How to know the power consumption of feedback resistor without knowing the internal resistance of battery?

sky999

How to know the power consumption of feedback resistor without knowing the internal resistance of battery? [Copy link]

 

This diagram is a 12.6V/charging circuit with a current of 1.5A. If you don't know the internal resistance of the battery connected to VBAT, how much power should be selected for the 66.7mR resistor RS, and how much power consumption should be calculated?

maychang

The charging current cannot be very large, and the power dissipation of Rs depends on the maximum charging current.

I don't know if the chip you are using has output current limitation (most of them do). If so, determine it according to this current limit value.

sky999

maychang posted on 2022-7-10 19:12 The charging current cannot be very large. The power dissipation of Rs depends on the maximum charging current. I don't know if the chip you are using has an output current limit ( ...

Is this it? According to this, how to calculate it?

sky999

maychang posted on 2022-7-10 19:12 The charging current cannot be very large. The power dissipation of Rs depends on the maximum charging current. I don't know if the chip you are using has an output current limit ( ...

Why is it determined according to this current limit value? Isn't it calculated based on the charging current of 1.5A?

maychang

sky999 posted on 2022-7-10 19:34 Why is it determined according to this current limit value? Isn't it calculated based on the charging current of 1.5A?

The charging current of 1.5A is the actual situation and the basis for calculating the power consumption of Rs under normal working conditions. 3.8A is the maximum value and the basis for calculating the power consumption of Rs under fault conditions (for example, the battery suddenly short-circuits and the chip has not had time to limit the current).

Of course, overcurrents of several PWM cycles generally do not damage Rs (resistors are the most robust for short-term overloads), so the power consumption of Rs does not need to be as large as 3.8A. However, it cannot be calculated based on 1.5A, and a certain margin should still be left.

maychang

sky999 posted on 2022-7-10 19:34 Why is it determined according to this current limit value? Isn't it calculated based on the charging current of 1.5A?

At more than 60 milliohms, the power consumption of this resistor will not be too large, and no matter how large it is, it will not exceed 1W.

sky999

maychang posted on 2022-7-10 19:49 More than 60 milliohms, the power consumption of this resistor will not be too large, and no matter how large it is, it will not exceed 1W.

How is the power consumption of this resistor calculated? How is it achieved that it will not exceed 1W?

maychang

sky999 posted on 2022-7-10 20:23 How is the power consumption of this resistor calculated? How is it not more than 1W?

How to calculate the power consumption of this resistor?

The square of the current multiplied by the resistance. This calculation method was taught in junior high school.

maychang

sky999 posted on 2022-7-10 20:23 How is the power consumption of this resistor calculated? How is it not more than 1W?

"How did you get it to be no more than 10,000?"

That’s my estimate.

However, actual calculations also show that it will not exceed 1W. The current is 3.16A, the square is 10 square amperes, multiplied by 0.05 ohms, which is only 0.5 watts.

sky999

maychang posted on 2022-7-10 20:26 "How is the power consumption of this resistor calculated?" The square of the current multiplied by the resistance. This calculation method was taught in junior high school.

OK, thank you very much. I have returned the things I learned in junior high school to the teacher.

nian1206

maychang posted on 2022-7-10 19:12 The charging current cannot be very large. The power dissipation of Rs depends on the maximum charging current. I don't know if the chip you are using has an output current limit ( ...

Same question, learn

qwqwqw2088

This ME4078B is a 3-cell lithium battery charger with a maximum of 1.5A

The 66.666 milliohms of Rs is configured for a maximum charging current of 1.5A.

Power is naturally calculated according to the current flowing

chunyang

There is no such thing as "battery internal resistance" here. The charging current is controlled by the charging circuit and has nothing to do with the battery internal resistance, so you don't have to worry about it.

The power of the sampling resistor is determined by the peak charging current with a certain margin, and the power consumption is determined by the actual charging current. Both are calculated using P=I^2R.

shuangchafeng

During the insurance period, the calculation is P = 3.8A * 3.8A * 0.0667 ohms = 0.963W, and a 1W resistor is sufficient.

芯片老兵

The square of the current multiplied by the resistance, just bring the actual parameters into the formula. The specific calculation shall prevail.