Dumb DC electricity question

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mb82

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#220325 6-Aug-2017 13:58

What would happen in the circuit above the blue line?

What would happen in the circuit below the blue line?

Basically want to connect a 9v battery directly to my car radio 'constant 12v' and 'ground' wires so I don't need the reset code when I disconnect the battery and can't get my head around whether it will blow up or not.

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Awrrr
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#1839859 6-Aug-2017 14:16

If you are lucky, you will only destroy the 9 volt battery. If you are not, there will be an explosion and you will destroy your car.

In both circuits the 12 volt battery will try to discharge through the 9 volt one because of the difference in potential. The 9 volt one won't like this at all.

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Rikkitic

Awrrr
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#1839861 6-Aug-2017 14:21

If you put a blocking diode into the circuit to protect the 9 volt battery, it might work. I don't know.

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geocom

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#1839862 6-Aug-2017 14:21

The 9V battery will explode. You need a circuit that will change from 1 source to another. However even then it may not work as the car is designed for 12V+ a bit more for charging the battery.

Geoff E

mb82

223 posts

Master Geek

#1839880 6-Aug-2017 14:39

I saw a device online that used a 9v battery but assumed it had a diode or something in it

What if i swap the 9v battery for another 12v car battery? Or would they have to be exactly the same voltage?

sbiddle

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#1839883 6-Aug-2017 14:46

You can simply hook a 9V battery up to the battery in parallel by attaching it to the terminals before you remove the old battery inline with a diode for protection.

Devices that plug into the 12V accessory / cigarette adapter are cheap (called "settings savers" or similar) and very readily available. These will also have a diode to protect them. Most places that sell car batteries will use one of these when swapping a battery to avoid the complexities of having to reset things for people.

tripper1000

1618 posts

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#1839888 6-Aug-2017 14:52

Yeah, you just need a diode - such as this. https://www.jaycar.co.nz/1n4004-1a-400v-diode-pack-of-4/p/ZR1004

A diode is a one-way valve for electricity - it will let 9 volts out of the 9v battery but won't let the car 12 volts reverse charge it and blow it up.

Or you can go to Super Cheap Auto and buy a ready made one:

http://www.supercheapauto.co.nz/Product/9V-Memory-Minder-SCA/381962

There are wiring diagrams all over the internet if you google memory saver.

mb82

223 posts

Master Geek

#1839895 6-Aug-2017 15:04

sbiddle:
You can simply hook a 9V battery up to the battery in parallel by attacking it to the terminals before you remove the old battery.

surely you mean with a diode or did you mean to say 'you can't'

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sbiddle

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#1839903 6-Aug-2017 15:07

mb82:
sbiddle:

You can simply hook a 9V battery up to the battery in parallel by attacking it to the terminals before you remove the old battery.

surely you mean with a diode or did you mean to say 'you can't'

I've just edited my post to make that clear - I only had the reference to the diode on the 2nd line and didn't make it clear you needed one regardless.

Rikkitic

Awrrr
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#1839930 6-Aug-2017 15:20

mb82: I saw a device online that used a 9v battery but assumed it had a diode or something in it

What if i swap the 9v battery for another 12v car battery? Or would they have to be exactly the same voltage?

Questions like this should be answered. A second 12 volt battery in parallel would work, but batteries are often not exactly the same, especially if they have different sources and ages. I think the diode is probably a better solution, and it is certainly easier.

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cadman

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#1839947 6-Aug-2017 15:51

So you want to put a battery on so you can disconnect the starting battery and not lose the stereo settings etc? What is the reason for wanting to disconnect the starting battery? There may be a better solution for the circumstances.

Aredwood

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#1839993 6-Aug-2017 16:36

If it is a job that you need to get done today, but you can't get a diode today. You can connect the 9V battery directly to the accessory wire, but make sure that you don't turn the ignition on while it is connected. Use the interior light to check that the new battery is connected. Then disconnect the 9V battery before starting the car.

mugs2000

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#1840201 6-Aug-2017 19:53

Would 9V be sufficient to hold the radio settings?

Also, the 9V battery, depending on how it is wired in may well supply power for any other ancillary

items in the vehicle. You will need to ensure that all lights/clock/god-knows-what-else etc are turned

off/disconnected to avoid current drain depending on the time you will have the 12V battery disconnected for.

richms

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#1840265 6-Aug-2017 21:25

Aredwood: If it is a job that you need to get done today, but you can't get a diode today. You can connect the 9V battery directly to the accessory wire, but make sure that you don't turn the ignition on while it is connected. Use the interior light to check that the new battery is connected. Then disconnect the 9V battery before starting the car.

Accessory line wont keep the memory I would expect tho. Its just a signal into the stereo that it can power on or off. Unless they specifically have it able to take power from both of the supplies to keep the microcontroller running I would expect it to still lose settings.

Richard rich.ms

raytaylor

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#1840289 6-Aug-2017 23:19

When using a diode dont you loose something like 0.7 volts?

So a device expecting 12v is now being supplied with 8.3 volts if the 9v battery is fully charged.

I would have just suggested using 8x 1.5v AA batteries in series.

An AA battery pack would last approx 4x to 6x as long as a 9v battery.

If the battery is powering multiple other devices, such as your clock or the car standby load, then definitley go with the AA.

However most car stereos should have their own dedicated positive power input for standby consumption.

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Aredwood

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#1840292 6-Aug-2017 23:26

richms:

Aredwood: If it is a job that you need to get done today, but you can't get a diode today. You can connect the 9V battery directly to the accessory wire, but make sure that you don't turn the ignition on while it is connected. Use the interior light to check that the new battery is connected. Then disconnect the 9V battery before starting the car.

Accessory line wont keep the memory I would expect tho. Its just a signal into the stereo that it can power on or off. Unless they specifically have it able to take power from both of the supplies to keep the microcontroller running I would expect it to still lose settings.

All of the stereos that I have opened up have an internal diode that allows the microcontroller etc to take power from the ACC line if the B+ line doesn't have power connected.

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