We have come a long way since the only rechargeable battery option was lead acid. Even lead acid batteries come in various formats – firstly the traditional ‘wet’ (or ‘flooded’) lead acid battery with lead plates in sulphuric acid which can be topped up as necessary as water becomes used-up through the process of hydrolysis (often incorrectly termed ‘boiling away’). ‘Gel batteries’ are a development of lead acid batteries, where the electrolyte (the liquid – in this case a solution of sulphuric acid) is present in a gel form, rather than a liquid. This provides a slightly higher battery voltage, higher capacity. a more compact battery, better resistance to vibration, better handling of higher depth of discharge and a battery that does not leak if it is turned upside down and is tolerant of a wider temperature range – all useful characteristics. However, there are a few disadvantages. The main one is reduced ability to handle high currents, so this type of battery is not useful for starting engines, and it needs to be charged more carefully to avoid damage – less useful with intermittent or unpredictable charging as you might experience when using renewable sources. AGM lead acid batteries have the electrolyte absorbed into glass mats (‘Absorbed Glass Mat’), and therefore performs similarly to a flooded lead acid battery, and can provide high currents for engine starting, but still retains many of the advantages of gel batteries, with the added advantage of cheaper manufacture. The main disadvantage of AGM compared to Gel batteries is a shorter lifespan in terms of discharge cycles, though this is compensated for by much greater tolerance of less-than-ideal charging regimes.
Although Nickel-Cadmium (‘Nicads’ or ‘NiCd’) cells were first invented in the late 1800s, it was not really until the early 1980s that they became commonplace as rechargeable batteries for the general population – making their appearance as replacements for disposable 1.5V alkaline cells or batteries intended for small portable equipment. The voltage of NiCd (and the later NiMH cells) is slightly lower at 1.2V, but close enough for many portable electronic items. These batteries were fairly expensive, but their cost was quickly recouped as they could potentially be reused 500-1000 times instead of using that many throw-away single-use batteries. Their disadvantages included high self-discharge rates, so stored batteries did not retain their charge for very long. More serious downsides include the high toxicity of the heavy metal Cadmium, their tendency to leak, and the environmental consequences when they are disposed of at the end of their lives. NiCds were eventually superceded by Nickel Metal Hydride cells (‘NiMH’), which did away with the toxic cadmium component, and as technology improved, the storage capacity of the cells improved dramatically, so much so, that the best cells on the market now equal or exceed the high-capacity of disposable alkaline cells. NiMH cells still suffered from a high self-discharge rate, but a newer variant of the technology is now available which has much lower self-discharge, so in theory a charged cell can be stored for several months or even a few years, and still retain the bulk of its charge for use when needed.
The next big jump in battery technology, has been achieved by scaling-up and reduction in costs for manufacturing Lithium Ion Cells. These provide very high capacity, and with a significantly higher cell voltage of 3.7V. Therefore lithium ion cells cannot be used as direct replacements for alkaline (1.5V) or NiMH (1.2V) cells, but are useful for portable electrical equipment which has been designed to incorporate lithium ion battery packs, either internally or as interchangeable battery-packs. The same technology is used for home batteries – where households store energy generated by renewable means during the day, for use at nighttime when solar energy, for example, is not available. Lithium ion batteries have the ability to store large amounts of energy in compact batteries with much lower weights than for other battery technologies. It is able to cope with high charge / discharge currents, and is also very efficient – returning something in the region of 96-100% of the energy used for charging – compared to 65-90% for NiMH, and 70-85% for lead-acid batteries. Very large lithium ion batteries (25kWh – 150kWh) are used for electric vehicles, some of which have quite remarkable performance figures. Lithium ion batteries need very careful charging to avoid imbalance between individual cells in a battery, and care must be taken to avoid overcharging or charging with excessive current which can cause battery overheating and thermal-runaway chemical reactions, with potentially catastrophic consequences.
Battery technology is ever-evolving, but the basic technology choices have not changed a great deal over many years. Batteries are expensive, regardless of technology if you wish to store a decent amount of energy, and it is always far cheaper to improve your own efficiency – in terms of using more energy-efficient devices, using power when power is available, rather than storing it for use later, and by cutting back on energy processes which are not necessary, or can be scaled-back without any significant consequences.