A Mg/MnO2 primary cell yields specific energy higher than a conventional Zn
/MnO2 cell. Additionally, the shelf life of the Mg/MnO2 cell is extremely h
igh. These cells, which were more than a decade old, were investigated for
their discharge capacity, delay-time behaviour and impedance characteristic
s. The values of discharge capacity and the delay-time of an aged Mg/MnO2 c
ell were comparable to those of a fresh cell. The voltage dip on initiation
of a galvanostatic current, however, was rather large. This was attributed
to the presence of a thick, and highly resistive, surface passive film on
the Mg anode. The complex plane electrochemical impedance spectrum of a par
tially discharged cell consisted of two semicircles whose sizes decreased w
ith decrease of state-of-charge of the cell. The a.c. frequency correspondi
ng to the maximum value of the imaginary part of the high frequency semicir
cle was shown to be a useful parameter for estimation of the state-of-charg
e of the cell. The resistance parameters of a partially discharged Mg/MnO2
cell increased linearly with open circuit ageing time. This feature was att
ributed to growth of a passive surface layer on the Mg anode.