flat battery question

dodgyme

right will be brief. A long time since I did this but on the net the advice when recharging a battery is very contradictory. Firstly the AA site says to rev the boosting car for upto 10 mins and then turn both cars off. However most other sites say to leave the flat car running and turn off the boosting car. Essentially removing the cables with the previously flat car still running. Then drive the flat car to refresh the battery.
Some sites say to remove the black cables first, other say to remove the red cables first. Some say to connect negative to negative and other say to go negative on the boosting car and clip to the engine block on the flat car.

Mr Magnolia

Hook up leads. Let the car idle for a while with leads connected. After a few minutes start the car to be 'jumped' while rev'ing running car.

Remove leads with both cars running. Leave the car with the dodgey battery running for a while to charge the battery back up.

Works for me anyway.

Avns1s

What's your question......... because as far as I can see all of the options you describe will work.

Stark

dodgyme wrote:

Some say to connect negative to negative and other say to go negative on the boosting car and clip to the engine block on the flat car.

Both have worked for me.

I think the best way is to leave the flat car running when you remove the cables. If you switch off the engine and remove the cables, you might not have enough charge to start it. Then take the car for a spin (about 20 mins) to charge.

dodgyme

Avns1s wrote:

What's your question......... because as far as I can see all of the options you describe will work.

That is my question why are there so many options for such a simple task?

Healyc

Mr Magnolia wrote:

Hook up leads. Let the car idle for a while with leads connected. After a few minutes start the car to be 'jumped' while rev'ing running car.

Remove leads with both cars running. Leave the car with the dodgey battery running for a while to charge the battery back up.

Works for me anyway.

Do what he said!!!!!;)

Fey!

As Mr Magnolia said.

Don't turn off the "boosting" car, in case it's battery is weak and you end up with 2 non-starters!

Also, I tend, out of habit, to connect the negatives first, then the positives. I remove the positives first, and then the negatives. Stops me touching the leads together or connecting negative to positive by mistake.

Related: on some cars the battery is in the boot with connection points in the engine. Should you only use the points in the front to boost to/from another car, or can you link direct to battery? (some old wives tales say you can't connect to battery on these cars)

Avns1s

Fey! wrote:

As Mr Magnolia said.

Also, I tend, out of habit, to connect the negatives first, then the positives. I remove the positives first, and then the negatives. Stops me touching the leads together or connecting negative to positive by mistake.

You should actually do this the other way around as if you have the negatives and you drop a posative on some part of the engine you'll short out possibly ruining your alternator in the process.

Always connect the posatives first and disconnect them last.

Avns1s

dodgyme wrote:

That is my question why are there so many options for such a simple task?

OK I see your point but "there's more than one way to skin a cat" ... as they say.

Some points depend on the situation.

It BeeMee

dodgyme wrote:

Some say to connect negative to negative and other say to go negative on the boosting car and clip to the engine block on the flat car.

It's the same thing really: the negative on a battery goes to the chassis, in effect turning the whole body into one bit negative terminal. Sometimes it can be easier to put the jump lead directly to the engine block or the chassis, as the position of the battery may make it's own terminal difficult to access.


I heard somewhere before there is a danger of triggering the airbags on the running car, something to do with a spike of current coming back up the cables that triggers the sensors?
Anyone heard of this, or is it an urban myth?

cantdecide

Stark wrote:

I think the best way is to leave the flat car running when you remove the cables. If you switch off the engine and remove the cables, you might not have enough charge to start it. Then take the car for a spin (about 20 mins) to charge.

Not to cause more consternation but I always thought the last bit to be true but on many cars, taking it for a spin when you're done won't do too much apparently-

I know of 2 recent occasions in my family where a presumably knackered battery lasted at least 6 mths and 12mths respectively.

I've always been of the impression that under normal circumstances, ie, unless you've left your lights on, or the there is a specific reason for the battery going flat, when you need a jump start, your battery's number is up.

But after taking them out of the car and getting them charged overnight by 230V charger, both cars were traded in without any signs of weakness in the battery.

Del2005

It BeeMee wrote:

I heard somewhere before there is a danger of triggering the airbags on the running car, something to do with a spike of current coming back up the cables that triggers the sensors?
Anyone heard of this, or is it an urban myth?

Not only airbags. Most manufactures recommend that you don't jump start their cars or jump start another car as you can blow the ECU and other expensive electronic parts. But there are jump leads available now that are supposed to cover this, don't know how good they are though.

cantdecide

I've heard of ECU problems when 'bump' starting.

If you were to connect it up for a wee while and then disconnect the flat car from the boost car, assuming it has charged enough to get it to start, would this be a way of avoiding damage/ malfunction?

Esel

THe correct and safe way is:

+ good battery to + flat battery

- good battery to any good earth in engine bay of car with flat battery.

Never connect to - flat battery. There is a danger of explosion, apparently.

JUst because lots of people do it and the battery does not explode does not mean it is right or safe.

colm_mcm

the way I remember it is "GaBLE" read it in Auto Express years ago.

Remember GaBLE - Good and Bad, Live (+) then Earth (-). Connect the 'good' car to the bad (flat battery) car, using the live (red) lead before the earth.

daedalus2097

Right, from an engineering/electrical point of view:

- The positive should be connected first, battery + to battery +.
- The ground should be connected second to avoid shorting out by dropping or twisting a cable.
- The negative should be connected to the bood battery's negative terminal, but to a piece of bare metal on the chassis/engine on the flat car to avoid sparking around a battery which has possibly been leaking (explosive) hydrogen gas.
- The good car should be running when the leads are connected as the dead battery can place a large strain on the good battery and the good car might not start with the extra load.
- When connecting or disconnecting, both cars should have their headlights off to avoid spikes damaging the bulbs, and rear demisters and heater fan on to absorb any spikes in the power lines.
- Good car doesn't have to be revved hard as modern alternators produce enough current at say 2000 RPM as they do at 6000.
- After running for a few minutes, the dead car should start no problem.
- Leave both cars idling for a minute or so.
- Disconnect negatives first, for the same reasons you connect them last.
- Disconnect positives last.
- Some cars (e.g. some Volvos & BMWs) have batteries elsewhere. These generally provide a covered positive terminal in the engine bay which should be used for jumping.
- There is always a risk of surges and spikes damaging stereos, ECUs, alternator regulators, airbags and so on, but following the points here will reduce the risk. Most other methods will work as it's a simple matter of sending power across a cable, but carry a greater risk of damaging sensitive electronics.

demanufactured

lol we always just bump start a car with a push or down a hill , never did any harm.

run_Forrest_run

colm_mcm wrote:

the way I remember it is "GaBLE" read it in Auto Express years ago.

Remember GaBLE - Good and Bad, Live (+) then Earth (-). Connect the 'good' car to the bad (flat battery) car, using the live (red) lead before the earth.

My Haynes manual states to connect the bad to the good using the live (red) lead before earth..and I'm still alive