Early automotive battery technology was not brilliant and accumulators and battery cells needed regular testing to ensure that they had not gone flat. The batteries used by military vehicles were no exception and the vehicles of the 1940s were limited by the technology available at that time. Tonight we have a pair of War Department Accumulator hydrometers used to test the charge in a battery cell:Both these boxes were originally green, but have suffered from wear and paint loss over the years. We can tell they are military issue by the large ‘WD’ and /|\ mark indicating War Department which is stamped into the lid:A small sheet metal latch is fitted to secure the lid shut:Inside the boxes are a range of glass vials, rubber bulbs and tubing:To explain how these were used, we turn to the 1953 Army manual of Mechanical Vehicle Training:
Testing the Cells
The state of the cells in batteries should be tested regularly, to give timely warning of any trouble developing.
The most satisfactory way to test is by means of a hydrometer. This consists of a small glass float, weighted at the bottom of the bulb, and with a hollow glass stem containing a paper scale graduated in degrees of density, or specific gravity. The scale usually reads 1.300 at the bottom to 1.100 at the top. The float is contained in a glass tube fitted with a rubber bulb at the top and a rubber nozzle at the bottom, through which the electrolyte can be drawn up into the glass tube…
When the battery is fully charged, the electrolyte is at its densest, and the float will ride high giving a reading of about 1.290.
The lower the state of charge, the deeper the float will sink, since the electrolyte gets less and less dense as the battery discharges and the voltage drops. A reading of 1.150 represents a fully discharged battery.
Using a hydrometer to test a charge sounds like a dangerous affair: