How is a hydrometer used?

[John Richard]

I recently bought a hydrometer from Pep Boys. It is the type that has a bulb on one end that sucks up the battery fluid. How is it used? There were no directions.John Richards/v Jack's Place

 

[Mike I]

Hydrometer

A hydrometer measures the specific gravity of the electrolite. If yours is the one float type, suck up enough electrolite to raise the float, and read the level of the electrolite on the side of the float. Most are color coded (red, yellow, and green with green being charged). Make sure the float is free floating and not resting along the side of the glass tube. Hold it straight up and down. Some hydrometers use small balls instead of one float. If all three balls are floating in the electrolite, battery is charged. Don't add water before checking, it will dilute the electrolite and give a false reading. If the electrolite is too low to suck up enough to raise the float, add some water and charge the battery then check.

 

[Gary Wyngarden]

Hydrometer Use

Hi John,The hydrometer can be a useful tool for assessing the charge of your batteries when it's used properly. I would recommend purchasing Nigel Calder's book, Mechanical and Electrical Systems for the Boat. It's very useful on hydrometers and many other topics.The hydrometer as previously mentioned will read the specific gravity of fluids. Water has a specific gravity of 1.0. As the battery discharges the specific gravity of the battery fluid declines toward that of water. You can gauge the charge of the battery by comparing the specifc gravity of your battery fluid to a chart (in Calder's book and other places). Two words of caution: The specific gravity of your battery fluid is temperature sensitive and needs to be adjusted for proper comparison to the chart. Also, each cell of the battery needs to be separately tested. Like a chain and its links, a battery is only as strong as its weakest cell.Hope this helps.Gary WyngardenS/V Shibumi H335

 

[Don Evans]

Specific Gravity vs State of Charge

Although these values can be found elsewhere I thought I'd include them for you. I measure each cell in the series, identifying each cell's SG, and recording them in a batt log I keep. This is to moniter each cell seperately, as they can behave independantly as they age. Ideally, you would measure 24 hrs after a good charging, and as pointed out these values are at 26.7 C, or around 80 F.Charge% SG100 1.265-1.27575 1.225-1.23550 1.190-1.20025 1.155-1.1650 1.120-1.130Don

 

[Jim Rushing]

When Batteries fail

Do what Don says and you will eventually notice one of your cells fail. They will all take a full charge except one. After a period of time, one cell will always go bad first. Of course, you have to replace the whole battery. This is one of the advantages of the golf cart battery approach. It takes two batteries in series to equal one boat battery. So one bad cell only cause you to replace half of the batteries in your 12 volt system.