What is a battery Indicator?

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The car battery green light is a small round window on top of automotive batteries, which changes color from green to black to white as the condition of a battery degrades. It can give you an idea of your battery power and reveal some simple battery issues. It is usually a feature of most completely sealed maintenance-free lead-acid batteries. However, it may also be seen on non-sealed lead-acid batteries.

Firstly, the “Battery Green Light” or Magic Eye is not a light. The Eye is actually a colored ball in a small tube which moves according to the specific gravity of the acid in the battery. In effect, it acts as a really simple hydrometer for one cell of the battery.

The car battery green light is also called the:

• Battery Eye
• State of Charge Indicator
• Battery Status Indicator
• Magic Eye

We cannot open sealed batteries, so it is impossible to use a hydrometer to check the condition of the cells of your sealed maintenance free battery. The state-of-charge indicator (SoC) is a quick way to check the condition of your lead acid battery when you have no other means of testing at hand.

It can give three battery condition readings but needs a little interpretation to understand if you really have a faulty battery.

Most battery manufacturers advise that when the Battery Indicator/ Magic Eye is:

• Green the battery is fully charged
• Black the battery needs to be recharged
• White the battery is dead.

The most frequent question I get in regard to the battery green light is:

“Why is my battery light green, but the battery is not starting?”

Answer: The battery status indicator only gives you information about the state of charge of one cell (out of six cells) of the battery. Because none of the cells in a battery shares its electrolyte (battery fluid) with the other cells, there is no way for the status indicator to check the condition of other cells which might have problems.

Also, the state of charge of one cell will not tell you anything about the actual health of the battery. There are many other battery faults or potential issues which cannot be shown by the battery charge indicator.

Finally, maintenance-free batteries contain electrolyte fluid. If the fluid levels of your battery go down too low, the Battery Status indicator will simply stop working and may remain stuck on the green side, thus giving a false reading.

The correct way to interpret the Battery Indicator

If the car is not starting but the battery indicator is green. Check that the battery terminals are clean and tight. If another battery starts the car fine or if jump starting works, then the most likely scenario is that your battery has a fault and should be replaced.

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If your battery indicator is black or white, don’t throw it away, get it tested by a qualified battery shop. The use specialised battery testers and battery analysers, which are much more accurate than the magic eye on your battery. They may need to recharge and retest your battery. Only a proper test will show if your battery just needs a good charge or has irreparable damage.

Investigate why the battery died so that you don’t have a repeat of the same problem. A faulty alternator can discharge your battery, but there are other potential causes of a discharged battery. Check out our article on the Eight (8) Things That Can Kill Your Car Battery.​