Doctor Diesel

Back to Basics – The Car Battery

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Web_b2bPerhaps the original black box, the car battery is an indispensable component, unless someone one day replaces it with a compressed air tank, or a flywheel, for energy storage. In 1860, a French scientist, Gaston Plante, produced the first rechargeable lead acid battery, which relied on electrodes of pure lead metal and lead dioxide, using sulphuric acid as the electrolyte. Using this same wet cell technology and chemistry, the lead-acid battery was developed into a device capable of producing a high current, primarily for engine starting, which was continuously recharged by an engine powered generator. Improvements in the lead acid battery have been continuous and many, and it still remains, almost exclusively, the automotive battery for internal combustion engine powered cars.

Absorbent Glass Mat 12 Volt batteries are the current type now used in most cars, where the sulphuric acid electrolyte is safely contained within the glass mat between the electrodes of the six cells that generate a voltage somewhat in excess of 12 Volts. Whilst not strictly completely sealed (they are valve-regulated, to avoid any explosion dangers), they are generally spill-proof and require no maintenance, or topping up with acid, as in past times. These latest battery types are only compatible with the sophisticated alternator charge control systems of today’s cars and, whilst essentially maintenance-free, it’s impossible now to test the acid electrolyte with a hygrometer to establish the state of charge. Most charging systems will generally provide a fixed voltage of typically 13.8 to 14.4 Volts and, with today’s intelligent alternators using the increasingly specified braking regeneration, significant charging will mostly take place during braking, when no engine power will be drawn, thereby assisting fuel economy. When heavy current-drawing items, like air-conditioning compressors, entertainment systems, windscreen wipers and lighting are running concurrently, older cars would often be in a battery discharge situation, where alternator output did not match the total load. Most modern vehicles, are matched with alternators that provide enough current for all loads and significant battery power is only drawn for engine starting. Engine speed has little effect on charge or discharge and the battery voltage and charge state will be maintained almost regardless of the other electrical loads, even at idling speeds.

All this sophistication, along with other complex car electrical systems, means that once simple tasks like charging a flat battery, or jump-starting a car, or using a portable battery booster, are fraught with danger for the unwary. There’s a possibility of severe damage if jump leads are incorrectly or carelessly attached, and some manufacturers instruct owners to seek professional assistance when a flat battery is evident. Jump-starting involving any hybrid car as donor or recipient is absolutely forbidden for owners, for example. Loss of battery charge today is not an infrequent event though, and most frequently due to auxiliary circuits that drain battery capacity when the car is parked up for long periods; whilst there are many model-specific reasons and solutions for charge loss, in such circumstances use of an intelligent compact “trickle charger” may be advisable, if not a total solution.

Lead-acid batteries are relatively cheap to produce, but are heavy and bulky as a method of energy storage; so hybrid and electric cars have turned to more compact, but costly, NiMH and lithium-ion or lithium-polymer batteries for storage of their traction power. Yet both these types of vehicle still use small, relatively conventional 12 Volt batteries, whose charge is maintained by voltage-adjusted power drawn from the main battery pack, for auxiliary systems like lighting, heating, cooling, and windscreen wipers. Lightweight and compact lithium-ion batteries will possibly replace lead-acid batteries in time, as the cost comes down, and are already available for motorcycles and some exotica, like Porsche  911 GT3s, albeit at a shocking price. More likely, in the relatively immediate future, we may see car systems, including hybrid and electric vehicles, turn to 24 Volt systems as used in HGVs and commercial aircraft, to reduce weight by using lighter wiring gauges, which then might accelerate any moves away from the lead-acid battery.

One Response

  1. I think understanding the basics of a car battery is important. It would give you a better idea of what might be wrong if anything happened. I’d imagine that it would come in handy if you come across car problems and can’t take your vehicle in to get repairs right away.

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