What diodes are you using?

I’m glad you asked that question -

i mentioned in my last post that I couldn’t get the one you recommended - so I asked the guy at the store to give me a suitable alternative. I’ve just noticed in the spec sheet that he sold me one with a drop of almost a whole volt! (0.95v).
Considering that I’ve seen the Apple TV switch off at 11.9v - this diode won’t be suitable because I’d be giving the Apple TV 11.6v

I need to go back to the store and find a diode with less than 0.6v drop

I’m learning so much about this from what you are teaching me - that I’ve just realised that the diode used is going to determine which value resistor I need to use.

 

I’m currently using a 47R resistor and two of these diodes

As you noted, those are standard diodes and have a higher forward drop than a Schottky, so are not as well suited for your requirements.
Look for a 2A or greater Schottky diodes, such as a 1N5822.
It has about a 0.4V forward drop at 1A.

so I asked the guy at the store to give me a suitable alternative.

Unfortunately many of those guys at electronic shops don't know beans about electronics.

 

As you noted, those are standard diodes and have a higher forward drop than a Schottky, so are not as well suited for your requirements.
Look for a 2A or greater Schottky diodes, such as a 1N5822.
It has about a 0.4V forward drop at 1A.
Unfortunately many of those guys at electronic shops don't know beans about electronics.

Ok - an update...

I went to the store and got some 1N5822 Diodes and a 1ohm resister.

I connected it into the car and left it disconnected from the car battery so that I could see if the circuit now has enough voltage to power the Apple TV. The Apple TV powered on , but only for about 30 seconds before shutting back down with the light flashing on the front of the Apple TV.
So again - I’m pretty sure it’s getting power - but the voltage must still be too low.

I did buy a battery charger earlier today - but it was telling me that the battery was fully charged and my multimeter was telling me 12.6v across the batteries terminals. Right now I’m leaving the battery sit for a couple hours before checking it again in case the reading was wrong and the battery isn’t fully charged.

mother than that - I’m confused by the voltage issue. Unless 5 meters of cable is giving enough of a voltage drop to be an issue. But using online calculators I’m assuming that the cable should only be giving me 0.125v loss at the most. The diode spec sheet says it’s a 0.525v loss. All up that takes the voltage down to 11.95v -

hopefully that’s not too much of a loss.

But what I don’t understand is that my main car battery can power the Apple TV fine with the engine off - and that should be no more than 12.6v from the battery and also passing through a diode and the same 5m cable run.

so the new battery not putting out 12.6v is the only thing I can really see as possibly being the issue...

 

The car battery voltage will not drop measurably when supplying 1A, but the battery pack voltage will drop some voltage (unknown, you'll have to measure it to know) at 1A;

 

The car battery voltage will not drop measurably when supplying 1A, but the battery pack voltage will drop some voltage (unknown, you'll have to measure it to know) at 1A;

Yeah I was thinking about how I really need to take some actual measurements . It’s just a pain because the wiring is all soldered together with heat shrink up at the Apple TV and also requires removing a bit or interior trim to get to it. But it would be a good idea to check the voltage at the Apple TV

 

Ok - here’s an update..

I had a bit more of a guy trying to figure out the charging thing and after letting the battery sit for a couple hours - it did accept some charge for about half an hour or so before saying it was fully charged again. The charging thing is confusing me a bit with these batteries.

I removed the Apple TV from the car so that I could test it properly running off the new battery (standalone - but with the diodes and resistor in place). The apply tv ran perfectly!

so I install it back in the car and found that the Apple TV is now running fine in the car too being powered off the battery (tested without the Cars main battery connected).

so that’s a win - only thing now is it turns out that I don’t have the relay sorted out.Seems that two 2200uf capacitors don’t seem to be holding the relay on (or at least on long enough to keep the relay open for the during of the car engine starting.

im nearly at a point of giving up on the relay and just buying a proper timer relay - but they are expensive and it will feel like I’ve come so far and quit at 90%....

 

What's the purpose of the relay? I see no reason for one.

Draw a diagram of exactly what you have.

 

What's the purpose of the relay? I see no reason for one.

