
At top of the image yo can see positive main busbar and negative at bottom, they are done with squashed copper pipe, the kind that was used for home water instalations.
The right side 3 strings are connected in parallel at pack level, right side willl be when i replace the black cells string.
Battery main cables are connected to the middle point:

Here is a video of the building proccess:

I uploaded a customizable model of the holder to thingiverse, it is possible to change bus bar size and some other parameters:
https://www.thingiverse.com/thing:3018655