Batterien Engl

7/30/2019 Batterien Engl

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Pb battery

Chemical equation:

discharge>

Pb+2 H2SO

4+ PbO

2


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Acid density:

Acid density (ad) determines:

Voltage (rule of thumb: Vo=ad+0,84)

=> state of charge can be determined by the acid density when the

battery is in a `rest state`

Capacity

=> maximum acid density is at 1,28 kg/l. Above this value reactions

without external circuit will take place.

It also tends to increase corrosion


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Processes and problems during

operation: At the moment Lead Acid Batteries are the

cheapest and most practical solution for

photovoltaic systems.

But:

Lead Acid Batteries are sensitive to overcharging

and deep discharge Other chemical reactions will lead to problems

and damage


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Gassing:

When the battery is charging, the charge current will lead to a

electrolytic breakdown of the water in the electrolyte.

=> Electrolyte gets lost, the acid concentration increases

=> During high gassing, some particles will be blasted off the plates.

=> The particles will settle on the bottom of the battery cell. In

extreme cases this can lead to a short circuit between the plates.

Result:

capacity lossas particles get blasted off the plate maintenance is required e.g. replacing the water in the electroyte

Destruction of cells caused by short circuit


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Acid density layers:

Dense acid, created in the plates during charging `flows` along theplates towards the bottom of the cell.

=> The acid density at the bottom part of the plate is higher than on

the upper part.=> Theres a voltage drop between the upper and lower part of the

battery plates.

=> The voltage difference leads to higher discharge of the lower part

of the plates.

This results in:

Sulfation of the lower parts of the plates

capacity loss

Remedy: Circulation of acid with a um or b assin .


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Corrosion:

The structure which holds the battery plates together is often made ofLead which has been oxidized on the surface. These `holders willbe attacked by the acid and slowly destroyed.

The speed at which this destruction takes place is dependant on theacid density and the temperature.

Remedy:

restriction of charge voltage lowering the nominal acid density (particularily at high ambient

temperatures)

In solar systems the consequences of corrosion are often

overestimated.


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Acid concentration gradient:

During discharge water will be developed, during chargingsulphuric acid.This will lead to a difference in the acid concentration between the

inside of the Pb-plate and the electrolyte around it.

=>Open circuit voltage adjusts itself according to the inside acid

density, the voltage of the cell will be shifted accordingly.

=>In operation it is inaccurate to determine the state of charge via theterminal voltage of the cell. However it is a rough guide.


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Sulfation:Main problem in stand alone

solar systems!!!!!!Pb sulphate which develops during discharging is

crystalline.

After discharing the crystalls are small but have a largesurface area. Lead sulfate disolves in the electrolyte andtends to recrystallize in areas which already contain Lead

sulfate crystalls.

=> crystalline growth=> smaller crystalline (reaction) surface

=> Pores of the plate will get closed


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Consequences of sulfation:

=> Higher internal resistance caused by smaller

reaction surface

=> Higher acid concentration differential caused by

closed pores.

This results in:Cells develop a high resistance and stop working.


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Precautions against sulfation

and remedy:

Prevent a low state of charge using deep charge protection. (speed

of sulfation is dependent on the amount of Lead sulphate)

Higher concentration of acid in the cell as the solubility of Leadsulphate will increase with lower acid density.

Fully charge the battery as often as possible using

=> Hybrid systems with additional sources (e.g. Diesel generator)

=> Intelligent (but restrictive) Charging technology


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Movement caused by mass

change:During discharging of the Battery PbSO

4( lead sulphate) will be

developed by the reaction of Pb (lead) and PbO2

( lead oxide). The

volume of PbSO4

is 1,5-times higher than the volume of PbO2

and

3-times higher than the volume of Pb.

=> Each discharge cycle causes a mechanical movement of the plate.

=> With time the structure of the plates will be destroyed

Remedy:

Discharge protection to restrict the discharging

Selection of a stable plate


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Different plate types for Lead

Acid Batteries:

Plate with a large surface area: (positive plates)

Consists of pure solid Lead. The surface is rippled to create a large

surface area for the chemical reactions to occur on.

Characteristics: High cycle life, small capacity, expensive

box plate: (positive and negative plates)

Lead box holds the active mass together

Characteristic: high cycle life, expensive, very rare


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Most common plate type:

Grid plate:(+)(-)

The active mass will be pasted into the grid made of pure Lead.

