There are lots of reasons for building your own LiPo pack:
- special form factor needed
- cheap single cells
- recycling old cells
Its quite easy, if you know what you are doing and work carefully. Basically its just connecting all the cells in serial, with a separate balancer port on every bridge. Nevertheless, I go into more detail here and document the building of a pack out of old cells I took out of a MacBook battery.
Lithium Polymer (LiPo) cells can be quite dangerous. The really dislike being shortened, mechanical pierced, bend or overheated and can react by catching fire and releasing poisonous nano particles which carry a uneraseable smell. Please be careful around them at all times.
I'm not responsible if you are destroying or damaging yourself, your home or anything or anyone else. The risk is yours.
Step 1: Materials and Tools
You will need:
- LiPo cells (LiFePo work as well): 2 - infinite
- I use two 606168 I recovered out of an old MacBook battery pack.
- A one cell LiPo pack is possible, but you do don't need this tutorial to create this.
- I use a T-connector
- If you want to use your pack for high power usage (like flying a copter), please use connectors that are rated for the maximum power draw. I really like XT-60 connectors.
- Find our that fits your charger
- Again, you need to use wire of a thickness according to the maximum power draw. I use 2.5 mm^2 which is properly to much. The thickest I've ever used was 4 mm^2 for a 6S - 5000mAh pack.
- Silicon wire is great! You can't overheat it, its very flexible, its save, use it!
- DON'T use stuff like speaker cable. You will start a fire! I warned you.
- To isolate stuff
- You need some that fits over your main power cable
- And some that fits over the entire pack. to keep your cells together and add a bit of protection
- Soldering iron with a broad tip and a bit of power. Don't use a small SMD one, that will not work.
- Multimeter does help the check connections and get the polarity of unmarked packs.
Step 2: Check Out Your Cells
If you use store bought ones you can properly skip this.
I want to use very old cells without polarity marks, so I need to check out if they are still good and find out where is plus and minus.
Use your multimeter for that.
You can see on the picture that the cell only has 1.8V left. Thats not good, that means the cell is way below its minimum charge. There are techniques to refresh cells that are that far gone (like single cell charge them for a bit with a external power source) but this can be dangerous and you will not get the original capacity back.
Well, my cell not working has nothing to do with you, so I just continue. =)
Step 3: Think About What to Do and Cut the Cables to Size
You have to think about how to setup your pack.
Look at the diagram, the electrical connections need to be:
- Serial connection between cells (minus to plus)
- Main power connection from the endpoints of the serial connected batteries
- Balancer port connected to every bridge between the cells and the endpoints.
After you groked that, you have to decide how you do the bridges and where the cables exit.
If you have loose cells you can stack them back-to-back and just solder two of the solder taps together to create the bridge. You have to careful not to short the other two though.
Again, I have a bit of a problem with mine, as both cells are glued over each other and can't be separated. So I use another piece of wire to build a long bridge across.
The cables can come out of the same side other different, your call.
'After thinking about it trice, cut the cables to size.'
Step 4: Prepare the Cables
Remove the isolation from the tips an put solder on all around them.
Tip for the big wires: Let your soldering iron sit on them for a while, when they really get hot the solder will draw itself inside the wire.
Pre solder the connector as well.
Step 5: Build the Main Power Connector
Solder the power cables to the connector. Check for correct polarity!
Having something to hold the connector is a must on thicker wires.
After letting it cool of after soldering pull on the wires to see if they are properly stuck. Better now then after you closed everything off.
But the shrink wrap on the wires and shrink with hot air gun or lighter.
Step 6: Add the Bridges
Fix the pack in an upright position and prepare the solder taps on the packs with some solder.
Carefully attach the bridges one after the other. When you are done with one bridge, isolate carefully with electrical tape to prevent accidents.
With bigger pack you should add in the balancer ports now, as you don't want to remove the electrical tape again.
Step 7: Add Main Power Connector
Add the main power connector last.
Step 8: Add the Balancer Port
Connect every bridge and the endpoints to the balancer connector.
Be careful not to loosen your bridges again.
Step 9: Close Up and Label
Close the whole top of the pack with strips of electrical tape.
Label your pack with voltage and capacity.
Step 10: Shrink Wrap All the Things!
Put the shrink wrap around the pack and shrink down. Don't use a lighter for that, that only works on smaller pieces.
A normal hairdryer gets hot enough though.
Gratulations, you just build your own LiPo pack. =D