New crystallographic data prove that CaPb 3 and Sn 3CaL1 2 compounds belong to the same single-phase Ca(Pb,Sn) 3 domain in the ternary PbSnCa equilibrium phase diagram. A tentative calculation of this diagram shows that about 2 wt.% Sn, introduced in the α-Pb matrix, allows a decrease in the solubility of Ca at 400 K from 10 t-2 to 10 −7 wt.%. Simultaneously, the L1 2 compound, in equilibrium with the α-Pb matrix, crosses rapidly the x Ca=0.25 line, continuously from the PbCa border to near the SnCa border, when increasing the Sn content in the α matrix from zero to 8 wt% Sn at 400 K. A very large triangle formed by the three phases α-Pb (8 wt% Sn), β-Sn (1.7 wt.% Pb), Ca 25Pb 5Sn 70L1 2 appears in this diagram at 400 K. This triangle provides a ternary eutectic, calculated at 424 K. This calculation brings to light some reasons for the process transition for the hardening of PbCa grid-battery alloys, from discontinuous to continuous, when increasing the tin content.