OPEN CIRCUIT BATTERY MEASUREMENT TABLE
By: Don McClatchie
Measuring battery State of Charge can be a tricky business especially if the battery is in service connected to a source or drain of current. I am sure that many of you have measured a battery and found the voltage to be good but when you try the battery under a load it appears to be dead. When you connect the battery to a load and measure the terminal voltage that is a Shunt Based measurement and that type of measurement is the best way to check a battery for service.
Batteries are much like a sponge, you cannot know how much juice there is in it without squeezing it dry. The fastest and most effective way to check the state of charge of a flooded lead acid battery is to check the Specific Gravity of the electrolyte but that requires a battery that has cell caps that allow access to the electrolyte. This method also allows you to check each cell of the battery separately as one bad cell will affect the state of charge for the entire battery.
However, under certain circumstances you can get an approximation of the state of charge or how much energy is left over in the battery by carefully measuring the At Rest terminal voltage of the battery with a digital voltmeter that has an accuracy of at least 1%. The accuracy is required because of the very small differences in battery terminal voltage that must be measured ranging over only about 1.25 volts total.
At rest means that the battery must be disconnected from any source of charge or drain of current for at least 6 hours and 24 hours is better. The accuracy of this type of measurement is dependent on the battery temperature and age of the battery. If a battery has lost its ability to take a charge this method of reading will not be useful.
The following table indicates the readings you should get when measuring an (FLA) Flooded Lead Acid battery sealed or not, (GEL) Gelled Electrolyte and the (AGM) Absorbed Glass Mat type batteries.
|OPEN CELL LEAD ACID BATTERY|
|MEASURE||STATE OF||SPECIFIC||MEASURE||MEASURE||MEASURE||STATE OF|