Jump-Start Fail

[Gnome]

Wouldn't make a difference either way. If the jump leads are tick enough (wire gauge wise) and the battery sizes aren't substantially different between the cars you don't even need to start the other car. The battery will be doing most of the work anyway, most car alternators are rated to about 45-90amp. A small car starter alone pulls 83-85amp. The x-trail probably more. Furthermore alternator ratings are the maximum current output, that means you'd have to rev the car to the limit to get that power from the alternator. On top of that the amount of current required is increased because the cars alternator is sapping power (that is the reason a starter motor has the distinctive whir whir whir sound, it's a showdown between the alternator and starter motor).

Revving the car will help a little but not much, that is why cars specifically use batteries that can handle huge amounts of current for short periods of time.

Lead-acid batteries for automotive use are made with slightly different construction techniques, depending on the application of the battery. The typical battery in use today is of the "flooded cell" type, indicating liquid electrolyte. AGM or absorbed glass mat type batteries have electrolyte immobilized as a gel. This article deals with the flooded type of car battery.

The starting (cranking) or shallow cycle type is designed to deliver large bursts of energy, usually to start an engine. The SLI batteries usually have a greater plate count in order to have a larger surface area that provides high electric current for short period of time. Once the engine is started, they are recharged by the engine-driven charging system. See Jump start (vehicle).

The deep cycle (or motive) type is designed to continuously provide power for long periods of time (for example in a trolling motor for a small boat, auxiliary power for a recreational vehicle, or traction power for a golf cart or other battery electric vehicle). They can also be used to store energy from a photo voltaic array or a small wind turbine. They usually have thicker plates in order to have a greater capacity and survive a higher number of charge/discharge cycles. The specific energy is in the range of 30-40 watt-hours per kilogram.[3]

Batteries intended for starting, lighting and ignition (SLI) systems are intended to deliver a heavy current for a short time, and to have a relatively low degree of discharge on each use. They have many thin plates,thin separators between the plates, and may have a higher specific gravity electrolyte to reduce internal resistance. Deep-cycle batteries have fewer, thicker plates and are intended to have a greater depth of discharge on each cycle, but will not provide as high a current on heavy loads.[4]

Some battery manufacturers claim their batteries are dual purpose (starting and deep cycling).

Also sparks when you connect the jump leads are normal. There are critical systems that are directly wired to the battery (IE. no way to turn them off), they draw current and that will cause a spark when you connect the lead. For example the ECUs, the car radio (even when you radio is off it pulls power), car clock, security systems, etc. etc.

 

[Colin62]

I been thinking about it. Your flat battery might have saved you a bit, It would have drawn the major charge from the other car.

Not if the jumper leads were connected the wrong way.

For the record, the car should no spark when you connect the jumper cables.
The correct procedure is as follows.

Make sure everything is off in the car to be started. EVERYTHING! Radio, aircon, Garimin, lights. The works.
Start the car to do the jump, connect the positive of the jumping car to the positive of the car to be jumped. Then connect the negative in the same order.

The flat battery will draw more current than your Garmin by a long way, and you will almost always get a spark. Even if you connect a fully charged battery to a flat battery with neither connected to anything else, you'll get a spark. Switching everything off is the correct thing to do - it minimises the current going to waste so that all of the current travelling through the cables is available for charging the flat battery and starting the car.

Most manufacturers recommend clamping the negative directly onto the chassis of the car being started, and not onto the battery terminal. It should make little difference if it's just the battery that's flat, but if there is corrosion and increased resistance on earth link, it can make a difference.

Then while the jumping car (also with all non essential stuff off) revs, you try and start the car.
If the car to be jumped's battery is too flat, you can jump untill you are blue in the face, it will not start.

It's best to rev the donor car at about 2000 rpm for a little while. This places some charge in the flat battery which means that it can contribute some current towards getting started.

Note!! Driving a car with a flat battery will NOT charge the battery. It needs to be put on a charger. The alternator can only put back what has been taken out by starting the vehicle. It cannot charge a flat battery.

The alternator can charge a flat battery. That's exactly what it is there for. It won't charge a completely flat battery on a short trip, but if you drive for an hour or longer, it should charge your battery. If it doesn't, then your alternator isn't working properly. Many alternators today can deliver well over 100 amps. That is plenty to both run the car's electrics and to charge the battery too.

 

[Colin62]

Wouldn't make a difference either way. If the jump leads are tick enough (wire gauge wise) and the battery sizes aren't substantially different between the cars you don't even need to start the other car. The battery will be doing most of the work anyway, most car alternators are rated to about 45-90amp.

Most alternators will be able to supply their full current at about 2000rpm. The idea behind revving the donor car for a while before jump starting is to place some charge into the the flat battery, so that it will assist in starting the car (and at the very least not take current away from the starter motor). Just using the battery, with the donor engine not running may work, but the alternator runs at a higher voltage than a battery (especially a battery under load) and is therefore (from Ohms law) able to provide extra current against the same resistance. This may make no difference if the flat battery is not completely flat, but it can make the difference between success and failure.

On top of that the amount of current required is increased because the cars alternator is sapping power (that is the reason a starter motor has the distinctive whir whir whir sound, it's a showdown between the alternator and starter motor).

