Car battery jump-starting gizmo?

[Andrew Gabriel]

Nowadays, the alternator is often controlled by the engine management (which is essential for some batteries) and it won't produce any output at all unless the EMU is already working. Indeed, some (maybe even all) EMUs won't let EMU controlled alternators start generating output until they are sure the engine is started, to reduce the load on the engine whilst it starts and the starter is pulling the battery volts down, which would otherwise cause a heavy alternator load.

With a flat battery, it takes less than 5 minutes with a 4A charger to get enough into it to start the engine, providing the engine normally starts first time. One problem you might have is that many EMU's lose their learned memory of the optimum engine settings when they lose power, and that can mean that an engine that normally starts first time might not in this case as the EMU falls back to initial factory settings until it relearns the engine characteristics.

[Fredxx]

An alternator doesn't spin fast enough to generate meaningful volts whilst an engine is being cranked. However depending on the type of alternator, it does make sense not to provide field current until the engine is running, so at least the drain on the battery is minimised before and during cranking.

[Dave Plowman]

Must be cheap ones then. The minimum should be 16mm and decent ones are

1. But cost.

[Dave Plowman]

The one I bought from Lidl for 20 quid started my V-8 Rover. Of course it might depend on how flat the battery is and how quickly the engine normally starts. And how thick the oil is. However, the battery in them doesn't last long if subjected to high discharge.

[Dave Plowman]

Every EFI system I know requires a signal from the ignition before the injectors will work. For that reason.

[Harry Bloomfield]

Dave Plowman laid this down on his screen :

It's really a matter of how long you are prepared wait, for enough charge to be put into the flat battery, to be able to start the car. Connect with thick enough cross section cables and good clips and you would be able start as if the good battery were in the car. Most jump leads with a completely flat battery need the engine running and a few minutes, before there is enough to start the car.

Rather than full size jump leads I carry in my boot for emergencies, a pair of home made leads. I used four of the large clips as used to be used on old battery chargers and for leads coax cable - not TV coax, but commercial coax which is double copper screened. Very flexible and they do contain quite a lot of copper, but not enough to provide an instant start - five minutes of charge and even my biggest engine will start.

[Laurence Payne]

can't be just put in the boot and forgotten. But keep it charged up at home and the day your battery dies it will get you going. There's enough further starts in it to get you through the day.

[Derek Geldard]

There's a big danger that will ocurr. The one time that they are used is the one time they tend to get left in the boot.

You just have to get a taxi ride to Halfords for a new battery.

If that's what you call it. :-)

It's 90 quid for 20 AH. My new battery was 36 quid for 40 AH.

Derek

[NT]

I did it enough times with 2 vehicles I owned, so it does at least on some. They weren't modern designs though.

NT

[Dave Liquorice]

at a cost of about =A3100 for the fare? The nearest Halfrauds is > 20 miles away... I think phone the local garage and get them to order one (if they didn't have one) then if I had no transport ask them to drop it up to me.

Must be a tiddly little car, my car is 110AH ('tis a 2.5l diesel mind...) but I suspect the poster was refering to an SLA for thse booster packs not a car battery. Still =A390 for a 20AH SLA is steep.

[Dave Liquorice]

Hum, so a less than 10AHr SLA can fully charge a 60AHr car battery in

6hrs, interesting... I think dennis has just solved the world energy crisis.

[dennis]

You are TMH ICMFP as you can plainly see that I said nothing of the kind.

[The Medway Handyman]

"Such a device could probably supply 10A, which would fully recharge a typical battery in about 6 hours" - sounds pretty much like teflon Dennis is at it again...

Explain the difference between your statement at that of Mr Liquorice halfwit.

[Bill Wright]

On the face of it the difference is in the units. He said 10A, not 10Ah. With no other information he could have meant that the device could produce

10A for 6 hours.

But this suggests that he unaware of the difference between amps and amp/hours. Surely that isn't the case. Or is he unaware that the total number of Ah available is limited by the Ah capacity of the device's battery, and that these devices have quite small batteries. But he must know these things.

So it's a mystery.

Bill

[Roger Chapman]

My battery died March 2008. I checked the price with Halfords before calling out the AA just in case the sudden deadness was something more serious. AA man sold me a battery at somewhat less than Halfords price which saved me the problem of hitching a ride to Halfords with a neighbour.

My new battery was £78.99 but I can't remember what the modest saving was.

[Laurence Payne]

And if you keep a spare at home, charged and ready, it can compete with one of these boxes.

The last time my battery died I was lucky enough to be able to borrow one. It got me to work on time, and then on to my second appointment (my job is one where I just CAN'T be late.) Well worth it.

[dennis]

No mystery at all, you know the difference and therefore know that I never claimed the device could charge the battery.

Even in English the bit that says "which would" refers to the last "object" talked about, "10A", so says exactly what I intended, that 10A would charge a typical battery in about 6 hours. It had already been stated that the unit would only run for a few minutes. It is typical of TMH to misread everything I post just so he can be an arse.

Even his question "Explain the difference between your statement at that of Mr Liquorice halfwit." is insulting Mr Liquorice by calling him halfwit. He really doesn't understand English and his lack of punctuation is worse than mine. At least I would have put the comma after Liquorice. I gave up trying to explain anything to TMH as he is incapable of understanding even basic stuff.

I don't want to read what he posts anymore, please don't quote him.

[Dave Plowman]

I have a spare which doubles as the standby for the central heating should we have a power failure. Since doing that installation we haven't had one. ;-)

[Clint Sharp]

In message , Dave Plowman writes

Oh yeah, definitely cheap to make but not necessarily cheap to buy and most definitely in the majority. I've rarely seen a set of retail jump leads that were anything like man enough to start a car straight away.

Having said that, I'd rather let the 'dead' vehicle get some charge from a cheap set of leads and have it start from its own battery. Much kinder to my alternator and battery.

If the person asking for help hasn't got enough patience to wait for

5-10 minutes then they can get stuffed, I usually find most are grateful just to sit in a nice warm car for a while after being stranded anyway.

[Dave Plowman]

16mm meter tails can be bought from TLC for a couple of quid a metre. So say 8 quid for the wire and 5 for decent clips. So should be on Ebay for this sort of cost. Very flexible ones will cost more.

Indeed. But if the battery is very flat you'll still need decent leads. There's a danger of cheap ones melting.