Batteries and Fuel Cells Questions and Answers

Answer: A

Cells in series raise the voltage and cells in parallel raise the current capability, and a battery is any such assembly. Common usage blurs the distinction, but a single 1.5 volt unit is strictly a cell rather than a battery.

Answer: B

Correct answer: cannot usefully be recharged

A primary cell’s reactions are not practically reversible: the products cannot be converted back to reactants by passing current, and attempting it generates gas and heat. A secondary cell is built so that the same reaction runs both ways, which imposes real constraints on the chemistry and the construction.

Answer: C

Correct answer: kept apart until the cell is needed

A reserve cell stores the electrolyte separately and is activated when required, so nothing can react and self-discharge is essentially zero. This gives shelf lives of years or decades, which is why reserve cells are used in munitions, emergency beacons and torpedoes - devices stored for a long time and used once.

Answer: D

It states the charge the cell can deliver, and multiplying by the nominal voltage gives the energy in watt hours. The figure depends on the rate of discharge, so the rate at which it was measured must be quoted with it.

Answer: A

Correct answer: polarisation and internal resistance drop the voltage sooner

At high current the ohmic drop across the internal resistance is larger and the electrodes polarise more, so the terminal voltage reaches the cut-off while active material remains unused. The material is not consumed any faster; the cell simply becomes unable to deliver at a useful voltage. This is why capacity must always be quoted with the discharge rate.

Answer: B

Correct answer: recovered on discharge to that put in on charge

Coulombic efficiency counts charge alone, so it measures how much of the current went into the intended reaction rather than into side reactions such as gassing. A good lithium ion cell is above 99 per cent; a lead acid cell nearer 90, since some of the charging current electrolyses water.

Answer: C

Correct answer: the charging voltage exceeds the discharge voltage

Energy is charge times voltage, and a cell must be charged above and discharges below its equilibrium potential because of polarisation and internal resistance. Even at 100 per cent coulombic efficiency the energy efficiency would therefore be less than one, and the difference between the two figures is what appears as heat.

Answer: D

Internal reactions consume the active material whether or not the cell is used, so a cell may be flat before it is ever connected. A low self discharge rate is what makes a cell suitable for a smoke alarm or a remote control.

Answer: A

The zinc can is both the container and the negative electrode, so it thins as the cell discharges and eventually leaks. The graphite rod is only a current collector; the manganese dioxide is the actual cathodic reactant.

Answer: B

Correct answer: the cathodic reactant, taking up hydrogen

Manganese dioxide is reduced at the positive electrode, taking up hydrogen from the electrolyte to become MnOOH. It is often described as a depolariser because that reaction consumes the hydrogen which would otherwise accumulate and polarise the electrode - but it is a reactant that is genuinely consumed, not an inert component.