How to measure the capacity of a lithium-ion battery at home

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Hello friends!

Determining the capacity of a lithium-ion battery is quite an important measure, since it allows you to assess the compliance of the declared characteristics of the battery with the real ones.

Such a need may arise if your electric bike or electric scooter began to pass on a single charge.

The very principle of measurement is to discharge a fully charged battery through the load, and at the same time measure the number of watt-hours, or ampere-hours.

An energy consumer of suitable power must be used as a load. They, of course, can be

In this case, for a battery with a capacity of 960 Wh (20 Ah 48 V), the discharge time of the engine at idle with this consumption will take almost

It is much more convenient to use a set of lamps connected in series as a load. Due to the fact that the nominal voltage of most e-bike batteries is a multiple of 12 (24 V, 36 V, 48 V, 60 V), they are perfect for this purpose.

Necessary

Power

Of course, other energy consumers can be used as a load, but it is important to make sure that their power

So we got to the most interesting thing — the process of determining the capacity.

The first method, which will be considered, is suitable if you do not have any devices, but only a load of known power and the battery itself. It will not give an exact value, but it will allow

After fully charging the lithium-ion battery using the charger supplied with the bike, connect it to the load and time it.

As soon as the battery is discharged (BMS inside the battery breaks the circuit and the lights go out), look

For example

This is just

If you have a multimeter, you can get a more accurate reading.

Let’s continue with our example with a 48 volt battery and four bulbs. Let’s estimate the current that will flow in the circuit, for this you need to divide the power by the voltage.

I = P / U = 260 W / 48 V = 5.4 A

Even a simple multimeter, as in the image below, can measure current up to 10 A, for this it has a separate socket, so it suits us.

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We will need

As the battery discharges, the current will decrease as the voltage across the battery will drop. For example, at the very beginning, the discharge current was

Now we count

To get the capacity, we multiply this value by the discharge time, expressed in hours (from the previous example, 1 hour 50 minutes, or 1.83 hours):

Capacity

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Absolute capacity

We got a more accurate value than in the first example, assuming that our relationship is linear. But to get an even more accurate value of the capacity, we need a wattmeter.

On the Aliexpress marketplace, there is a fairly wide selection of wattmeters, but as an example, in this article, we will consider the HiDANCE model.

This is a fairly compact wattmeter, measuring 48 x 29 x 20 mm, designed for voltages up to 150 V and current up to 20 A. If necessary, the link is not for him

Such a wattmeter measures the capacity continuously, during the entire discharge of the battery, and retains the accumulated value even when the battery is disconnected.

On the reverse side of the wattmeter there are terminals, to one pair of which the battery is connected, and to the second — the load, as shown in the figure.

We fully charge the battery, connect a wattmeter to it, and reset it to zero values by long pressing the only «Key» button. After that, we connect the load.

When the discharge process is completed (the BMS board disconnects the load and the lamps go out), the accumulated capacity value will be saved on the wattmeter

To view the measurement results, turn off the load and put the battery on charge. Voltage will appear at the output of the battery and the wattmeter will turn on, displaying the capacity. To get the value

This is the simplest and

And if you have acquired such or a similar device, you can

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