How Many Times Can I Charge My Battery?

500 million lithium batteries are in use today. A very big number indeed and the chances that you are one of them are quite high. You could have a laptop, PDA, MP3 or even a cell-phone, all of which more likely than not has a lithium ion or a lithium polymer chemical based battery system. If so then one question that you will have eventually is how many times will I be able to charge the battery before it is effectively dead? Is it 300 times, 400 times, or 500 times? The answer is between 300-500 times.

500 million lithium batteries are in use today. A very big number indeed and the chances that you are one of them are quite high. You could have a laptop, PDA, MP3 or even a cell-phone, all of which more likely than not has a lithium ion or a lithium polymer chemical based battery system. If so then one question that you will have eventually is how many times will I be able to charge the battery before it is effectively dead?  Is it 300 times, 400 times, or 500 times? The answer is between 300-500 times.

But what does that answer mean? As this article will explain the charge cycle is quite complex and involves the replenishment of electrons. In order to get a beginning understanding of what actually is taking place during a charge and discharge cycle we need to understand: what a battery is, how it works, what it produces, and finally what happens when you charge and discharge.

What is a Battery?

As I have written in other articles a battery is a device that converts chemical energy into electrical energy. Batteries have two electrodes, an anode (the negative end) and a cathode (the positive end). Collectively the anode and the cathode are called the electrodes. What is positve and what is the negative terminal? It would be great to simply say that the anode is negative and the cathode is positive, however, that is not always the case. Somtimes the opposite is true depending on battery technology. In between the battery’s two electrodes runs an electrical current caused primarily from a voltage differential between the anode and cathode. The voltage runs through a chemical called an electrolyte (which can be either be in a liquid, solid, or gel state). This battery consisting of two electrodes is called a voltaic cell. Most batteries today are advance forms of the voltaic cells and have additional technology packed into the battery casing to support the overall system and its connected  device. These controls include the connector, fuse, charge and discharge FETs, the cell pack, the sense resistor (RSENSE), the primary and secondary protection ICs, the fuel-gauge IC thermistor, pc board, and the EEPROM or firmware for the fuel-gauge IC.

How Does a Battery Work and What Does It Produce?

We know that the result of a battery converting chemical energy into electrical energy allows us to turn on our laptop, PDA, MP3 or even a cell-phone. But how does the conversion process take place? As stated above the batteries we use today are variable changes of the voltaic pile. In addition to the controls I listed above today’s batteries are made up of plates of reactive chemicals (Li-ion, Li-po, NIMH, NICD) separated by an electrolyte barrier (which can be either be in a liquid, solid, or gel state), and subsequently polarized so all the electrons gather on one side. The system was designed to separate both positive and negative electrons. Then after separation an electron exchange occurs and a current of electron flow moves electrons to and from the anode and cathode. Simultaneously an electrochemical reaction takes place inside the battery to replenish the electrons. The effect is a chemical process that creates electrochemical energy.

Now the electrochemical reaction that is taking place is a chemical change that is necessary in order to create electricity. One factor that needs to be understood is that electricity is the flow of electrons. Specifically, electricity is a property of subatomic particles which couples to electromagnetic fields and causes attractive and repulsive forces between them. This repulsive force between the subatomic particles creates an electric current; the flow of electric charge transports energy from one atom to another. This electrical current is measured in amperes, where 1 ampere is the flow of 62,000,000,000,000,000,000 electrons per second!

Electricity therefore is a created energy source. All electricity in fact is a created source made or converted from coal, natural gas, oil, nuclear power, wind, heat, sun, water, biomass and or other chemicals. In batteries today electricity is created by two chemicals in a solution for example: {a Solution of Lithium hexaflourophosphate (LiPF6) – a mixture of Organic Solvents: [Ethylene Carbonate (EC) + DiEthyl Carbonate (DMC) + DiEthyl Carbonate (DEC) + Ethyl Acetate (EA)]}

Charge cycling a battery means to completely discharge (or drain) a battery’s created electricity to where there is a charge of less than a 1% capacity remaining. At this point the power to the device will cease and your device will power off. Then after the power is off you recharge the battery to 100% capacity using a power adapter either from a wall socket for example. Regardless of how you charge the battery that process of discharging and charging represents one complete charge cycle.

