Is it bad to jump your car with too many amps (i.e., with a 1000 amp jumper vs. a 400 amp jumper)? Or does the car regulate how many amps it will take?

EDIT: I was told 400 amps is enough for most cars. Is this true?

  • The specific numbers are probably different in the parts of the world where 110 V AC is standard, but in Europe with 230-240 V AC, 16 A is a common mains service feed fuse rating, and individual circuits may be fused to 6, 10 or 16 A. Even so, it's possible to hook up a LED light or cell phone charger that draws on the order of milliamps despite the fact that the circuit can provide a thousand times more. – user Jul 11 '15 at 12:46
  • RE- ABOVE -- Be aware This does not apply to voltage. If you have a 6 V bat and you connect to 12 V source, you can blow the battery up. THIS happened to Kevin 54 long Yrs ago.. In 1,963 -I hooked a truck's 24 V to a Stubborn FJ Holden Short {RECOND] engine. Luckily Front-Facing side of the 6 V bat went out of the Front of the Car, as it was able, due Only to BAT sitting high, on Fire- wall. But the Bat side tavelled Abt 10 Meters, in front of Car.. It is like Yesterday. -- Also a Bat whc was "known to be dead", -did Blow its water Out V Fast, into my Face, when i shorted across the posts, wit – Kevin Thorpe Jul 25 '17 at 6:41

Those numbers are the max that they can supply. Your starter will "automatically" use as much as it needs up to the max. If the starter needs more than those units can supply, the jumper will only provide its max.

If your battery is only partly dead, i.e. it won't start, but the interior lights come on, you can use a smaller jumper than if your battery was completely dead (and likely damaged).

400 amps is probably enough to start most small to midsize CARS and the smallest of SUVs and trucks. To start "bigger" vehicles you will probably need the 1000 amp unit.

The battery for your car is picked out by the engineers so it's cold cranking amps (CCA) number is bigger than the needed amps to start the engine. My truck battery says it produces 700 CCA so I can assume that I would need a jumper unit that can provide 700 amps or more if my battery is completely, hopelessly dead. I could use a smaller unit if my battery still had some juice.

Sidenote: If you're looking at one of those portable, "save yourself" units, don't buy it. They hardly ever work. Invest in a thick pair of reasonably long jumper cables.

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  • This is a good answer. I do have one small issue with your first statement. The unit will only supply as much power as it can give. There really isn't need for a fuse or circuit breaker in this case because it cannot be over drawn. I'm not saying they don't exist on the jumper box, but they really aren't needed. – Pᴀᴜʟsᴛᴇʀ2 Jul 10 '15 at 20:23
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    @Paulster2 Oh, I didn't know that. I just assumed I'd never used one past it's capacity, i.e. blow a fuse, but now that I think about it, your point is probably why I've never seen a blown fuse, haha. Thanks! Edited. – Zach Mierzejewski Jul 10 '15 at 20:28
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    I must disagree with your final comment "Sidenote: If you're looking at one of those portable, "save yourself" units, don't buy it. They hardly ever work. Invest in a thick pair of reasonably long jumper cables." - Firstly, I've seen a 'Save Yourself' kit jumpstart a Rover saloon that hadn't moved, or even turned-over, in a decade. I had bet it would never start and would need to be towed - and lost. Secondly, the implicit assumption with jumper cables is that there's something to connect them to - and thus they're totally useless if you're in an isolated spot with no way to summon help. – Eight-Bit Guru Jul 11 '15 at 13:39
  • One problem with the "save your self" jumper batteries is people carry them in their trunk in the winter. While it might be a lead acid battery inside, it doesn't have the cold cranking amps of a good battery. They generally aren't much use in the winter if kept outdoors. But in the summer or if kept under your desk at the office they can be quite handy. – bobpaul Feb 3 '17 at 20:12
  • Yes, the 'save yourself' comment belies a fundamental misunderstanding the way voltage, current , and charge are related. They don't need very much charge (the ability to sustain 14V & high amps for a very short time), so yeah even though they're small and light they can pack a huge punch, but you'll only have a few times to get it started until it needs recharging and then you'll wish you had good jumper cables. – davea0511 Nov 21 '19 at 16:59

The starter circuit should only draw whatever current it needs so the excess shouldn't cause any problem.

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The amp count tells you the maximum amount of current that the equipment can supply.


  • Your car needs 600 amps to start: The 1000 amp jumper can handle it.
  • Your car needs 1200 amps: The 1000 amp jumper can't do it. You'll have to find a bigger one.

Be aware that this does not apply to voltage. If you have a 6V battery and you connect it to a 12V source, you can blow the battery up.

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Think of electricity as water, where voltage is the pressure that exists due to height of the water column above the point where you are taking it (known as "head" in fluid dynamics), and current is the flow-rate (just as you would think when you think of water current - not a coincidence water-current is flow rate). Electrical current, i, like Water-current is dependent on a lot of stuff ... voltage (reservoir height) of course, but also the resistance of the wire (diameter of pipe), but mainly what is on the end of that wire ... if connected to ground, that's like nothing at the end of the pipe, connected to a resistor (like a nozzle) then ground (open end pipe), connected to a motor (like a waterwheel).

