Is a B&D Simple Start safe to use?

[HarryM1BYT]

Originally Posted by kaiser

There is a direct correlation between the voltage measured and the charge of the battery. Two batteries of the same type and the same condition, yet one charged and one not, would display two different voltages.
If you would care to go and verify that from any graph depicting charge vs voltage, you could perhaps go and look at the link in the previous post?
It is perfectly sensible to expect two batteries connected together to equalize their charges, and that is indeed what happens.
What would be unreasonable, is to expect the one battery to charge the other fully, and I have never claimed that. What I have claimed, and what is indeed true, is that if you connect another battery, that is charged to your discharged car battery, that a flow will go from the charged battery into the uncharged battery and that will persist, until the voltage difference is nil.

So the case is, in this story, it will help to connect any other battery with a higher charge to your car battery.

It just doesn't work like that. The flat battery rapidly develops a surface voltage, which prevents any flow of current. If you then disconnect them, the surface charge voltage will rapidly decline again from the flat battery.

The charge versus volts charts can only be relied upon if that surface charge has dissipated.

The only real way to charge one battery from another, of the same voltage is to boost the voltage output via an inverter.

I do suggest you try it and let us know how you get on, because it would be very useful if batteries could be charged that way - but we all know they cannot :-)

[Forestgreen]

Interesting discussion. I've often wondered whether so-called battery boosters and other devices that claim to get a car with a flat battery going again are worth carrying around or not.

The one and only time I had to call the AA out to start my car was last winter and indeed that was for a flat battery. In fairness to the car I hadn't used her in a couple of weeks (don't like driving my precious R75 in ice and snow) and for a few weeks before that I'd noticed she took longer than usual to turn over when starting. So I didn't resent forking out for a new battery and got the most expensive one I could with the longest warranty from Halfords. I suppose it was bad luck that the battery on my second car went the very next week!

But as for a spare...is it actually worth carrying a booster around? Wouldn't it make more sense to carry a spare battery and swap that over if necessary, like you carry a spare tyre round with you (but hopefully never need to use it?) And regardless of whether you are carrying a spare battery or a booster, surely the extra device itself needs charging every so often otherwise it won't start the car the one time you need it. Furthermore, what's its expected life expectancy? Lead batteries don't last forever, and I'd be a bit annoyed if I got a booster only to find that I had to throw it out and replace it every couple of years even if it never came into its own for successfully starting the car.

For those of us who have to have on-street parking this discussion is particularly helpful since we don't have the luxury of being able to trickle-charge our car overnight in the garage if necessary.

On a related rant, I have a UPS (uninterruptible power supply) to my home PC so that in the case of a power cut the PC would still keep on working for long enough for me to shut it down. I bought a cheap model and the one time I had a power supply, a year later, the lead battery inside turned out already to be defunct and so of course it didn't work when I needed it to! I now have a much more expensive one that self-tests the battery once a week (the battery for the other one cost more to replace than the original purchase price so it went off to the council tip).

[berkshirelad]

If you connect the "booster" to the dead battery, does the charge not equalise between them?

Anyway, wouldn't work on mine as the cigar lighter is only connected with the ignition on

[kaiser]

Originally Posted by HarryM1BYT

It just doesn't work like that. The flat battery rapidly develops a surface voltage, which prevents any flow of current. If you then disconnect them, the surface charge voltage will rapidly decline again from the flat battery.

The charge versus volts charts can only be relied upon if that surface charge has dissipated.

The only real way to charge one battery from another, of the same voltage is to boost the voltage output via an inverter.

I do suggest you try it and let us know how you get on, because it would be very useful if batteries could be charged that way - but we all know they cannot :-)

I will finish with an example that might be familiar to quite a few.
Some people (garages, car dealers, auctions)have a battery charger, but it is often limited in reach to an area close to the socket. Some have movable boosters, (which obviously have built-in batteries) but other places have a normal big battery on a small trolley, which they will move up to a car that does not start.
A lot of the car auctioneers have that.
This works quite well. But the best method is, to leave the booster battery attached to the flat battery in the car for a couple of minutes, to let the charge move to the flat battery, before you attempt to start.
This is surely something that must be familiar to many on the forum, and it shows without doubt the process in action.

[VMax1000]

Have a read here http://wiki.answers.com/Q/What_will_...attery#slide=1

Yes, if you connect a fully charged battery to a battery with lesser charge there will be a voltage difference. Fully charged roughly 12.7 volts vs 11.9 at 40%

Difference 0.8 volts. Current that will flow is based on the voltage difference and the resistance of wiring in place and the internal receiving battery resistance, but I'd expect a few ohms. (say 2)
As U = I x R this mean that I = U/R

So you'll be charging your battery with 0.8/2 = 0.4 Amps at best

Seems rather pointless to me to be honest

PS. And for the Dutch, unit of electrical charge normally is in Coulombs.
Basically 1 C = 1 A per second. So Ah is a derived unit of charge

[SD1too]

Originally Posted by berkshirelad

Anyway, wouldn't work on mine as the cigar lighter is only connected with the ignition on

Yes, I seem to remember others reporting this. Yours is a 2002 car, yet my 2003 wiring diagram shows the cigar lighter wired directly from the battery!

The lighter must draw a pretty hefty current. Is there a relay feeding it in your car? I wonder if this is what MG Rover have labelled an 'auxiliary socket'?

Simon

[HarryM1BYT]

Originally Posted by kaiser

This works quite well. But the best method is, to leave the booster battery attached to the flat battery in the car for a couple of minutes, to let the charge move to the flat battery, before you attempt to start.
This is surely something that must be familiar to many on the forum, and it shows without doubt the process in action.

Of course it has nothing to do with the rather heavy duty leads, linking the car to the very large battery on a trolley?

Yes of course, leave the two connected for a few minutes, so the car's battery gets some small charge into it and brings it up from completely flat, but it is the large leads which make the difference when you turn the key.

[HarryM1BYT]

Originally Posted by VMax1000

So you'll be charging your battery with 0.8/2 = 0.4 Amps at best

Seems rather pointless to me to be honest

Pointless and a complete waste of money buying such a device, but lets not let Ohms Law and common sense get in the way.

Even my 3.5amp charger would need several hours to bring a flat battery up enough to start the engine. Yet here we are discussing an initial 0.4v, which rapidly falls off as the flat battery gains in voltage, being able to start the engine in minutes. Absolutely impossible I say, without it having to defy basic physics.

[VMax1000]

Maybe this would be an option http://www.eurocarparts.com/ecp/p/ca...0321&0&cc5_824

In all the discussions we have assumed that the Black & Decker unit supplies 12 Volts. Well, it doesn't.

According to the spec it has 3 x 6 Volts batteries = 18 Volts
http://www.blackanddecker.co.uk/auto.../catno/BDV040/

Maybe it will help a battery that just fails to start a car enough based on this.

[SD1too]

Originally Posted by VMax1000

In all the discussions we have assumed that the Black & Decker unit supplies 12 Volts. Well, it doesn't.

According to the spec it has 3 x 6 Volts batteries = 18 Volts
http://www.blackanddecker.co.uk/auto.../catno/BDV040/

But the product description clearly states that the output is 12v, not 18v.

"Curiouser and curiouser, said Alice."

Simon