12v car batt paralleled with 9v

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    12v Car
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Discussion Overview

The discussion revolves around the feasibility and safety of connecting a 9V battery in parallel with a 12V car battery to power a circuit. Participants explore the implications of such a connection, including potential risks and protective measures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant questions the safety of connecting a 9V battery in parallel with a 12V battery, expressing concern about the potential for the 12V supply to damage the 9V battery.
  • Another participant strongly advises against the proposed connection, suggesting it could lead to dangerous outcomes.
  • A different viewpoint suggests that using diodes could allow for safe operation by preventing current from flowing back into the 9V battery when the 12V supply is connected.
  • One participant describes implementing a protection diode to prevent current from the 12V supply from damaging the 9V battery, seeking confirmation of this approach.
  • Concerns are raised about the voltage levels in a car's charging system, with one participant noting that the voltage can exceed 12V, which could potentially harm devices designed for 12V operation.
  • Another participant mentions that while using a diode can protect the 9V battery, it may reduce its effective life due to voltage drop across the diode.
  • Questions are posed regarding the absence of a resistor in series with the battery to prevent overcurrent, leading to a discussion about the circuit's current draw in different modes.
  • Some participants express confidence in the OP's ability to modify the circuit safely, while also warning about the risks associated with the car charging effect.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the safety of connecting the batteries in parallel. There are multiple competing views regarding the use of protective components like diodes and resistors, as well as differing opinions on the risks involved.

Contextual Notes

There are unresolved concerns regarding the voltage levels in a car's charging system and the potential for damage to the circuit. The discussion also highlights the need for further verification of the circuit design by a qualified individual.

Who May Find This Useful

This discussion may be of interest to individuals working with battery-powered circuits, automotive electrical systems, or those seeking to understand the implications of connecting different voltage sources in parallel.

n0_3sc
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I have a circuit that can be powered by either a 9V battery or a 12V battery.
Can I connect a 9V battery in parallel with a car cigarette lighter 12V battery plug, so that the supply's are now in parallel with one another and they can both power the circuit?

I'm a bit scared because 12V outputs in a car are generally regulated and can spit out enough current to cook the 9V battery...
 
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Can you say BOOM? Do not do this!
 
Although wth a couple of diodes you can arrange that it would be powered by the 9v when the 12v is not connected and vice-versa.
 
Ok so what I did was place a simple protection diode from the +ve part of the 9V to the +ve rail, and just connected the +ve part of the 12V cig. lighter plug to the rails too. This should prevent ANY current going through the diode into the +ve 9V right?
 
...just a thought for safety. Try using a proper adapter because a car charging system works on voltage differentials tocharge the battery. say a battery at 12v is being charged and the charging circuit is operating at 16 volts. Your cigarette lighter output will be at 16 volts . sorry i changed this
 
Last edited:
If your car is hitting 16v you need a new battery or regulator (or both).
However, it is normal to approach 14v.

If the device in question was made for 12v battery operation, then having it plugged into a running car could possibly overheat and damage it. This may not happen immediately, so unless the manual specifies auto operation it would be best to use an adapter as noted.

Using the diode will keep the 9v battery from exploding but can cause a shorter effective battery life on the 9v as there will be a voltage drop associated with the diode.
If this turns out to be a problem then you can use a power jack that disconnects the 9v when the external power cable is plugged in.
 
quick question for diode advisor

why did you not advise a resistor in series with the battery to protect from overcurrent.
i would advise him to have a qualified person verify his circuit
sorry
 
klystronman said:
why did you not advise a resistor in series with the battery to protect from overcurrent.
Homework questions.
1) If the OP circuit draws 5ma in idle mode and you pick a resistor to drop 2v.
What will be the voltage drop across the resistor when the devices active mode current it 50ma.
2)Why did I pick 2v.
3) Why did I pick 5ma and 50ma?

klystronman said:
i would advise him to have a qualified person verify his circuit
1) The OP knows enough electronics to modify their circuit with a switch diode.
That makes him/her somewhat qualified in their own right.
2)I did warn them that they could damage it due to the car charging effect.
 

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