At a time when many of us are considering making the change to owning an electric vehicle, it might be pertinent to look at some of the issues surrounding EV battery health and charging. EVs have been around in hybrid and fully electric forms for some time. However, as attractive as the idea of a self-charging car may seem, most hybrids are ultimately powered by internal combustion and hence require fossil fuels. To be truly kind to the planet, and assuming that they will be charged from renewable energy sources, there's no escaping the fact that, sooner rather than later, more and more of us will need to take the EV 'plunge'!

If the internet is to be believed, a common source of confusion amongst the general public in relation to EVs seems to be the frequent use of both kW and kWh. Watts (W) are the units of power, energy per unit time, and this is commonly used to describe the rate of flow of electrical energy from a charging point. For instance, domestic charging points, that an EV owner may have installed at home, typically have a power rating of 10 kW, whereas so-called public 'fast-charging points' may deliver power in the order of 100 kW. Kilowatts are also used to denote the power output of the cars themselves and will obviously depend on the make, model and engine size.

This brings us neatly to the kWh (kilowatt-hours), a unit of energy itself, which is used regarding a car's electrical storage capacity, analogous to the size of the fuel tank in a traditional petrol, or gasoline, fuelled car. Most EVs are now fitted with banks of lithium-ion cells, which together make up the battery. These cells, which are almost self-explanatory in that the charge carriers are the lithium ions, have the advantage, over previous technology, of improved energy density (energy per unit volume), they are relatively quickly charged, have a low self-discharge rate (i.e. when not in use) and are comparatively low in mass. Nevertheless, they are prone to overheating and require a charge management system to maximize their useful life span.

All battery systems have their pros and cons, however, given that advances in cell technology increasingly indicate that a battery could well outlive the car itself, for most drivers the overriding concern remains the time needed to charge an EV, along with the availability of charging points. Batteries need to be charged with a direct current. Yet, because the national grid of most countries provides an alternating current unless you are charging your vehicle from a public fast charging point (most of which are DC), the current from a domestic charging point will need to be converted from AC to DC by the car's onboard rectifier. This is the thing that really slows down the domestic charging process, taking many hours for a full charge as opposed to mere minutes at a public charging point. There are good reasons for national power grids to produce AC, not least due to the type and means of electricity generation, but also the ease with which transformers can step the voltage up or down as needed: the alternating current provides a changing magnetic flux in the secondary coil.

Rectifiers come in many configurations. A full wave rectifier typically works by using an arrangement of diodes (basically one-way valves) to convert a sinusoidal voltage (where the peaks and troughs correspond to positive and negative half cycles of AC) to a series of positive-only half cycles with twice the frequency of the original supply. The rippling effect of the rising and falling voltage can be smoothed out using a capacitor which stores and releases electrical energy as required. The battery can now be charged from this essentially direct current.

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Other issues surrounding EV charging relate mainly to maintaining a steady battery temperature and avoiding over-charging. Most manufacturers of EVs recommend keeping fast DC charging to a minimum as the increased current can cause overheating, which potentially shortens a battery’s life span. Also, unlike a traditional fuel tank, the charging rate depends on how much charge the battery already has - analogous to the speed at which one might pour liquid into a vessel to a specifically prescribed limit.

In the meantime, whatever the future holds, EVs are clearly here to stay. Let's hope that as the market for these vehicles grows, they become more affordable and that the charging infrastructure expands so that even more of us have the opportunity of ecologically responsible driving.