Gel cell and absorbed glass mat batteries are common in these roles, collectively known as valve-regulated lead–acid (VRLA) batteries. First invented in 1859 by French physicist , it was the first type of rechargeable battery ever created. Because the electrolyte takes part in the charge-discharge reaction, this battery has one major advantage over other chemistries: it is relatively simple to determine the state of charge by merely measuring the of the electrolyte; the specific gravity falls as the battery. PlatesThe lead–acid cell can be demonstrated using sheet lead plates for the two electrodes.
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