Frequently Asked Questions

Solar Charge Controllers - General Information

  1. What does SOC mean?
  2. Why is SOC calculation important?
  3. How does the Steca SOC algorithm work?
  4. Which chargers from Steca carry the optimised algorithm?
  5. State of charge control: SOC - state of charge. Example.

1. What does SOC mean?

The performance of our products is shown by the accuracy of the state of charge (SOC) measurement, which results in the long life-time of the battery. The SOC (State of Charge) indicates the actual charging status of the battery. If the battery is fully charged the SOC is 100 % - if it is completely empty the SOC is 0 %. All values in between are possible, but a lot of battery types should not reach SOC values less than 30 %. It is important not to confuse the SOC with the capacity of the battery. The SOC does not reflect the remaining capacity of the battery. The actual remaining capacity of the battery is influenced by a lot of parameters besides the SOC. Multiplying the SOC with the nominal capacity of the battery results in information about the residual capacity of the battery. This value does still not reflect the remaining capacity accurately due to various other parameters including the age of the battery.

2. Why is SOC calculation important?

If a battery is charged, the charge controller needs to know if it is full to prevent battery damage due to overcharging. While discharging, the controller needs to know if the battery is empty in order to prevent dangerous deep discharging. There are several possibilities to determine if the battery is full or empty. The most common criterion is the voltage of the battery. A certain fixed voltage is set to disconnect the load and protect the battery. Unfortunately this criterion is improper. Especially in solar systems, low discharging currents are common and lead to improper battery maintenance if a fixed voltage for load disconnection is used. Better solutions also take the charging / discharging current into account to determine if the battery has to be disconnected from the load. But also this method does not allow an adequate load disconnection to protect the battery optimally due to a very low accuracy and a high error rate. A lot of additional parameters, like temperature, the age of the battery, the user behaviour and other values, influence the battery. Only an accurately calculated state of charge allows disconnecting the load correct according to the properties of the battery. This is why Steca developed a powerful and precise algorithm to determine the actual state of charge of a battery.

3. How does the Steca SOC algorithm work?

The Steca state of charge algorithm is a combination of different methods in order to ensure a precise calculation combined with a stable long time performance. Cost optimised product realisation is additionally another important point for Steca. Years of experience in this field and important research activities led to a self learning "fuzzy logic"algorithm. It takes into account the user behaviour and the ageing of the battery. The voltage of the battery, as well as all battery currents, is watched closely by the charge controller in combination with the temperature. The charger approximates the SOC, during a learning period which takes place in the first cycles. By monitoring the battery and adapting parameters to the changes, a self learning algorithm results that is also able to take the use of the battery into account. This characteristic makes the Steca SOC algorithm a powerful and reliable function, which will ensure the correct monitoring of the battery. The user benefits from a fast and precise information about the battery status that is displayed on the charge controller. Finally the user benefits from the most important advantage to enlarge the life-time of the battery with the help of optimised battery maintenance.

4. Which chargers from Steca carry the optimised algorithm?

The Steca product range is divided into two lines. One is optimised for use in simple applications with less demand and equipped with the minimum necessary features. The other line is designed to cover high-end demand to sup-ply a good communication interface to the user and optimised battery maintenance features. For both lines there exist charge controllers in a wide power range. All chargers equipped with the special Steca State of Charge algorithm are marked with the SOC symbol in this catalogue.

5. State of charge control: SOC- state of charge.Example.

The graph shows the properties of a 28 Ah lead acid battery in relation to the charging/discharging current, the voltage and the state of charge. If the full battery is discharged with 50A and a load cut off voltage of 1.85 V/cell is applied (equal to 11.1 V for 12 V battery) the load will be disconnected at around 70 % state of charge (point 1). This means the battery is still quite full but the load can no longer be supplied due to deep discharging protection. If it is discharged with 5 A, the voltage of 11.1 V will lead to a disconnection at 10 % state of charge which is already a dangerous deep discharge for the battery (point 2). Only having a discharge current of 25 A the battery would be disconnected at 30% SOC (point 3).With the Steca SOC algorithm the load will be disconnected along the line of 30 % SOC in dependence of the discharging current at the cross with the discharging cur-rent line. Only this complicated procedure can ensure optimal battery maintenance.

Click to enlarge the image