Hydrometer (Specific Gravity) – How to Test a Battery

Dec 13
Battery Hydrometer

Battery Hydrometer

Here at BestBatteryTips.com we get asked this question numerous times in a week. “What is a Hydrometer, and how do I test my battery with one?”

A hydrometer is a float-type device used to measure the concentration of sulfuric acid or ( The Specific Gravity) of a battery electrolyte (”battery acid”). After reading this post you can easily and accurately determine a non-sealed battery’s State-of-Charge. A hydrometer is a glass or plastic container with a rubber nozzle or hose on one end and a soft rubber bulb on the other. Inside the barrel or container, there is a float and calibrated graduations used for the Specific Gravity measurement. We are going to explain step by step procedure on how to use a battery hydrometer, to find out and know the condition of your battery.

Rubber Gloves

Rubber Gloves

Safety Goggles

Safety Goggles

Remember, Safety first!! Always wear rubber gloves, and goggles before performing any maintenance or while handling batteries.  Its a good idea to wear goggles over safety glasses, because of the seal goggles gives you around your eyes in case of an explosion.

If the fluid in the battery (electrolyte) is above 115 degrees, let the battery cool down before proceeding forward.

If the battery has been charged in the past 5 hours, you will have to remove the surface charge to get a accurate reading. There are a couple of ways you can accomplish this. Put the battery in a car and turn on the high beam lights for 5 minutes, and then wait 10 minutes. You can let the battery sit at room temperature for 6 to 12 hours to allow the surface charge to dissipate.

Battery Hydrometer Example

Hold the Hydrometer vertically, squeeze the rubber bulb on the end of the Hydrometer and insert the other end of the Hydrometer with the rubber hose on it in the battery cell closest to the positive post or flag post. Then release the rubber bulb and the electrolyte will then begin to be sucked up into the Hydrometer. It may be necessary to tap on the Hydrometer to remove any air bubbles that are in the Hydrometer, these bubbles can interfere with a accurate reading if not removed. Once you get a reading then you press the rubber bulb to force the electrolyte back into the cell of the battery. It is recommended to take several reading of each cell for accuracy. When you get two different readings of the same cell, average the two readings for a accurate specific gravity. At eye level and with the float steady, read the Specific Gravity at the point the surface of the electrolyte crosses the float markings. The Specific Gravity reading should be between 1.100 and 1.300.

Repeat the process for each individual cell. The Specific Gravity reading should not have a difference of more than 30 “points” (.030) between the lowest and highest reading or 10 “points” (.010) below the battery manufacturer’s recommended temperature value with the battery fully charged. If so, try and equalize the battery by following the battery manufacturer’s procedures. If equalizing does not help, replace the battery.

When finished taking all of your Hydrometer readings, be sure to rinse the Hydrometer out thoroughly with water.

Specific Gravity vs. Temperature
at Various States-Of-Charge (SoC)
for a Wet Low Maintenance (Sb/Ca)
or Standard (Sb/Sb) Car Battery Table

Electrolyte Temperature (Fahrenheit) Electrolyte Temperature (Celsius) 100% SoC 75% SoC 50% SoC 25% SoC 0% SoC
120° 48.9° 1.249 1.209 1.174 1.139 1.104
110° 43.3° 1.253 1.213 1.178 1.143 1.108
100° 37.8° 1.257 1.217 1.182 1.147 1.112
90° 32.2° 1.261 1.221 1.186 1.151 1.116
80° 26.7° 1.265 1.225 1.190 1.155 1.120
70° 21.1° 1.269 1.229 1.194 1.159 1.124
60° 15.6° 1.273 1.233 1.198 1.163 1.128
50° 10.0° 1.277 1.237 1.202 1.167 1.132
40° 4.4° 1.281 1.241 1.206 1.171 1.136
30° -1.1° 1.285 1.245 1.210 1.175 1.140
20° -6.7° 1.289 1.249 1.214 1.179 1.144
10° -12.2° 1.293 1.253 1.218 1.183 1.148
-17.8° 1.297 1.257 1.222 1.187 1.152

For example, if the electrolyte is at 20° F (-6.7° C), the Specific Gravity reading would be 1.289 for a 100% State-of-Charge because the liquid is more dense at the colder temperature. At 100° F (37.8° C), the Specific Gravity reading would be 1.182 for 50% SoC and a reading of 1.104 or lower at 120° F (48.9° C) would indicate a discharged battery.