#Batteries for mobile devices
Batteries for mobile devices and portable devices. Condition assessment.
VASILIEV Vladimir Yuryevich
It is no secret that the trouble-free operation of mobile devices or devices with backup (or uninterruptible) power supply directly depends on the condition of their batteries. In this regard, the correct and timely determination of the actual characteristics of batteries during operation is of particular importance.
The article examines the main parameters of the battery, by which its condition can be assessed, as well as the methods and devices that allow such an assessment to be made.
When writing the article, materials were used kindly provided by Mr. Isidor Buchmann, the founder and head of the Canadian company Cadex Electronics Inc.
A detailed article about batteries can be read on the following page: Batteries. History, technology, reality.
As an electrical device, a battery is characterized by the following main parameters: the type of electrochemical system, voltage, electrical capacity, internal resistance, self-discharge current and service life.
Moreover, depending on the area of application, some parameters or others come to the fore.
For example, a cell phone battery should be assessed based on the totality of its three main characteristics: real capacity, internal resistance, and self-discharge current, while a home radiotelephone battery can be assessed based only on its capacity and self-discharge.
If you underestimate or ignore any parameter or exaggerate the importance of one of them (usually capacity), you may end up in a «nothing but a broken trough» situation.
Let's briefly dwell on the main parameters of the battery.
Electrochemical system.
The following types of batteries are currently most widely used to power mobile devices and equipment: lead-acid (SLA), nickel-cadmium (NiCd), nickel-metal hydride (NiMH) and lithium-ion (Li-ion).
Lithium-polymer (Li-Pol) batteries are beginning to appear.
Each of them has certain advantages and disadvantages.
Comparative characteristics of batteries depending on the type of electrochemical system are given in [1].
Voltage.
Battery voltage is determined by the device for which it is intended to power.
If the required voltage value is not provided by one element, the battery is assembled from several elements connected in series.
The nominal voltage of a single element of NiCd or NiMH batteries is 1.2 V, SLA – 2 V, Li-ion – 3.6 V.
Electrical capacity.
The nominal electrical capacity is the amount of energy that the battery should theoretically have in a charged state.
The amount of energy is determined by discharging the battery with a constant current during the measured period of time until the specified threshold voltage is reached.
Capacity is measured in ampere-hours (A h) or milliampere-hours (mA h).
Its value is indicated on the battery label or encrypted in the designation of its type.
The actual capacity of a new battery at the time of its commissioning ranges from 80 to 110% of the nominal value and depends on: the manufacturer, storage conditions and period, and the technology of commissioning.
Theoretically, a battery, for example, with a nominal capacity of 1000 mA h can deliver a current of 2000 mA for 30 minutes, 1000 mA for one hour, 100 mA for 10 hours, or 10 mA for 100 hours.
In practice, at high discharge current values, the nominal capacity is never reached, and at low currents it is exceeded.
During operation, the battery capacity decreases.
The rate of decrease depends on the type of electrochemical system, maintenance technology during operation, chargers used, conditions and service life.
Internal resistance.
The internal resistance of the battery (the resistance of the current source) determines its ability to deliver a large current to the load.
With a low value of internal resistance, the battery is able to deliver a higher peak current to the load (without a significant decrease in the voltage at its terminals), and therefore a higher peak power.
The relationship between the values of the load current, the voltage at the battery terminals and its internal resistance is determined by Ohm's law.
A high resistance value leads to a sharp decrease in battery voltage with a sharp increase in load current.
Such a collapse (decrease) in voltage characterizes the “weakness” of an apparently good battery, because the stored energy cannot be fully supplied to the load.
The internal resistance of the battery depends on the type of its electrochemical system, capacity, the number of elements in the battery connected in series, and increases towards the end of its service life.
Self-discharge.
The phenomenon of self-discharge is characteristic to a greater or lesser extent for all types of batteries and consists of their loss of capacity after they have been fully charged.
For a quantitative assessment of self-discharge, it is convenient to use the value of the capacity lost over a certain time, expressed as a percentage of the value obtained immediately after charging.
