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Battery Management

Complex battery design – handy tips before starting out

Most electronic design engineers are reluctant battery designers' principally because most of the design effort falls beyond their comfort zone and requires a specialisation that they simply do not have the time to acquire. VARTA Microbattery has introduced a new service called CellPac Plus which takes care of the entire battery solution starting with the design specifications to a complete battery pack that can be dropped into the OEM's end product on the assembly line.

The major benefit of this approach is that designers can focus all their time and energy on their design and a household name synonomous with quality, reliability and integrity, VARTA, will deliver the battery system

Following are a few hints and tips to guide electronics design engineers in a direction that should make their battery design easier and allow them more time to concentrate on the areas where they are more comfortable to operate.
 
If you take nothing else from this short guide please take away the message that not all batteries are equal and even batteries with ostensibly the same datasheet specification can behave very differently from each other over time. Remember too that a battery is at its heart a chemical system and its performance tomorrow will depend at least to some extent how it is stored, charged and discharged in the lead up to today.
 
Take your battery design seriously and consider the specification early in the design cycle to avoid any problems and get a battery that meets your requirements throughout the lifetime of the product.

What should be done
 
When beginning the design of a complex rechargeable lithium bespoke battery system you should:
 

• Put safety, quality, reliability and long term performance at the top of the list of priorities;
  
• Treat your bespoke battery system as a standalone product design and employ the services of a professional battery company at the earliest point in the design cycle;
  
  
• Consider how the performance of your battery will affect the performance and perceived value of your product;
  
  
• Consider when and by whom the battery can/will be replaced - should the battery be removeable or embedded?
  
  
• Start from a blank sheet of paper and define in detail how the battery should perform and in what circumstances before considering a solution;
  
  
• Define your charging regime and the hardware that will be used to charge the battery in parallel with your battery design;
  
  
• Define your acceptance and test criteria at the outset of the design;
  
  
• Define your certification requirements, such as UL, CE, IEEE, ATEX at the beginning of your design;
  
  
• Consider when, where and how the battery will be delivered to you and to your end customer - consider samples, pre-production and volume.

What should NOT be done

When beginning the design of a complex rechargeable lithium bespoke battery system you should NOT do the following:

• Think that all batteries are equal and that you can leave the battery selection until last;

• Start with an existing battery and try to modify it to suit your application;
  
  
• Go on the internet and find a cell that looks like it will fit the bill and then design the rest of your system around this footprint. Batteries that are in high volume production today are likely to be used in one or more consumer applications and as you know the consumer market moves quickly which can leave you high and dry if things move contrary to your expectations;
  
  
• Wait until the battery is designed and then start thinking about what kind of charger you will use;
  
  
• Treat the battery as just another component.

 
Pitfalls
 
Taking the view that the battery is not important and that it can wait until the hardware and software are defined, designed and built before thinking about a battery solution can be very dangerous. It is wise to consider early in the design cycle what exactly the battery will be expected to deliver and in what circumstances it expected to operate. The chemistry of a battery not only defines its open circuit voltage and capacity but many other parameters as well.

For example a lithium primary coin cell with o/c voltage of 3 V and capacity of 560 mAh will deliver a maximum pulse of 20 mA while a rechargeable lithium cell having o/c voltage of 3.7 V and capacity of 560 mAh may be capable to deliver a pulse in the range of 2 Ah. However if the protection circuit that has been fitted to the rechargeable lithium cell is limiting the current to 1 A then you will not see the 2 A pulse that you expect.

Consider therefore all parameters and under what circumstances before selecting a solution, do not select a solution and then try to fit your application around what you have.

Points to consider

1. System Voltage – this determines how many cells you will require in series in order to achieve your requirement.
   
   
2. Run Time or Capacity – in a typical application the run time and capacity is determined by three key components i.e. the display, micro, and communications. Typically the micro and the display will largely determine the quiescent current required and the communications, WiFi, GSM, GPRS, Bluetooth and such like will determine the pulse load requirements. Of course you have to consider the pulse load profille, duration and duty cycle of the application.
   
   
3. Cut-off Voltage – although every Lithium Ion rechargeable battery will have a protection circuit fitted to protect against over charge and over discharge it is usually necessary to build in a low voltage cut-off into your application to completely switch off the application and protect against driving the battery into deep discharge.
   
   
4. Temperature - all battery chemistries are affected by changes in temperature and it is important to specify at the outset the maximum and minimum temperature range under which the battery will be both charged and discharged as this can have a significant impact on the performance of the battery. If there will be extremes of temperature for example < 0 degrees C or >45 degrees C then it is wise to also consider the durations of exposure to these temperatures as well.
   
   
5. Charging – how and how often will the battery be charged? Consider especially the maximum current available for charging and how much time will also be available. Will you employ a fuel gauge, will you have a smart battery interface and if so what is your preferred chip set? You should also consider that in most cases this will also require that specific parameters be programmed into the chip set and these will all have to be defined and implemented at the outset. Will you employ a smart charger and if so what will be the specification for this?
   
   
6. Storage Conditions – a battery is a chemical system and therefore its performance depends at least to some extent on its history. Therefore we also have to consider the storage conditions of the battery especially in relation to temperature as this will affect the self discharge of the battery however the relative humidity of the environment can also have a detrimental affect on things. You should consider for how long the battery will be in storage and under what conditions i.e. maximum, minimum and average temperature. If the battery will also be on the shelf for a time thereafter then this too should be considered.
   
   
7. Mechanical specification – it is normal for engineers to specify the space envelope available to accommodate the battery but you should also consider that each and every rechargeable lithium ion battery will expand and contract during the charge/discharge cycle and also during the life of the battery.
   
   
8. Whether or not the battery will be removable from the application and by who will also determine what type of packaging is required. If the battery will be embedded in the application and never be removed or only removed by a technician then it is likely that a softpack solution where the battery is packaged in shrink-sleeve will be the best solution. In this case you have to define the length of wire required together with the connector type to suit your mechanical and electrical specifications together with the pin assignment and specific wrapping instructions that are required will also need to be defined.
   
   
9. In the event that the battery will be replaceable by the end user then some form of plastic housing will be required. This can take several forms either it is a simple plastic housing, a complex plastic housing or if cost is more important then maybe a plastic frame with a label of a metal/plastic combination may offer a more economic solution. In any event it will be necessary first of all provide a sketch of your requirements, followed up with 3D drawings and you will need to specify a contact method for the battery that is in line with your mechanical and electrical specification. There are several options the most common of which are to mount the connector directly on the battery electronics or to use contacts or to have a connector mounted mechanically in some other way.
   
   
10. Certification – there are many certification requirements to consider and what you require will be determined by the application and market segment in which you will participate. First of all there are two levels of UL certifiication UL1642 applies to the cell and UL2054 applies to the battery itself. In addition for the battery there is potential for requiring a CE mark, TuV certification, RoHs, CSA and others. All of these need to be known and defined at the outset as they will affect the selection of components and the design of the battery.
    
    
11. UNIATA – all rechargeable lithium batteries require a UNIATA certificate in order that they can be shipped by air. Without this certificate transportation is difficult if not impossible and without this certificate you will have difficulty to ship your product to your end customers. This certificate is specific to each and every battery and in the event that any changes are made to the battery then irrespective of how minor these changes may be, it could be necessary to re-certify the battery again.

Related links and articles:

www.cellpacplus.com

VARTA Microbattery launches battery system integration for handheld electronic devices

VARTA Microbattery unveils new battery system integration service for OEMs of portable devices

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