When the battery is fully charged, the charger needs to timely block the charging current, otherwise the battery will appear a large number of overcharged reactions such as gas, water loss and temperature rise, which will endanger the service life of the battery. Therefore, it is necessary to monitor the charging of the battery at any time to ensure that the battery can be fully charged and not charged. The main control methods are:

First, timing control When the constant current charging method is selected, the charging time required by the battery can be easily determined according to the battery capacity and the size of the charging current, so it is only necessary to set the charging time in advance, once the time is up, the timekeeper can issue a signal to stop charging or reduce to trickle charging. The timer can be used by a time relay, or by a single chip computer. This method is simple, but the charging time can not be automatically adjusted according to the condition of the battery before charging, so the actual charging may appear sometimes undercharged, sometimes overcharged appearance;

Second, battery temperature control for Ni-cd batteries, normal charging, the temperature change of the battery is not significant, but when the battery is overcharged, the internal gas pressure will increase rapidly, the oxidation reaction on the negative plate makes the internal heat, and the temperature rises rapidly (can rise several degrees Celsius per minute). Therefore, investigating the change in battery temperature can determine whether the battery is overflowing. Two thermistors are usually used to detect the battery temperature and the ambient temperature separately, and when the temperature difference between the two reaches a certain value, the charging stop signal is announced. Because the dynamic response speed of the thermistor is slow, the full charge of the battery cannot be accurately detected in time.

In general, when the battery is fully charged, its terminal voltage will show a decreasing trend, according to which the time of negative growth of the battery voltage can be used as the stop time. Compared with the temperature control method, this method responds quickly, in addition, the negative increment of the voltage has nothing to do with the absolute value of the voltage, so this stop-charge control method can be adapted to the battery pack charging with a different number of cells. The defect of this method is that the general detector sensitivity and reliability are not high, at the same time, when the ambient temperature is high, the voltage reduction after the battery is sufficiently charged is not significant, so it is difficult to control;

Under normal circumstances, the polarization voltage of the battery appears after the battery is just full, usually in the order of 50mV ~ 100mV, and the relevant patented technology is used to measure the polarization voltage of each single cell battery, so that each battery can be charged to the degree it needs. Research has shown that because each battery has at least some subtle differences in geometry, chemical properties and electrical properties, it is more appropriate to determine the charging level required according to the characteristics of each cell battery than to control the battery pack as a whole.