Noob Question - Car Wiring

[vinter93]

Hello forum!
First post here and electrical noob, please bear with me!
Im looking into making a new fuse box in my car just to power accessories like dash cam, parking sensors, a tablet etc.
My idea is to run a wire directly from the battery of the car to the custom fuse box, protected by a 100amp fuse.
My problem, and potentially lack of electrical knowledge, is that I wanted the custom fuse box to only power on when the ignition key is on, preventing the battery from draining.
My idea, if such a thing exists, is to have some kind of switch that would block the power from the battery to the custom fuse box unless it detects that the ignition is on.
I could easily grab the signal from the radio (it only turns on when the ignition is on) to serve as the switch to allow power to the custom fuse.
Does such a thing exists?
If not is there any way of making something that would suit my needs?
It probably exists, I just don't have enough knowledge on electrical components etc so I'm sorry if this is a dumb question!
Thanks for the help!

 

[M-Ty]

A relay or starter solenoid might fit the bill.

 

[vinter93]

If I got it correctly, analysing a relay diagram, I should connect pin 30 to the battery positive and pin 87 to the fuse box, this circuit between 30 and 87 is always open unless there in power on pin 86, in my case the radio power.
If there is power on pin 86 the connection between 30 and 87 is now closed. Pin 85 for ground.
Did I get it right?

 

[Megawatt]

If I got it correctly, analysing a relay diagram, I should connect pin 30 to the battery positive and pin 87 to the fuse box, this circuit between 30 and 87 is always open unless there in power on pin 86, in my case the radio power.
If there is power on pin 86 the connection between 30 and 87 is now closed. Pin 85 for ground.
Did I get it right?

Why don’t you just add an on and off switch but make sure the contacts will be able to take the loads you want

 

[vinter93]

Why don’t you just add an on and off switch but make sure the contacts will be able to take the loads you want

I didn't want to have the trouble of turning it on and off every time I turn the car on or off. Besides, I could forget to turn it off and drain the battery :/

 

[StephenRowley]

A relay is what is what your after with a the relay coil powered from a switched ignition source.

You idea sounds about right.

Make sure you've 100Mo relay correct wire rating ect.

 

[James]

Something like this might do.

LC1D38JD | Schneider Electric TeSys D LC1D Contactor, 12 V dc Coil, 3-Pole, 38 A, 3NO, 690 V ac/dc | RS

uk.rs-online.com

 

[Lucien Nunes]

Did I get it right?

Yes., exactly.

You mention a 100A fuse. What accessories are you planning to power? A dashcam might take half an amp and the parking sensors likewise. A 35W tablet charger at max charge rate will need about 4A. If you genuinely need in the order of 100A (20 halogen driving lights) then you must look at the rating of the alternator and battery for suitability.

Something like this might do....

It might do very well, but something like this should be fine too, at lower cost:

Durite 100A relay

Or even this, which I think is the modern equivalent of a relay I used in the hundreds under pretty arduous conditions many moons ago:

Bosch 75A relay

Amongst high current automotive relays, beware of solenoid type relays and 'starter' or 'diesel preheat' types as they are often only intermittently rated (e..g 20 seconds in a minute). They have powerful coils packed into a tiny box that will soon overheat if energised continuously. For example, the Bosch relay above (0 332 002 168) has a 48 ohm coil, but change the last digit to a 1 and you get a 15 ohm starter relay that looks identical but is not continuously rated. Some unknown brands may have rather optimistic current ratings too!

 

[Megawatt]

I didn't want to have the trouble of turning it on and off every time I turn the car on or off. Besides, I could forget to turn it off and drain the battery :/

Add another battery in you vehicle

 

[vinter93]

Thanks for all the replies. I know the 100a is a bit over the top. I was thinking in feeding an amp from this as well. The custom fuse box is rated for 100a as well so I started from there. I'll probably pick up a 75a or less since I won't be able to put an amp at the moment.. I've got a diagram ready with all the wiring, hopefully it will work out I need to get all the materials now!

 

[vinter93]

Thanks for all the replies. I know the 100a is a bit over the top. I was thinking in feeding an amp from this as well. The custom fuse box is rated for 100a as well so I started from there. I'll probably pick up a 75a or less since I won't be able to put an amp at the moment.. I've got a diagram ready with all the wiring, hopefully it will work out I need to get all the materials now!

Actually I found a problem. I won't be able to feed the relay with the radio wire since I just found out that it is always powered.
Would it be ok if I use a piggyback fuse on the cigarette lighter fuse and feed the wire to the relay?

 

[telectrix]

Actually I found a problem. I won't be able to feed the relay with the radio wire since I just found out that it is always powered.
Would it be ok if I use a piggyback fuse on the cigarette lighter fuse and feed the wire to the relay?

should be. although the radio has 2 inputs. 1 permanent to retain it's memory, and 1 from ignition/accessory to make it play. mine has 3 inputs. the 3rd one is to warm the valves.

 

[vinter93]

should be. although the radio has 2 inputs. 1 permanent to retain it's memory, and 1 from ignition/accessory to make it play. mine has 3 inputs. the 3rd one is to warm the valves.

Thanks. I'll go with the piggyback fuse easier to do.