Circuits are provided whereby rechargeable batteries and cells are charged very fast by a controlled current, and substantially at a rate never exceeding the ability of the battery or cell to accept current--i.e., to convert electrical current to stored chemical energy. The resistance free terminal voltage of the battery or cell is detected during an interval when the charging current is interrupted, and compared against an independent reference voltage to control the charging current when a difference between the reference voltage and the sensed resistance free terminal voltage exists. The reference voltage may be altered at any instant in time, as a function of the ambient temperature, or the internal temperature or pressure of the battery, as a function of the charging current when it is within a predetermined range, or if a particular change in the value of the charging current occurs over a predetermined period of time. Charging characteristics may also be controlled and altered either through the passage of time or as a consequence of predetermined conditions occurring. The various provisions preclude thermal runaway for example, and assure very rapid and complete charging operations no matter what may be the condition of the battery--provided it is capable of accepting charge--and for various kinds of rechargeable batteries and cells including nickel cadmium, lead acid, and so on.