7.2V Li-ion Battery Assembly — Process Detail


Hello friends!

Today I want to talk about assembling a lithium-ion battery with a nominal voltage of 7.2 V. Such batteries are used, for example, to power bicycle lights. But before proceeding directly to the assembly, a little theory.

The battery is assembled according to the 2S2P scheme, that is, it will consist of two series-connected blocks, each of which has two parallel-connected cells.

Since lithium-ion cells operate in the range of 3V to 4.2V, the average value of 3.6V is the nominal voltage. When we connect two cells in parallel, their nominal voltage remains the same (3.6 V), and the capacity doubles.

If we connect two cells in series, the capacity does not change, but the nominal voltage doubles, and it turns out 7.2 V. In this case, the operating voltage range also doubles, and in our case it will be from 6 V to 8.4 V.

Overcharging a lithium-ion cell above 4.2 V or over-discharging below 3 V will lead to its degradation and failure, therefore, a BMS (battery management system) is used on lithium-ion batteries, which monitors voltages and interrupts the charging process or discharge at the right time.

In addition, BMS has a balancing function (although not always). It is needed in order to equalize the voltages on the battery cells. If the voltage on one of the cells is higher than the rest, a balancing resistor is connected to it, which discharges it. On the picture
the balancing resistors are labeled «101» for 100 ohms.

If the BMS does not have a balancing function, and the cells are not of the highest quality, when charging, the battery will be disconnected according to the most charged cell, and when discharging — according to the most discharged, and as a result, the operating voltage range of the battery will decrease, which means its capacity will also decrease.

This is found in cheap e-bike lithium-ion batteries that are assembled in China. In addition, these batteries are often not designed for the load with which they are operated, and degrade from overheating of the cells rather quickly — in 1-2 seasons. But this is a completely different story, worthy of a separate article.

To assemble the battery, I will use new cells with a capacity of 3000 mAh and an internal resistance of less than 30 mΩ, which is used by

If you use a used cell, you need to make sure that they have the same capacity, internal resistance, and the same voltage at the time of assembly.

To measure the capacity and equalize the voltage on the cells, a charger for 18650 batteries, for example, LiitoKala Lii202, is suitable.

But to measure the internal resistance, you need a more serious device, for example, the iCharger X8. Although, if the battery is planned to be used with a relatively low load, for example, with a flashlight, then the internal resistance of the cells no longer plays such an important role.

So, let’s start assembling. The main components we need are 4 18650 cells and a 2S BMS board.

First you need to glue the cells. For this we will use cyacrine (aka cyanoacrylate, or superglue), purchased in Leroy. A few drops are enough at the points of contact of the cells.

The cells are glued together so that there are two pluses and 2 minuses on one side, as in the photo.

Since I have access to a spot welder, I will use it. Some craftsmen at home use a powerful soldering iron (from 100 W), but I would not recommend soldering because of the danger of overheating the cell, and, as a result, ruining it.

We prepare contacts: two on the left will go to the plus and minus, the square one — to the connection between the cells, and the narrow one — for the balancing output.

We weld on the contact connecting all four cells. In order for spot welding to provide a reliable connection, the settings of the welding machine must be correctly selected, and the contacts to be welded must be clean and free of grease.

Then we turn the cells over and weld the positive and negative contacts. In this case, you need to be careful so as not to accidentally close plus and minus.

In order for the BMS to be able to balance the cells (equalize the voltages), we weld on the balancing contact.

Then we glue the syntoflex, a special insulating material, to place the BMS board on it.

We solder the balance wire, as it will pass under the BMS, and later it will be inconvenient to solder it. For greater reliability, we put on a glass-reinforced insulating tube on the wire.

We estimate the location of the BMS board and make sure that the B + and B- contacts fall on the positive and negative contacts of the cells, respectively.

We solder the contacts B +, B- and the balancing contact, marked on the board with the marking «BM».

Now we are preparing the output connector — I used the XT60, which worked well. In order to prevent the contact from melting the plastic during soldering, we put on a mating connector.

Batteries use female connectors to reduce the likelihood of accidental short circuits, while consumers use male connectors.

After soldering, we put on heat-shrinkable tubes (red for a plus, black for a minus) and glass-reinforced tubes on the wires, for greater reliability.


We glue the battery on all sides with syntoflex …

… and shrink a heat-shrinkable tube of a suitable diameter on top.

This completes the assembly of the battery, it remains to check it in operation. To charge such a battery, you need a 8.4 V charger (twice 4.2 V).

In this case, a 10A BMS was used and the battery will be used for the children’s toy car. In the case of a bicycle light, a 3A BMS will suffice, which, moreover, is much more compact and cheaper.

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