Characteristics: Extremely efficient structure, susceptible to corrosionand movement caused by change of mass, high peak current output.

Tubular plate:(+)

The Lead active mass will be pressed into a porousplastic tube. Characteristics: very resistant to mass

movement; high cyle life


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Common Battery types andtheir use in solar systems

There are many different types of Lead Acid Batteries available onthe market. Each has advantages in different applcations.

For every application you have to consider the following criteria

Capacity

Field conditions (Industrial usage, rural electrification, leisure....)

Ease of maintenance and maintenance cost

The choice of which Battery will be used influences the cost structure

of the whole system significantly. In most cases the user has to

make a compromise between the different battery characteristics

and the cost.


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Lead accid battery types:

Starter battery

Starter batteries are designed to start engines. They predominantly usegrid plates.

Characteristics

short cycle life (about 50 complete cycles)

susceptible to sulfation and mass movement

high self discharge

very cheap

To sum up:

Starter batteries are unsuitable for solar systems!!!

In cases where only starter batteries are available Oversize the

Solar generator and the battery to minimise depth of discharge.


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Solar batteries are modified Starter batteries.

Ideally the following measures have been implemented:

Thicker, more solid plates (-> higher cycle life)

Less antimony in the grid (less self discharge)

More electrolyte with less acid density (less sulfation, maintenanceand corrosion)

Cost: about 30-50% more than starter batteries for similar capacity

Use:

Leisure market and rural electrification

Size of the systems 30 - 500Wp

Lead acid battery types:

Solar batteries


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In sealed Batteries the acid will be held in an Absorbed Glas Mat(AGM) or in Gel. The plate mainly consists of grid plates. In rare

cases the negative plate in Gel Batteries is a tubular plate.

Only limited gassing is allowed in Sealed Batteries.

Maintenance is not possible.

Small self discharge. relatively deep discharge tolerated.

Lead Acid Batteries:

Sealed Batteries


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Difference between AGM and Gel

Gel Batteries are resistant to sulphation.

=> They can be stored for up to 1,5 years without charging

minor acid stratification will develop in AGM Batteries

=> Danger of sulfation exists, limitted warehouse shelf life

Cost: about 200-300% more than Starter Batteries

Use: Solar systems (Solar gel Batteries are available in sizes from 60Ah

and above)

High grade solar systems

Systemsize: from 5Wp up to about 1000 Wp

Lead Acid Type Batteries:

Sealed Batteries


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Stationary Batteries are designed to have high life time, high cyclelife and are reliable in use. Some have tubular plates. In rare casesspecial high surface area plates are used.Some stationary Batteries are available as Gel batteries.

Characteristics:

small self discharge

high cycle life

=> Suitable for big solar systems

Cost: about 200-300% more than Starter Batteries

Use:

Big industial solar systems, telecom systems

Systemsize 200Wp to 50KWp

Lead Acid Battery Types:

Stationary Batteries


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Lead Acid Battery Types:Cycle Life of different types


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Sizing of Lead Acid Batteries:

A common mistake is to design the system with too little batterycapacity.

=> The batteries tend to sulfate

Rule of thumb

Not more than 1Ah Per Peak Watt of array capacity, (northern

China);Closer to the equator 2 Ah per Peak Watt is possible.


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Interconnecting Pb-Batteries:

Connecting Batteries together in parallel will causeequalising currents to flow between them.

Avoid parallel connections when possible

Galvanically isolated Batteries can be connected

in parallel using special regulation technology.

When stringing many Batteries together in Series

the voltage can be monitored and controlled in

blocks of 12V or 24V.


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LiFePO Batteries

Chemical EquationAnode (Grafit):

discharge

Li1C

6C

6+ Li+ + e-

charge

Cathode:

Li+ + e- +FePO4 LiFePO4


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LiFePO Batteries

Structure of LiFePOBattery


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Ageing mechanisms of LiFePO

High Temperature

> Binding material ingraphite get destructed

> high resistance

> loss of capacity if singlegraphite sectors get

disconnectedConsequence:

> prevention of hightemperature


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High Voltage

> Oxidation of nanostructure

> loss of capacity

Consequence:

> limitation of charge voltage


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Deep Discharge

> Copper-Dendrite growth

> Short circuit of electrolyte

Consequence:

> Prevention of lowvoltages


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Charging at negativetemperature

> Li-Dendrite growth

> Short circuit of electrolyte

Consequence:

> Reduced charge current

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