In the paper the upgrade procedure for high-power thyristor rectifiers in the thermal power plant "Nikola Tesla A" is described. Originally, thyristor rectifiers were not designed for operation without batteries. Thus it may happen that, in a critical situation, battery failure would induce the thyristor rectifier switch off, leaving all the consumers without a direct voltage. In such a situation, it is very important for rectifier to stay in operation, continuing power supply of the critical consumers with a stable direct current, even if the extremely heavy electrical disturbance occurs in a local direct current grid. Improvements of the thyristor rectifiers based on the digital controller of type "DRI 07" were described in detail. Responses of the rectifier digital regulators were presented in the cases of the battery disconnection and connection, in circumstances of permanent operation in the voltage regulation mode, as well as during the alternation of the voltage and current regulators in a trickle charge mode. The effect of the adaptive PI regulator on dynamic characteristics of rectifier was analyzed during the abrupt load current variations. Applied procedure for the battery availability detection was described, as well as its influence on the characteristics of the storage battery.