Battery SoC monitoring methods

The main problem when charging batteries is to find a parameter that allows determining their state of charge (SoC) with sufficient accuracy.
When charging sealed alkaline batteries, several parameters change, such as voltage, temperature, and internal pressure. Figure 1 shows the characteristics changes during the charge of a sealed nickel-cadmium battery. These parameters provide different sensitivity and have different limitations when used. The changes of characteristics when charging nickel-metal hydride batteries are similar, but batteries of this type are more sensitive to overheating during overcharging.


Fig. 1. Characteristics changes of a sealed nickel-cadmium battery during charging

The standard mode of charge is regulated by certain time. Voltage monitoring method here is ineffective because voltage at the end of charging at low charge current (Ucon) changes a little. Monitoring voltage, selected in accordance with the manufacturer's recommendations as typical for this type of current source, can lead to undercharging of some batteries and overcharging of others (depending on their individual charging characteristics).  The passport voltage of the whole battery pack is a statistical parameter. The voltage spread among the cells of one particular batch can be significant. In addition, this value depends on temperature and battery service life.
With a fast charge, using voltage as control parameter proved to be more effective. The shape of charging curve shows it. In this case, there is no need to focus on a specific value of maximum charging voltage; you just need to set the moment when it reaches its maximum. This requires monitoring devices to determine periodically dU/dt or dU2/dt2. You can observe the maximum charging voltage when overcharging the battery up to 110-120% of rated capacity.
If you stop charging the battery at this moment, the subsequent discharge in standard mode can remove about 95% of the rated capacity. To ensure a greater overcharge (up to 140-160%), you must either keep the charge at the same current for the required time, or switch on to a safer charging mode with a lower current.
Nowadays, another criterion is used to control the fast charge: the charge is stopped after the battery voltage decreases by ΔU after reaching its maximum, thus providing a correct level of battery recharge.
Such monitoring is recommended for fast charge (within 1 hour) of cylindrical alkaline batteries, if the manufacturer allows such charging for a specific type of battery. In literature, it is called ΔU detection. The ΔU of batteries made by different manufacturers can range from 5-10 to 10-20mV. For battery’s SoC monitoring it is more often suggested to use 10mV, when charging the battery at temperature from 0 to 30°C. At the same time, at the beginning of charge (within 5-10 minutes), it is recommended not to measure the voltage of the current source in order to avoid triggering the monitoring system due to a possible voltage surge (and its subsequent slight drop) after long-term storage.
Another parameter of modern sealed alkaline batteries SoC monitoring is temperature. Temperature control is most needed for charging nickel-metal hydride batteries. The temperature sensor is not installed on each cell, but on one of them in the battery pack. Of course, the influence of battery constructive features and the implemented heat exchange conditions make SoC monitoring with absolute temperature T very difficult, because it is not easy to determine this parameter definitely enough. GP specialists, for example, studied in detail the charging process of a battery with capacity 2.5Ah with current 0.5C at ambient temperature (Tat) 15-45°C. The battery shut down when the temperature (Tbat) increased to 55 and 60°C. This showed that if the ambient temperature is above 35°C, then at Tbat = 55°C a significant battery undercharge takes place. At Tbat=60°C the undercharge is slightly reduced. It is impossible to increase the controlled parameter (Tbat > 60 °C) higher without increasing the risk of battery failure.
As a rule, all manufacturers recommend the maximum temperature for fast charging not more than 55°C. It should be understood that using temperature SoC monitoring method at elevated ambient temperatures, it is impossible to avoid low battery. It is more rational to control another parameter: the speed of temperature change (ΔT/Δt), which makes possible to diagnose the intensification of side processes during battery overcharging at any ambient temperature. The ΔT/Δt value, at which various manufacturers recommend to disconnect sealed alkaline batteries from power sources, is in the range from 1 to 2°C/min with charge current 1C and 0.8°C/min if the current is less.
The majority of manufacturers believe that the best results might be achieved by controlling the charge according to two criteria (ΔU and ΔT/Δt) simultaneously. This method of monitoring is universal both for different types of batteries and for different levels of charge. It should be noted that the second parameter provides better conditions for long-term battery life.
Another electrical parameter changes more significantly and is more visible than voltage. This is the response of the current source to the AC test signal.
To control the degree of charge of lead-acid batteries, you can use the open-circuit voltage, which varies from 2.05-2.15V/ac when charged (depending on the acid concentration) to 1.95-2.03V/ac after full discharge. Figure 2 shows this relationship.


Fig. 2. Dependence of the open circuit voltage of a lead-acid battery on the level of charge

When SoC monitoring of a lead-acid battery the charge is considered complete if the charge current (with the standard charge voltage unchanged) remains unchanged for 3 hours.
When charging lithium-ion batteries, battery SoC is monitored by voltage. In the beginning when lithium-ion batteries with graphite system were first introduced, the charge voltage limit 4.1V per cell was required. Currently, lithium-ion cells can be charged to voltage of 4.20V. The voltage tolerance is only about ±0.05V per cell. Figure 3 shows the standard charging process for a lithium-ion battery.


Fig. 3. Voltage and current dependence on time when charging a lithium-ion battery

STEP 1 - The maximum permissible charge current flows through the battery until the voltage reaches the threshold value.
STEP 2 - The maximum voltage on the battery is reached, the charge current gradually decreases until it is fully charged. The moment when the charge is complete is when the charge current decreases to 3% of the initial value.
STAGE 3 - Periodic compensating charge conducted during storage of the battery, approximately every 500 hours of storage.