Technical data

ManualsBrandsAeca ManualsBattery ChargerXTH 3000-12

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

U [V/cell]

100

80 A

60A

100 A

50A

40A

30A

20A

15A

10A

7.5A

5 A

20 A

40 A

50 A

60 A

70 A

80 A

90 A

10 A

1 3

State of Charge (SOC)

Deep Discharge Protection (SOC = 30%)

Constant Discharge Voltage

I = 25 A

Current

Capacity of battery

Voltage of battery

Charging Technology

10|

|11

Figure on the left-hand side

... shows the characteristics of a lead-acid battery with a

rated capacity of 28 Ah. Its voltage changes in relation to

the charge and discharge currents and the state of char-

ge. If a fixed discharge cut-off voltage of 11.1 V is now

specified, this means that, at a discharge current of 50 A,

a full battery is disconnected when its state of charge is

still 70% (point 1). This is represented in the diagram by

the green line. The majority of the capacity which is still

available cannot be used in this case.

If the same battery is discharged with 5 A, however, the

system disconnects it at the same fixed voltage of 11.1 V,

which in this case means at a state of charge of around

10% (point 2).

This is already a dangerously low state of charge which

can result in significant damage to the battery. Only with

a discharge current of 25 A would the battery in this case

be correctly disconnected at an SOC of 30% (point 3).

Using the Steca state of charge algorithm the charger is

able to disconnect the battery at the correct threshold

with any discharge current. The cut-off voltage is deter-

mined by the point at which the 30% line crosses the

discharge current line (Steca SOC deep-discharge pro-

tection). Only a method of this kind can ensure that the

battery is maintained correctly, and thus has a long service

life.

How does Steca‘s state of charge determination

work?

Steca‘s algorithm for determining a battery‘s state of

charge is a combination of various methods which

ensure that the SOC is calculated accurately enough

and delivers reliable, stable values over a long period

of time. Furthermore, attention is paid to making a

calculation method which can be carried out simply

and at a low cost in various solar charge controllers.

Years of experience in the research and development

lutions, for the battery currents are taken into ac-

count alongside the voltage. But this method does

not allow the state of charge to be determined ac-

curately either, since many important factors are not

considered. Only if the state of charge is calculated

precisely is it possible for the solar charge controller

to treat the battery correctly, to end a charge cycle

using the solar module at the correct time and to

switch off a load neither too early nor not too late.

For this reason, Steca has developed a high-perfor-

mance algorithm with which the state of charge can

be calculated with a sufficient degree of accuracy

and the battery can be optimally protected.

What does SOC mean?

SOC means the current ‚state of charge‘ of the bat-

tery. This is given as a percentage. A battery is ful-

ly charged when the SOC is at 100%. The lowest

value which can be reached is 0%. In theory, all

other values in between can be reached, but most

types of batteries should not reach state of charge

values of less than 30%. Such values can quickly lead

to dangerous deep discharges which decrease the

service life of the batteries or destroy them directly.

A battery‘s state of charge should not be confused

with its remaining available capacity. The actual re-

maining capacity depends on many parameters such

as the temperature, age and history of the batte-

ry and many others. It is possible to gain a rough

estimate of a battery‘s current remaining capacity by

multiplying the correct state of charge of the bat-

tery by its rated capacity. As the age of the battery

increases, however, the rated capacity can change

significantly, which means that the prediction of the

available capacity can be strongly distorted.

Steca‘s charging technology

The Steca products stand out thanks to an

optimal state of charge determination. This

is the key to the batteries having a long

service life.

Why is a state of charge determination

so important?

During charging, the solar charge controller has to

know when the battery is fully charged so that it

can protect it against overcharging at the right mo-

ment and in the correct manner. When discharging

the battery it is equally important to know the sta-

te of charge in order to protect the battery against

harmful deep discharge. In order to carry out this

function, there are various criteria which can indi-

cate how full the battery is at a given time. Some

of these criteria are better suited than others. The

simplest and most common criterion is the voltage

of the battery. With this method, a fixed charge

cut-off voltage is defined. When this voltage is re-

ached, charging is stopped. A fixed deep dischar-

ge threshold is also defined. If the battery voltage

falls below this value, the load is switched off. This

method is simple, since the voltage of the battery is

easy to measure precisely, yet it is not ideal for most

types of batteries because their state of charge does

not change in direct proportion to the voltage. Low

discharge currents are common in solar power sys-

tems in particular. This leads to inadequate battery

maintenance if fixed voltage values determine the

charging or discharging processes. The full-charge

and deep-discharge thresholds provide better so-

of battery state of charge algorithms has led to an

auto-adaptive ‚fuzzy logic‘ algorithm. This includes

the age and usage history of the battery in the cal-

culation as well as the other important parameters.

The battery voltage and its currents and the tem-

perature are constantly measured as accurately as

possible by the solar charge controller. During a lear-

ning phase, the solar charge controller estimates the

state of charge on the basis of experience values. At

the same time, the controller monitors the behavi-

our of the battery and adjusts various parameters

to the current system. The learning phase lasts for a

few cycles. The advantage of this method is that it

makes it possible to respond dynamically to the re-

quirements of the system and individually adjust the

battery maintenance to the requirements of every

individual system. This feature explains the high per-

formance and reliability of the Steca battery state of

charge algorithm. At the same time, this algorithm

guarantees optimum battery maintenance, which

is reflected in the long service life of the battery. In

addition, the user benefits from the fact that the

battery‘s current state of charge can be displayed,

which means the user constantly has optimal control

over the system.

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 ne-

cessary features. The other line is designed to cover

high-end demand to supply a good communication

interface to the user and optimised battery mainte-

nance features. For both lines there exist solar char-

ge 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 (see overview page 63).