# During battery charging how much is the voltage at its terminal?

Discussion in 'General Electronics Chat' started by Jas9, Nov 13, 2010.

1. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
hello friends,
Suppose my car battery current voltage is 10V and the alternator is charging my battery during the car run so if the voltage arriving at battery terminal is 13V then what value of voltage i should consider in my equivalent circuit of battery?
my equivalent circuit consists of a resistance in series with parallel combination of a capacitor and resistance. For this circuit i have the values of voltage and current generated by the alternator. So to calculate the total impedance of the circuit should i directly divide voltage(alternator) by current(alternator) or it would be something else?
The reason i am confused is because the battery current voltage is 10V and at time t=1s the alternator voltage is 13V and current is 35A. Then to calculate total impedance of the battery should i divide 10/35 or 13/35 or i should consider both the voltages that is 10V and 13V???
thanks

2. ### timrobbins Senior Member

Aug 29, 2009
318
18
What battery model references have you read up on - there are a lot around - this is a fairly extensively researched topic, and not an easy subject to simplify for a forum as it is modelling the complex combination of chemical and electrical non-linear parameters.

3. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
thanks a lot.....yes you are right that its more of research but i already have the values of capacitor, charge transfer resistance, the incoming voltage and current values from alternator. SO the only thing that i have to find is the series resistance that is the internal resistance.
So considering my first post can you tell me about the voltage considerations that is what voltage i should consider the alternator or the battery voltage or i should apply kirchoffs law?
thanks

4. ### timrobbins Senior Member

Aug 29, 2009
318
18
Jas9, there is a whole lot more to modelling a battery than what you have identified. Imho, I strongly recommend you identify a published model and make reference to it, rather than generate your own model from a very limited understanding of what is required.

5. ### iONic AAC Fanatic!

Nov 16, 2007
1,450
71
Simply put, Whatever the multimeter says it is. But it is constantly changing. I have seen two senaarios. 1) where a higher voltage applied to a lower terminal voltage start high and work its way down to a charged state(Probably not the optimal way) and 2) where the terminal voltage begins at the original terminal voltage and works its way towards the charging voltage or charged battery state.

6. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
well thanks again...but as i am doing a simulation so i have to design an equivalent circuit as my circuit is a reference of a standard randles circuit. I am only confused with the voltage value. I am calculating the internal resistance based on electrochemical spectroscopy. So i will calculate the total impedance of the circuit from which i would be able to calculate the internal resistance. Generally the total impedance is the total voltage across the battery terminal and the total current. So to calculate this i have alternator voltage and current values which are constantly varying. If i take the division of alternator voltage and current then it will give me impedance of the line from alternator to battery but not the battery total impedance. So after realising this i stucked up at the point that what will be the voltage of the battery and the current flowing through the battery. So if i get these two values i can calculate the total impedance.
I am thinking of subtracting the incoming voltage with the battery voltage and it will give me the actual voltage of the battery.
but i am not sure so i hope i get a real working solution very soon
thanks

7. ### iONic AAC Fanatic!

Nov 16, 2007
1,450
71
Sorry. My answer was too simple and not very constructive to your needs. Are you trying to determine this voltage without actually measureing it? At least the actual measurements will tell you how close your theory and calculations are coming.

iONic

8. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
i got your last point but if i use a multimeter at the battery terminals it will give me the battery voltage and where i can measure the alternator voltage then??

9. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
ok i am clear with the things now so if i have a set of values of voltage and current of a car battery during the car running then if i divide the voltage and current does it indicates the total impedance of the battery or i have to take other factors into the consideration?
the voltage and current signal that i have has lots of ripples that means its not a pure dc wave so is it the right to just have Z=V/I or i have to account for the ripple also??

10. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
If your vehicle's battery is less than about 11.5v, it's considered 100% discharged. A real automotive battery (conventional construction) won't tolerate such deep discharges well at all; it will quickly fall apart inside. They're designed for brief discharge periods (hundreds of Amperes for up to a minute or so) and then be immediately recharged.

If it's less than 11.5v even after charging, you likely have one or more shorted cells.

A typical automotive battery will measure 12.7v to 12.8v when fully charged. Alternators put out ~14v or so (newer alternators may go up to 15v if the battery is an AGM type), and generally settle down to ~13.8v after operating for a bit.

11. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
thanks friend but what about impedance if the measured voltage and signal waveform has ripples should i directly divide V/I to get battery internal impedance or something else to be also applied??

12. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Look at the battery's datasheet for the particular battery you wish to model. It should have a specification for internal resistance.

13. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
well i am creating my own model of calculation of battery parameters. I have reference readings of all parameters of the battery. I just want to see whether the calculated values of my model are nearby the reference values or not.
thanks

14. ### Rbeckett Member

Sep 3, 2010
205
32
Jas, the ideal charging voltage on a modern car is 14.2 volts with less than 1 volt AC on the positive terminal while the car is running at 1000-1500 RPM. The amperage is driven by the regulator and is usually around 10-15 on a properly functioning system soon after start up. As the engine runs and the system stabilizes 1-5 amps is sufficient to charge tha battery and run the engine electronics. If you are using a high current device such as an audio amplifier or high powered radio transmitter you should consider upping the regualtor to include that device. Most current autos do not allow for much adjustment in that system since it is all solid state and controlled by the ECU. Ac ripple and excess voltage drops across the battery and alternator are a constant source of most starting/charging issues. Hope this helps.
Bob

15. ### timrobbins Senior Member

Aug 29, 2009
318
18
Bob,

'Most current autos do not allow for much adjustment in that system since it is all solid state and controlled by the ECU.'

I'm pretty sure the alternator is not controlled by the ECU in any common auto. Can you point to any that have a controllable alternator/regulator?

Ciao, Tim