Can somone explain WHY positive first when jumping a car battery?

[invalid unparseable]

Mighty✅ Wannabe✅ wrote on 1/20/2023 12:10 PM:

Sorry, my bad. It should be I x I x R (energy = I^2 * R)

[invalid unparseable]

Mighty✅ Wannabe✅ wrote on 1/20/2023 1:17 PM:

Sorry, my bad. It should be I x I x R (energy = I^2 * R)

[Xeno]

Batteries should never be *quickly* recharged. If you flatten your battery, it is far better to slow charge the battery until it is fully charged at a rate of 4 - 6 amps - depending on battery size. Using the alternator to charge a dead flat battery will result in 35-80 amps being pumped into the battery which will cause overheating and maybe plate buckling - as well as gassing the battery of course. All this tends to shorten the battery life.

Starters can draw 150 - 500 amps of 12 volt DC current. If the negative leads aren't making a good connection, the car bumpers might make the perfect substitute. In the days of metal bumpers, having them weld themselves together is probably not the best outcome. With plastic bumpers, not so much of an *electrical* issue but you'll be scuffing the paint on them.

The negative cable - the last connection to be made - to the dead car should not be made to the battery terminal. Instead, it should be connected to a metal part far removed from the battery. Batteries give off hydrogen gas, especially when they are flat and have been fast charged by the jump cars alternator. The last cable to be connected is the one most likely to create sparks. Best not to be near the potentially gassing battery. Ditto when disconnecting that earth, might spark, again best not near that potentially gassing battery.

If you need to constantly add water, I suggest you check your charge output. Your battery is being overcharged and is gassing the battery. Doesn't do a lot for the longevity of the battery either.

As long as the electrolyte covers the top of the cell plates, all will be well.

Nope. The electrical system of a car is *14 Volts*, not 12V as is commonly believed. You need that voltage to charge a 12 Volt battery at its nominal 12.6 Volts. After charging, the battery will show more than that but it is only a *surface charge* and will quickly dissipate back to 12.6. What people don't realise is that the battery is *only* for starting the engine. Whilst ever the engine is running, the car electrical system is running off the alternator - hence a 14 Volt system. NB, alternators typically output between 13.8 and 14.2 Volts. Any less and the battery may not fully charge, any more and you will get gassing.

Ask yourself what heating water does to its volume and you'll have answered that question.

You shorten the life of your battery.

[Carlos E.R.]

The "bad" battery will start charging when connected, and there will be no current limiting.

[Michael Trew]

The last time I had a really flat battery, and jumping it didn't work (perhaps the cables weren't heavy enough gauge), I started it with another battery installed (cables semi-tight pushed-on)... then with the alternator keeping it going, I disconnected that battery, and hooked up the flat battery. You could instantly hear the engine bog down from the alternator kicking into high-gear. This was a 1960's car, BTW, with a modern 12v battery. Perhaps this isn't the hottest idea with a new car using expensive electronics, but it works fine on older rigs with a good alternator.