How to Check a Battery with a Hydrometer

Step by Step Hydrometer Procedure

Hydrometer Parts

If you’ve ever used an eyedropper, you are capable of using a hydrometer to test your battery state of charge.

The picture to your right shows the basic parts to a hydrometer.  You will want to familiarize yourself with these parts so that you understand this procedure.

Types of Tests

There are basically two types of tests.  Choose the type you’re going to run.

  1. The first type is one where you charge the battery to see if all cells accept a full charge.
  2. The second is where you allow the battery to become discharged and make your measurement at that point.  This allows you to see if one or two cells are discharging faster than the others.  This is a fairly good test to see if a cell has lost capacity.

Preliminary Steps

You’re about to determine whether an expensive item is good or bad.  Make a mistake, and you will be forking out money for something you didn’t need.  You’re also about to do something that can be risky to your health. Do pay attention to step one.  If you’re going to be a cowboy, at least do so with your eyes wide open.

  • Charge or discharge your battery
  • Disconnect your battery to ensure that it is not under load (being drained by a device connected to it).  Disconnecting the negative lead is often the smartest and best way of doing this.
  • Allow your battery sufficient time to settle while disconnected.  Twenty four hours is generally the most conservative and best approach.  Otherwise, an overnight rest will usually do.
  • Make your measurements between temperatures between 60 and 80 degrees F.  Temperature affects the specific gravity of the measurement.  Getting much beyond this range could give you some pretty crazy results.
  • Do you test in an area that has good ventilation and running water.  The battery fumes are dangerous.  The running water will be used if you accidentally get the acid onto skin or into your eyes.  If you do so, flush with the water generously

Materials Required


Step 1 – Put on Goggles and Gloves… Have Some Baking Soda Around

Acid burns. Acid blinds.  Asking you to be safe is not about making your life difficult, but about making sure you can continue to live a healthy life.

Use the baking soda to neutralize any spills immediately.

Note – Safety glasses and face shields are not goggles. Acid splashes and neither safety glasses or face shields provide adequate protection.  That said using a face shield and goggles do increase your protection.

Step 2 – Open Battery Cell Caps

A 12 volt battery has six cells.  A six volt battery has three cells.

Sometimes the cells each have their own cap and sometimes they share a large rectangular cap with other cells.

Sometimes the caps twist off, and sometimes they need to be gently pried of.  Take your time. Don’t force anything.

Step 3 – Position Hydrometer Over Cell and Squeeze Bulb

There should be no liquid in your hydrometer after performing this step.

Step 4 – Insert Hydrometer Nozzle into the Cell to Be Tested

You will want to get the nozzle well into the liquid. This liquid is known as electrolyte.  You’re measuring the specific gravity of this electrolyte.

Step 5 – Release Bulb to Suck Electrolyte into the Chamber

You will want enough electrolyte in the chamber to cause the float to be freely suspended in the liquid

Step 6 – Allow Liquid and Float to Settle – Take A Reading

You’ve been moving stuff around. The float and electrolyte are likely bouncing. Give it a little time to settle.  Take your reading.  If you have a common automotive hydrometer, the float will be marked with green, red and white zones.  These are generally good reliable indicators of the current specific gravity of a cell.

Step 7 – Insert Nozzle into Cell and Squeeze Bulb

You’re replacing the electrolyte you took out of the cell.  This is a pretty important step.

Step 8  – Repeat for Remaining Cells.

You will be repeating steps 2 through 7.  You should probably make a note of the results of each cell.

Step 9 – Think About and Interpret Your Results.

The biggest mistake with this measurement is a misinterpretation that causes a good cell to be declared bad.  Was your battery really charged before the test?  Did you allow it time to settle in an open circuit condition after charging?

Now, the one thing you can be sure of is that you have bad a cell if there is a dramatic deviation between your readings.

Step 10 – Clean Up

You may have little drops of acid in or nearby your work area. This is a good time to clean it up.  You really don’t want loved ones to accidentally put their hands in it.