Ultracapacitor
26 Aug

Introduction

Ultra-capacitor is also called a super-capacitor. It is an energy storing device and it stores the energy in the form of electrical field. The Ultracapacitor is electrochemical device where-in no chemical reactions are involved in the storing of its electrical energy. Ultra-capacitor is capable of holding hundreds of time more electrical charge quantity than the conventional capacitor.

A conventional capacitor contains two electrically conductive surfaces, separated by an insulating layer called a dielectric. The Capacitance of the capacitor depends on surface area of conductor. However, a practical limit exists as to the capacitance obtainable with this design. The structure of ultra-capacitor is different than the conventional capacitor. The ultra-capacitor employs a porous medium between the two electrodes. In ultra-capacitor the electrodes are made from graphite carbon in the form of activated conductive carbon, carbon nanotubes or carbon gels. The porous material passes the positive ions and blocks the electrons. The porous material and electrodes are saturated with the liquid electrolyte – aluminum.

The electrodes and porous material are very thin, but when coiled up together then their capacitance value goes up to hundreds of farads. In order to further increase the capacitance of an ultra-capacitor, we need to increase the contact surface area, A without increasing the capacitors physical size, or use a special type of electrolyte to increase the available positive ions to increase conductivity.

The typical capacitor cell is working on voltage range between 2 to 3 volt dependent upon the electrolyte used.  If we want to increase the cell terminal voltage of ultra-capacitor then cell must be connected in series and to increase capacitance value cell must be in parallel.

Charging/Discharging

At the time of charging/discharging of ultra-capacitor no chemical reaction is happening, so the charging/discharging of the ultra-capacitors can occur at the same rate. A number of methods are possible for charging of the ultra-capacitors. This may be either through constant current or constant power charging via a dc source or through ac charging methods. Ultra-capacitor is capable to take millions of charge/discharge cycle.

Cell Balancing

For most of the applications we need to connect the cells in series and parallel combinations. In the manufactured cells there are several factors like  tolerance in capacitance, resistance and leakage current that will imbalance the cell voltages of series stack. It is important to ensure that the individual cell voltage do not exceed with its maximum limit as this could result in electrolyte decomposition, ESR (Equivalent Series Resistance) increase and ultimately reduced life.

In the cell balancing the most important factor is cell capacitance between two cells and leakage current. The proper balancing can eliminate the imbalance. There are two types of balancing technique

  1. Passive Balancing – To dominate the total cell leakage current, we need to place the resistor in parallel with each cell. This will reduce the variation in of equivalent parallel resistance between the cells which is responsible for the leakage current.
  2. Active Balancing – If the energy sources are limited then the active balancing is preferred and it draws less current in steady state and requires large amount of current in case of cells imbalance.

Temperature Effect on Ultra-capacitor

The main advantage of ultra-capacitor is its wide temperature range. The typical temperature range of ultra-capacitor is from -40 deg C to 80 deg C. The life of ultra-capacitor reduces when it operates at higher temperature range. The internal resistance will increase reduces when it operates at lower temperature range.

Application

  1. Hybrid Buses Accelerate

Capture the Energy from Regenerative Braking System and release power to help hybrid buses accelerate.

  1. Provide cranking power to START/STOP system
  2. Burst power for lifting application

Advantage

  1. High Power Density
  2. Instant charging
  3. Very long lifetime
  4. Low ESR
  5. Low temperature performance

Disadvantage

  1. Low voltage per cell
  2. Cannot be used in high frequency application.

 

By – Santosh Ghorpade

 

Lead Acid Battery Charger for Lithium-Ion Battery
08 Jul

There are several questions about the use of Lead Acid battery charger for charging Lithium ion battery:

Q: Is it possible to charge the Lithium ion battery with lead acid battery charger?

Q: What is the difference between lead acid and lithium ion battery charger?

Q: Why people insist to use lead acid charger for charging lithium ion battery instead of lithium ion battery charger?

Q: What will be the impact on lithium ion battery if lead acid battery charger is used for charging?

Yes it is possible to use  Lead acid battery charger for charging Lithium ion battery.

The lithium ion battery charger can modulate or regulate the current as per the demand. The lithium ion charger can communicate with BMS and modulate its output as per the BMS commands. But the lead acid battery charger it can not regulate the current and can not communicate with BMS.

The only reason is lithium ion battery chargers are pretty expensive than the LA battery chargers. But if we think about the outcomes, with the dedicated Li-ion battery charger the battery life of Li-ion Battery will get prolonged for sure.

The impact of the Lead Acid charger on the Li ion Batteries

“But the problem is that all the lithium cells do not get self equalized when on top of the charge as the lead acid cells do”.

The lead acid charger for charging lithium ion battery is good up to the 70 to 80% of the SoC but if we used further it can generate the cell imbalance. Because lithium ion cells do not get self equalized when on top of the charge as the lead acid cells do. After several cycles the cells will go out of balance and each progressive charge cycle will get worse cell imbalancing. And such an imbalance definitely impact battery life.

For lithium ion battery charging, balancing is the biggest worry. So ideally the charger should operate in CC mode so that the battery can get charged rapidly

And when it reaches to top of the charge or near to cell target voltage the charger should operate in the CV mode to make sure that the battery can achieve the balance.

But if anyone insist to use the Lead Acid battery charger for Li ion batteries and without compromising on balancing of the cells.

That can be made possible by using the charger with controlled voltage output but this will take very long time to get the battery fully charged.

The only reason is lithium ion battery chargers are pretty expensive than the LA battery chargers. But if we think about the outcomes, with the dedicated Li-ion battery charger the battery life of Li-ion Battery will get prolonged for sure.

Conclusion

We can not go for regular use of lead acid battery charger to charge the Lithium ion battery. We can use but only if the dedicated Lithium ion charger is really not available and this is also with controlled voltage manner so that the balancing can be ensured.

By – Janardan Chavan

 

Logic Optimization
07 Jul

Do you think having a cup of tea at 5th min depends really on good hands working for it?  In reality, all it depends on predefined steps followed up for making it testier. The same phrase is applicable for electronics industry as well. Building embedded logic for a bit complex system is not very difficult task to proceed but the main challenge here is to do the same with least effort in logic, So as to make it more robust with its functionality in all its test cases. While designing any c logic for a complex system, one should think a more on the concept by all mean so as to understand small bottlenecks that may arise in later phase of time.

Steps for proceeding any logic:

Ø  Understand all requirements for the system from customer, suggest them if any improvement can be done which you think of more importance to share and come out with proper list of requirement with which embedded developer should also be well aware so as to avoid bottleneck arising in later phase of time.

Ø  Designing a top level algorithm for flow of source code logic along with all necessary flowchart.

Ø  Discuss this top level algorithm with all your colleagues or team member and update your algorithm to make it more optimized and then share with your customer so as to be on track.

Ø  Work out with your pseudo code for pre designed algorithm.

Ø  At each and every step of logic in your pseudo code, try executing more suitable and best optimized way of c programming logic fulfilling all your test cases.

Ø  Once you are done all above steps, again have a common discussion with all team members regarding your best optimized and more robust pseudo code logic.

Ø  Here comes the implementation phase of c logic and step by step testing of c logic after uniform interval of time throughout your logic building process.

Conclusion:

  • Thinking more on the concept before you implement it is always better to make it more versatile. Before implementing any of the logic, one should be get discussed among your team so as to find if further improvement is required in your logic or not.

References:

 

By – Pavan Jadhav