Deep Cycle Battery Maintenance Part 2: Checking Specific Gravity

In this article, we'll show you how to check the charge of your deep cycle battery using a Hydrometer.

More battery banks are lost due to poor maintenance than any other cause. Regular maintenance should take place every three months, and ensuring your battery's charge level is full is an important step in that process.
To ensure your deep cycle battery is fully charged you'll need to measure the specific gravity with a Hydrometer or Refractometer.

Keeping an accurate log of battery maintenance will also protect your warranty and let you know when issues arise.

How to use a Hydrometer

demonstrating how to use a hydrometer to test the SG of a deep cycle battery


Before proceeding please refer to the safety section at the bottom of this post.

Clean your work area before you begin. Once that's done, remove the port caps on the top of your battery. Insert the hydrometer's tube into the electrolyte and squeeze the black ball. Release, and you'll see the window fill with liquid.

hydrometer reading solar battery's charge

Once the window has filled completely, you'll see your reading. This is the specific gravity, or the density of the electrolyte compared to the density of water. In the photo above, our reading is at 1.260, which is within the 100% range. You can see that 1260 lines up with the black mark on the left of the window. Compare your reading to our chart below. If it looks like your number is low, it's time to charge your battery.

% Charge Specific Gravity* (SG)
100 1.255 - 1.275
75 1.215 - 1.235
50 1.180 - 1.200
25 1.155 - 1.165
0 1.110 - 1.130


  • Disconnect charge controller and main amp battery breaker.
  • Wear goggles when you check the specific gravity level of your battery. The electrolyte in the battery is sulfiric acid, so it's important to protect your eyes with safety goggles.
  • Wear gloves that withstand exposure to sulfiric acid. Latex gloves work, but only for a few minutes. If you notice a drop or spill on the gloves, remove them and use another pair. Neoprene gloves, although harder to find, offer upwards of an hour of protection.
  • Wear old clothes with long sleeves, long pants, and closed-toe shoes. Cover as much skin as possible.
  • If the electrolyte comes in contact with your skin, wash the area immediately.
  • Once the job is done, dispose of all used gloves and rags.
  • Return to top.

For more information, see the other 2 post in this series:

Deep Cycle Battery Maintenance: Part 1 - Assessing Water Levels

Deep Cycle Battery Maintenance: Part 3 - Checking Battery Voltage

HES is proud to offer Canadian's a selection of Surrette batteries. Surrette has a well-earned reputation for building reliable batteries with strong capacity, cycling performance, and long life. For more information, view our Surrette inventory, or contact us.

Want to know more about battery maintenance?

Batteries 101

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