Draw a diagram of exactly what you have.

the relay is to turn the Apple TV off and on - controlled by the power window circuit of the car. The relay is activated when I unlock the car with the key remote and is deactivated after I get out of the car, shut the door and lock the car again. I’m using it simply because I don’t want to have to install some kind of ugly toggle switch to turn the Apple TV on/off (even if the Apple TV had a power switch on it, it’s hidden inside the dashboard of the car).

the relay being triggered by the power window circuit is perfect for my needs except for one issue:

** The Car’s main computer cuts power to the power Window circuit while the engine is cranking!**

so the original issue I was trying to solve is two-fold:

a) the Apple TV losing power during engine cranking because of voltage drop - SOLVED
b) the apply TV losing power during engine cranking because the relay switches off - NOT YET SOLVED

 

b) the apply TV losing power during engine cranking because the relay switches off - NOT YET SOLVED

I'm confused.
That's why I need a diagram.

Can't the battery keep the power to the TV when cranking?

Why can't you just connect the TV diode circuit to the Accessory power without the relay?

 

I'm confused.
That's why I need a diagram.

Can't the battery keep the power to the TV when cranking?

Why can't you just connect the TV diode circuit to the Accessory power without the relay?

because the car doesn’t exactly have a traditional “accessory power” circuit.

the stereo in the car is turned on/off intelligently via CANBUS, and so is all the other accessories. Nothing has the traditional 2 wires (constant + acc) that you used to see in cars.

I had to create my own “acc power circuit” by wiring the Apple TV direct to the battery and then placing a relay between the Apple TV and the battery. The relay then has its control input wired to the power window circuit of the car.

the relay is now sitting between the Apple TV and the new battery (otherwise the battery would just end up flat after turning the car off

i will draw you a diagram - I’m just not in a position to be able to do it yet so was trying my best to explain

 

the relay is now sitting between the Apple TV and the new battery (otherwise the battery would just end up flat after turning the car off

OK, that makes sense.
I was thinking you would be turning the TV off manually.

So what is delay circuit configuration for the relay?
What is the relay coil resistance?

Is it not possible to activate the relay from the ignition power so it doesn't turn off during the start, thus requiring no relay delay circuit?

 

OK, that makes sense.
I was thinking you would be turning the TV off manually.

So what is delay circuit configuration for the relay?
What is the relay coil resistance?

Is it not possible to activate the relay from the ignition power so it doesn't turn off during the start, thus requiring no relay delay circuit?

Are you talking about powering the relay from the ignition AND the power window circuit?

 

Are you talking about powering the relay from the ignition AND the power window circuit?

No, the relay would be activated by the ignition only.
It activates when the ignition is on and is off when the ignition is off.
The circuit could be powered from the window circuit.

Would that not do what you want?

 

No, the relay would be activated by the ignition only.
It activates when the ignition is on and is off when the ignition is off.
The circuit could be powered from the window circuit.

Would that not do what you want?

No it wouldn’t -

* if it was controlled by the ignition circuit then the Apple TV would only be on when the key is turned to ignition - which is far less than what I already have (where the AppleTV is powered from the moment I unlock the car until the moment I lock it again)

* if the Apple TV was powered by the power window circuit it would still have the issue of dropping voltage while cranking - so I’m assuming you are suggesting connecting the battery to the power window circuit too?

If so that makes sense - but I don’t want the Apple TV to only be on when the car is in the ignition setting.

I’m actually thinking now - replacing the relay with a five pin relay and wiring it up so that it’s controlled by the ignition circuit - but what it does is switch between two inputs , switching between whether the Apple TV is powered by the new backup battery or by the power window circuit.
It would be wired up so that the Apple TV is getting power from the new battery when the ignition switches the relay on - and then switched over to getting power from the power window circuit when the ignition is turned off and not energising the relay anymore.

if that makes sense?

that may it would be running off the power window circuit when I first unlock the car. It would then switch over to be running off the backup battery circuit while the car is cranking and the whole time the car is running and then switch back over to being powered off the window circuit when I turn the car off. It’ll stay powered until I remove the keys from the car, get out and lock the car behind me.

I just don’t know if the Apple TV will drop out while the relay is switching or if the relay is fast enough at switching between the two that the Apple TV won’t shut off?

I could rig up something different using multiple relays so it’s only the main relay’s control input that’s being switched rather than the actual power to the apple TV being switched. But that’s more complicated (but entirely possible if my first idea won’t work)

 

It would be wired up so that the Apple TV is getting power from the new battery when the ignition switches the relay on - and then switched over to getting power from the power window circuit when the ignition is turned off and not energising the relay anymore.

Just use two diodes to the relay that controls the TV power, one from the ignition and one from the window circuit.
That way either (or both) will power the relay coil but the diodes will isolate the two circuits so they don't interfere with each other.

You can run the main power to the diode/battery circuit from the window power.

You battery is only large enough to operate the TV for a short time (such as during cranking).

How does that sound?