I noted above that an electrochemical reaction takes place inside the battery to replenish the electrons. The effect is a chemical process that creates electrical energy (electrochemical energy). Lithium is used, amongst other chemicals, as a battery anode material due to its high electrochemical potential. In fact the energy of some lithium-based cells can be five times greater than an equivalent-sized lead-acid cell and three times greater than alkaline batteries. Lithium cells often have a starting voltage of 3.0 V. This means that batteries can be lighter in weight, have lower per-use costs, and have higher and more stable voltage profiles.

Charging lithium can be thought of as the introduction of ions or movement of chemistry. To move the lithium chemistry (lithium-ion, lithium polymer, lithium iron phosphate, etc) you have to have a minimum voltage applied to the lithium. Most battery cells are charged to 4.2 volts with relative safe workings at about 3.8 volts. Anything less than 3.3 volts will not be enough to charge or move the chemistry. One thing to note here is that volts are an algorithmic measurement of current. So in a sense to create current through your battery you have to introduce current into your battery’s lithium .

Introducing current into your lithium is called intercalation. Intercalation is the joining of a molecule (or molecule group) between two other molecules (or groups). When it comes to charging your battery you are in effect pushing ions in and out of solid lithium compounds. These compounds have minuscule spaces between the crystallized planes for small ions, such as lithium, to insert themselves from a force of current. In effect ionizing the lithium loads the crystal planes to the point where they are forced into a current flow. The current flow is then channeled back and forth from anode to cathode and thereby creating an electrical flow to power on your device. Again this can done 300-500 times. In my next article we will look at why batteries have limited charge cycles.

Until next time Dan Hagopian www.batteryship.com

Lithium Ion Batteries Are Sensitive to Heat

Over 15 million students are enrolled in fall college classes across the United States according to a US Census Bureau study in 2004. Using that number as a base it can be projected that the fall of 2008 should see a slight up-tick in college enrollees. Interestingly the number of college students that are going to college with laptop computers have increased by 28% compared to 42% of college students in 2004. This means that nearly 70% of enrolled students are using laptop computers. In real numbers that represents 10,500,000 laptop computers.

Now listen up college students – your laptop battery is more than likely a Li-ion battery and if it is then there is a natural tendency to keep your laptop plugged into a wall outlet when you are close to one. You may also find that you are actually “plugged” in to a wall outlet more than you are not and there in lies a problem. When your laptop is plugged into a wall outlet your battery heats up big time and heat and lithium do not mix well together.

Hold on! You have to charge battery. Yes that is true, but you do not have to keep your laptop plugged into a wall outlet for the entire school year! But won’t that reduce my battery life if I’m constantly powered from the battery?

Your battery will diminish in capacity – the ability to charge and power your laptop. That is a fact and a natural consequence of batteries today. This diminishing power performance is called battery degradation and power loss. I have written on this topic before and you can read about it on my blog but on a high level a battery over time degrades and eventually stops working, this is no surprise, and it occurs due to the following technical processes: declining capacity, increasing internal resistance, elevated self-discharge, premature voltage cut-off on discharge.

So should you constantly keep your battery charged at 100% capacity? No you should not. Why? To answer that question let’s look at what is occurring when you charge a battery. When charging your battery you are forcing electrical current into a battery cell from a charger. The force of electrical current causes temperature increases.

Now it is true that contained within your laptop battery are integrated power management circuits that are designed to protect against over-voltage and under-voltage conditions that increase heat in the battery but one factor of how well a battery is being protected during a charge depends on the ratio of the heating rate versus the dissipation rate. If the heating rate is higher then the dissipation rate then thermal runaway will occur (leaking, smoking, gas venting, flames).

Now don’t go into panic mode since the integrated circuits are really good at keeping the heating rate lower than the dissipation rate and you are in extremely minimal danger of thermal runaway occurring.  But the practice of keeping your battery charged continuously can negatively affect your battery’s longevity. So charge your battery and then run your laptop on battery power until you have to charge it again.

Until next time Dan Hagopian www.batteryship.com

Digital Memory Effect on Batteries

Have you ever wished that you had an extra 20 minutes of battery life left in your portable device? How about an hour! A real difference exists between the life of your battery and the displayed battery charge meter on your device? How big of a difference? How often does it occur? Why does it occur? What can be done? In this article we will look at these questions and learn about what you can do to reduce power waste and maximize your battery life.

