A diagnostic device comprising: an acquisition unit configured to acquire data of only some battery cells of battery cells constituting a storage battery system; and a diagnostic unit configured to diagnose the storage battery system based on the acquired data, wherein the data includes a voltage value and an integrated current amount value, the some battery cells include: a low-state MIN battery cell acquired in data as a battery cell having a minimum voltage in a low state of charge, SOC, state; a low-state MAX battery cell acquired in data as a battery cell having a maximum voltage in the low SOC state; a high-state MIN battery cell acquired in data as a battery cell having a minimum voltage in a high SOC state; and a high-state MAX battery cell acquired in data as a battery cell having a maximum voltage in the high SOC state, the diagnostic unit includes at least one of: a balance determination unit configured to determine whether the battery cells are in a balanced state or an imbalance state; an imbalance amount calculator configured to calculate an imbalance amount between capacities of the battery cells; or a capacity calculator configured to calculate a value related to a capacity of a battery cell. an acquisition unit configured to acquire data of only some battery cells of battery cells constituting a storage battery system; and a diagnostic unit configured to diagnose the storage battery system based on the acquired data, wherein the data includes a voltage value and an integrated current amount value, the some battery cells include: a low-state MIN battery cell acquired in data as a battery cell having a minimum voltage in a low state of charge, SOC, state; a low-state MAX battery cell acquired in data as a battery cell having a maximum voltage in the low SOC state; a high-state MIN battery cell acquired in data as a battery cell having a minimum voltage in a high SOC state; and a high-state MAX battery cell acquired in data as a battery cell having a maximum voltage in the high SOC state, a low-state MIN battery cell acquired in data as a battery cell having a minimum voltage in a low state of charge, SOC, state; a low-state MAX battery cell acquired in data as a battery cell having a maximum voltage in the low SOC state; a high-state MIN battery cell acquired in data as a battery cell having a minimum voltage in a high SOC state; and a high-state MAX battery cell acquired in data as a battery cell having a maximum voltage in the high SOC state, the diagnostic unit includes at least one of: a balance determination unit configured to determine whether the battery cells are in a balanced state or an imbalance state; an imbalance amount calculator configured to calculate an imbalance amount between capacities of the battery cells; or a capacity calculator configured to calculate a value related to a capacity of a battery cell. a balance determination unit configured to determine whether the battery cells are in a balanced state or an imbalance state; an imbalance amount calculator configured to calculate an imbalance amount between capacities of the battery cells; or a capacity calculator configured to calculate a value related to a capacity of a battery cell.