mkove electronics

Charge Time Required and Charge Correction

Charging Time


If you look at this datasheet (page 19) or look at the image below, you can see that the charge time for a lead acid battery requires two hours at the end of the charge sequence.


charge time


There are two charging stages required to charge a battery to full. The first is called the bulk charging stage where the battery is charged from it's existing state of charge to approximately 90 percent full. The next stage is called the absorption stage where the battery is held at a fixed voltage until it is full.


There are other stages such as the float stage and the equalisation stage but these both happen after the battery is full and are not of concern here. The speed of the bulk stage depends on the power that can be provided by the charger, but regardless of the power of the charger the absorption stage generally takes around 1.5 to 2 hours. During this stage the power of the charger is reduced and the battery is held at a constant voltage while the battery 'absorbs' the remaining charge. The two hours specified in the datasheet above is the time required for the absorption stage.


The absorption stage explains why in many systems a battery will rarely get to a full state. Without solar panels or a constant charging system you would normally have to drive your car or motor your boat for at least two hours to charge the batteries to full.


The synchronisation problems for all other battery monitors that we are aware of is that they use full state detection, which occurs at the end of the two hour absorption stage, to maintain synchronisation with the battery. At other times they use coulomb counting which can lead to a large drift between the reported state of charge and the actual state of charge if the battery rarely reaches full.


Charge Correction


The MK70 uses a feature call Charge Correction to align the reported state of charge to the actual one during charging. This means that the battery can be partially charged and still maintain it's synchronisation with the battery.


It does this by using a number of staistical and derivative methods to calculate the state of charge during charging. To do this it uses parameters for a standard battery in good condition combined with the learned parameters of the actual battery in use to make it's calculations. It does this at all times during charging but usually has it's largest effect and accuracy during the typically lengthy absorbtion phase.


The majority of calculations are within 1 percent of the actual state of charge but the results of the calculations not applied fully to the overall state of charge. Instead they are added as a gentle pushing effect which slowly adjusts and aligns the overall state of charge.