ELEC5206 Chap.14 Rechargeable Batteries for Energy Storage
Rechargeable Batteries for Energy Storage
The battery lectures look at storage as an electrical engineer would. Part one covers cell, module and pack structure, the lead, nickel, lithium and sodium families with their voltages, strengths and weaknesses, the rating vocabulary of nominal, terminal, cut-off and open-circuit voltage, capacity and C-rate, state of charge and deep cycles, and constant-current, constant-voltage charging with the role of temperature.
Part two models the battery as an open-circuit voltage behind a resistance, identifies that resistance from datasheet curves, fits the open-circuit voltage as a polynomial or a straight line, and then turns to cell mismatch, passive and active balancing, and the comparison with supercapacitors. Calculations here are short but frequent: charge times, energy, resistance and terminal voltage.
Worked examples size a four-cell lithium pack, identify a battery's resistance from two readings and predict lithium terminal voltage at different discharge rates with the lecture's linear model.
What this chapter covers
- 01
Cells, modules and packs
- 02
Lead-acid, nickel, lithium and sodium-sulfur chemistries
- 03
Voltage terms and their limits
- 04
Capacity, C-rate, energy and state of charge
- 05
Constant-current, constant-voltage charging and temperature
- 06
The Thevenin model and resistance from datasheet curves
- 07
Fitting open-circuit voltage for NiMH and lithium cells
- 08
Cell mismatch, balancing methods and supercapacitors
Worked example · free
Charge time and usable energy of a lead-acid bank
- 1Bulk current: C/4 = 100/4 = 25 A.
- 1Ideal full-charge time at 25 A: 100/25 = 4 h, before the slower constant-voltage stage.
- 1Rated energy: 24 × 100 = 2400 Wh.
- 1Energy per deep cycle: (0.80 − 0.20) × 2400 = 1440 Wh.
Key terms
- C-rate
- Charge or discharge current expressed relative to rated capacity, so 1C empties or fills a battery in about one hour.
- Cut-off voltage
- The lowest allowable discharge voltage, below which a rechargeable battery can be badly damaged.
- Deep cycle
- A charge above 80% state of charge followed by a discharge below 20%, often the unit for cycle life.
- Thevenin battery model
- An open-circuit voltage in series with a resistance, valid over a short-term steady state.
- Cell balancing
- Equalising the state of charge and voltage of series cells by dissipation or by shuttling energy between them.
- Supercapacitor
- A high-capacity capacitor that bridges conventional capacitors and batteries, storing energy as half CV squared.
Rechargeable Batteries for Energy Storage FAQ
Why does charging slow down near full charge?
In the constant-voltage absorption stage the charger holds the upper voltage limit, so the current falls as the open-circuit voltage rises toward it. Charge ends when the current becomes small, which makes the last part of the charge the slowest; the lecture notes that 20% to 80% state of charge is the fast region.
How do I find a battery's internal resistance from a datasheet?
Read the terminal voltage at two discharge currents at the same discharged capacity and divide the voltage difference by the current difference. The lecture's NiMH example gives 0.216 V over 10 A, or 21.6 mΩ, and the open-circuit voltage follows by adding back the resistive drop.
When is a supercapacitor better than a battery?
When power and speed matter more than energy. A supercapacitor delivers power within milliseconds and lasts far longer, but stores much less energy, self-discharges quickly and needs an advanced converter. Hybrid systems use it as a short-term buffer and the battery as the long-term one.
What causes series cells in a pack to drift apart?
Manufacturing imperfection, temperature differences across the pack and uneven ageing give cells different capacities and resistances. Repeated charge and discharge then spreads their states of charge, and a weak cell can fail early and drag the pack down, which is why large series packs need equalisation.
Exam move
Build a one-page table of the four chemistries with cell voltage, strengths and weaknesses, and memorise the voltage terms in order from cut-off to upper limit. Practise C-rate, energy and deep-cycle arithmetic until it is instant, then the two-reading resistance method and a terminal-voltage prediction with the linear lithium model.
Close with a comparison answer on passive versus active balancing and on batteries versus supercapacitors. Learn the battery sign convention, positive current on discharge, before any model question.
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