Types of rechargeable battery
First some terminology. The word ‘battery’ comes to us via a French word meaning an array of artillery weapons, to which Benjamin Franklin compared his experimental array of Leyden jars. So strictly speaking a ‘battery’ should comprise more than one cell, but this distinction is only rarely made in everyday usage. The English term ‘battery’ covers both rechargeable and non-rechargeable types, the more precise terms ‘primary cell’, ‘secondary cell’ and ‘accumulator’ no longer being in common use. Now, with that out of the way, we can look at some of the more common battery types used today.
At over 160 years old, the lead-acid battery (see Figure 1) is certainly the oldest type of rechargeable battery
that still finds practical use. The cells, which typically have a nominal voltage of 2 V, have been used in automobiles since around 1900. In that year the celebrated Lohner-Porsche Semper Vivus  was unveiled, the first ever hybrid car (are you listening, Toyota?). Another early application was in telephone and telegraph offices. The lead-acid battery is still difficult to beat in terms of price and robustness, and so, despite its heavy weight and other disadvantages it is still used to power the starter motor for internal combustion engines.
The NiCd battery also has a long history, going back over 100 years. In particular, in the 1980s, small NiCd cells were the most popular rechargeables for powering consumer electronic devices despite their nominal voltage of only 1.2 V. Their ability to deliver large currents made them the battery of choice for cordless tools and radio-controlled models. Also, Toyota Prius models up to and including the Prius III use this battery technology for their hybrid drive. The biggest disadvantages are poor environmental credentials (cadmium is toxic) and the notorious ‘memory effect’.
The better is the enemy of the good: low-cost NiMH batteries (see Figure 3) came to replace NiCd batteries in consumer devices, having first been manufactured on an industrial scale some 35 years ago. Offering the same nominal voltage of 1.2 V they were an ideal substitute. They are also by and large more environmentally friendly, do not exhibit the memory effect, have reasonable energy density and are economical. On the other hand, they are a bit trickier to charge. Since about ten years ago NiMH batteries with very low self-discharge have been available.
More recent rechargeable battery technologies are based around the element lithium. These chemistries allow the construction of lightweight batteries with a very high energy density, which are important factors
in mobile applications such as laptops, tablets and smartphones as well as in electric vehicles. The three main types of practical importance are as follows: LiPo (‘lithium polymer’: see Figure 4) whose compactness makes them ideal for mobile applications; and LiCoO2 (see Figure 5) and LiFePO4 (see Figure 6) for electrical drives. Their nominal voltages, at 3.7 V for LiPo, 3.6 V for LiCoO2, and 3.2 V for LiFePO4, are significantly different from those of other chemistries. They do not exhibit the memory effect and have very low self-discharge. On the other hand they are sensitive to environmental conditions and there are strict rules to obey when charging and when discharging them. As well as a range of cylindrical formats lithium batteries are available in customized packs for mobile devices and in prismatic housings with higher capacity. It is interesting to note that the current range of Tesla electric vehicles use ‘batteries’ comprising many thousands of type 18650 cylindrical cells. The RAM (rechargeable alkaline manganese) battery (AA size shown in Figure 7) is certainly the newest kid on the block in the world of rechargeable batteries. These alkaline secondary cells have a nominal voltage of 1.5 V and so are ideal as direct replacements for zinc-carbon or alkaline manganese primary cells. However, they are only suitable for use in applications with low discharge current, must not under any circumstances be subjected to deep discharge, and require a special charger: they are decidedly not compatible with NiCd or NiMH chargers!