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Sizing batteries correctly

Batteries need to be sized correctly to be able to feed the required load for the required time, and a number of factors need to be decided to be able to optimize the battery for the duty expected. Some of these factors are fixed within the chemistry of each type of cell and, in some cases, the physical structure of the plates that make up the cell.

The performance is also influenced by the temperature and other location factors, and as an optimal combination of cells is needed to provide the required performance, the following important factors need to be considered:

  • Maximum and minimum system voltage
  • Correction factor
  • Duty cycle

System voltage (maximum and minimum)

The cells that make up any battery have a limited voltage range specific to the type of cell being used. In the case of lead acid batteries, the cell nominal voltage, which is the voltage of a fully charged cell without any input charge or load, is 2 V.

On the other hand, the minimum voltage that a battery cell can safely supply a load without damage is typically 1.7 V, although to give a margin of safety, it is more normal to use 1.75 as the operating minimum. Similarly, to be able to charge a battery, the voltage across each cell must be more than the nominal 2 V, and to keep a battery fully charged, each cell typically needs to be kept energized at 2.2 to 2.25 V, dependent on cell construction.

This is the float-charge voltage. Because individual cells in a battery can develop higher impedance than others when floated for a significant time, or after they are discharged, batteries only charged on “float” can result in some cells being less charged than others.

To overcome this condition, it is necessary to subject the battery to a higher voltage, the equalize charge voltage, which could be up to 2.7 V per cell.

Although the higher voltage would allow a faster recharge and would even up the charge on individual cells more quickly, this level of cell voltage would make the battery voltage range exceed the rating of most equipment that uses DC supplies.

It is therefore usual practice to keep the equalize charge in the range of 2.33 to 2.5 V per cell and extend the time required to equalize the battery.

Based on the above, the common battery size for a 125 V North American battery uses 60 cells with a battery voltage range of 105 to 140 V DC. This range is computed as follows:

  • Equalizing voltage = 2.33 V per cell
  • Maximum battery voltage under equalized charge = 60 V × 2.33 = 140 V
  • Minimum volts per cell = 1.75 V
  • Minimum battery voltage = 60 V × 1.75 = 105 V

Because the equipment fed by such a battery must also be operable with a level of voltage drop in the associated distribution cables, the operating range should cover the range 100–140 V. For international use, the typical lead acid battery consists of 55 cells having a battery voltage range of 96–128 V, resulting in a required equipment voltage range of 91–128 V.

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