Step 1: Source Your Batteries
One of the best places to get batteries that are no longer used but have large capacity and large power output is laptop batteries, these batteries are usually li-ion as they are the lightest per power output and are good in general. if you have any old laptops that are not used anymore then use them, if you have a laptop with a battery that says not charging, what is the point in keeping it, re-use it. phone batteries are good as well but usually don't have good capacity as they are designed for phones and although have the best power to weight ratio they don't output much power as it is small, these batteries are good for smaller projects.
Step 2: Take the Batterys Apart
I strongly recommend not to take phone batteries apart as under the metal is the cells itself with harmful acids inside.
So first, you take all of the stickers off (make sure to enjoy taking the warranty void sticker off)
After that use a sharp but strong flat head screwdriver to be able to pry away the plastic. i would recommend an old pair of pliers and wire cutters that you don't mind messing up slightly
Pry at the plastic until something breaks, this is one of the hardest parts. If you ever see sparks remove the tool immediately!
After taking the plastic off there will be a lot of other crap inside the pack, only start removing this until every battery pack has been dissembled.
Step 3: Rewire the Cells
So after you have cells and only just cells (I am using the term cells because a battery is more than one cell) decide what voltage you want your battery pack, this is the list of voltages per amount of cells:
Cell / Volage
1 - 3.7
2 - 7.4
3 - 11.1
4 - 14.8
5 - 18.5
Every cell you add the another 3.7v you add.
Be aware that lithium cells can go up to 4 volts and down to 2.9v and don't ever over-volt or under-volt sensitive electronics with specific voltages. to be able to get stable voltages use a voltage regulator like this one: http://www.conrad-electronic.co.uk/ce/en/product/1...
Now if you want more "power" in your battery pack but don't need volts and you have the same amount or more spare cells, wire them so they are in parallel, parallel is where the two or more cells are in + to + and - to -.
Step 4: Protect the Battery
If you have an abundance of electric tape then wrap them up in that, but if you don't have that duct tape will do perfectly fine, you just need to make sure the + and - do not touch as this will short circuit the battery, this could make it explode or catch fire but it usually lowers the capacity of it by quite a bit.
Check that everything is working fine and charge with the batteries rated power E.g. a 12v battery will need a 12v psu (power supply unit, this can be anything from the things that power computers to your phone charger, just remember to check both of the voltages)