Battery voltage state of charge SOC – From fully charged to discharged

Battery state of charge (SOC) is used to indicate rechargeable battery voltage at any given time. Non rechargeable (primary) batteries start off with their maximum voltage. As they are used to provide power, their voltage drops until it is too low to be used anymore.

Rechargeable (secondary) batteries can be charged up to a maximum voltage (100% SOC) and discharged to a minimum safe or recommended voltage (0%) and recharged again.

Never connect uneven voltage batteries in parallel diagram by electronzap electronzapdotcom
Never connect uneven voltage batteries in parallel diagram by electronzap electronzapdotcom

Diagram shows typical lithium ion voltages when fully charged and a common final discharge voltage.

Different battery chemistries have a different range of voltages, and different operating conditions. So ultimately, you always need to consult with the manufacturer datasheets/product information before attempting to charge/discharge them. Better yet, use a proper and well made charger for that type of battery. Ideally, use a charger provided by the manufacturer of the battery (pack) that you are using. Universal chargers will probably do just fine though.

No load (not powering something) voltages to estimate SOC

This is just a brief starting point for understanding how to recharge batteries. Always search for more information on the particular battery types you are working with. 

Li ion single cell rated at 3.6V (sometimes 3.7V is claimed instead). Typical SOC (may vary):

  • 100% = 4.2V (Never exceed for most li ion cells)
  • 50%  = 3.6V
  • 0% =  3V (never drop below 2.5V) best to recharge as soon as possible once you get around 3V or less.

Unlike lead acid batteries, It is OK to only partially charge Li ion batteries. The charger should stop putting in current once it is fully charged too.

Series li ion batteries: If you have 4 li ion batteries connected in series, for example, then the total voltage will be about 16.8V charged, to 12V discharged. Unfortunately series batteries charge and discharge unevenly. Primarily based on their actual capacity. Always use equal capacity batteries. Lithium based batteries always needs circuitry that monitors, and stops any batteries from being overcharged, and also cuts power when any battery gets low. It should also provide extra charge to series batteries that need it, and a number of other safety options.

Lead acid 12V rated (usually six 2V cells in series). Common SOC (may vary):

  • 100% = 12.73V (try to keep lead acid fully charged)
  • 50% = 12.1V (recharge as soon as possible)
  • 10% = 11.51V (recharge as soon as possible)
Some battery topics:
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