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How to rebuild a Li-Ion battery pack

Implemented for Fujitsu - Siemens Lifebook S-Series FPCBP25 battery pack
source: Electronics-Lab.com


Warning
If you are not completely sure what you are doing, don't apply what is described below. In any case www.electronics-lab.com does not have any responsibility in any way to the consequences that this guide may have. Read it at your own risk. Please don't continue reading if you don't agree with the above.
 

 

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   Did you recently notice poor performance of your notebook Li-Ion battery?. Don't be taken aback, this is happening even to the best battery! Now days Li-Ion batteries are widely used in portable devices due to there excellent energy to weight ratio and for the reason they are not suffering from "memory effect".

   These two reasons make them the best choice on portable devices, but not suffering from "memory effect" doesn't mean it will last for ever! Chemical conversions inside the battery make it to produce electric energy but these chemical reactions aim to attenuate as time and charge cycles pass over.

   When the battery life drops significantly it's time to search for a replacement, but you will recently find out that most of notebook batteries cost almost 1/3 to 1/2 the price of the laptop at the time you will need to replace the battery. So if you want to keep your laptop but don't want to spend much money for battery replacement it's time to think about rebuild it your own, replacing each individual cell inside the pack.

But before step on this you must consider many parameters messing around Li-Ion batteries, about the way they are charged and the way you must handle them.

 

Special precautions must be taken to avoid Li-Ion battery fire up or explosion that can cause serious injuries. This is because Lithium when comes in contact with water or air's moisture burns violently. Never try to extinguish such a fire with water! but only with suitable extinguisher.

  In this article we will discuss how to handle Li-Ion batteries to avoid any malfunction, the precautions you must take, the way Li-Ion batteries are charged, the protection circuits used and finally you can find a step by step guide on how to reconstruct a Fujitsu - Siemens Lifebook S-Series FPCBP25 battery pack. This guide can also be read as a tutorial on how to rebuild other kind of Li-Ion battery packs, except the one we will use here.

It's recommended to read the following details in order to understand how a Li-Ion battery must be handled to avoid any injury, before proceed to the reconstruction of the pack.

 


Overview

Li-Ion (and Li-Po) batteries are leading edge battery technology and consists ideal selection in use on portable computers and cellular phones due to their high energy density and high voltage. Laptop computers were some of the first products to take advantage of the technology, quickly jumping on board shortly after seeing the benefits to Li-Ion's long lasting battery life. A typical Li-Ion cell is rated at 3,6V and this is three times more than the typical NiCd or NiMH cell voltage (1,2V).

 

Structure

Li-Ion cell has a tree layer structure. A positive electrode plate (made with Lithium Cobalt oxide - cathode), a negative electrode plate (made with specialty carbon - anode) and a separator layer.

Inside the battery also exists a electrolyte which is a lithium salt in an organic solvent.

 Li-Ion is also equipped with a variety of safety measures and protective electronics and/or fuses to prevent reverse polarity, over voltage and over heating and also have a pressure release valve and a safety vent to prevent battery from burst.

Never short circuit, reverse polarity, disassemble, damage or heat over 100 degrees Celsius a Li-Ion cell. That can be really dangerous.

 

Working Principle

 


 

Lithium battery uses lithium cobalt oxide as positive electrode - cathode - and a high crystallized special carbon as negative electrode - anode.

Also an organic solvent specialized to be used with the specific carbon works like electrolytic fluid.

The chemical reaction that takes place inside the battery is as follows, during charge and discharge operation:

The main principle behind the chemical reaction is one where lithium in positive electrode material is ionized during charge and moves from layer to layer in the negative electrode (as illustrated to the left image).

During discharge Li ions move to the positive electrode where embodies the original compound.


Features of lithium Ion batteries

High energy density that reaches 400 Wh/L (volumetric energy density) or 160Wh/Kg (mass energy density).
High voltage. Nominal voltage of 3,6V or even 3,7V on newer Li-Ion batteries.
No memory effect. Can be charged any time, but they are not as durable as NiMH and NiCd batteries.
High charge currents (0,5-1A) that lead to small charging times (around 2-4 hours).
Flat discharge voltage allowing the device to stable power throughout the discharge period.
Typical charging Voltage 4,2 0,05V.
Charging method: constant current - constant voltage (CV-CC).
Typical operation voltage 2,8V to 4,2V
Recommended temperature range 0-40


Charging Characteristics

Charging method is constant current - constant voltage (CV-CC). This means charging with constant current until the 4.2V are reached by the cell (or 4,2V x the number of cells connected in series) and continuing with constant voltage until the current drops to zero. The charge time depends on the charge level of the battery and varies from 2-4 hours for full charge. Also Li-Ion cannot fast charge as this will increase their temperature above limits. Charging time increases at lower temperatures.


