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Pendulum clock driven by three weights as "gravity battery". An old and simple application is the pendulum clock driven by a weight, which at 1 kg and 1 m travel can store nearly 10 Newton-meter [Nm], Joule [J] or Watt-second [Ws], thus 1/3600 of a Watt-hour [Wh], while a typical Lithium-ion battery 18650 cell [2] can hold about 7 Wh, thus 2500 times more at 1/20 of the weight.
This method works only with batteries that offer access to their liquid electrolyte, such as non-sealed lead acid batteries. The specific gravity of the electrolyte can be used to indicate the SoC of the battery. Hydrometers are used to calculate the specific gravity of a battery. To find specific gravity, it is necessary to measure out volume ...
The alkaline electrolyte (commonly KOH) is not consumed in this reaction and therefore its specific gravity, unlike in lead–acid batteries, is not a guide to its state of charge. When Jungner built the first Ni–Cd batteries, he used nickel oxide in the positive electrode, and iron and cadmium materials in the negative.
Some battery designs include a simple hydrometer using colored floating balls of differing density. When used in diesel-electric submarines, the specific gravity was regularly measured and written on a blackboard in the control room to indicate how much longer the boat could remain submerged. [14]
Specific energy Energy density (W⋅h/L) Comment Zinc-air battery: 1.59 [48] 6.02 441.7 1 672.2: controlled electric discharge Lithium air battery (rechargeable) 9.0 [49] 2,500.0 controlled electric discharge Sodium sulfur battery: 0.54–0.86 150–240 Lithium metal battery: 1.8 4.32 500 1200: controlled electric discharge Lithium-ion battery
The electrolyte mixture of potassium hydroxide and lithium hydroxide is not consumed in charging or discharging, so unlike a lead-acid battery the electrolyte specific gravity does not indicate state of charge. [7] The voltage required to charge the NiFe battery is equal to or greater than 1.6 volts per cell. [18]
The state of charge of a lead-acid battery can be estimated from the density of the sulfuric acid solution used as electrolyte. A hydrometer calibrated to read specific gravity relative to water at 60 °F (16 °C) is a standard tool for servicing automobile batteries. Tables are used to correct the reading to the standard temperature.
A United States Navy Aviation boatswain's mate tests the specific gravity of JP-5 fuel. Relative density, also called specific gravity, [1] [2] is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material.