What is the difference between ‘Input Charge Voltage’ and ‘State of Charge Voltage’?

Charge Voltage: #

As the world transitions to Lithium Battery technology there is one big difference in the way that that the battery interacts with chargers. Learn about the difference between ‘Input Charge Voltage’ and ‘State of Charge Voltage’.

With AGM batteries there was no internal control circuits. They are basically a lump of lead plates, in either a wet or gel electrolyte, that stores and outputs stored electrical energy. All of the charging control is left to the circuits contained in the chargers. To compensate for the way lead acid batteries operated chemically  chargers had multiple stages. Some recent chargers had as many as 9 stage charging modes. Pulsing to remove sulphur build up on the lead plates, float values to maintain surface charge and other modes were a common part of AGM chargers. In summary the charger was in control of the charging and used some different modes  to help with the chemical issues in the battery. 

Lithium Batteries have an integrated BMS (Battery Management System) that is controlling the battery and monitoring the internal cells. The BMS makes the decisions on charging including High Voltage Disconnect (HVD) which is point where the battery is full and requires no more charge. Also the BMS does cell balancing which  ensures the individual cells are kept in balance. In summary in Lithium batteries it is the BMS that makes the charging decisions.

When a modern charger has a Lithium Mode what it does is turn the charger into  a very basic power supply and allowing the BMS in the battery to make the decisions for disconnect when charging.

When it comes to setting the values in a charger the following voltages are recommended for Lithium Batteries. If the charger is the type with no lithium profile the Bulk Charge Voltage should be set a 14.2-14.6V and the Float Charger Voltage should be set to 13.8V. So the optimal charging range of voltages is between 13.8V-14.6V with the sweet spot being 14.2V. If the charger has a Lithium mode then you would put it into that mode.

State of Charge Voltage. #

State of Charge (SoC) refers to how full the battery is at any point in time. 100% SoC is a completely full battery and 0% is a completely empty battery. 

When you set the charger to the voltages that are recommended above, what you will observe with a lithium battery is the following:

  • The charger will slowly bring the voltage up as you go through the charger cycle. It may start at say 12V for a fairly empty battery and you will see the voltage rise up to 13V. It will continue rising until you get to about 13.8V. Why you cannot predict exactly what this voltage is lies in the way the BMS operates. It is watching individual cells and will disconnect the charger when any single cell rises above 3.65V. This normally is around 13.8-14.0V.
  • When the battery is full you would see a sudden rise in voltage as it approaches this 13.8V because at some point the BMS will disconnect the charger and what you see is the voltage that the charger is set to So what you will see is the voltage suddenly go from 13.8V to 14.2V (If you set your charger to 14.2V)

Once a battery is full and sitting at 13.8V what you will observe that it will then drop back to 13.3-13.4V. Don’t be worried that you have lost any charge it is more the fact that the voltage between 13.4-13.8V is a small amount of surface charge and quickly goes away once the battery has a moment to settle down.

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