Typical charging characteristic

Charge current is recommended to be set at 0,7CmA (where C is battery capacity). If voltage is below 2,9V per cell it's recommended to charge at 0,1CmA. Charging environment must have a temperature between 0-40. Maximum discharge current must not exceeds 1.0CmA and discharge voltage must not go below 3,0V

 

Capacity

At a typical 100% charge level at 25, Li-ion batteries irreversibly lose approximately 20% capacity per year from the time they are manufactured, even when unused. (6% at 0, 20% at 25, 35% at 40). When stored at 40% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 respectively. Every deep discharge cycle decreases their capacity also.


Typical capacity characteristic over charge cycles

100 cycles leave the battery with about 75% to 85% of the original capacity. When used in notebook computers or cellular phones, this rate of deterioration means that after three to five years the battery will have capacities too low to be still usable.

 

Tip: To increase battery life store it at 40% level at low temperatures (even to the refrigerator but not below 0 degrees Celsius) and never discharge it full. Charge it early and often. Excess heat can damage the battery. Also charge once a year to prevent overdischarge.

 

Self discharge

One great advantage of Li-Ion batteries is their low self-discharge rate of only approximately 5% per month, compared with over 30% per month and 20% per month in nickel metal hydride batteries and nickel cadmium batteries respectively.

 

 Chemistry Type Ni-Cd Ni-MH Lead acid Li-ion Cylindrical Li-ion Prismatic Li-Po
 Nominal Voltage (V)
1.2
1.2
2,1 3.6
3.6 / 3.7
3.6
 Specific Energy (Wh/Kg)
50
70
30 80
100-160
140
 Specific Energy (Wh/L)
150
200
- -
250-360
-
 Cycle Life (Times)
500
560
-
1000
1000
-
 Environmental hazard low medium medium high high high
 Safety High High medium low low low
 Cost low medium low high high high
 Self-Discharge Rate (%/month)
25-30
30-35
-

6-9

6-9

-
 Memory Effect
yes
yes
yes
no
no
no

Comparison table of the most common batteries types

Precautions

Be sure to follow the safety rules listed below (PANASONIC recommendations):

Do not place the battery in fire or heat the battery.
Do not install the battery backwards so that the polarity is reversed.
Do not connect the positive terminal and the negative terminal of the battery to each other with any metal object.
Do not carry or store the batteries together with necklaces, hairpins, or other metal objects.
Do not pierce the battery with nails, strike the battery with a hammer, step on the battery, or otherwise
subject it to strong impacts or shocks.
Do not solder directly onto the battery.
Do not expose the battery to water or salt water, or allow the battery to get wet.
  Do not disassemble or modify the battery. The battery contains safety and protection devices which, if damaged, may cause the battery to generate heat, rupture or ignite.
  Do not place the battery on or near fires, stoves, or other high-temperature locations. Do not place the battery in direct sunshine, or use or store the battery inside cars in hot weather. Doing so may cause the battery to generate heat, rupture, or ignite. Using the battery in this manner may also result in a loss of performance and a shortened life expectancy

Use common sense precautions. Do not short circuit, overcharge, crush, mutilate, nail penetrate, incinerate, reverse polarity, heat above 100 degrees Celsius, solder directly on the metal can. Dispose them following local batteries disposal rules.

 

Safety circuits inside a Li-Ion battery pack

Inside a Li-Ion pack there is always a safety circuit that consists of four main sections:

1. The controller IC that monitors each cell (or parallel cells) voltage and prevents the cells to overcharge or overdischarge controlling accordingly the cutoff switches. Also the voltage across the switches is monitors in order to prevent  over current.
2. The control switches that usually comprises FET structures that cutoff the charge or discharge depending on the control signals of the controller IC.
3. The temperature fuse that cutoff the current if the control switches experience abnormal heating. This fuse is not recoverable.
4. The thermistor (usually PTC) that measure the battery temperature inside the pack. It's terminals are connected to the charger so it can sense the temperature of the pack and control the charge current until the battery it's full charged.


A typical structure of Li-Ion battery pack (block diagramm)
 

Battery packs made from Li-Ion cells always have protective circuits and PTC elements to monitor battery status any time. Never remove this circuitry as this will cause ignition.

 

Now you have read and understand all the above information you see that charging a Li-Ion pack can't be done with simple charge methods used in other type of batteries. So never charge a Li-Ion cell if you are not completely sure what you do.

Now it's time to continue to the step by step guide to see how we can safely reconstruct a Li-Ion pack.

   

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