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S O C – state of charge

State of

charge control

Steca‘s Charging

Technology

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.

What does SOC mean?

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 comple-

tely 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 ca-

pacity of the battery is influenced by a lot of pa-

rameters 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 remai-

ning capacity accurately due to various other pa-

rameters including the age of the battery.

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 over charging. While discharging, the control-

ler needs to know if the battery is empty in order

to prevent dangerous deep discharging. There

are several possibilities to determine if the batte-

ry 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 bat-

tery. Unfortunately this criterion is improper. Es-

pecially in solar systems, low discharging currents

are common and lead to improper battery main-

tenance 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 opti-

mally due to a very low accuracy and a high error

rate. A lot of additional parameters, like tempe-

rature, the age of the battery, the user behaviour

and other values, influence the battery.

Only an accurately calculated state of charge al-

lows to disconnect 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 batte-

ry.

How does the Steca SOC algorithm work?

The Steca state of charge algorithm is a combi-

nation of different methods in order to ensure a

precise calculation combined with a stable long

time performance. Cost optimised product reali-

sation is additionally another important point for

Steca. Years of experience in this field and im-

portant 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, are watched closely by the charge con-

troller 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 pre-

cise information about the battery status that

is displayed on the charge controller. Finally the

user benefits from the most important advan-

tage to enlarge the life-time of the battery with

the help of an optimised battery maintenance.

Which chargers from Steca carry the opti-

mised algorithm?

The Steca product range is divided into two

lines. One is optimised for use in simple appli-

cations 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 spe-

cial Steca State of Charge algorithm are marked

with the SOC symbol in this catalogue.

Example

The graph shows the properties of a 28 Ah lead acid battery in relati-

on to the charging/discharging current, the voltage and the state of

charge. If the full battery is discharged with 50 A 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 dischar-

ge current of 25 A

the battery would be

disconnected at 30%

SOC (point 3).

With the Steca SOC algorithm the load will be discon-

nected 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.

2.1

2.0

1.9

1.8

1.7

1.6

1.5

1.4

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30

Capacity of Battery

SOC = 30 %

Voltage of Battery

I = 25 A

State of charge (SOC)

Current

U [V/cell]

80%

70%

60%

50%

40%

20%

7.5

10A

15A

20A

30A

40A

50A

60A

80A

100A

90%

1.8

SOC = 30 %

Steca SOC

deep discharge protection

I = 25 A

SOC = 30 %

10%

constant discharge voltage

1.85