Question
Differentiate between primary and secondary batteries. Write the cell reactions for a lead-acid battery during discharge and charging. Why are lithium-ion batteries preferred over lead-acid batteries in mobile devices?
(CBSE 12 + JEE Main + NEET pattern)
Solution — Step by Step
| Feature | Primary Battery | Secondary Battery |
|---|---|---|
| Rechargeability | Not rechargeable | Rechargeable |
| Reaction | Irreversible | Reversible |
| Example | Dry cell (Leclanche), mercury cell | Lead-acid, Ni-Cd, Li-ion |
| Use | Torches, remote controls | Vehicles, phones, UPS |
| Cost | Cheaper initially | Higher upfront, cheaper long-term |
During discharge (use):
Anode:
Cathode:
Overall:
During charging: all reactions reverse. An external current converts PbSO back to Pb and PbO.
Notice: both electrodes form PbSO during discharge and HSO is consumed, so the acid density drops — this is how mechanics check battery health.
- Energy density: Li-ion is ~4 times higher (150 Wh/kg vs 35 Wh/kg) — lighter for the same capacity
- No memory effect: Li-ion can be partially charged without losing capacity
- Self-discharge: Li-ion loses ~2% per month vs ~5% for lead-acid
- Weight: lithium is the lightest metal (atomic mass 7) — ideal for portable devices
- Voltage: each Li-ion cell gives ~3.7 V vs 2.0 V for lead-acid
flowchart TD
A["Batteries"] --> B["Primary (non-rechargeable)"]
A --> C["Secondary (rechargeable)"]
B --> D["Dry cell: Zn anode, MnO₂ cathode"]
B --> E["Mercury cell: constant voltage"]
C --> F["Lead-acid: Pb/PbO₂ in H₂SO₄"]
C --> G["Li-ion: Li intercalation"]
C --> H["Ni-Cd: Ni(OH)₂/Cd"]
F --> I["Cars, inverters, UPS"]
G --> J["Phones, laptops, EVs"]Why This Works
A battery converts chemical energy to electrical energy. In a primary cell, the reactants are consumed irreversibly. In a secondary cell, the chemical reaction can be reversed by applying external current — the products are converted back to reactants, “recharging” the battery.
The lead-acid battery is elegant in its simplicity: both electrodes and the electrolyte participate in the reaction. As the battery discharges, both electrodes become coated with PbSO (a poor conductor), and the acid becomes dilute. Charging reverses this, restoring the original electrode materials and acid concentration.
Alternative Method — Comparing by Application
| Application | Best Battery Type | Why |
|---|---|---|
| Car starting | Lead-acid | High current capacity, cheap, heavy but weight is acceptable |
| Smartphone | Li-ion | Light, high energy density, rechargeable |
| Hearing aid | Mercury/zinc-air | Constant voltage over lifetime |
| Emergency torch | Alkaline (primary) | Long shelf life, no charging needed |
For NEET and JEE, know the overall cell reactions — not just the electrode reactions — for lead-acid and dry cell batteries. Also remember: lead-acid battery EMF is about 2 V per cell (car batteries use 6 cells in series for 12 V). The specific gravity of the electrolyte drops during discharge — this is a testable fact.
Common Mistake
Students write the wrong electrode for anode and cathode in lead-acid batteries. During discharge: Pb is the anode (oxidised) and PbO is the cathode (reduced). During charging, this reverses: Pb becomes the cathode and PbO becomes the anode. The key: during discharge, the more reactive electrode (Pb) is the anode. During charging, an external source forces current in the opposite direction.