PURPOSE: To ease retaining of electrolytic liquid in plate groups and improve battery efficiency by injecting dilute sulfuric acid again in such a condition that internal pressure of a battery is greater than the atmospheric pressure after injecting the dilute sulfuric acid into the battery.

CONSTITUTION: Using paste type positive plate and negative plate that have already been charged, the positive plate and the negative plate are piled up alternately through fine fiber of minute glass and they are accommodated in an electric tank. Before fitting a seal valve to this battery, dilute sulfuric acid is injected under the normal pressure in such a condition that there is no free electrolytic liquid. After 5min since the injection dilute sulfuric acid is injected again by an injection jig capable of measuring the inside pressure of a battery to the extent that the free electrolytic liquid does not flow out as visually observing in such a condition that the internal pressure is greater than the atmospheric pressure. Thereafter the injection jig is removed and the internal pressure of battery is returned to the normal pressure, and a seal valve is fitted and the first charging is performed. Thereby, discharging efficiency is improved corresponding to the internal pressure at the time of second injection and further, the second injection volume of the electrolytic liquid is approximately correlative to the internal pressure at the time of second injection. Thus, retaining of the electrolytic liquid on the plate group becomes easy and the battery performance is improved.