Switching between car battery and external battery to power backup camera

Thread Starter

Al_D

Joined Jan 28, 2023
4
Hi,

I've got a low current backup camera that I currently turn on using a cigarette port at the front of my vehicle. I want to be able to turn it on from the back also, using an external battery. The power from the cigarette port also supplies a monitor at the front, which I don't want when turning on the camera from the back. I have a second monitor I'm using for the back, whose power source is already sorted.

I've decided that to isolate the batteries from each other, I'd like to use an 8 pin, 12v relay capable of handling over 2x the anticipated current draw. The vehicle battery produces 12V nominal, but the external battery is wired for a stable 12V output. There are other devices on the external battery. I'd like to avoid ground loop issues by keeping both batteries grounded to their respective grounds, hence the isolation.

I have a small background in EE, but it's been a while. My question is this: will connecting my batteries to the appropriate terminals of the relay cause any forseen issues? I'm thinking backfeed from the coil, contact bounce from the relay arm, and maybe others? It doesn't seem like it's ever as simple as it aught to be.

Please see attached proposed drawing. There's a 24V to 12V converter controlled by a switch. This feeds a fuse box. A stable 12V is applied as the trigger to the relay as well as power to the camera when turned on from the back. The vehicle 12V nominal should supply the camera from the front and will be the normal operation until the back switch is turned on. Both switches won't be turned on at the same time. I'm not after circuitry that guarantees that.

Thanks,
 

Attachments

LowQCab

Joined Nov 6, 2012
4,075
No Relay is needed, and it would only create an unnecessary Battery-Load.

Just install a ~100uf Capacitor across the Rear-Camera power wires,
and use a DPDT-Switch to choose the Power-Source, ( switch both the Hot and the Ground ).
The Capacitor will keep the Camera powered-up for the
fraction of a second that the Switch is changing positions.

If You want to have this change-over to happen automatically,
use a DPDT-Relay whose Coil is only powered when the Alternator is working.
This may require a simple Electronic-Circuit to
detect the increase in Voltage from the Alternator.
.
.
.
Engine Run Detector 1 FLAT .png
 

Thread Starter

Al_D

Joined Jan 28, 2023
4
Hard to tell from your drawing but I think this is the way you want to wire the relay.
Why is there a 24 battery in the vehicle also?
View attachment 286317
Yes, I believe that is more or less what my diagram should look like. Thank you for cleaning it up for me.

The 24V battery powers other devices that run better on 24V instead of 12V. However, this camera is only spec'd for 12V.

So the question is really: are there other components that need to be added in order for this idea to run properly? Or can I just use a simple 12V 8pin relay?
 

sghioto

Joined Dec 31, 2017
5,391
A diode wired across the relay coil terminals is usually recommended.
Some relays come with the diode built in.
I'll modify the drawing to illustrate.
 

Thread Starter

Al_D

Joined Jan 28, 2023
4
Thanks. I was thinking a diode would be needed to control the back current. Glad to hear it from someone else.
 

Thread Starter

Al_D

Joined Jan 28, 2023
4
No Relay is needed, and it would only create an unnecessary Battery-Load.

Just install a ~100uf Capacitor across the Rear-Camera power wires,
and use a DPDT-Switch to choose the Power-Source, ( switch both the Hot and the Ground ).
The Capacitor will keep the Camera powered-up for the
fraction of a second that the Switch is changing positions.

If You want to have this change-over to happen automatically,
use a DPDT-Relay whose Coil is only powered when the Alternator is working.
This may require a simple Electronic-Circuit to
detect the increase in Voltage from the Alternator.
.
.
.
View attachment 286315
Thank you for your response.
 
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