My goal was to add an external power connector that would allow me to plug my battery up to whatever I wanted to while still being able to use it in my tools and the standard charger without any extra effort. This has been achieved.
Step 1: Gather your battery and take it apart.
For my project I used a Ryobi P108 battery. I was able to get several of them for a cheap price and I know these batteries are built using high quality Samsung INR18650-20Q cells. Also I have a large collection of Ryobi One+ tools so it was a win win.
So first, gather your battery. Figure out how it needs to come apart. My Ryobi is pretty simple. Has 4 Torx 10 security screws. One has a plastic cover that you have to remove by prying it out with a small screw driver or knife.
Step 2: Figure out the best place to add your new wires.
For the Ryobi and probably all batteries, I think the best place to add your new wires is to the battery contacts where the current wires are soldered on.
Step 3: Solder on your new wires.
The Ryobi battery has 18awg wires coming from the circuit board to the contacts. I added 16awg wire for my modification. I figure that if 18awg is safe for the stock length, using 16awg for my short extension should be adequate. I also added a piece of electrical tape to help protect the wires from the connections stick out of the board. I also added a ziptie to each wire remove strain from the soldered connections.
Step 4: Modify the case so the wires can get out, cut wires to length, add your favorite connectors, and put back together.
At first I was going to make one large hole for both wires to share, but then I realized the wires would fit better if they each had thier own hole. So I driled two matching small holes for the wires.
Once that was done I put the battery back together with the wires coming through their holes.
Next was to cut the wires to length, do them one at a time so you don't short the battery.
Then add your favorite connectors, again doing the wires one at a time. My favorite is Anderson Powerpoles.
Step 5: Testing and proof of concept.
In the first photo I have the battery connected to my power meter then that is connected to my hobby battery charger. The Ryobi battery is powering everything.
You can see that the power meter shows 20.63v which is around what a fully charged 5S lithium battery pack should be. Not the 18v it is advertised to be. (18v comes from the NiCD battery days and Ryobi has not changed that nomenclature).
You can also see the battery installed in my Ryobi vacuum. You can see that the extra connector does not cause any obstructions. The battery will still fit into any Ryobi One+ tool as well as all their chargers.
Step 6: Multiple Uses - only limited to your imagination.
This can pretty much power anything. I have a ton of projects that I've done in the past years that this can be used with.