As a rule, a time interval equal to one day and one month is taken as a time period.
For example, for serviceable NiCd batteries, self-discharge of up to 10% is considered acceptable during the first 24 hours after completion of charging, for NiMH — a little more, and for Li-ion it is negligible and is estimated per month.
It should be noted that self-discharge of batteries is maximum in the first 24 hours after charging, and then significantly decreases.
Self-discharge of batteries depends on the quality of the materials used, the manufacturing process, the type and design of the battery.
It increases sharply with increasing ambient temperature, damage to the internal separator of the battery due to improper maintenance and as a result of the aging process.
Service life (operating life) of the battery.
It is usually estimated by the number of charge/discharge cycles that the battery can withstand during operation without significant deterioration of its main parameters: capacity, self-discharge and internal resistance.
The service life depends on many factors: charging methods, depth of discharge, maintenance procedure or lack thereof, temperature and electrochemical nature of the battery.
In addition, it is determined by the time elapsed since the date of manufacture, especially for Li-ion batteries.
A battery is usually considered to be out of order after its capacity has decreased to 60 — 80% of the nominal value.
Assessing the condition of batteries
Let's imagine several different situations.
Situation 1.
An organization operates a large fleet of various mobile communication devices, on the reliable operation of which not only its successful activity, but sometimes even people's lives depend. Communication failures in such organizations are catastrophic failures.
To prevent them, it is necessary to periodically monitor the condition of all used batteries in order to promptly remove from service batteries that appear to be in good working order, but in fact do not provide the required reliability, and replace them with new ones.
Situation 2.
It is necessary to evaluate the parameters of batteries from different manufacturers and choose the best one. Or perform an incoming inspection of the purchased batch of batteries in order to check for compliance with the delivery conditions.
Situation 3.
Development and production of new types of batteries.
What do all three of the above situations have in common?
Of course, it is the need for a reliable and timely assessment of the state of batteries by checking the values of their main parameters.
How can you analyze the parameters of batteries?
With what devices? What should such a device be able to do, and what characteristics should it have?
To successfully organize a battery maintenance system, it is necessary to ensure:
- quick assessment of the battery condition;
- quick charging of the battery without damaging it;
- display of information on the charge level, capacity and internal resistance of the battery;
- simplified signaling of the battery test results for an untrained user;
- simultaneous maintenance of several batteries of different types;
- switching to automatic recovery mode for batteries of certain electrochemical systems if the measured capacity is less than that specified by the user;
- detection of short-circuited cells in batteries;
- detection of «soft» cells in batteries (accelerated voltage growth during charging);
- battery maintenance in various modes selected by the user;
- standard interface with a computer and printer;
- standby battery charging mode;
- protection from unauthorized access.
Obviously, these tasks cannot be solved using conventional charging and discharging devices and require the use of special devices.
There are two possible approaches: the first is the development and manufacture of specialized devices, usually designed for specific requirements, and the second is the use of universal battery analyzers.
Naturally, the scope of application of the latter is much wider, so we will focus on them.
Table 1 provides comparative characteristics of the most common types of analyzers.
Here is a photo..
Table 1. Comparative characteristics of some common analyzers.
Let's take a closer look at some of the main characteristics and functionality of the above analyzers, and assess their importance and necessity for the consumer.
Number of simultaneously serviced batteries.
This parameter directly affects the speed of battery servicing if it is necessary to assess the condition of a large batch of identical or different types of batteries.
Average service time.
Battery service time depends on many factors: the type and parameters of the service program, the battery capacity, its condition.
The table shows average values.
Actual values may differ from those indicated by two times, both up and down.
Types of serviced batteries.
The analyzer should support all the most common types of batteries according to the electrochemical system, and also have the ability to upgrade to support newly emerging types of batteries.
For example, at the moment, a new type is lithium-polymer batteries, which are already included in cell phones and laptops.
Battery capacity indication.
There are two options for displaying the final (real) battery capacity — in milliamperes (amperes) and as a percentage of the nominal value.
Moreover, the second option is more informative, since on many types of batteries the capacity value is encrypted in their designation and the indication of the actual capacity of the battery in mA hours does not tell the user anything about its condition if he does not know its nominal value.
