Your battery charger may explode!

There are a lot of electric toys for children at home. I sigh that although the current electric toys are only tens of yuan, they are not only postal, but also come with chargers and rechargeable batteries. They are really cheap and everyone can afford them.

In the picture below, I forget which toy comes with this charger. It looks decent, but it's a little light. At such a cheap price, I want to see what circuit scheme it is

Back appearance: the font on the back is not easy to see: (sister's hand)

After image processing, you can see it clearly: it says, this is a nickel cadmium and nickel hydrogen battery charger, No. 5 and No. 7 batteries can be charged.

USB power supply, nominal input is 5V 1a.

Charging time 3-5 hours.

Remove 4 screws and disassemble the back cover

At this time, you can button out the circuit board

Focus on the circuit board, there are few components: according to the real object, restore the circuit diagram, the circuit is very simple:

When the battery is not put in the battery bin, the red LED light is not on when the battery is connected to the battery bin: (the same is sister paper's hand) because there is no battery, the circuit where the LED light is located can not form a loop: when the battery is put in the battery bin, the LED light is on: at this time, it is charging.

When charging, what is the voltage and current of each point in the circuit?

Direct calculation with circuit simulation, simple and intuitive: (take the circuit charging the battery with 1.3V voltage as an example)

As can be seen from the figure above:

1. The forward voltage drop of red LED lamp is 3.15v - 1.3V = 1.85v, flowing 4.67ma current.

2. The charging current of the battery is 175ma.

See here, you found no, this charger does not detect the battery charging state of the circuit, so the battery will always be charged, never stop!

Red LED light will always be on and never go out!

Users who don't know why must be surprised that they haven't been charged all day, because the red light is still on. So I continued to charge.

So the problem, such a simple and rough charging circuit, will not be a problem?

Next, take the charging of Ni MH battery as an example to continue the analysis.

The charging curve of Ni MH battery is as follows: the ordinate is the charging voltage.

Abscissa is how much electricity has been charged, measured as a percentage of the nominal capacity of the battery.

What do 0.1C and 0.2C mean?

C represents the battery capacity. If the battery capacity is 1200mAh, then 0.1C represents the charging current as follows:

0.1 x 1200mA = 120mA

0.2C is:

0.2 x 1200mA = 240mA

It can be seen from the charging curve that the charging curve is different with different charging current. Here are four charging curves.

The expensive "smart charging" is to judge whether it is fully charged according to the characteristics of the charging curve, so as to stop charging.

There are several ways to judge:

1. When it is detected that the charging voltage does not rise for a period of time, it is considered to be fully charged.

2. When it is detected that the charging voltage is decreasing instead, that is, when the "negative slope" of the voltage appears, it is considered that it has been fully charged.

3. Some people judge that when the battery voltage reaches the specified value, they stop charging, but the accuracy of this method is poor.

The charger disassembled this time is simple and crude. Apart from the charging function, it has nothing. It will be charged all the time. At the end of charging, the voltage of the battery will not go up, and the continuous charging current will be used to generate heat.

Assuming that the maximum charging voltage of the battery is 1.53v: for a single battery, the power used for heating is:

1.53V x 163mA ≈ 0.25W

The three batteries are:

0.25 x 3 = 0.75W
iPhone Adapter
It's not small!

It should be said that Ni MH batteries and Ni Cd batteries are very easy to manufacture. Generally speaking, the problem is not big. This is also the reason why businesses dare to produce and sell such chargers.

iPhone 12 charger port
If the battery continues to charge after it is fully charged, it will cause the battery to continue to heat, or even lead to the decline of battery performance, capacity and life.

More serious, may appear the problem of battery explosion (also related to the quality of the battery)!

So when you use cheap chargers, you must pay attention to:

Generally, there is no problem with charging;

When charging, do not have flammable and explosive materials near the charger, pay attention to heat dissipation;
iPhone Adapter
When charging, someone must be at home;

Charging time should not be too long. It can be charged all day, but overnight charging is not recommended.

Just be careful when you use it. I'm really worried. You can buy a better charger, and it's not expensive.