Consider the case where you are draining water at only 1 inch below the surface (1" of head, or lets liken this to 1 Volt). It does not matter if you're draining from a reservoir the size of a lake or a gallon of water if all you need is a quart of water ... the pressure and flow rate will be the same from both reservoirs.

Similarly a 12V system that only needs 10A but is supplied 10000A won't damage your system ... the current (flow rate) and voltage (pressure, or "head") will be the same. If however you need a gallon though and you're draining from a reservoir that's only 1 quart of water ... well then you have a problem.

So you should always use a source that has more amps than you need, but it does not matter how much more. Voltage is different ... too high of voltage is like too much pressure and you're sure to damage your system.

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  • Great answer! Visual of it makes it great! Thanks for posting :o) – Pᴀᴜʟsᴛᴇʀ2 Nov 3 '16 at 16:00
  • I like this answer, but I think current is more analogous to the inner diameter of the pipe/hose, and not the volume of the reservoir. (I think the volume of the reservoir would be 'Watt-hours'?) – Ryan V. Bissell Nov 3 '16 at 21:35
  • I also sort of like the answer, but I would add that too low of voltage means also trouble. The answer mentioned only too high a voltage as cause of a trouble. – juhist Jul 26 '17 at 13:56
  • Yeah, in retrospect, the analogy does break down a bit at the edges. I kind of like thinking of leaving the capacity to capacitance, and max current more as a nozzle at the interface of the reservoir ... which doesn't cause a problem unless the nozzle restricts the flow more than the circuit requires. – davea0511 Nov 20 '17 at 23:13
  • I wrote the original answer and what I wrote was misleading (current being compared to water reservoir size). Current is really analogous and not surprisingly is also intuitive to the FLOW-RATE of water (not surprisingly in hydrodynamics water current is flow-rate), not the capacity (reservoir size) behind it. Capacity is actually potential energy. When a charger lists the rated current, that is not what it will deliver, it is the maximum it is capable of delivering. It would be more intuitive & accurate if they called it "max current" instead of just "current" for chargers and batteries. – davea0511 Nov 21 '19 at 16:53

These are the maximum capacity of the leads. You should use the highest you can bear to carry or purchase, simply because the resistance should be lower (theoretically) and they will sustain a short circuit for slightly longer before melting.

When we speak of current, it is not the leads we worry about, it is the load we are putting on the "source", typically a charging circuit can supply 30A at capacity, and some batteries around 1000A. In reality when we jump a car, we are charging the local battery and the current required drops off as the voltage rises, and then surges again upon cranking. You should probably not be running a modern car without a functioning battery.

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It doesn't matter how many amps a battery can put out. Your starter motor is based on wattage (volts multiplied by amps). At 12 volts, your engine will require roughly 200 amps. As your voltage drops in your battery while cranking the amperage goes up to compensate, so at 9 volts you may be using 250 amps. So if you have a battery with 300 cranking amps, it should start any gas automobile.

For another example, your hair blower is based on wattage. If it is plugged into an outlet with a 20 amp breaker and you switched out the breaker for a 50 amp breaker, would you expect your hair blower to blow harder? It will draw 10 amps no matter how many you offer it. Put a 20,000 amp battery in your car and it will crank the same. The only time you need more amperage is when your battery is crap and the voltage falls off, or your starter is toast, in that case they can draw up to as many amps as the battery can supply.

For a 1.7 kilowatt (1700 watts) starter, if your starter falls to 10 volts when cranking, the volts multiplied by the amps will give us 1700 watts. So 10 x amps = 1700, meaning amps = 170. If your battery won't hold the voltage and it falls to 5 volts then we will be needing 340 amps.

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As silly as this is one thing a lot of people misunderstand, yea the 400amp unit to boost a vehicle that requires 700 amps to star "shouldn't" work, but most times your battery is 100% completely dead, is because you left something like a light on over night. In that case, you don't need to boost your car, your 400amp power pack is more than enough to charge your battery. I've had this issue before, if you're not in a super hurry, just connect up your 400amp unit like you would to boost your vehicle and leave it for 2-3 minutes, you'll then be able to start your vehicle no problem, because you'll have the combined output of your (weak) battery and your booster pack.

It's also note worth that if your battery is completely dead, even if your booster pack is more than powerful enough, you may still have to wait a minute to charge up the battery, if your pack puts out 700amps and you need 700 amps to start, well, sorry to say your battery is going to draw a part of that power away to charge it's self while it is completely dead. I use a 400amp pack, and I've boosted my 2011 Ford Escape (3.0L turbo) with a dead as dead can be battery (left a light on) in -35°C by simply letting the pack charge up my battery first (and Yeah I did try to start it right away, no dice).

I will say if you have the cash to drop for a 1200 amp unit do it! But my $60 400amp pack from Walmart has lasted me two years now and given me and friends about 30 or so effortless boosts, as long as you understand how your battery affects the system. And finally yes, if your battery is pooched, end of life and won't charge again, then you probably just need to invest in a new battery, and not a bigger booster.

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  • Surely on some conditions it may works. But it can also damage the jumper leads, the batterry plugs, both batteries (boiling, dropping hydrogen, overheating while out of range), the starter motor, the alternator, the ecu unit.. , following the ohm law. Because the voltage will drop much more under 12V while dropping 700amps, the amperage himself will drop up, to more than 700amps in few seconds. – user3312 Jan 12 '17 at 17:50

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