Auxiliary battery charging apparatus

An auxiliary battery charging apparatus (1) is configured to comprise: a main battery (41) provided with a plurality of rechargeable battery cells (7-1 to 7-n, 6) connected to each other in series; an external generator (2); a transistor (13) provided between the external generator (2) and an auxiliary battery (43); an operational amplifier (12) with a positive input terminal, a negative input terminal, and an output terminal, wherein an output voltage of one rechargeable battery cell (7-n, 6) of the plurality of rechargeable battery cells (7-n, 6) is input to the positive input terminal, a voltage applied to the transistor (13) is input to the negative input terminal, and the output terminal is connected to a control input terminal of the transistor (13); a switch (14) provided between the external generator (2) and the transistor (13) ; and a control circuit (5) to turn on the switch (14) to supply a current from the external generator (2) via the transistor (13) to the auxiliary battery (43) during charging of the auxiliary battery (43).