Cells and Batteries

Tip

Exam Focus

Key points: primary (non-rechargeable) vs secondary (rechargeable) cells; terminal voltage $V=E-Ir$; series adds EMF ($V=nE$, $r=nr_0$) while parallel adds current capacity ($E$ unchanged, $r=r_0/n$); capacity in ampere-hours; and the telecom -48 V lead-acid/Li-ion battery plant.

Cell EMF and Internal Resistance

Caution

Key Formula — Terminal voltage

$$V=E-Ir,$$

$E$ = EMF (open-circuit), $r$ = internal resistance, $I$ = discharge current.

	Primary cell	Secondary cell
Rechargeable	No	Yes
Reaction	Irreversible	Reversible
Examples	Dry (Leclanché), alkaline, Li	Lead-acid, Ni-Cd, Li-ion
Use	Remotes, clocks	Vehicles, UPS, telecom plant

Primary versus secondary cells.

Cells in Series and Parallel

Cells in series add EMF; cells in parallel add current capacity.

Note

Key Point

Series increases voltage (EMFs add) — used to reach a required supply voltage. Parallel increases current/capacity (EMF unchanged) — used to supply larger load current and share discharge. Capacity is rated in ampere-hours (Ah): a 100 Ah cell nominally supplies 5 A for 20 h.

Telecom exchanges run a -48 V DC plant: a rectifier float-charges a lead-acid or Li-ion battery string that instantly backs up the load on mains failure (no break), a key reliability feature.