Introduction: Simple Powerbank From Old Lipo Battery

About: Engineer, retired. Have always had an interest in electronics, often related to radio control. This evolved into a part time business that I still practice. I do some voluntary work in schools to encourage …

In my quest for a better powerbank from old batteries I have made one using an old Lipo battery. I use a smartphone for navigation when cycling and need a powerbank to extend the battery life for a full days touring. This one more than meets my needs.

The key parts are the old Lipo batteries and a powerbank electronics module bought for $2-3 on ebay (search for "Lithium Battery Charger Module" and check it has both charging and output capability). The battery was one I bought more than 10 years ago but has had little use. It didn’t quite push out the power needed for a model helicopter (quite demanding). However it had held its voltage with very infrequent charging and the cells showed no signs of puffing. So looked like a good chance to work in this lower current application. The case for the powerbank is cardboard, with a heatshrink cover to make it look nice. This worked well to make a low cost, light and snug fitting enclosure. The heatshrink was the much thinner PVC type, as used for covering batteries.

Specification of the powerbank built here:

  • Cell capacity: 4x 1000mahr (i.e. 4000mahr)
  • Charge current: 0.6A at 5v
  • Discharge current: 1A at 5v
  • Size: 100 x 60 x 10 mm
  • Weight: 110g

The picture above shows the main parts required:

  • Old Lipo Battery and Electronics module
  • Cardboard and heatshrink (75mm flat width, PVC)

The key tools required are:

  • soldering iron
  • modelling knife

If you want a powerbank with the capacity to fully charge your mobile then the powerbank needs cells of roughly double the mobile battery capacity. The reason for the higher capacity is that the powerbank cell voltage (around 3.7v) has to boosted to 5v to supply the charger. At say 80% efficiency we will end up with 0.8 * 3.7 / 5 or about 60% of the mahr. There is also a small loss in the charging process.

A word of warning before going further . Lithium batteries hold a good deal of energy and if mishandled or overcharged or short circuited they will overheat and can explode and catch fire. De-soldering and soldering a live battery must be done with great care to avoid a short. There is also always a point in assembly where one is soldering a wire to a live battery – so first check polarity, double check and check again.

Step 1: Battery Disassembly

The cells need to be wired in parallel. The lipo battery had 6 cells in a 2p3s configuration (three parallel pairs in series). Hence the first stage is to disassemble the battery.

Carefully remove the heatshrink wrapping from the battery. If possible avoid using metal tools. Then use a soldering iron to remove the wires. A 50W iron with a 6mm bit is able to deliver the heat required. At all times remember the battery is live and avoid shorts. The battery tabs, especially the positive, are fragile and hence often soldered to a PCB. We need the cells separated so they can be wired in parallel and hence this PCB has to be removed.

I found that the battery tags could be lifted by the iron tip when the solder had been melted. The positive tags are quite weak, made of some sort of alloy, with solderable tags spot welded to them. Again take great care to avoid any short circuit. The first two photos show the tags freed from the PCB. You may notice that one of the tags broke when unsoldering. However later checking showed this to have been corroded and the cell dead. Not a problem here as I just wanted to use two of the three pairs of cells

The cells are usually stuck together with double sided tape. I have separated these by carefully pulling them apart, taking my time. First separate the pair of cells that is the other way around to the rest, and remove the PCB when these cells have been separated. Next split the remaining 4 cells into two pairs.

Now, with two of the pairs of cells (or all three if you want a bigger powerbank), connect the pairs of positive terminals and the negative terminals. I aimed to get one on one side of the middle line and the other on the other side so I could separate the connecting wires. See photo above. Do the positive tags first as these are the more fragile and will need bending so that the solderable parts can be placed and soldered together.

Step 2: Wiring

The cells need to be wired in parallel so the pack works like a single high capacity cell. First solder a wire to connect the negative terminals of each pair of cells. Before connecting the other ends it is important to check they are the same voltage. Otherwise a high current will flow when the higher voltage cell quickly charges the low voltage cell. I targeted them to be within 0.05v. If they are not - then connect a resistor of around 1ohm between the unconnected terminals to bring them into balance. When balanced connect a wire between the positive terminals.

I then added short leads to the electronics module. I used wire with 16x0.2mm conductors that has a 3A rating. Smaller 7x0.2mm wire could have been used. The higher current wire is just a bit more robust.

Next I connected the negative wire from the module to the negative battery terminal. After double checking I had the battery polarity correct I connected the positive lead. Doing this sequence means that the final connection is away from the electronics module and hence less chance of the wire touching something it shouldn't before being fixed.

We now have a functional module. I did a test charge at this stage to check the cutoff voltage. This was 4.2v which is fine.

Step 3: Preparing for the Enclosure

Finding a standard enclosure the right size is always difficult. Hence the approach to use cardboard. I used an old tissue box. A cereal box would also have been similar thickness. I haven’t done ‘cutting-out’ for decades! The design is simple, with a full overlap on the underside and open at the connections end. I added 0.5mm to the 10mm wide 'sides' to allow for the thickness of the cardboard.

The cell end of the unit will be covered snugly by the cardboard enclosure. I felt that the open module end needed some support. So I made a surround from some scrap balsa. This could also have been made from layers of thick card. See the two photos above. I also cut off the protruding pins on the PCB underside to make this flatter for a better fit.

Step 4: Enclosure

This worked out better than I expected. I held the case together with sellotape and used some double sided tape to hold the electronics module in place. I then covered the lot with some heatshrink (75mm flat width). Some parcel tape would have worked just as well. I allowed only 4mm each end for lengthwise shrinkage. This was barely enough, even with shrinking the sides first. 6 or 8mm overage each end would have been better.

Step 5: Testing

First the powerbank was charged. A red LED shows while charging. This changes to green when fully charged. Note that the 5v output circuit is not activated until the unit has seen some charge.

I discharged my phone battery to 20% and then charged it from the simple powerbank. A blue LED shows current being drawn. It stops automatically (and turns the LED off) when the powerbank cells are low. It took the battery up to 82% in just over an hour, delivering around 2/3 capacity. My phone has a reasonably high capacity 2600mahr battery.

This is great for my needs. I now have a slim and light powerbank I can take on long rides to give my phone a lunchtime boost.

I hope this shows how easy it is to recycle old lipo batteries into useful items.