The alternator draws mechanical power from the engine and converts it into electrical power. I'm not sure which car's alternator you're talking about sapping power, but neither are (much). The donor car's alternator is delivering current. The other alternator is using a small amount of mechanical power in being turned, but at starting speeds, it is not generating any current, so this will be to overcome friction, which is trivial compared to the amount of friction to be overcome to spin the engine. There is no showdown between the alternator and the starter motor when starting a car. The surges in speed you hear are from the differences between turning the motor in a compression cycle compared to any of the other three cycles.

When the battery is flat and you hear a loud clacking noise, that is the starter solenoid. It closes contact when you turn the key, and the starter motor draws power, which causes the battery voltage to drop. If this drop is big enough (on a flattening battery) the voltage will be too low to keep the solenoid in, and it releases, which disconnects the starter motor. The battery voltage recovers and the cycles repeats.

 

[SlinkyMike]

...battery ran dead because a door was left open.

You serious?

 

[assagai]

You serious?

yeah for about 24hours

 

[Mars]

May I stick my 2 cents in, I don't believe turning everything off is good practice, I think that something should be on like the radio in order to consume power from a slight surge? Surely by having everything off the chances of blowing something are higher? The radio would act as a release to the 'pressure' of a boost in power to the car?

Well, not really, the reason everything should be off is because the flat battery will draw most of the power straight away, you don't want to be sharing current with other stuff.

The flat battery will draw more current than your Garmin by a long way, and you will almost always get a spark. Even if you connect a fully charged battery to a flat battery with neither connected to anything else, you'll get a spark. Switching everything off is the correct thing to do - it minimises the current going to waste so that all of the current travelling through the cables is available for charging the flat battery and starting the car.
Most manufacturers recommend clamping the negative directly onto the chassis of the car being started, and not onto the battery terminal. It should make little difference if it's just the battery that's flat, but if there is corrosion and increased resistance on earth link, it can make a difference.

Agreed, but technically when a car is off, the current draw should be minimal. Yes the flat battery is minimal, and the negative should be connected to part of the body, but in my experience people have trouble connecting the negative to a part of the body. Sparking always happens, but when you connect the cables correctly you can avoid most sparking, to the point of not seeing any, even on a flat battery.

It's best to rev the donor car at about 2000 rpm for a little while. This places some charge in the flat battery which means that it can contribute some current towards getting started.

Not actually. Reving will not really make a difference to the current that the alternator outputs because its regulated, unless your alternator is faulty. The current will come from the fully charged battery. if you leave the batteries connected for a bit, the charge will move from the full one to the empty one. This will benefit the flat battery, but not so much the fully charged one. Revving will not make a difference to the target battery.

The alternator can charge a flat battery. That's exactly what it is there for. It won't charge a completely flat battery on a short trip, but if you drive for an hour or longer, it should charge your battery. If it doesn't, then your alternator isn't working properly. Many alternators today can deliver well over 100 amps. That is plenty to both run the car's electrics and to charge the battery too.

In theory it could, but actually? No it won't. This is a misconception I deal with every day. A car battery is what is called a shallow cycle battery, or a battery that operates within 82% to 100% of of ts charge capacity. It should not nor is it made to be drawn flat. In fact drawing a car battery flat will damage it as the active material will come off the plates. Do this a few times and the battery will not longer take charge.
Within that 18% area the alternator will charge it easily. Drop below that and the alternator starts to struggle. This can cause weird effects in cars, especially your higher end ones. I had a customer with an A class merc that started perfectly, but while driving would cut out intermittently. After spending a few days at the auto electrician he bought a new battery and it sorted out all the trouble.

So in a nutshell,the flatter the battery the more current it needs to charge. The charging cycle works in a bell curve. For a completely flat battery, the alternator just does not generate the current needed to get it past the lowest part of the curve. In addition, the flat battery then draw the current away from the other components and could possibly damage the electrical system.

 

[pb41]

An auto electrician once told me to turn on the headlights of the car with the flat battery. This would avoid potential damage to electrical components when jump starting. Not sure about all the technical reasons, either a surge or drop in voltage or current, but he seemed quite convinced about it. Seems counter intuitive as well because the lights would drain more power.

 

[assagai]

anyone know where i can get the part?
it's a 100A fuse, tried some nissan dealers but they don't have.

 

[HavocXphere]

ok - just checked the fuse box which is situated next to the battery. The biggest fuse is labelled 001 - BATTERY and it is blown

Lucky.

For the record, the car should no spark when you connect the jumper cables.

I don't see why not? I know if the battery is disconnected & you reconnect it it sparks so I think it would do the same if you connect jumpers.

Note!! Driving a car with a flat battery will NOT charge the battery. It needs to be put on a charger. The alternator can only put back what has been taken out by starting the vehicle. It cannot charge a flat battery.

Why? Surely there alternator can apply juice to the battery regardless of its state? Not trying to be difficult btw...really want to know.

An auto electrician once told me to turn on the headlights of the car with the flat battery. This would avoid potential damage to electrical components when jump starting.

Heard a similar story for BMWs. Though I recall that its the car w/ charged battery that supposed to have the lights etc on. Something about protecting the ECU.