A battery pack design could be for a single or group of cells arranged in series and parallel, depending on application this may include: thermal control, electrical switching and management system. In it’s simplest for a phone battery pack design is a single cell that has temperature and voltage monitoring. The battery pack design for an electric vehicle is more complex and it is likely you will need access to battery pack design software.
However, let’s start looking at the basics of battery pack design.
There are fundamentally 2 types of battery cell:
- Primary Cell – A cell or battery which is not intended to be recharged and is discarded when the cell or battery has delivered all its electrical energy.
- Secondary Cell – A galvanic battery which, after discharge, may be restored to the fully charged state by the passage of an electric current though the cell in the opposite direction to that of discharge. eg Lithium Ion Battery
The main parameters considered when selecting batteries:
- usable energy capacity
- system operating voltage range
- power capability
- charge and discharge
- time dependent power capability
- charge time
- temperature environments
- weight and / or volume
I will give a brief overview of each element and then discuss some of these in more depth in further postings.
A fundamental requirement and we have to look at this from an electrical, mechanical and thermal viewpoint. The first thing is to look at the specification of the individual battery cell as this will specify the limits:
- Maximum and minimum operating voltage
- the voltage needs to be measured for most applications to ensure you do not go beyond these limits
- Maximum and minimum temperature range
- depending on the application it is likely that you will need to monitor the temperature of the cells
- once operating above 60V you will need to make it finger proof and provide a system that can monitor and switch off the output if an issue is detected
- external shorts could result in very high currents that can then result in cells getting very hot very quickly, a fusing strategy is required to protect at this basic level
Usable Energy Capacity
The energy capacity of a cell at a maximum is determined by the maximum operating voltage to the minimum operating voltage. The ampere-hours discharged multiplied by the average voltage will give the energy capacity in Wh.
However, this operating voltage range might be too wide for the system. Or the operating range might be limited to extend the lifetime of the pack. In the case of “self-charging hybrids” the usable energy capacity can be just 30% of the total capacity of the pack. This reduction in usable capacity being required to increase the charge/discharge cycle capability.
System Operating Voltage Range
Battery cells have an open circuit voltage that changes with the state of charge. In addition, real battery cells have an internal resistance and this means that the voltage drops as the discharge current increases. On complex battery packs there are additional sources of resistance associated with busbars, fuses, relays and other electrical connections in the pack. All of these mean the voltage will be dependent on charge/discharge current.
Therefore, it is very important to define the operating voltage range of the complete system very carefully.
The maximum power will be a product of voltage and current. The voltage will change with the state of charge of the battery cell. The maximum current will change with temperature, age and state of charge.
For primary cells the shelf storage time or calendar ageing discharge rate is the most important factor with respect to lifetime as this will determine how long you can store the cell before using it.
For secondary cells or rechargeable cells we are interested in the calendar ageing and the cycle ageing. The calendar ageing will show how the capacity reduces with time, even when the battery cell is not being used. The cycle ageing will show you how many cycles the cell can operate for at a given charge and discharge rate.
This is normally quoted as the time required to recharge a secondary cell and normally applies to the recommended usable energy window.
Depending on application a battery pack will be subjected to a number of inputs: vibration, electrical noise, thermal gradients, shock along with a number of possible fault conditions. All of these need to be considered when designing the pack.
Battery cells are a bit like humans in terms of the temperature range over which they are happy to operate, ie -30°C to 60°C.
However, below 0°C a lot of the cells get extremely sluggish in terms of being able to deliver current and sometimes extremely poor in terms of charge acceptance. Hence it is quite normal to have to heat battery packs.
Above 60°C and we can see everything from faster ageing of the cell to a breakdown in some of the internal components and then failure of the system.
Weight and / or Volume
In model aircraft the weight of the battery pack is extremely important and quite often more important than the volume. In model cars and tanks the volume of the battery pack is more important than the mass. This factor might change the type of battery chemistry that you select.
The cost of a battery pack is a bit like “how long is a piece of string?”. Fundamentally a battery pack can be a single primary cell such as a CR2032 in a simple clip housing or as complicated as the battery pack on a back-up energy system where several thousand cells are joined in parallel and series.
These are the fundamental parameters associated with the battery pack design, but of course these are complex systems and each aspect of the design can open up a huge range of options.