An attempt to walk you through the battery basics from a single cell to multiple cells.
The history of the battery goes back a long way, but perhaps the significant step is the Voltaic pile invented by Alessandro Volta in 1800. This multiple layer zinc and copper plates, with cardboard soaked in brine between the layers formed a battery that produced a steady electric current.
Since then many different battery chemistries have been invented and developed, some more successful than others. For this introduction we will concentrate on the Lithium Ion battery invented by John Goodenough and Stanley Whittingham, a commercial Li-ion battery was developed by a Sony and Asahi Kasei team led by Yoshio Nishi in 1991. The fundamentals of the battery are the same in that we have an anode, a cathode and an electrolyte.
The cathode is a lithium transition metal oxide, eg manganese or cobalt or a combination of transitional metals. The anode is a graphite-based material, which can intercalate or release lithium.
When discharge begins the lithiated carbon releases a Li+ ion and a free electron. Electrolyte, that can readily transports ions, contains a lithium salt that is dissolved in an organic solvent. The Li+ ion, which moves towards the electrolyte, replaces another Li + ion from the electrolyte, which moves towards the cathode. At the cathode/electrolyte interface, Li+ ions then become intercalated into the cathode and the associated electron is used by the external device.
These cells can be charged and discharged within certain limits, eg:
- Voltage between 2.5V to 4.2V
- Temperature range of -20°C to 60°C
- Charge and discharge at a continuous current over 2 hours (hence a C-rate of 1C in each direction)
- Need a pressure applied to the anode, separator and cathode layers to ensure they maintain contact
If we don’t apply any load to the battery the voltage profile versus State of Charge (SoC) varies between 4.2V when fully charged down to 3.3V when completely discharged.
This no load voltage is known as the Open Circuit Voltage (OCV).
The State of Charge (SOC) is the percentage of the full capacity of the battery cell as measured in Ampere-hours (Ah) compared to the cell capacity in Ah.
Format of the Cell
There are many different cell formats. There is no cell format that is better or worse than any other, it is down to design, integration and application.
The primary formats are: Cylindrical, Pouch and Prismatic. The cell format can offer different levels of electrical, thermal and mechanical performance. Therefore, it is really important to understand the requirements of your application before selecting the cell.
A battery cell is not a perfect current source as it also has an internal resistance.
Symbolically we can show a cell with the internal resistance as a resistor in series.
Rint is the DC internal resistance, sometimes abbreviated as DCIR.
The DCIR is not just a single number for any given cell as it varies with State of Charge, State of Health, temperature and discharge time.
For most simple peak power calculations we will be interested in the DCIR value for a new cell at 50% SOC, 25°C and for a 10s pulse.
If we have an OCV of 3.7V @ 50% SOC and an internal resistance of 0.025Ω and we draw 10A from the cell the voltage will drop 0.25V This is simply Ohms Law.
V = 3.7V – 10A x 0.025Ω = 3.45V
Hence the voltage of the cell under a 10A load will be 3.45V
We can also calculate the maximum current we can draw taking the cell down to the minimum voltage:
2.5V = 3.7V – I x 0.025Ω
I = (3.7V – 2.5V) / 0.025Ω = 48A
These numbers are quite typical of a 5Ah NMC cell. Peak discharge is around 10C.
If we want more power then we need more voltage or more current. We could:
- use a large battery cell
- put more cells together in series / parallel
Series and Parallel
Cells added together in series (S) and in parallel (P). Hence we get the shorthand 96S46P configuration of cells that we might see in a Tesla pack. This means 46 cells are connected together in a parallel group and this is then connected in series with 95 more of these groups.
If each cell was 5Ah then we would have a total capacity of 46 x 5Ah = 230Ah
and the total nominal voltage of the pack would be 3.7V x 96 = 355.2V
the total nominal energy content of the pack = nominal voltage x capacity = 355.2V x 230Ah = 81,696Wh or 81.696kWh
Watts are defined as 1 Watt = 1 Joule per second (1W = 1 Js-1)
time is simple 1 hour = 3600 seconds
Hence 1 Wh = 3600 Joules
So the Watt hour (Wh) is a strange unit as it is energy use per unit of time multiplied by time.
The voltage and temperature limits of the cell are critical to maintain safety and longevity. This means we need a control system that measures the voltage across each cell and the temperature of the cells. This control system can then limit charge and discharge as necessary or demand heating/cooling as required.
When we connect cells in series the resultant pack is only as strong as the weakest cell in the string.
- Discharging, first cell to hit 2.5V stops the discharge of the whole pack.
- Charging, first cell to hit 4.2V stops the charging of the whole pack
- If one cell hits 60°C then pack has to stop charge and discharge
The impact of this is you need to ensure:
- All cells are made identical
- same capacity
- same internal resistance
- All connections between cells are identical
- Cooling and Heating of all cells is identical
This means we need:
- Quality control in cell manufacturing
- Pack engineering that can design, model and validate all functions of the pack
- Battery management system (BMS) that can control the pack over it’s lifetime
There is just one cell in an IPhone, it is expected to last around 2 years, we keep it close to us and so it is kept at an optimum temperature and it discharges quite gently at C/24. The electric car might have 6000 cells, it is expected to last at least 10 years in all conditions and can be discharged in C/2. One cell can weaken this whole pack.BatteryDesign.net
The temperature of a cell determines how well it can deliver power and how easily or if at all it can be charged. The weakest cell in the pack can just be the hottest or coldest cell. Hence, it is important to control the cell temperature within limits and control the temperature gradients across a cell and across a pack.
There are many different cooling systems used in automotive battery packs. The selection of the right cooling system will be down to many factors, including:
- performance requirements
- cell heat output
- cell operating temperature
The safe working limit for DC is less than 60V. More than 14 cells in series and we will be above 60V when fully charged.
Therefore we need to isolate the pack, check the isolation (BMS) and control the inputs and outputs to the pack to maintain safety in any situation (another BMS function).
This also means we need to contain the pack to protect people and to maintain the system in a clean and dry environment.
Failure and Abuse
When things go wrong with the battery it is important that we can interrogate the control system to understand if it is isolated and safe. Otherwise we will need to test the isolation and ensure that the pack is in a safe condition.
When cells fail completely and go into thermal runaway they can generate a lot of gas, heat and debris. This means the pack needs to be able to vent the gas safely and contain any fire long enough for anybody to get to safety.
The EUCAR Hazard Levels define the outcome of cell level safety testing. These levels are normally used to describe the outcome of tests such as overcharge as part of the cell specification.
End of Life
Finally and really firstly we need to think about:
The 4 R’s as they are abbreviated to in the automotive industry and these are all about extending the life, using the packs or cells in another application and finally recycling the materials.
This has been a blast through the Battery Basics to give you an overview. We will add further links, details and references over time.