Battery Fundamentals: Anodes, Cathodes, Electrolytes, and Voltage Losses

What is the anode, and what happens there during discharge?

The anode is the negative electrode. Oxidation (loss of electrons) happens there during discharge.

What is the cathode, and what happens there during discharge?

The cathode is the positive electrode. Reduction (gain of electrons) happens there during discharge.

What does the electrolyte do in a battery?

It lets ions move between the anode and cathode but blocks electrons, forcing electrons through the external circuit to do useful work.

What is Open Circuit Voltage (OCV)?

The voltage of a battery at rest, sitting at its thermodynamic equilibrium potential, with no current being drawn.

What is overpotential (polarization)?

The drop in terminal voltage below the OCV that occurs as soon as current is drawn from the battery.

How does overpotential relate to efficiency?

Higher overpotentials mean greater energy losses, which lowers the battery's round-trip efficiency.

What does a typical car battery system look like, and roughly what current does the starter motor draw?

Battery to Solenoid to Starter motor. The starter motor can draw around 200 A, and a dead/weak battery's voltage can drop below 10 V under this load.

What are the three main types of voltage loss (overpotential) in a battery?

Activation losses, ohmic losses, and concentration losses.

What causes activation loss, and where does it show up on the discharge curve?

It comes from the chemical kinetic energy barrier needed to transfer electrons at the electrode-electrolyte interface. It causes the initial voltage drop at very low current densities, as the reaction struggles to start.

What is ohmic loss, and what equation governs it?

Ohmic loss is the conventional electrical resistance to charge flow through the battery (electrolyte's ionic resistance plus plates/contacts' electronic resistance). It follows Ohm's law: V = IR.

Where does ohmic loss dominate on the discharge curve, and how does it scale with current?

It scales linearly with current and typically dominates the long, steady linear middle portion of the discharge curve.

What causes concentration loss?

Mass-transport limitations: ions near the electrode surface get consumed faster than new ions can diffuse in from the bulk electrolyte, creating a localized concentration gradient.

What happens to voltage at high current densities due to concentration loss?

Reactant concentration at the electrode surface approaches zero, causing a sharp, sudden collapse in terminal voltage and making the remaining capacity inaccessible.

How does larger internal resistance affect current, and vice versa?

Larger resistance leads to slower (lower) current; smaller resistance allows larger current.

What is the difference between amps (A) and amp-hours (Ah)?

Amps measure current, how fast charge flows right now. Amp-hours measure capacity, how long the battery can sustain a given current.

A battery is rated 2 Ah. Name three current/time combinations that would use up that capacity.

2 A for 1 hour, 1 A for 2 hours, or 4 A for 0.5 hours, all equal 2 Ah.

What is the formula for energy in watt-hours (Wh)?

Energy (Wh) = Voltage (V) x Capacity (Ah). Equivalently, Energy = V x I x T (time in hours).

Why can comparing two batteries by Ah alone be misleading?

Ah ignores voltage, so two cells with the same Ah rating can store very different amounts of actual energy if their voltages differ.

Compare a 1.2 V, 2 Ah cell to a 9.6 V, 2 Ah battery pack in Wh. Which stores more energy, and by how much?

1.2 V x 2 Ah = 2.4 Wh. 9.6 V x 2 Ah = 19.2 Wh. The 9.6 V pack stores 8 times more energy despite the identical Ah rating.

What does the "C" rating of a battery describe?

How fast a cell can be safely discharged relative to its capacity, without overheating. The "C" refers to the cell's capacity value in Ah.

A 2.2 Ah cell has a 20C rating. What is its maximum safe continuous current?

20 x 2.2 Ah = 44 A.

A 2.2 Ah cell has a 40C rating. What is its maximum safe continuous current?

40 x 2.2 Ah = 88 A.

Why does effective capacity drop at high discharge rates?

Internal resistance wastes some stored energy as heat when drawing high current, so faster discharge lowers the usable (effective) capacity.

A 2 Ah battery is discharged slowly at 0.4 A vs. aggressively at 4 A. What effective capacity do you get in each case?

At 0.4 A (slow), you get the full 2 Ah. At 4 A (aggressive), effective capacity shrinks to around 1.7 Ah.

What are the typical capacities of AA, AAA, and C alkaline batteries?

AA is about 2870 mAh, AAA is about 1150 mAh, C is about 7.8 Ah.

What does the mnemonic OIL RIG stand for?

OIL: Oxidation Is Losing electrons. RIG: Reduction Is Gaining electrons.

What is a redox reaction?

A chemical reaction that consumes or produces electrons, involving both oxidation and reduction happening together.

What is a galvanic cell?

A cell in which the energy released by a spontaneous redox reaction is converted into electrical energy (this is how batteries discharge).

What is an electrolytic cell?

A cell in which electrical energy is used to drive a nonspontaneous redox reaction (this is how batteries are charged).

What is equivalent series resistance (ESR) in a battery, conceptually?

The combined internal resistance (from activation, ohmic, and concentration losses) that acts like a single resistor in series with the battery, causing voltage to drop under load.