Ah efficiency and Wh efficiency.

This article will explain how to determine the Ah efficiency and Wh efficiency of a battery.

In a nutshell, Ah and Wh refer to a battery’s efficiency. A cell’s efficiency can be measured in two ways:

  1. The ampere-hour (Ah) efficiency.
  2. The watt-hour (Wh) efficiency is a measurement of energy efficiency.

The Ah efficiency does not account for the varying charge and discharge voltages. During discharge, the e.m.f. drops from roughly 2.1 to 1.8 volts, but during charge, it climbs from 1.8 to 2.5 volts. The Wh efficiency, on the other hand, takes into consideration the varying charge and discharge voltages.

Ah efficiency = \frac{Amphour discharge}{Amphour charge}

A lead-acid cell’s Ah efficiency is usually between 90 to 95 percent, which means that for every 90-95 Ah taken out, around 100 Ah must be put back into the cell. The Ah available for delivery from the battery reduces due to gassing that occurs during the charging. It also decreases as a result of the self-discharge of the plates caused by local reactions, as well as current leakage caused by inadequate insulation between the battery’s cells.

Alternatively, Wh efficiency ranges from 72 to 80 percent. If Ah efficiency is known, the following relationship may be used to calculate Wh efficiency:

Wh efficiency=Ah efficiency*\frac{average volts on discharge}{average volts on charge}

As can be seen from the above, everything that raises charge voltages or decreases discharge volts reduces Wh efficiency. Because high charge and discharge rates reduce Wh efficiency, it’s best to stay away from it.

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