Batteries (part 1) Flashcards
What are primary cells
Non-rechargeable. The electrode material is irreversibly changed when you discharge it.
What are secondary cells
Recheargable. Electrode material can be changed back to its original state using a reverse current through the cell.
What is a wet battery
The electrolyte is liquid and free flowing in the cell. Must be used in the correct orientation
What does VRLA stand for
Valve-regulated Lead Acid
What defines the state of health of a battery
The ability of the battery to repeatedly provide its rated capacity over time
What are the 4 main types of battery nowadays?
PbA (lead acid)
NiMH (Nickel Metal Hydride)
Lithium based
NiCd
What is the general explanation of how a battery actually works?
Two DIFFERENT metals are used as the cathode (+ve terminal) and anode (-ve terminal) and are separated by the electrolyte (an acid). The anode wants to give up electrons and the cathode wants to gain them. Electrons can’t flow straight from anode to cathode through the electrolyte so they flow through the connected circuit. Therefore conventional current flows from the cathode to the anode.
Give 4 advantages of lead acid batteries
+ Low cost
+ High voltage per cell
+ High surge current capacity
+ Recyclable
Give 4 disadvantages of lead acid batteries
- Heavy
- Poor low temperature characteristics
- Low energy to weight and energy to volume ratios
- Damage occurs if left uncharged for some time
Give 7 advantages of Nickel Cadmium (NiCd) batteries
- Low cost for low power applications
- Low internal resistance
- High charge rates
- High surge current capability
- Wide temperature range (up to 70 deg C)
- Can be stored in charged/discharged state without self-damage
- Electrolyte is commonly available
Give 8 disadvantages of Nickel Cadmium (NiCd) batteries
- Memory effect (main problem)
- High cost for high power applications (~4* more than lead-acid)
- Low voltage per cell
- High self-discharge rates
- Easy to damage on overcharge
- Cadmium is high cost and environmentally damaging
- Requires specialist recycling methods
- Flat discharge pattern means it is difficult to determine SoC
Describe gassing
Because of overcharging, the electrolyte dissociates (e.g. in a lead acid battery, it dissociates into hydrogen and oxygen). This causes bubbles on the plates, which grow and then move upwards and out of the vents. This means the battery loses electrolyte material.
NiCd batteries can get around this using an active material which makes the positive electrode reach full charge BEFORE the other one. Therefore released gas is reabsorbed (equilibrium can be achieved).
Give 9 advantages of Nickel Metal-Hydride (NiMH) batteries
- High energy density
- High cycle life (Not as high as NiCd)
- Low internal resistance (not as low as NiCd)
- Fast-charge capable
- Environmentally friendly and recyclable
- Wide thermal operating range
- Can be stored in charged/discharged state without self-damage
- Electrolyte is commonly available
- High power per kg (almost 2* higher than NiCd)
Give 6 disadvantages of NiMH batteries
- High self-discharge rate (main problem) – 4% per day in some cases
- Some memory effect
- Low voltage per cell compared to PbAcid or Li-ion
- ~20% more cost compared to NiCd
- More complex charging regime (trickle charging to prevent pressure venting)
- Repeated high discharge rates reduce cycle life
Give 8 advantages of Li-ion batteries
- High energy and power density (25% of the weight of PbAcid)
- Long cycle life
- High cell voltage
- Fast-charge capable
- High discharge rates
- Environmentally friendly and recyclable (ish)
- Low self-discharge rate – shelf life of 5-10 years is possible
- Polymer gel electrolyte formats mean spillage impossible and often flexible
Give 8 disadvantages of Li-ion batteries
- Key disadvantage is cost
- High internal resistance compared with NiCd
- Safety issues and self-damage if over-charged/over-discharged
- Complicated charger requirements and need for protection circuitry/management system
- Very flat discharge curve therefore SoC hard to measure
- Thermal runaway and venting possible on puncture/crushing
What is a string
A group of a small number of cells within a large battery pack
What is passive balancing
Cells (or strings) are only balanced using dissipation
What is active balancing
