mkove electronics

Detecting Full


The following applies to a 12 Volt 6 cell lead acid battery. Voltages below are double for a 24 Volt system.


Detecting full is arbitrary and dependant on a definition. A commonly accepted definition, and the definition used by the MK70 is that a battery is full when it's charging current has reached 1 percent of it's capacity.[1][2] For a 100 amp hour battery this would be at 1 amp and likewise for a 40 amp hour battery this would be at 0.4 amps. It is possible to slightly charge the battery beyond this point and this capacity would be available for use, but in practice almost all charging beyond this point is overcharging.


But what about the charging voltage? It turns out that if the charging voltage is reasonably high, the charging voltage affects the charging current only slightly during the last few percent of charging. Changing the charging voltage anywhere from say 13.5 Volts to 14.5 Volts during the very last stages of a change process will barely change the charging current. If it does change then it will only change briefly and then return the the previous current. This is convenient as it means that a percentage of capacity as a full state indicator will work reliably regardless of the charging system and voltage (within reasonable limits). At anything other than almost completely full however changing the charging voltage will affect the charging current proportionally. This is why systems that use a low requirement for detecting full such as a high amperage or a low voltage can result in inconsistent full state detection. This is also why setting the full detect trip points lower to reduce charging time is not effective.


It is not quite as simple as this. If a low current source is connected, such as a solar array on a cloudy day, the current may be just under the 1 percent requirement and a full state could be triggered without the battery actually being full. Also some chargers can switch to a fairly low float voltage, for example 13.4 Volts which although this will eventually charge the battery to full takes a longer time to do so.


A key voltage is 13.5 Volts. Above this voltage if the current falls below 1 percent of the batteries capacity then the battery will be full or very close to full.


The MK70 uses the following conditions for full detection;


If the battery is held steadily above 13.5 Volts for five minutes with the current at or below below 1 percent of the batteries capacity it will set the battery as full.


Between 13.0 and 13.5 Volts it requires that the current be held below 1 percent of the capacity for a longer period of time. At 13.0 Volts it requires two hours and at 13.5 Volts it requires five minutes. In between 13.0 and 13.5 Volts the time required is reduced proportionally from two hours to five minutes.







[1] Power Sonic battery datasheet.

[2] Lifeline battery datasheet.