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Confused over charging system

04-14-2017, 03:58 AM
As an amateur and a newcomer to the forum, this is a question about auto-electrics which I am trying to get my head around. I'm very confused as to what's going on when the 12v lead-acid battery in a car is being charged or not charged. Say, I switch on the ignition and turn the lights on, a current flows in one direction from the battery and around the lights' electric circuit. As I understand it, if the engine is then started, the alternator will charge the battery by reversing the current flow through the battery. Does this mean that the current through the light circuit and any other circuit in use is also then reversed? As the alternator has to supply a higher voltage to the battery to 're'charge it, there seem to be two opposing potential differences - the one due to the existing charge in the battery and the one applied by the alternator. I am told that a voltmeter put between the two battery terminals while the engine is running (and the battery is being charged) should read around 13 -15 volts, if all is well. So thinking in terms of the 'voltage' of the battery pushing in one direction and the 'voltage' of the alternator pushing in the opposite direction, is the voltage read by the voltmeter the difference of the two. Therefore does the alternator actually supply a 'voltage' of 25 - 27'? I would be very grateful if someone can help me to understand better. Thanks

04-14-2017, 10:54 AM
The output of the alternator is positive, and is supplied to the battery positive in parallel circuit.

No reversing polarity is occurring during charging (or any other time)

04-14-2017, 02:01 PM
Thanks Copytech99. What you say sounds sensible and I would assume it also means that the voltage supplied by the alternator is 13 -15 rather than my conjectured 25 - 27. Nevertheless I'm not entirely convinced on everything. I have looked on the internet previously and have seen 'reversed current' mentioned during the charging session e.g. look on this link - http://www.ausetute.com.au/pbbattery.html Also I'm old enough to remember when voltmeters were standard equipment on some vehicles and then the voltmeter pointer moved from negative to positive sides of the dial and vice versa, dependant on rpm and load. Surely that would mean the current going in the opposite direction too. Admittedly those were the days of dyamos rather than alternators. Even so? Cheers John

04-14-2017, 02:45 PM
That was likely an ammeter, which would act that way.

04-14-2017, 03:11 PM
Sorry, you are right - they were ammeters not voltmeters! To try your patience ,further when you say ammeters would act that way, what do you mean and what did you make of that link which talked about reverse current? Thanks John

04-14-2017, 04:47 PM
That is just the definition of how an alternating current works.


04-15-2017, 02:42 AM
Thanks Stealthee - but in my vehicle circuits everything is DC because of the rectifier in the alternator, so I don't see your point? Cheers John

04-15-2017, 11:16 AM
The alternator charges in AC, but converts the power to DC before going to the battery.

04-15-2017, 11:44 AM
"requires an input of slightly more than 2 volts per cell to drive the spontaneous reactions in the reverse direction"

The process.... not the voltage is reversed. When output is above 12V, the battery will charge but output is always + DC.

04-16-2017, 04:49 AM
Thanks Copytech99. You caused me to do more thinking and I can see that the voltage is not reversed and in its parallel circuit the alternator provides just enough excess voltage to recharge the battery as necessary. Still it seems to reverse the chemical reactions at the terminal plates by reversing the flow of electrons inside the battery and that makes me cling to the notion that the direction of the current must change too - back to behaviour of the ammeter!

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