How to fix a monowheel lithium-ion battery?

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Hello friends!

Increasingly, on the streets you can meet people moving on mono-wheels — self-balancing unicycle means of individual mobility.

And today I came across a battery from just one of these devices. Its owner reported that it began to discharge very quickly.

On the sticker we see all the main characteristics: this is a lithium-ion battery with a capacity of 7 Ah for a voltage of 60 V (57.6 V), an absolute capacity of 403.2 Wh.

We remove the heat shrinkage.

The BMS (battery management system) board is the size of the battery itself. This is done for the convenience of connecting the balancing contacts — they are soldered directly to the BMS board, and not connected via a connector, as is implemented on most electric bike batteries.

There is an insulating material on the top of the board. Peel off the tape and remove it.

Before us is the BMS board in all its glory: large balancing resistors, charging and discharge transistors are visible. The balancing contacts are labeled V1 … V15 (the 16th is the «P +» load output).

By the way, in the lower right corner you can see a row of holes for soldering an additional balancing connector — apparently, for the case when not one but two batteries are used in the electric unicycle.

By marking, we determine which elements were used to assemble the battery — this is the LG MJ1 with a capacity of 3500 mAh. Since each of the 16 cells consists of 2 cells connected in parallel, a capacity of 7 Ah is obtained, which is indicated on the sticker.

Using a multimeter, we measure the voltage across each of the 16 cells. At all, except for the 12th, the voltage was 3.3 V, and at the 12th = 0 V, that is, it is faulty.

To replace a faulty cell (two elements), you first need to remove the BMS board. We sequentially unsolder all the balancing contacts and bend them up.

Unsolder contacts «B-» and «B +» with particular care, since they are only a couple of millimeters apart and can be accidentally short-circuited. And since I didn’t plan fireworks for today, I used a piece of PCB to avoid short circuits.

When all contacts are soldered, remove the BMS board from the battery pack

This is what the battery pack looks like. On the side of the wires for additional insulation, an electrical cardboard is glued. Insulation materials are also glued on the side ends and between the blocks.

We split the block into two halves. To make it more convenient to work and not to confuse anything, I decided to sign all the contacts — I will be guided by them when assembling the battery.

Once again, we make sure that this particular cell (12th) is faulty, and mark it with a marker.

We remove the end insulators and get access to the contacts connecting the elements to each other.

Using a flat screwdriver, carefully tear off the contact plates and clean up the remnants of the spot welding.

The elements are interconnected using cyacrine (cyanoacrylate, superglue) and are firmly held together, therefore, it will not work to separate them without damaging the shell.

And if this is the case, you can sacrifice the shell of faulty cells. Cutting it with a knife, we separate the elements and replace them with new ones, which we also fix with cyacrine. For this we use elements of the same capacity — Sanyo GA 3500 mAh.

It is necessary to make sure that these two elements have the same voltage, otherwise during welding, when they are connected in parallel, one element will start charging the other, and if the difference is significant, the process may be accompanied by heating.

We cut out the suitable contact plates and go to the spot welder. In addition to the contact plates themselves, we need narrow balancing contacts, which will later be soldered to the BMS board.

The elements are welded, the balance contacts are welded over the contact plates.

Before continuing, it is necessary to compare the voltage of the new cell (12th) with the voltage of the remaining cells, and if it differs by more than 0.1 V, recharge or discharge the 12th cell to the voltage of the remaining cells.

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We place the BMS board in its original position and solder the balance contacts to it.

Now you need to perform a final capacity test — charge the battery with a charger, and then discharge it through a wattmeter under load.

It is important to take into account that the BMS boards of monowheels do not turn off the discharge when the lower threshold voltage is reached — this is done so that during operation the monowheel does not turn off at the wrong moment and the driver does not get injured.

In this regard, when discharging, you need to monitor the voltage yourself, for example, by turning on the sound signaling of reaching the lower threshold on the wattmeter.

We glue the top insulator. It remains to sit on top of the heat shrink tube and seal the ends.

That’s all for now, I hope you were interested.

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THANKS

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