Looking on your currency data described relations:

hour_rates (h) = capacities (Ah) / currents (A)
capacities (Ah) = hour_rates (h) * currents (A)
currents (A) = capacities (Ah) / hour_rates (h)

These are not met explicitly in the data you presented. I've created the data which are exactly like the presented results:

capacity_data_corr = capacity[['hour_rates', 'capacities']]
capacity_data_corr['currents'] = capacity_data_corr['capacities']/capacity_data_corr['hour_rates']

Interpolation is almost ideal

This means, that the interpolation obtained can be good, but the data does not meet assumed relations. If these relations are only approximate, in such long horizon error like this should not be as bad as it looks.