Summary of Electricity Calculation Method
Fuel gauge is a very important power management chip, which is mainly responsible for estimating the remaining capacity of the battery; its main function is to monitor the battery voltage, the charge and discharge current of the battery, and it can monitor the battery temperature. The battery charge can be estimated through the above information and some algorithms. Electrical state SOC. There are currently two main methods for calculating the state of charge of a battery on the market: open circuit voltage (OCV) and coulomb measurement.
- Open circuit voltage method
It is relatively easy to calculate the remaining power of the battery by the open circuit voltage method. It is generally obtained by looking up the table of the battery's open circuit voltage and the corresponding state of charge. The open circuit voltage generally refers to the battery terminal voltage when the battery is idle (neither charging nor discharging) for about half an hour;
however, the voltage curve of the battery will change under different loads, temperatures and aging conditions. Therefore, a fixed table cannot represent the state of charge in different situations. In other words, using this method to obtain the state of charge will produce a large estimation error.
To sum up, the open circuit voltage method is only suitable for systems that require relatively low battery state of charge; - Coulomb metering method (ampere-hour integral method)
The main principle of coulomb metering method is realized on the premise that the real-time current of the battery can be monitored. Generally, a detection resistor is connected in series with the battery charging and discharging path. The ADC detects the voltage across the resistor, and then the voltage can be converted into the current value of the battery being charged or discharged, and then the current is integrated over time (RTC) to get the charge or discharge coulomb;
this method can be accurate Calculate the real-time state of charge of battery charging or discharging. According to the previous remaining battery capacity, the current remaining power RM and the full charge capacity FCC are calculated. Thus, the remaining capacity RM and the full charge capacity FCC are used to calculate the state of charge SOC, that is, SOC=RM/FCC. In addition, it can also estimate the remaining time, such as power exhaustion (TTE) and full power (TTF).
There are two main reasons for the accuracy deviation caused by the coulomb measurement method: the
first is the accumulation of deviation in current monitoring and ADC measurement. Any precision ADC has accuracy problems, which will cause this under long-term operation. Accumulation of error, if it has not been eliminated, will cause a big deviation; in order to eliminate the accumulation error, there are 3 possible time points in normal battery operation: end of charge (EOC), end of discharge (EOD) and rest ( RELAX). The end of charging means that the battery is fully charged and the state of charge of the battery is 100%. The condition that the discharge ends means that the battery has been completely discharged, and the state of charge of the battery is 0%. The first two are generally represented by the voltage information and battery current information of a battery. For example, the condition for full battery voltage to be fully charged is generally that the battery voltage is greater than a certain value and the current charging current is less than the cut-off current; while the resting state refers to Approximately no charging and no discharging, also called light load state, generally keep this state for more than half an hour, the battery voltage at this time is also similar to the open circuit voltage of the battery, if you want to use this method to eliminate the error, you need to match A table of the open circuit voltage and power of the battery;
the second reason for the deviation of the accuracy of the coulomb measurement method is the error caused by the full charge of the battery, which is mainly the difference between the designed capacity value of the battery and the true battery capacity of the battery . And the full charge capacity will also be affected by factors such as battery temperature, aging and load. Therefore, the re-learning and compensation of the full battery capacity is very important.