Different battery types exhibit different ageing behaviour. The ageing behaviour can usually be derived from an understanding of the fundamental working principles. Possible mechanisms for NiCd and lead-acid batteries in different applications will be reviewed first. Then ageing is discussed for lithium-ion cells.
NiCd batteries suffer from memory effect if they are not frequently fully discharged. This can be related to the ongoing growth of crystals over time. Cycling keeps the crystals smaller and smaller crystals provide higher surface area and hence lower internal impedance and better power
Lead-acid batteries conversely exhibit the opposite behaviour if compared to NiCd batteries. Lead-acid batteries degrade due to sulphation if kept at a low state of
charge. Sulphation even occurs if the battery is not subject to regular full recharges. There is usually not sufficient time to achieve a full recharge in cyclic applications. This undercharge is the main cause for degradation of lead-acid batteries in electric vehicle applications [66]. Charging currents should be as high as practically possible and charging voltages can significantly exceed gassing voltages. Opportunity charges should be applied whenever possible [43].
The ageing behaviour also depends on the type of application. Different applications exhibit different typical usage characteristics. Lead-acid batteries in stand-by
applications for example suffer from the opposite problem if compared to traction applications, which were mentioned earlier. In stand-by applications (e.g. un-
interruptible power supplies), the battery is kept fully charged at all times. The cells rapidly age due to corrosion or dry-out, which is a result of electrolysis and gassing; in these cases, the charging voltage should not be too high. The optimal charging voltage of lead-acid batteries not only depends on the application but also on the temperature and the ageing status of the cell. Considering that a pack consists of several cells connected in series, charging lead-acid batteries with the correct charging voltage can be very challenging, in particular, in the presence of temperature
differences within the pack [48]. The series hybrid electric vehicle application considered in this project requires large battery packs and one key advantage of the specified drivetrain is the possibility to optimise the weight distribution. However, this usually means, that the battery cannot be mounted in one central place and this makes it difficult to keep all the cells at the same temperature.
The working principle of lithium-ion batteries does not permit overcharging and, hence, lithium-ion batteries are not meant to be fully recharged. Apart from lithium- ions moving back and forth, there is no material movement and there are no solution / dissolution processes which could lead to ageing. Ageing mainly occurs due to one or more of the following processes, not considering any abuse conditions:
• Stresses in the electrode material as a result of expansion / contraction due to lithium-ion intercalation during cycling [14].
damage. A thickened SEI results in poorer performance. The apparent
capacity decrease due to impedance increase may be more significant than the true capacity decrease by consumption of lithium, especially in high-power applications [7; 38; 58].
• Solvent molecules may ‘co-intercalate’ together with the lithium-ions into the host structure and hence damage the surface structure and prevent further intercalation of lithium-ions. This will result in reduced capacity as well as performance loss due to decreased active surface area [61].
• Electrolyte decomposition [14].
• Lithium deposition and formation of SEI at the negative electrode towards the end of charging [75].
In essence, there is no known ageing process that involves the growth of crystalline structures, which is the cause for sulphation in lead-acid batteries. There is no known reason for why lithium-ion cells should degrade when not kept fully charged and hence no specific tests were conducted here to investigate this issue.
Prolonged float charging as applied in uninterruptible power supply systems (UPS) is likely to cause ageing of lithium-ion cells. The SEI for example will grow more rapidly due to the constantly elevated cell voltage [7; 58]. Prolonged float charging can be avoided in the application considered in this work. Hence, this aspect was not part of the conducted tests.
Up to today, the longevity of the small lithium-ion batteries in most portable
applications has not been an issue for manufacturers or suppliers. For large expensive battery packs, ageing and life expectancy however are important issues. Unlike lead- acid batteries, where it is relatively well known how long they last in certain
applications, ageing of lithium-ion batteries is still poorly researched [12; 14]. This is mainly due to the fact that large lithium-ion cells have not commonly been available. Only a few manufacturers are able to provide information for certain usage conditions and this information is not based on a huge number of tests. Cell designs change faster than tests can be conducted. A generic ‘equation’ for predicting the ageing or battery life, taking into account the full range of conditions and also the requirements of a particular application does not exist
The working principle of lead-acid batteries must cause ageing because material goes into solution, is then transported and crystallisation takes place. [66]. In lithium-ion cells there is no such mass transportation and lithium-ions simply move back and forth, travelling through the SEI and intercalate into the electrodes without substantial volume change [38].
The above ageing mechanisms are not very well researched as mentioned earlier, but it would appear that low temperatures and lower terminal voltages will prolong battery life. Furthermore, from the literature review, it appears that the high
frequency impedance is affected less by ageing than is the low frequency impedance [50]. Ageing is carefully considered during testing throughout this project and rigorous measures are taken in order to detect and quantify ageing.