This paper addresses the problem of estimating SOC-imbalance between two battery cells connected in series. Particularly, the effectiveness of using force measurements for the SOC-imbalance detection against pack/total voltage measurements is studied. SOC imbalance estimation during charging using pack voltage measurement was previously demonstrated for the LiFePO4/graphite battery chemistry. However, the Li-ion battery with LiNiMnCoO2/graphite, which is of great interest in a hybrid electric vehicle application, exhibits an almost linear relation between SOC and voltage when battery SOC is greater than 15%. This characteristic makes SOC imbalance estimation using pack voltage challenging. The use of other novel measurands, related to volumetric change of the electrode materials during battery operations, make the problem feasible. To estimate SOCs of two batteries connected in series, a Moving Horizon Estimation (MHE) technique is applied and three different measurement sets are considered: (1) total voltage, (2) force, and (3) both voltage and force. Simulations results show that, for the batteries of interest, the inclusion of force measurements significantly improves the estimation of SOC-imbalance.