Car wont jump start

NeverEnough

You might be able to revive it with a good charge off a CTEK type charger with a desulphating program, but an 8 year old battery in freezing conditions is likely to be at the end of its useful life, to be honest.

---lee---

Dr._Shoe wrote: »

It doesn't actualy matter. My ex has jumped my petrol car with his diesel and vice versa.

Indeed, but I would expect a diesel to have a bigger battery than a petrol.

AdrianC

---lee--- wrote: »

Indeed, but I would expect a diesel to have a bigger battery than a petrol.

A diesel may well have both a higher-capacity battery and a higher-capacity alternator. Neither will particularly matter, because any old battery and alternator is likely to be delivering far more current than the average jump leads will be able to deliver to the flat battery.

Joe_Horner

Not many jump lead sets are up to starting a car "directly" from the other car.

The stall torque of a small petrol starter motor can be 500 amps - that's the initial current draw for the fraction of a second before it starts turning the engine. On a diesel it can easily top 1000 amps.

Now, a set of (for the sake of illustration) Halfords 35mm2 3m jump leads have a resistance of about 0.0015 ohms if they're pure copper, which is unlikely. You can also expect at least the same again at each of the clamps, giving a total resistance of around 0.005 ohms.

That's not a lot of resistance but, when you try to pass that initial 1000 amps through it, ohms law tells us that you'll drop a full 5 volts across the leads. And that's ignoring any other losses in the system like the car's own wiring, internal resistance of the battery, and so on.

No matter how good the "donor" battery is, that's going to drop the voltage at the dead car end of the leads to about 7 or 8 volts, which is not enough to crank very much at all, and the starter solenoid will normally simply drop out giving an annoying "click click click" but no cranking.

All "home" jump leads are actually designed to charge the dead battery in order to attempt a start, not to supply the full starting current themselves.

What you're meant to do with them is connect the leads, start the donor car (keep all other electrics off), and let the full 70 amps or so of goodness available from its alternator to flow down the leads into the dead battery. Give it 10 minutes or so and you'll be good to go.

That's why it really doesn't matter what size car you're jumping off (I've happily jumped 2 litre diesels off our Daf 66 with 35 amp alternator before - you just have to wail a little longer for them to build up enough in the dead battery!)

The good news in doing this is that even a completly screwed battery will usually take enough surface charge on the plates to get the car started.

Alternatively, get a set of very serious industrial jump leads, but it'd probably be cheaper to replace your battery and buy a spare to carry round for next time!

Marktheshark

abc123456 wrote: »

Thanks Mark...what is supposed to happen anyway ?

If the earth is bad clipping a jump lead between the negative and engine either temporary cures it or rules it out.
Can save buying a new battery when it is just an earth lead that needs cleaning

bartelbe

Having just repaired the electrical system on my old diesel, it is a myth that you need special jump leads to jump start a diesel. Neither will a dead battery cause any problem, mine was flat as a pancake.

Joe_Horner

bartelbe wrote: »

Having just repaired the electrical system on my old diesel, it is a myth that you need special jump leads to jump start a diesel. Neither will a dead battery cause any problem, mine was flat as a pancake.

Absolutely agreed if you use them as they're intended by leaving the other car running for a while to get charge into the dead battery rather than relying on the leads to take the whole strain.

abc123456

I have had the battery on charge since my last post and have reconnected it to the vehicle. It started first time so I have taken it for a half hour run on the motorway.

However, when I disconnected it from the battery charger, before putting it back into the vehicle, I measured the voltage between the terminals for a 2 minute period. During this time, the voltage dropped 1/20th of a volt. Now, by my calculations, in the space of an hour, the voltage will have dropped from around 12v to 10.5V.

Is an initial rapid discharge normal when a car battery is diosconnected from a charger, and should it even out (if not being drained), ie in a stationary vehicle?

Or is it likely that the battery (being 8 years old) is just dying a normal death? :A

Marktheshark

Overnight, it is likely to be flat again in the morning.
New battery time by the sounds.

AdrianC

abc123456 wrote: »

Is an initial rapid discharge normal when a car battery is diosconnected from a charger

Yes, perfectly normal.