Month: October 2008

How Many Cells Are In A Battery

Question from a reader: “How many cells are in the Toshiba Satellite M105-S3041 Laptop Battery? The battery is for a Toshiba Satellite M105-S3041 and its technical ratings are 10.8 volts 8800 mAh, Lithium Ion

The question from the reader is a great question for it gets to the heart of battery manufacturing and begins to unravel the power of a battery. The answer is simply: 12 cells. But what does the answer mean and how does the number of cells relate to the overall value of a battery? Let’s find out…

What is a Battery and How Does it Create Energy?

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 liquid or solid). This battery consisting of two electrodes is called a voltaic cell.

Therefore batteries in effect create electrochemical energy. In order to convert chemical energy into electrical energy there is a chain of events that have to occur prior to the creation of electrical energy. Key to the creation of electrochemical energy in batteries is that electrical energy is injected into two chemicals in a solution. Electricity is introduced into a battery via a charger. The charger acts as a conduit of the pushing electrons that are forcing their way into the chemical lithium. This charge process involves intercalation: the joining of a molecule (or molecule group) between two other molecules (or groups). Intercalation is the process of ions being pushed by electrical current into solid lithium compounds. Lithium is one of the chemical components used to create electrical energy in batteries. Lithium compounds have minuscule spaces between the crystallized planes for small ions to insert themselves from a force of current. Ionizing lithium loads the crystal planes to the point where they are forced into a current flow. Intercalation replenishes, in effect, lithium but the net result of ionization is the ultimate depletion of the lithium reactive property. You could say if you use it you will lose it!

But what makes lithium good for batteries is that lithium is one of the metals in the alkali group (the other metals include Sodium, Potassium, Rubidium, Cesium, and Francium). Lithium is a highly reactive metal. Lithium has only one electron in its outer shell (two electrons in its inner shell), which makes it chemically “ready” to lose that one electron in ionic bonding with other elements. Lithium is used as a battery anode material (due to its high electrochemical potential). Electrochemical potential is the sum of the chemical potential and the electrical potential. The higher the electrochemical potential the better the electrical current yields. In some lithium-based cells the electrochemical potential can be five times greater than an equivalent-sized lead-acid cell and three times greater than alkaline batteries. One other core advantage that lithium has is that it is soft and bendable which allows for tight configurations in small cell designs (PDAs. Laptops, Cameras etc…).

What Type of Lithium Cell is Used in Laptops

Now this brings us to battery cells and our original question from the reader. Lithium based battery cells are good but there are a variety of lithium based battery cells. For example the lithium based cell identified as 18650 is one of the most common battery cell on the market for laptops. 18650 is manufactured by many manufacturers including many private branded companies to public companies like LG, Sony, Sanyo, Samsung, Panasonic.

18650 is a 3.6V cylindrical Li-Ion cell. 18650 has no memory effect (distinguish between digital memory effect) and longer storage life than NiMH battery cells. 18650 is light weight and has a high energy density. It is in effect perfect for building batteries for laptop and other portable power devices.

The additional technical specifications for the 18650 battery cell include:

  • Nominal Voltage Average 3.7 V – the concept of nominal voltage is that voltage range exists depending on the number of cells in the battery.
  • Nominal Capacity – 2200 mAh (above 2200 the stability of lithium based cells is called into question)
  • • Max. Charging Current – 2.4 Amps Max.
  • • Max. Discharging Current – 4.6 Amps max.
  • • Dimensions (DxH)  18.3 mm (Max 18.4) x 64.9 mm (Max 65.1)
  • • Weight   46.5 g (1.64 oz) 
  • Internal Impedance  Internal Impedance: less or equal to 90 ohms 
  • Cycle Performance is 80% of initial capacity at 300 cycles

Now as stated above the reader asked how many cells were in the Toshiba Satellite M105-S3041 Laptop Battery? The battery is rated at 10.8 volts and a capacity of 8800 mAh.

As I mentioned above the nominal voltage average is 3.7V. Some manufacturers may use 3.6V and some may use 3.7V. In the case of a laptop battery with 10.8V the nominal voltage rating used is 3.6V. Thus if we divide 10.8V by 3.6V we get 3. Thus 3 cells in a series. We also know that the batteries capacity is 8800 mAh. We know the nominal capacity is 2200 mAh. Therefore if we divide 8800 mAh by 2200 mAh then we get 4 in parallel.

