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I have a 3.0 litre V6 Diesel Jaguar XJ which has a 850amp battery.

A fair number of people on this 400amp Amazon jump starter say it started their 3.0 litre cars fine.

http://www.amazon.co.uk/PORTABLE-STARTER-COMPRESSOR-BATTERY-BOOSTER/dp/B00IDCMLFE/ref=zg_bs_4923209031_9

Can something with half the amperage still crank the engine? If its 12v x 400amp surely its a lower wattage/energy output.

Also, I read some jump starters can damage your electrical components, but surely if its a low amperage than your battery, its not going to surge it?

6

The cranking amps required to start an engine are not the same as the cranking amp rating of a battery. I don't know the amps required to start your car's engine, but if it is less than 400 amps, then it will work.

Higher cranking amps in a battery yields additional benefits. The battery can be used for more starts because less capacity is used on each start. The battery can be used for starting in colder temperatures because the reduced capacity caused by the cold weather leaves enough left over to still crank the engine. Since these things are probably not required for the jumpstarter, the manufacturer may choose to use a smaller battery

Additionally, the car's battery is expected to power loads without an alternator, so it is performing more duties than just cranking the engine. Since battery cranking amps and capacity are directly proportional in a lead acid battery, the higher capacity may be for powering other loads, giving it a higher cranking amp rating as a benefit. Since the jumpstarter is a single purpose device, the battery does not need to have such a high capacity, thus only needs to produce enough cranking amps to start a vehicle.

That jumpstarter is rated at 12 VDC, which I assume is the same as your car's battery. It's possible that if someone connects the leads in reverse, they may damage their vehicle's electrical system. However, the same would be true when using jumper cables.

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The idea isn't to replace the amperage of the battery, but to boost (add to) what charge is available from your battery. The lower amperage output is fairly typical. For this reason and for the simple reason that these devices are made to be portable. If it had the same amperage as the battery in your car, it would weight about the same amount. Since your vehicle has a diesel fueled engine, it will require more amperage from the start (due to higher starter need and glow plug requirements). 400 amps may not meet that requirement. If what the amperage of the battery in your vehicle plus the amperage of the jumper box is not sufficient to meet that need, it won't be able to start your vehicle (that may be fairly obvious at this point). The description on the Amazon page says that it will "start any vehicle ...". If you take them at their word, you should be golden. Unfortunately that's one of the risks you take when purchasing anything, whether online or not, if it's going to work. Thank goodness for the Amazon return policy.

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