An equalising charge is a charge that is applied on top of a charge that brings all cells to a full-charge and balanced state which is critical in fleet or series string stationary battery applications where a number of equal voltage and capacity batteries (an example of this is 4 x 12 volt batteries = 48 volts) are connected to achieve a desired output voltage. An imbalanced state occurs in a battery pack where an individual batteries capacity and float voltage are not matched with the other batteries within the pack. The imbalanced battery discharges quicker, reducing the whole battery pack performance and it will charge quicker, leading to an overcharge state and gassing.
Figure 1 -
Imbalanced Battery in a Series String Pack
Stationary batteries remain on continuous float charge and are only seldom used on demand in an emergency occurrence such as loss of supply power to a electrical auxiliary system. Stationary batteries require periodic maintenance, one of which is equalisation charging. An equalisation charge applies a controlled higher voltage to the battery (usually 10%) for a determined length of time. This 'overcharge' removes the reversible sulphate crystals that build up on battery plates over time, sulfation reduces battery performance and life expectancy and is the primary cause of most battery failures. Equalisation charging also reverses battery acid stratification condition, where the concentration of acid is greater at the bottom area of the battery when compared to the top. Equalisation charging must be administered with care because excessive charging will cause harm to the battery.
Poor battery performance and reliability for passenger rolling stock operators and maintainers is a common issue throughout Australia on new and old train fleets.
Technobility engineers has identified through engineering investigations that the on-train Electrical Auxiliaries Battery Charging Systems causes imbalances across series string battery packs which are used on all rolling stock with as many as eighteen 6 volt batteries series connected to achieve the 110 volts required. . SSC Series String Charging creates excessive charge imbalances across the battery pack and will lead to premature battery cell failures and reliability issues.
Testing and measurement of both series string and equalisation charging methods shows the amplitude of charge imbalance when comparing the two charging methods. The resultant charge applied to the same set of batteries using equalisation charging and series string charging is shown below as the charged battery voltage per battery.
The histogram below highlights the extreme spread of charged battery voltages between equalisation and series string charging.
It is apparent series string charging of numerous battery cells, creates excessive charge imbalance over time. The imbalance will gradually get worse over time, resulting in some batteries over charging and under charging. Equalisation charging will help reduce this charge imbalance, ensuring all batteries receive a full and proper charge, which will translate to extended battery life.
Technobility engineering staff was awarded a contract by Sydney Trains to design and build 16 Battery Equalisation Chargers, connection harnesses and test equipment that would improve their fleet battery reliability as well as give significant cost savings.
Our extensive experience on passenger rolling stock, rail electrical systems and batteries ensured that we delivered a solution that meet our customers requirements.
The Battery Equalisation Chargers were designed to meet Sydney Trains specifications and standards, and were subjected to rigorous type testing, both on and off train, to confirm equipment performance.
The Battery Equalisation Chargers consists of 9 x isolated chargers that are rated and limited to 7.35 Volts DC and 6 Amperes. Safe operation is ensured with key design features that include:
Reverse polarity protection and indication
Fused outputs on every channel with indication
Charger ON/OFF status indication
Emergency stop switch
Isolation of output from 240VAC input supply on every channel
Timer for automatic shut-off after completion of a 4 hour of charging cycle
Channel output voltage and current meter displays
The Battery Equalisation Chargers were designed to satisfy these Operational requirements:
The Battery Equalisation Charger shall be easy to use and maintain
After repair, performance of the charger and components used shall not deteriorate with time (at least for 10 years).
The charger shall not be adversely affected by conditions typical to the railway depot environment.
The charger shall be protected against faults on both inputs and outputs.
The charger shall not be adversely affected by incorrect connection or operation.
The Battery Equalisation design incorporates high quality materials and components to satisfy harsh operating environments and exceptional workmanship and build quality to satisfy these requirements:
Operational minimum temperature of -5°C.
Operational maximum temperature of 50°C.
Operational humidity 0 – 95%
All components of Battery Equalisation Charger including electronic components, connectors, enclosure etc. shall be suitable to be used in the depot environment.
All components inside the Battery Equalisation Charger shall be mounted correctly and with sufficient amount of heat sinking where required. Wires shall be neatly secured.
The Battery Equalisation Charger and test box enclosures and build quality are rated for heavy duty.
The Battery Equalisation Charger and associated equipment does not have any sharp edges or parts which might result in injury.
The Battery Equalisation Charger design has appropriate ventilation to dissipate heat generated inside the charger.
The Battery Equalisation Charger design demonstrated that the materials used are fire resistant. This includes providing specification sheets of the selected materials.
The toxicity of materials used for the manufacture or repair of the Battery Equalisation Charger is low.
The connection harnesses were designed using rail grade materials, such as PMA conduit, Radox 4GKW cabling and mil-spec Connectors, to ensure equipment reliability and safety within the harsh depot environment.
All of the equalisation charging units and test boxes were mounted within Pelican cases ensuring portability, while offering equipment protection.
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