Therefore we have 3 cells in series X 4 cells in parallel equals 12 cells in total.

Until next time Dan Hagopian www.batteryship.com
Copyright © BatteryEducation.com. All rights reserved.

Voltage Failure Modes

Question from a reader: "Hi Dan, What is the failure mode for Lithium Polymer batteries? I understand that Nickel Cadmium batteries fail as a short circuit usually. Therefore, a pack of NiCads will continue to operate with a shorted cell but with a voltage that is one cell lower. Do LiPo's fail…resulting in the loss of the entire pack? Thanks

Answer: 

First of all the reference to a "failure mode" is not necessarily a singular event. It is in effect a catch-all phrase representing potential problems. I wrote an article series on the concept of Battery Failure Mode and Effects Analysis. But this concept is not new but is manufacturing quality assurance analysis, in essence, be applied to every industry.

The article series can be accessed at:

Part 1
http://www.batteryeducation.com/2008/07/battery-failure.html
Part 2
http://www.batteryeducation.com/2008/07/battery-failu-1.html
Part 3
http://www.batteryeducation.com/2008/07/battery-failu-2.html

Specific to your question on lithium polymer batteries there could be a number of "failure mode(s)" and sub-failure modes related to the manufacturing and personal usage of a battery including Li-Po chemistry based cells. For example:

  • Batteries can have faulty cell design
  • Batteries can be manufactured under uncontrolled processes
  • Batteries can be operated in uncontrolled conditions
  • Batteries can be abused
  • Batteries can degrade and lose power

For more information on these examples please see:
http://www.batteryeducation.com/2006/06/what_causes_bat.html

Heat and water for example are not good for any battery. But it is surprising to me how many people use their battery and device in both and of course that represents several failure modes. An example is when one of our customers several years ago always took his iPod mini into the sauna at his gym. Well that is not the best place for a battery or device. Eventually his battery died and so did the device. That is a bad failure mode brought about be a user. Another example is when there are metallic electrode shavings evident in the batteries electrolyte. When current is running from one electrode to another through the electrolyte the shavings cause any battery to short circuit. The short circuiting could result in the battery smoking, bubbling or stopping. That is an example of bad manufacturing and of course a failure effect. Another example of a failure mode is when you open the battery casing. If you open the casing once it is sealed hermeneutically then you run the big risk of causing a failure mode to occur.

In any event – any failure mode will cause a battery to either operate at a lower performance than originally designed or stop working period. One interesting side note is that devices can operate with a battery that is at a lower voltage (but not higher voltage) as the device originally manufactured. The reason is because a battery's voltage is not necessarily a static measurement.

Volts – or V – are an electrical measure of energy potential. Voltage can also be thought of as the amount of "pressure" of electrons that pass from a negative connector to a positive connector. Or V can be defined as the measure of the strength of an electrical source of power for a given current level.

Voltage can also be defined as the Electrical Potential difference – a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. In the fields of electronics the electrical potential difference is the amount of work per charge needed to move electric charge from the second point to the first, or equivalently, the amount of work that unit charge flowing from the first point to the second can perform.

Mathematically voltage is commonly measured by V= I x R; where V=Voltage, I=Current, R=Resistance.

Beyond the definition what challenges many is the confusion that a battery contains four unique types of voltage measurements.
Each voltage measurement type residing in a battery effects battery life.

  • Float Voltage – is battery voltage at zero current (with battery disconnected).
  • Nominal Voltage – is battery voltage range 3.7V, 5.2V, 10.2V, 12V etc that says that a voltage range exists depending on the number of cells in the battery. For example a 12 Volt battery is made of 6 cells and has a Float voltage of about 12V.
  • Charge Voltage – The voltage of a battery while charging.
  • Discharge Voltage – The voltage of a battery while discharging. Again, this voltage is determined by the charge state and the current flowing in the battery. 

So in effect it is possible that a device will operate on a battery at lower voltage. I hope this answers your question.

Until next time Dan Hagopian www.batteryship.com
Copyright © BatteryEducation.com. All rights reserved.