The general characteristics of the battery are given in Table 3.8. A few manufacturers have claimed to produce electrically rechargeable zinc–air batteries, but the number
of cycles is usually quite small. The more normal way of recharging is as for the
aluminium–air cell, which is by replacing the negative electrodes. The electrolyte, containing the zinc oxide, is also replaced. In principle this could be taken back to a central plant
and the zinc recovered, but the infrastructure for doing this would be rather inconvenient.
Small zinc–air batteries have been available for many years, and their very high energy
density makes them useful in applications such as hearing aids. These devices are usually
‘on’ virtually all the time, and thus the self-discharge is not so much of a problem. Large
batteries, with replaceable negative electrodes, are only available with great difficulty, but
this is changing, and they show considerable potential for the future. Use of a replaceable fuel has considerable advantages as it avoids the use of recharging points – lorries
delivering fuel can simply take the spent fuel back to the reprocessing plant from where
they got it in the first place. The high specific energy will also allow reasonable journey
times between stops.
An important point that applies to all battery types relates to the process of ‘charge
equalisation’ that must be done in all batteries at regular intervals if serious damage is
not to result.
A problem with all batteries is that when current is drawn not all the individual cells
in the battery lose the same amount of charge. Since a battery is a collection of cells
connected in series, this may at first seem wrong – after all, exactly the same current
flows through them all. However, it does not occur because of different currents (the
electric current is indeed the same) – it occurs because the self-discharge effects we have
noted (e.g. Equations (3.4) and (3.5) in the case of lead acid batteries) take place at
different rates in different cells. This is because of manufacturing variations, and also
because of changes in temperature – all the cells in a battery will not be at exactly the
same temperature.

The result is that if nominally 50% of the charge is taken from a battery, then some
cells will have lost only a little more than this, say 52%, while some may have lost
considerably more, say 60%. If the battery is recharged with enough for the good cell,
then the cells more prone to self-discharge will not be fully recharged. The effect of doing
this repeatedly is shown in Table 3.9.
Where IC engine efficiency is to be optimised by charging and supplying energy from the
battery, clearly a battery which can be rapidly charged is desirable. This tends to emphasise
batteries such as the NiMH which is efficient and readily charged and discharged. An
example of this would be in the Toyota Prius and the Honda Insight, both very successful
hybrids that use NiMH batteries although on recent versions LIBs will be used. A zinc–air
battery would be no use in this situation, as it cannot be electrically recharged.
This type of hybrid electric vehicle – an IC engine with a battery that cannot be
recharged from the mains – was until recently the most common. It seems that the majority
of such vehicles currently use NiMH batteries, with a storage capacity typically between
about 2 and 5 kWh. (Note that the energy stored in a normal car battery is between about
0.3 and 1.0 kWh.) The very latest hybrids, such as the mains rechargeable Chevrolet Volt,
use LIBs.