Battery adapter.
The most convenient adapter is the one that allows you to connect any battery with identical design connecting elements to the analyzer, regardless of its electrochemical system and capacity.
Connecting with wires is very inconvenient, since many types of batteries have contacts recessed into the case or “flush” with it.
Wires also introduce additional error when measuring the internal resistance of the battery.
User programming.
This parameter may have two possible interpretations.
The first is broad – setting the parameters and program for battery maintenance.
For example, setting the type of electrochemical system, nominal capacity, charge and discharge currents of the battery and its maintenance program: charge, discharge, preparation for operation, restoration.
The second interpretation is literal – programming (creation) of a non-standard battery maintenance program.
Service management.
Using a computer connected to the analyzer provides:
- storing a database of different types of batteries from different manufacturers;
- storing reports and maintaining a database of serviced batteries;
- building graphs of changes in current battery parameters during servicing in real time;
- automatic installation of parameters of service programs based on the battery type designation from the database;
- using a barcode scanner to enter the battery type (original batteries from well-known manufacturers are supplied with a label with a barcode on it, which contains complete information about the battery);
- easy creation of your own maintenance programs based on individual needs and experience.
Saving data in the event of a power failure.
The function is certainly necessary and convenient, allowing to ensure continuous multi-hour battery testing during a short-term power failure.
Error messages.
Information about errors (reasons for failure) of the battery being tested, along with displaying its actual capacity, allows to obtain a more complete picture of the battery condition and predict its further behavior during further operation, if possible.
Display.
The required minimum information simultaneously displayed on the display should include:
- type of electrochemical system of the battery being serviced;
- current battery voltage;
- current current value;
- current internal resistance value;
- time elapsed since battery servicing began;
- current operating mode.
Service programs.
The availability of ready-made service programs for performing typical battery maintenance operations and taking into account the features of their specific types greatly simplifies and speeds up the maintenance process.
In this case, a ready-made service program is understood to be a program that already contains all the main parameters and the sequence of actions for battery maintenance.
The user only needs to select the type of electrochemical system, the nominal capacity of the battery and run the program.
Charging method.
As a rule, each analyzer manufacturer notes that its method is the best.
There are many different methods of charging NiCd or NiMH batteries, which can be divided into three main groups: standard charging — charging with a constant current equal to 1/10 of the nominal capacity of the battery for about 15 hours; fast charging — charging with a constant current equal to 1/3 of the nominal capacity of the battery for about 5 hours and accelerated or delta V charging — charging with an initial charging current equal to the nominal capacity of the battery, in which the voltage on the battery is constantly measured and the charge ends after the battery is fully charged.
Charging time is about 1 hour. And then each manufacturer begins to modify these methods.
In particular, the Cadex 7000 and CASP/2000L (H) analyzers use short discharge pulses between long charging pulses.
This charging method is believed to restore the crystalline structure of cadmium anodes, thereby eliminating the «memory effect».
Control capacity value.
This value is understood as the capacity value at which the battery is considered suitable for further use.
By default, it is equal to 80% of the nominal value of the battery capacity and can be changed by the user.
This value is usually used as a control value in two operations: during the incoming inspection of new batteries and when assessing the need to switch to the recovery cycle when checking NiCd and NiMH batteries.
The ability to set this value by the user is of fundamental importance for automating the battery maintenance process.
In what direction will battery analyzers develop in the near future?
This summer, Cadex announced a new, cheaper model of the Cadex C7200 battery analyzer for independent simultaneous analysis of two batteries.
The new analyzer is notable for its comprehensive battery condition assessment mode in just five minutes.
This feature is provided by the new Quick Test program built into the analyzer, which allows you to accurately determine the battery capacity in 5 minutes.
Together with the OhmTest program, which measures the internal resistance of the battery in 5 seconds, this makes the C7200 an ideal incoming control tool when checking large batches of batteries.
Literature.
1. Vasiliev B.Yu., Petrov N.N. Batteries. History, technology, reality//Special equipment No. 6, 2000