Help my batteries dead and I can’t power on!  The cultural expression of a “dead battery” is the habitual practice that occurs in place of the more technically appropriate reason of declining capacity. Declining capacity is when the amount of charge a battery can hold gradually decreases due to usage, aging, and with some chemistry, lack of maintenance. Declining capacity is inherent in the ultimate design of a battery – due to limitations with technology -  you could consider it the natural side effect or wear and tear of the battery (other wear and tear aspects includes increasing internal resistance, elevated self-discharge, and premature voltage cut-off on discharge).

But there is a real problem with declining capacity and that is the capacity that is measured by your device and displayed to you on your battery charge meter is not always correct. Your device could be reading a digital imprint instead of the actual hardware that transmits capacity back to the device. The digital imprint (digital memory effect) causes your device to use the incorrect reading as its base measuring capacity. This action results in forcing a premature voltage cut-off on discharge, which is when a device does not fully utilize the low-end voltage spectrum leaving unused power in the battery. Another fancy word for leaving unused power in your battery is “waste”. Let’s find out what can you can do reduce power waste and maximize battery life by looking at:

• What is the Digital Memory Effect?
• What Can Be Done To Correct the Digital Memory Effect?

What is the Digital Memory Effect?

The digital memory effect is a failure mode (see my article series on Battery Failure Mode and Effects Analysis) whose effect results in the transmission of improper calibrations of the battery’s fuel gauge to a device.

Now let’s unpack that answer to discover its real meaning.

First we must distinguish between memory effect and digital memory effect. A memory effect is the concept that was derived from cyclic memory. Cyclic memory is the thought that a battery could “remember” how much energy was used up on previous discharges. Cyclic memory only affects nickel-cadmium batteries.  Since we are strictly focused on lithium ion and lithium polymer chemistries I don’t want to get into the chemical change that occurs at the molecular level (crystal growth and concealment of active electrolyte material) but simply will state that the memory effect is the common term people use when there is a voltage depression problem with a battery. Voltage depression causes the inaccurate measurement and subsequent unnecessary charging of a battery.

Inaccurate measurement of capacity is the only similarity between memory effect and digital memory effect since digital memory effect has nothing to do with molecular chemical change. Instead digital memory effect is the improper calibration and reading by the device and the battery’s fuel gauge.

More specifically, inside a battery (or more correctly stated smart battery) is specialized hardware that provides calculated on demand current as well as predicted information to and from the device and includes:

• the connector
• the fuse
• the charge and discharge FETs
• the cell pack
• the sense resistor (RSENSE)
• the primary and secondary protection ICs
• the fuel-gauge IC
• the thermistor
• the pc board
• the EEPROM or firmware for the fuel-gauge IC.

In addition to the above advanced chip components information flows from these components to the device through the System Management Bus (SMBus) control – a two-wire interface through which simple power-related chips can communicate with rest of the system. The SMBus allows a device to transfer manufacturer information, transfers model or part number to and from the device and battery, save its state for a suspend event, report different types of errors, accept control parameters and return its status.

Now with that back drop of information we can address the digital memory effect. As alluded to above the fuel gauge integrated circuitry calculates remaining battery capacity (power) and transmits that calculation to the device operating system through the SMBus connectors. The fuel gauge also stores present cell capacity characteristics and application parameters within the on-chip EEPROM (electrically erasable programmable read only memory). The calculated capacity registers a conservative estimate of the amount of charge that can be removed given the current temperature, discharge rate, stored charge and application parameters. Capacity estimation is then reported in capacity remaining and percentage of full charge to the device.

But sometimes the reported information is not correct. The incorrect report of capacity remaining and percentage is caused by the fuel gauge not recalibrating its circuitry automatically. The digital memory effect is then a false reading for maximum capacity and thus results in lower battery run time.

What Can Be Done To Correct the Digital Memory Effect?

To correct the digital memory effect and properly recalibrate the fuel gauge circuitry simply do a full cycle discharge/recharge every several dozen charges. There is no real hard number. If you have never done a complete discharge then do so now. By performing a complete discharge you will cause a manual reset of the fuel gauge circuitry and will eliminate the digital memory effect.

Until next time – Dan Hagopian www.batteryship.com