Charging current - jumpstarting a car

In summary, when two batteries with different voltages are connected in parallel, the resulting voltage is the average of the two batteries' voltages. The total resistance is the sum of the internal resistances of both batteries. To find the charging current, use the formula I=V/R, where V is the resulting voltage and R is the total resistance. In this case, the charging current is 1.5/0.17 = 8.8 Amps. This method does not require the use of Kirchhoff's Rules, which are only needed for circuits with junctions of more than 3 branches.
  • #1
tony873004
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A 12-V battery with internal resistance of 0.02 Ω is used to charge a battery with an emf of 10.5 V and an internal resistance of 0.15 Ω. What is the charging current?

We don't have any examples in the book or class notes about connecting 2 batteries of differing voltages in parallel.

My guess would be to average the batteries' voltages to 11 V, add the resistors, then use I=V/R = 11 / 0.17 = 1.6 amps. Another student got 11.53 amps, but I'm not sure of his method. Any thoughts?
 
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  • #2
You connect the batteries in parallel so the 12V is trying to push current into the 10.5V - this is a potential difference of 1.5V
 
  • #3
Thanks. In that case, I get I=1.5/0.17 = 8.8 Amps. Is that the way to do it? The only examples in the book have junctions with 3 branches, needing Kirchhoff's Rules.
 

1. What is the importance of charging current when jumpstarting a car?

The charging current is the amount of electricity flowing from the donor car's battery to the recipient car's battery. It is important because it determines how quickly the recipient car's battery will charge and how long it will take to jumpstart the car.

2. How do you measure the charging current?

The charging current can be measured using a multimeter. Set the multimeter to measure DC amps and connect the positive lead to the positive terminal of the donor car's battery and the negative lead to the positive terminal of the recipient car's battery.

3. What is the ideal charging current for jumpstarting a car?

The ideal charging current for jumpstarting a car is between 30 and 50 amps. This is enough to quickly charge the recipient car's battery without risking damage to either battery.

4. Can a higher charging current damage the car's electrical system?

Yes, a significantly higher charging current can damage the car's electrical system. It can cause the fuses to blow or even damage sensitive electronic components. It is important to use a charging current within the recommended range.

5. How long should the charging current be applied when jumpstarting a car?

The charging current should be applied for 5-10 minutes when jumpstarting a car. If the car does not start after this time, there may be other issues that need to be addressed. It is also important to disconnect the cables in the correct order to avoid damaging the car's electrical system.

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