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Battery Testing

Lead Acid Batteries

It is critical that accurate battery cell SG measurements be taken.  A cheap automotive style hydrometer is worse than the bargain dial thermometers you see on display with most indicating different temperatures.  The cheap automotive style hydrometers only provide a rough approximation of charge and cannot be relied upon.  A quality hydrometer and a thermometer should used to determine the accurate charge level of lead acid batteries.  The temperature of the acid solution is extremely important as it affects specific gravity as well.

If you have a battery monitor such as a Trimetric by Bogart, or a battery monitor by Blue Sky or Xantrex it is essential you can provide information or compare the information these monitor provide against specific gravity, the single most accurate measurement of battery charge.

Most flooded batteries are in the range of 1.260 to 1.280 Volts when fully charged at room temperature (220 C).

Most deep cycle batteries should be in the 1.277 to 1.280 Volt range at 250 C (770 F)

Approximate values
(check with your battery manufacturer, there are differences between batteries)
Full 100% charged = 1.280
75% charged = 1.240
50% charged = 1.200
25% charged = 1.160
Dead 0% charged = 1.120.

There are few really good battery acid hydrometers available on the market.

We recommend and can supply you with the Freas Glassworks #98 Hydrometer Set with Thermometer, 1.1401.300 Specific Gravity and 30o120oF thermometer.  Their hydrometers are Precisely calibrated; all floats re-tested and guaranteed to be accurate.  The scientifically shaped barrel is proportioned to give free and accurate readings continually. The true inside diameters are manufactured to .005 inch tolerances.  Only the very highest grade acid-resistant rubber with a durable rubber ended tip is used.

The second most accurate method of determining battery capacity is by voltage.  Modern digital voltmeters are extremely accurate however it is sometimes difficult to tap individual cells for voltage. In order to Let batteries stabilize allow them to idle for at least an hour after charging or a heavy discharge.

Most fully charged cell flooded batteries are 2.14V per cell fully charged. (12.6 - 12.8V for a 12V battery) A flooded lead acid battery is at about 2.03V per cell (12.2V for a 12V battery) fully discharged. Remember to check with your battery manufacturer, there are differences between batteries.

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Last modified: January 12, 2023