Quote:
Originally Posted by kaiser
this is of course not true. It is like linking two buckets of water. The level in both buckets will eventually be the same. So you can of course charge a battery, but not fully, from another battery. You will even the charge on both.
However you can leave the battery connected via the cigar socket, and the car will charge this and the car's own battery fine.
It will in essence work as if you had two batteries installed in your car, which is indeed the case i some cars.
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Unfortunately you fail to understand how batteries work...
Batteries are not buckets. They fill up very quickly with volts, then the current and charging stops. The difference between a flat battery unable to crank and engine and a fully charged one, can be as little as one volt. Two batteries in parallel will quickly equalise their voltages, with little actual current charge being transferred. The 12.7v of the small battery, will at most put maybe 2 to 3% back into the car battery; 13.8v will charge a battery to 90%; 14.5v will put a full charge in. So very different from two buckets.
If you read the reviews for these gadgets, most say they don't work. Those few who have had success, have had success because the main battery has almost enough power in them to start the engine and just need a fraction more. If your car battery is just lacking that 2 to 3%, then you are in luck.
If you need to insure yourself against a flat battery, then the only way is to carry a decent sized, fully charged spare battery and some jump leads with a good amount of copper in them. The 'good amount of copper' means that the starter motor can draw current from the charged battery, rather than the flat one.
Instead, I carry a pair of home made jump leads which have a reasonable amount of copper in them, but nowhere near being able to crank the car over. I would hope to be able to use them to be able to jump start mine, from a car with its engine running. Idea is that the other car would be putting both volts and current into my battery, to enable it to start. In 30 years, I have never needed them.