In this article, sub-C size cylindrical NiMH cells were constructed for the performance investigation under long-term overcharge and elevated temperatures. The influence of operated conditions on cell performance and electrode materials was examined. The experimental results revealed that the NiMH cells had good tolerance for 1-month overcharge at a 0.2C rate, remaining the capacity and capacity-rate performance and showing promise as a candidate for backup power sources. However, the sub-C size NiMH cells did not exhibit good performance at elevated temperatures, and the cycling stability with 100% DOD (depth of discharge) at temperature of >50°C is necessary to be further improved. The deterioration of electrochemical capacity and cycling stability at elevated temperatures was analyzed and the reasons were discussed.