 

Just use two diodes to the relay that controls the TV power, one from the ignition and one from the window circuit.
That way either (or both) will power the relay coil but the diodes will isolate the two circuits so they don't interfere with each other.

You can run the main power to the diode/battery circuit from the window power.

You battery is only large enough to operate the TV for a short time (such as during cranking).

How does that sound?

the diodes on the relay sound like a great idea - I think that’s going to be the final solution!

the other part I’m a little confused about. I’ve already got the backup battery wired directly from the battery. The reason I did this was because the best location to put the new battery was right next to the main battery in the boot - and because of the access to the power window circuit wires (they don’t run to the boot and there is not enough space for the new battery close to those wires). Can you see any problem with just having the backup battery running off the main car battery? I’m assuming it won’t be charging when the car is not running anyway as the main car battery voltage won’t be high enough to charge the backup battery.
Also - if the Apple TV is 1 amp - I’m assuming that the backup battery (at 1.3ah) would probably give the Apple TV a good 20mins or so before it drops low enough stop working. And if I’m running the battery low constantly - is the trickle charging going to be enough to keep the battery charged enough to run the Apple TV? Seems to me like I’m better off only having the battery running the Apple TV alone for the 5 seconds or so needed while the car is cranking over

 

I’ve already got the backup battery wired directly from the battery.

Yes, that should be fine, but I would recommend adding a fuse (say 2A) from the main battery in case the backup battery goes bad or develops a short.

I’m assuming it won’t be charging when the car is not running anyway as the main car battery voltage won’t be high enough to charge the backup battery.

That would be true.

Seems to me like I’m better off only having the battery running the Apple TV alone for the 5 seconds or so needed while the car is cranking over

Okay, here's another idea for that.
Below is a modification that may work (if you can tap into the starter voltage signal).
The Window voltage powers the TV except when the Start voltage energizes the relay to power it from the SLA battery during the start time.
C1 may be needed if there too much delay between the Start voltage and the time the relay closes.

 

Yes, that should be fine, but I would recommend adding a fuse (say 2A) from the main battery in case the backup battery goes bad or develops a short.
That would be true.
Okay, here's another idea for that.
Below is a modification that may work (if you can tap into the starter voltage signal).
The Window voltage powers the TV except when the Start voltage energizes the relay to power it from the SLA battery during the start time.
C1 may be needed if there too much delay between the Start voltage and the time the relay closes.

View attachment 219890

Thank you so much for your help again.

At the moment I’m trying to figure out where to get the starter signal from.

I’m having a lot of trouble finding wiring diagrams or pin outs for it , and I have a feeling that it’s a bit more complex than a tradition ignition switch

 

At the moment I’m trying to figure out where to get the starter signal from.

You could use a meter to determine which ignition switch terminal is active when the starter is activated.

Worst-case you could get it directly from the starter solenoid.

 

You could use a meter to determine which ignition switch terminal is active when the starter is activated.

Worst-case you could get it directly from the starter solenoid.

I’ve got some wonderful news...

it works!

it’s all connected as per your last schematic (I didn’t use a capacitor as it doesn’t need one).

I spent Hours last night reading through the service manual for the car and was getting a bit disheartened by the fact that it looked like the ignition switch sends a low voltage data signal to the “body control module” which then sends a low voltage data signal to the “Engine control module” - which then sends out a 12v signal to the crank relay to allow the starter motor to be powered. This is all because the car has a fairly complicated anti-theft system where the ignition switch gets security codes from the key fob and passes it on the the various modules which then do a two way communication to exchange passcodes etc before the ECM is allowed to activate a range of relays to allow the car to start.

And then finally I found a schematic showing that the ignition switch does in fact have a 12v signal that’s activated when the key is in the “start” position in addition to the low voltage data signal (I think to let the BCM know that it is ok to start communicating security data). So armed with the information of which colors The wires are that send and receive the 12v at the ignition switch - i used my multimeter to check it. I found that the 12v signal was passed when the key was in BOTH the RUN and START positions. I figured this was an added bonus - as it would open my relay when I turn the key to the “run” position” a second or so before I turn the key further to crank the engine. I feel like this helps to prevent any delay that we were thinking we might have to use a capacitor for.

so it appears to be working flawlessly for what I was hoping to achieve.

The last question I have to make sure that everything will continue to be great - is about checking the voltage that the SLA is trickle charging at (just to make sure it is charging at 13.5v and not less making it go flat - and not more - damaging the battery). Is that as simple as putting the multimeter across the two battery terminals while it’s all connected up and the car is running?