# Current reversal in 2 connected car batteries?

I sometimes have to jump start my old car when its battery is too weak to start it.
Some people say I should connect the "donor" battery to my battery and
then to LEAVE IT FOR A FEW MINUTES so that my weak battery takes in some charge.
But wouldn't this result in the donor battery forcing a current through the weak battery such that it flows through the weak battery in REVERSE?
If so, then would this result in any damage to the weak battery?
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No.
Tim..
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Let me expand this discussion slightly. If the "fresh" battery is connected to a charger such as a car alternator, then leaving it for a few minutes to get some charge into it is a great idea.
However, if it is just a fully charged battery it cannot under any circumstances charge the dead battery. You need at least 12.9 volts to get the battery chemistry flowing in the right direction, but the voltage of even a fully charged battery will not be this high.
Best regards mark
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| | snip
|> |> > I sometimes have to jump start my old car when its battery is too weak |> > to start it. |> > | snip | | Let me expand this discussion slightly. If the "fresh" battery is | connected to a charger | such as a car alternator, then leaving it for a few minutes to get some | charge into it | is a great idea. | | However, if it is just a fully charged battery it cannot under any | circumstances charge | the dead battery. You need at least 12.9 volts to get the battery | chemistry flowing in | the right direction, but the voltage of even a fully charged battery | will not be this high.
I disagree. I have accompished charging one battery that was drained by paralleling it with another battery of like construction. But for the most part, what you say is valid. The drained battery was not overdrained to where it might be damaged, and it didn't get anywhere near a half charge from the connection. But it did charge enough to see some voltage from it again. Of course, the ideal is to have enough voltage to keep charging current flowing up to full charge. But to say such charging simply cannot even get started from battery to battery is just not consistent with my experience.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Alex Coleman wrote:

Erm, rechargeable batteries, such as car batteries, rather depend on current going in as well as out.
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Sue

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Alex Coleman wrote:

yes, it will "force" current through the battery. The technical term for this is "CHARGING"
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Alex Coleman wrote:

Some people would say you should get a new battery...
Andrew
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The reason I leave the jumpleads for a minute or so after jump starting, is to make sure the flat battery actually gets some charge, if it was disconnected too soon and the battery was well knackered, the alternator may overvolt and may (at the best) just blow bulbs but worst case scenario may blow something expensive.....
Do not even concern yourself with reverse discharge, the internal resistance of a battery will be higher than the resistance of a pair of jump leads!!
Des
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What's knackered mean?

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English and Merkin?
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Steve H 'You're not a real petrolhead unless you've owned an Alfa Romeo'
www.italiancar.co.uk - Honda VFR800 - Hongdou GY200 - Alfa 75 TSpark
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English = Language of the country England.
It's our language and we'll make the decisions over who can and can't call themselves 'English speaking'.
Americans don't fall into that category.
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Steve H 'You're not a real petrolhead unless you've owned an Alfa Romeo'
www.italiancar.co.uk - Honda VFR800 - Hongdou GY200 - Alfa 75 TSpark
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wrote:

I thought you lived in Wales? :-)
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wrote:

Bollucks. Most "English" these days can't speak their own language, merely a debased variant.

Correct, inasmuch that American isn't the same as English. FFS the Queens' English originates from Scotland.
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The DervMan
www.dervman.com
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But the yanks don't claim to speak American, they claim to speak English, which they clearly do not.
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And the Limeys claim to own their little language, so what?
Talk to most Americans and they'll say they speak American English.
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The DervMan
www.dervman.com
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DervMan wrote:

Talk to most Americans and they won't hear you. After September 11 2001, they didn't hear a word of the responses to the query "Why do they hate us so?"
John
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Is this from a limey newspaper?
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The DervMan
www.dervman.com
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DervMan wrote:

I don't know, I neither live there nor read their papers.
John
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wrote:

England squandered its world dominance and became a has-been second-world economy between 150 and 60 years ago. Does this *still* make you so bitter that you need to go around sniping at how other people speak, to make up for it? Well over 420 million people call English their first language, and fewer than 1/7 of those people "deserve to", in your eyes. Perhaps it's really time to finally consider getting over yourself.
Eric Lucas
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I can only assume you're American.
This will be the good old USA, where your industry is about to go the same way as ours has over the last 20 years.
--
Steve H 'You're not a real petrolhead unless you've owned an Alfa Romeo'
www.italiancar.co.uk - Honda VFR800 - Hongdou GY200 - Alfa 75 TSpark