Battery Hydrometer 150ml

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Battery Hydrometer with PVC bulb and spout, for topping up and testing density of battery electrolyte.

What is a hydrometer?

Hydrometers are instruments used to measure the specific gravity or relative density of a liquid.  The hydrometer consists of a sealed glass tube with a weighted bulb to make the hydrometer float upright in fluid. Inside the stem is a paper scale.  As the bulb floats in the liquid, a reading can be taken from the level at which the surface of the liquid corresponds directly to the scale on the paper inside the hydrometer and this determines the density of the liquid relative to the density of water.  If the liquid has a low density, the hydrometer will sink deeper into it and if it has a high density it will float higher.  The way the reading is expressed depends upon the type of hydrometer used; hydrometers are used extensively in many commercial operations to determine the relative density of liquids such as alcohol, sugar solutions, milk and petroleum products.

A battery hydrometer is used to gauge the state of the charge in the battery by measuring the relative density of the electrolyte in each cell.

How do I use a battery hydrometer?

Locate the filler caps on the battery. 

If you can't see any filler caps you may have a maintenance free battery which has its own built in hydrometer eye - when the battery is charged, the ball floats in the eye, or window, turning it green, but when the battery has lost its charge, the ball drops and turns the window yellow.  The problem with this is that the hydrometer eye gives an indication of the electrolyte level in just one of the battery cells, so you may never know that another of the cells has gone bad leading to battery failure.  Use a Battery and Alternator tester to check the battery voltage is at least 12 volts with the engine off (modern car electrics won't work with less than 12 volts - 10 volts means one dead cell).  Maintenance free batteries are not designed to give access to the cells for topping up, but sometimes you will find filler holes hidden under a label.

Pry open the filler caps carefully - use a rag to cover the caps and use protective eye gear to prevent splashes into the eye - battery acid is highly corrosive and can damage or destroy clothing, paintwork, electrical connections and eyesight!  Using the hydrometer, suck enough fluid out of each cell in turn to allow the bulb to float freely within the outer tube, noting down the reading on the hydrometer scale for each one before squirting the electrolyte back.  The readings on each cell should correspond within 0.05 of each other, and in any case above 1.250 on the scale, to tell you if the battery needs to be recharged or is in good health. If just one cell has gone bad, showing a lower reading than the others on the hydrometer, the plates in that cell may have become sulphated. Unfortunately, in this case, the battery might not retain a charge and may be past saving. 

If necessary, top up the fluid in each cell to the indicator level (usually a plastic lug seen above the level of the plates) with distilled (not tap) water.  Even when the fluid level in a cell has dropped significantly, you only need to top up with distilled water - the sulphuric acid element remains in the battery to automatically replenish the electrolyte within the water.

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