ITíS A NOISE we all learn to dread over time, particularly if youíre messing around with old cars. You know the one: that too-slow whirring sound that turns into a groan as a failing battery struggles to turn over your engine. It slows, it grinds, it stops. Nothing.

Listening to someone else do it drives me crazy. Iíve learned over time that when you hear that slowing sound, just switch it off. Turning it until life is extinct just does far more harm than good. The odds against the engine starting under those circumstances are high, meanwhile youíre straining starter gear and washing cylinders with unburned petrol.

There are a few things you can do: get a set of jumper leads and another car with a healthy system; put the battery on a charger and wander off to do something else for a few hours; hook up a booster battery. You may recall seeing the common workshop version of the latter, which generally was a full-size car battery on a trolley, with a set of jumper leads already hooked up. A great device, but hard to justify for most people.

Thanks to the development of lithium technology, the whole concept of booster batteries has changed. Now something that will start a car is light and and roughly the size of a paperback book, or smaller. They hold their charge for very long periods of time and can be recharged in many cases by a USB port.

There are several offerings out there, and most include a torch, plus the ability to charge up other devices such as smart phones.

Weíve been playing with two over recent months: a NOCO Genius Boost GB40 1000A and Cygnett 1200mAh. The former costs anywhere from $170 to $240 depending on the retailer, while the latter is priced at $200.

One of the issues youíll come across is how makers describe the ratings on their product Ė itís a little like the different units for torque and horsepower for your engine. The obvious solution is to find a common measurement. NOCO rates its product at 7000 joules, while Cygnett (after doing some conversions using some readily available online factors) works out to 7800 joules. Not a massive difference Ė but I guess every bit helps.

Cygnett says its product will start a dead petrol engine of up to five litres capacity. Iíd take this as an advisory rather than a hard and fast rule.

Since Iíve had these units, Iíve used them a lot, particularly on the mighty Kingswood. The issue there is the battery is probably a little tired and the car sits for long periods.

Yes, I do put it on a charger, but the difference between starting with the existing lead acid battery and with the booster attached is huge. The additional grunt means it has no trouble turning over enough to get fuel pumped into the carburettor and then it fires without hesitation.

Both the units here work well, so whatís the difference? The NOCO has larger battery clamps. The overall kit is simpler Ė just the unit, the clamps and a USB charger cable. The whole lot comes in a bag with a manual.

As for the Cygnett, it comes in a hard case with a much wider array of cables and attachments, including gear to recharge it from an old style cigarette lighter power socket. It claims a little more grunt and has slightly smaller battery clamps that might be better in tight situations.

Both have built in lamps that can be used in various modes, including as a torch or emergency beacon. Iím pretty happy with both.

These days, I just use them as a matter of course on anything that hasnít been run for a while Ė it makes life a whole lot easier.

Iíve also taken to throwing one in the boot whenever I go on a trip with an old car. Itís easy insurance.

Cygnett and NOCO both make a range of boosters. If you have a modest fleet of vehicles, youíre likely to find one of these things is invaluable.