This can be printed with up to a 1mm nozzle. Quite the boon if you ask me.

How to upgrade a pack:

1. Run wires (14/12 gauge depending how pedantic you are) down the pack to new rows of cells.
   This parallelizes those cells with the 5s pack that is already there. There is a photo of my, admittedly sloppy work in the photo. Grinding off the ends of the cells, and using very high heat minimizes exposure of the cells to heat. One can use a spot welder, but I'm not made of money, so I did it this way.
   2.After the battery pack is constructed, bend all original springs straight, minding where the tabs will mate. I glued in my spring tabs to the case with epoxy, There was not room to add a slot (for shame, I know).
2. Simply slide all the components into the shell, add a spacer to keep the board snug to the top of the casing. Photo of the offending stuff included.

Congratulations, larger battery. This is for the 18650 type packs. Not sure if they all are. This probably works with most tool batteries, however I only have Ryobi so this is the casing I created.

Why:
This has been a bit of a pet project for me as I hate how power tool companies will absolutely rip you off for big batteries. I have refined a methodology of extending the pitiful batteries that come with drills stock. I have created batteries larger than (easily) available. A 12ah battery notably. Impractically large however very inexpensive. Better cells, or more cells could make larger ones conceivably, but all my cells were recycled from modem batteries. 6 cells for about 3.50 plus shipping. In my case, I recycled previously not useful batteries, and for about 7$ a pop, plus the original battery, I end up with a 9ah battery. Good for the environment, good for my wallet, its a win win.

Cell selection.
Total current output of a battery is theoretically 15amps. There are 5 cells, in series, creating ~18v. 4.2v per cell max, 3.7v per cell nominal.
For each new layer, a new set of 5 in series, are parallel with the initial set of batteries.
This means each cell, individually, must be able to handle a different max current depending on how many you add.

YOU MUST ADD A FULL LAYER. If you have additional questions as to the math, I would refer you to "18650 battery pack calculations" / "18650 battery calculator" searches, or feel free to email me, [email protected]

Additional cells:
5 = 7.5amp rating each cell
10 = 5amp rating each cell
15 = 3.75amps

Math:
1 original parallel (series set of 5), 2 additional or 10 extra cells
15amps / 3 parallel sets = 5amps
1 original parallel (series set of 5), 3 additional or 15 extra cells
15amps / 4 parallel sets = 3.75amps
So on and so forth.

This is somewhat convoluted, and I am not explaining all that well. But the larger a battery you build the crappier and cheaper your individual cells can be. The output of the tool is limited by the resistance of the tool itself, so there is no risk of burning out a tool by providing too much amperage. (This has been tested extensively) Again, email me to argue if you must.