# 12v car batt paralleled with 9v

• n0_3sc
In summary: 3) 5ma and 50ma are just examples, there is no right or wrong answer here, just choose something that makes sense for your situation.
n0_3sc
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...

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.

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.

## 1. Can I parallel a 12v car battery with a 9v battery?

Yes, it is possible to parallel a 12v car battery with a 9v battery. However, it is not recommended as it can lead to inefficient charging and potential damage to the batteries.

## 2. What happens when you parallel a 12v car battery with a 9v battery?

When you parallel a 12v car battery with a 9v battery, the batteries will share the load and the voltage output will be the average of the two batteries, resulting in a 10.5v output. This can be problematic as most car electrical systems require a minimum of 12v to function properly.

## 3. Is it safe to parallel different voltage batteries?

No, it is not safe to parallel different voltage batteries. The batteries may have different internal resistances, which can lead to uneven charging and potential damage to the batteries. It is always best to use batteries with the same voltage and capacity when parallel charging.

## 4. What are the risks of paralleling a 12v car battery with a 9v battery?

The main risk of paralleling a 12v car battery with a 9v battery is potential damage to the batteries. The mismatch in voltage can lead to overcharging or discharging, which can cause the batteries to overheat and potentially explode. It can also damage the electrical system of the car.

## 5. Can I use a 9v battery to jumpstart a car?

No, it is not recommended to use a 9v battery to jumpstart a car. The 9v battery does not have enough power to turn over the engine and may lead to damage to the battery and electrical system of the car. It is best to use a proper jumpstarting device or a set of jumper cables connected to another vehicle's battery.

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