Why is my hydrometer telling me that my fully charged batteries are only partially charged?

Dear Car Talk

Dear Car Talk | Feb 01, 1995

Dear Tom and Ray:

I have always checked the condition of my battery by checking the specific gravity with a hydrometer. I recently purchased an Exide battery. The specific gravity of the battery stays about 1.225 whether the battery is fully charged or completely discharged. How is this possible?

TOM: It's possible with a broken hydrometer, Wade. It's time to buy a new one.

RAY: I'm sure you can find some mechanic who's dying to sell you his, since 99.9% of all batteries sold these days are maintenance-free and never need to be (and in most cases CAN'T be) checked.

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