You take the energy from the overcharged cell/string and use it to charge an undercharged one
Give 2 disadvantages of active switched capacitor balancing
- Slow equalisation speed
- Expensive DPST switches are required
Give 1 advantage and 2 disadvantages of active switched inductors balancing
+ Faster equalisation time
- Can have high current stresses in the switching devices
- Complexity and cost since each inductor needs 2 switches and a damping resistor
SoC
The amount of charge available in the battery at any time to sink/source current, usually given as a % of its rated capacity
SoH
The ability of a battery to repeatedly provide its rated capacity over time. May
be expressed as a percentage of its original rated capacity
Specific gravity
Another term for relative density - the ratio of the density of a material to a given reference
Give 2 disadvantages of using specific gravity to determine SoC
- Doesn’t take into account degradation of the electrodes or contamination of the electrolyte
- Only works for flooded cells that are not sealed
Describe coulomb-counting/current integration
Uses the idea that you can keep track of current going into/out of the battery to assess its SoC. Record the input current required to fully charge a discharged battery, then you can integrate the current coming out of it as it discharges to assess SoC (in coulombs. To get Ah, divide by 3600).
What are the 2 drawbacks of coulomb counting
- Not every electron that goes into the battery when charging is available to use when discharging (due to gassing, electrochemical reactions and thermal effects which cause electrolyte ageing)
- The current measurements need to be very accurate because a slight error will cause a ‘drift’ as you integrate. There are correction methods which correct for temperature and current rates and use charge/discharge cycles to recalibrate the SoC but these methods are time-consuming.
What is the problem with using the terminal voltage to determine SoC
The battery has an internal impedance which varies for different charge rates and temperatures, so the only time you can get an accurate measurement is when the battery is not connected to a load, which is not helpful.
What is an EIS
An Electric Impedance Spectroscopy testbench.
It allows you to evaluate the impedance, capacitance and conductance of a cell (over a freq range).
Uses sinusoidal stimuli
What is PRBS
Pseudo Random Binary Sequences is an alternative to EIS used for impedance analysis. You use logic hardware to generate a binary sequence current injection that goes between 0 and 1 at (ideally) random points in time, at random frequencies, making it look similar to noise.
What is the main trade off with PRBS
You need a large test signal to get a good signal to noise ratio, but it can’t be too large or it will discharge the battery itself
What is the SEI of a Li-ion
Solid Electrolyte Interface. Is on boundary between cathode and electrolyte. Stops lithium ions from reacting with the electrolyte
Describe ageing at the cathode (lithium based batteries)
If the SEI is not perfect, Li+ ions will leak through it. This can then cause reactions with the anode metals which can release metallic ions that can cluster in the SEI. This can increase electrical impedance. Li+ ions being lost and unable to move back and forwards also means reduced capacity
Describe ageing at the anode (lithium based batteries)
Usually caused by high temp. Again, electrical impedance goes up and capacity goes down
Describe ageing at the electrolyte (exclusive to lithium based batteries)
Deposites can form within the electrolyte solvent. This increases internal resistance and inhibits Li+ transport. Eventually capacity decreases.
What causes non-energetic failure of a Li-ion
Usually a result of long term exposure to ageing. But can also be from other things like mechanical damage causing loss of electrolyte.
What are the 5 causes energetic failure of a Li-ion
Usually results from operation outside the norm, and can be catastrophic.
1) Overcharging
2) Overdischarging
3) Overheating
4) Short circuits
5) Physical abuse
What are the 6 overall things that the BMS does
- Monitoring (V, I, temp, SoC)
- Protection (V, I, temp)
- Load management (control relays to add a resistance circuit)
- Computation (keep track of usage)
- Communication (for smart battery packs)
- Optimisation (charging and balancing)
