Rechargeable battery has the characteristics of high performance price ratio, large discharge current and long service life. It is widely used in various communication equipment, instruments and electrical measuring devices. However, different types of batteries such as nickel cadmium battery (NiCd), nickel hydrogen battery (NIMH) and lithium-ion battery have different charging characteristics and processes. Different batteries shall adopt different charging control technologies. Common control technologies include negative voltage increment control, time control, temperature control, maximum voltage control, etc. Among them, negative voltage increment control is recognized as one of the more advanced control methods. During charging, when the negative increment of battery voltage is measured, it can be determined that the battery has been fully charged, so as to convert charging into trickle charging. Time controls the predetermined charging time. When the charging time reaches, make the charger stop charging or turn to trickle charging. This method is safer.
The temperature control method is to measure the battery temperature or temperature change when the battery reaches the full state, so as to determine whether to stop charging the battery. The maximum voltage control is to judge the charging state according to the maximum allowable voltage of the rechargeable battery, which has good flexibility. This paper introduces an intelligent charger, which can charge nickel cadmium battery (NiCd), nickel hydrogen battery (NIMH) and lithium ion battery, and has the ability of automatic detection of rechargeable battery.
Charger design idea
When designing a universal intelligent charger, it is necessary to fully consider the charging characteristics of three batteries, and give different charging modes and corresponding algorithms according to the characteristics of each battery
Ni MH / Ni Cd battery charging mode
The two nickel batteries have similar charging characteristic curves, so the same charging algorithm can be used. The main charging control parameters of the two batteries are- Δ V and temperature θ．
The criteria for the conversion from precharge to standard charge of Ni MH / Ni Cd battery are as follows: ① the voltage level of single battery is 0.6 ~ 1V; ② The battery temperature is - 5 ~ 0C. The criteria for battery saturation charging are: ① battery voltage drop or near zero growth – Δ V = 6 ~ 15 MV / section; ② Maximum battery temperature θ max>50℃； ③ Battery temperature rise rate D θ/ dt ≥1.0℃/min。 Since the change of temperature is easily affected by the environment, the variables actually used to distinguish each charging stage are – Δ V、 θ Max, where – Δ V detection requires sufficient a / D resolution and high current stability. - △ V measurement has the following relationship with a / D resolution, charging current stability and battery internal resistance: when the battery internal resistance is equal to 50 Ω (close to saturation charging), charging current = 1200Ma, current drift is equal to 5%, and the maximum charging voltage of a single battery is 1.58v, Then the possible battery voltage change caused by current drift is 3 MV.
Lithium ion battery charging mode
During the charging and sampling of lithium-ion battery, the measured voltage is the on-line voltage of the battery. Generally, the on-line voltage is higher than the static voltage (related to internal resistance). In the charger design, the only measurement parameter for the conversion judgment of each stage of lithium-ion battery charging is the on-line voltage, and the voltage sampling deviation is less than 0.05 v
