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.


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?

3 Answers 3


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.


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.


Wait for half a minute after connecting and before starting the engine and it'll work just fine. Not all current has to come from the jump starter. The jump starter can rapidly charge the car battery, and then the jump starter and car battery work together to provide the needed cranking amps. But before doing so, please be certain that your car battery is working and just flat, instead of being faulty.

The problem with jump starting cars having faulty batteries is that the jump starter or the donor car battery provides enough buffer for the electrical system. Once you disconnect, the only buffer is the faulty battery in the recipient car. If it's faulty, it can't act as a good enough buffer, meaning you will have voltage spikes after disconnecting the donor car or jump starter, and those voltage spikes can do expensive damage to car electronics.

If the battery is flat, it can't operate the starter motor. By once you connect the jump starter, it rapidly charges the battery. If you charge the battery in 30 seconds and then for 3 seconds operate the starter motor, it essentially multiplies your current by 10. The 400 amp jump starter would effectively work as a 4000 amp jump starter (except the car battery can only give 850 amps, limiting the performance).

Actually, if you don't know if the battery is only flat or faulty, it might be a good idea to charge it for a few minutes with the jump starter, remove the jump starter, and try starting the engine on its own. That eliminates any chance that a faulty battery could be too little buffer for the electrical system, causing expensive damage from voltage spikes.

Also the 850 amperes of the battery are probably needed only during very cold days. If the day is not that cold, the 400 amperes might be enough without needing any more amperes from the flat battery.

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