CORDIAL RELATIONS

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Keep your mobile’s battery topped up and ready to go when you’re out and about. Rick Maybury has some handy hints and tips for buying a car cord

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A car power cord is one of the cheapest and one of the most useful accessories you can get for your mobile phone. Car cords, also known as talk and charge adapters, connect the phone to a vehicle’s electrical system, which provides an almost unlimited source of power, to operate the phone and charge its battery pack. They’re not just for cars either, they’re equally useful in boats and caravans.

Used with a dashboard holder or ‘hang-up cup’, it can form the basis of a simple car kit. At this point we feel obliged to tack on an important health and safety warning and remind you of the danger of using a mobile phone whilst driving. If you want to make or take calls with this kind of set-up you should pull over and park. You really should get a ‘hands-free’ car kit, if you plan to regularly use your phone on the move. Nuff said...

Most car cords look pretty much the same. They generally comprise a cigarette lighter-type plug attached to a small black box. A curly lead emerges from the back of the box; this is terminated in an accessory plug, that fits into a socket on the base of the phone. There’s usually a red LED indicator on the module, that shows it is working. On some models this changes colour to green or amber, or flashes, to indicate that charging is in progress, or has finished. Inside the module is a charging circuit. It’s job is firstly to regulate the supply voltage, and secondly, to control the output current, drawn by the phone and its battery.   

Regulation is vital. The voltage on your car’s electrical system can fall to below 11 volts on a cold day, and rise to more than 15 volts on a run. Twelve volts is far from standard either, many large vans and lorries have 24 volt systems. A car cord also has to filter out or suppress high voltage ‘spikes’,  generated by other electrical devices in the vehicle. Most phones require a stable 6 volt DC supply, and whilst they normally have built-in over-voltage protection systems, too many volts are bad for them and delicate microcircuits can be destroyed by surges. Accurate current control is just as important. If a flat battery is allowed to draw an excess current it can overheat, which may lead to permanent damage to the phone, or battery, or both.

The vast majority of car cords on the market are designed to work with nickel cadmium (nicad) and nickel metal hydride (NiMh) type battery packs; if your phone uses Lithium Ion batteries you should consult the manufacturer first, to see whether suitable car cords are available. Although most car cords look alike, internally there can be some important differences. These mostly concern the type of charge circuit, which can affect the life expectancy of your phone’s battery.

Ideally a car cord should work in exactly the same was as the desktop charger supplied with your phone. That means initially supplying the battery with a large current, that gradually reduces as the battery reaches full capacity, after which it reverts to a trickle-charge mode. The charger circuit determines the battery’s condition by carefully monitoring the voltage; there’s a small drop in voltage at the point when the battery reaches full charge (a technique known as negative delta V). At least one type of car cord uses a stepped-current pulse charging system, that is supposed to be better for the battery.

Another option is a trickle-charger, that deliver a constant low current. These take a lot longer to charge a battery from flat --  15 to 20 hours, depending on the capacity -- so they’re more suitable for top-up charging and long journeys... Whilst we’re on the subject, you may also come across ‘battery savers’, though there’s not so many of them around these days. They’re easy to spot, they normally have standard cigarette lighter plugs  and instead of the phone accessory plug, there’s a dummy battery, with a regulator circuit inside. This replaces the phone battery, powering it directly from the car’s electrical supply. Clearly the battery won’t receive a charged and quite frankly we can’t see the point, but they’re available if you need one.

So how do you sort the wheat from the chaff? Check the build quality; poor quality plug/charger modules are sometimes glued together and some we’ve seen have been put together so badly that they were literally coming apart at the seams, even before they were removed from their packaging. Colour change or flashing LED indicators are a good sign that the charger has a more sophisticated  charging circuit. Models rated for 12 and 24 volt supplies suggest that the regulator circuitry can handle a wider than normal voltage variation. Adapters that can cope with differently sized cigarette lighter sockets always get a few extra Brownie points.

Check that charger has an easily replaceable fuse; we have come across a couple of models with ‘automatic’ fuses or cut-outs; there nothing necessarily wrong with that, but it should be clearly mentioned on the packaging or instructions. The final piece of advice is mostly common sense, and that’s to stick will well known brands, brought from reputable suppliers. There’s an awful lot of cheap and occasionally nasty car cords kicking around at the moment, turning up in street markets, car-boot sales and the more dubious phone suppliers. You can tell a lot from the price, the vast majority of car cords sell from around £20 upwards, any costing significantly less should set your alarm bells ringing.  Some of them are probably okay, but do you want to take the chance? If they fail or fry your phone you’ll have very little comeback and it will almost certainly invalidate your handset’s guarantee.

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MORE HANDY HINTS

* There’s no such thing as an universal car cord,  you must get the right one for your make and model of phone.

* Check that the operation of LED indicators is fully explained in the instructions; one car cord we’ve tested assumed the user knew what flashing amber and green lights meant...

* Engine starter motors can create transients of several hundred volts but not all car cords have effective surge protection, so it’s good practice not to connect  the phone, until after the engine is running. 

* If your phone is a recent model, powered by a high-capacity lithium ion (Lio) battery your options are very limited, usually to manufacturers own products.

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Ó R. Maybury 1997 1204