Battery Glossary – Float Voltage

Float Voltage – the voltage a battery should be kept at if used in a standby application (such as emergency lighting).

To understand float voltage its important to know:

  • The voltage of a battery decreases as it is discharged and increases as it is charged
  • All batteries gradually discharge over time even if they are not in use via a process known as ‘self-discharge’
  • The voltage stated on a battery’s label is usually lower than the voltage the battery can achieve when fully charged

In most cases we are used to the idea that we fully charge a battery and then use it. We charge up our phones to 100% so they will last as long as possible. A camper will charge their camper battery to the top so it keeps the lights on the maximum amount of time.

But what about batteries that spend most of their time unused, such as emergency lighting batteries or uninterruptible power supply batteries that back up items such as computer systems? Keeping them fully charged over long periods is actually damaging and can reduce their lifespan. If they are not connected to a charger, they will gradually self discharge and not be able to operate at their best when needed.

As such, every battery has what is known as their ideal float voltage. The voltage it should be kept at if it is being maintained at a near full state of charge over a long period of time.

Float voltage calculations are complex as they are heavily influenced by the temperature in which the battery operates and the battery chemistry. A 12 volt lead acid battery in a room where the temperature is 86°F (30°C) has a float voltage of 13.28. If the room temperature were to drop to68°F (20°C) the float voltage required would increase to 13.52.

Many modern smart chargers are designed to charge batteries until they are full and then relax the charge to the float voltage until they are disconnected.

You can find the full calculation on the Wikipedia Page about float voltage.

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