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?

Glad to know it's all working as desired.

Yes, you can check the battery voltage that way.

You can also measure the voltage across the 1 ohm resistor to determine the charge current (1V equals 1A).

 

Glad to know it's all working as desired.

Yes, you can check the battery voltage that way.

You can also measure the voltage across the 1 ohm resistor to determine the charge current (1V equals 1A).

it will be fairly impossible to actually get probes on either size of the resistor While it’s connected (due to where it’s all mounted) but i will be able to get probes on the terminals of the battery

 

Glad to know it's all working as desired.

Yes, you can check the battery voltage that way.

You can also measure the voltage across the 1 ohm resistor to determine the charge current (1V equals 1A).

i measured the voltage of the SLA while the car was running and it’s showing 14.15v
The documentation with the battery specifies these two charging voltages:


Cycle:
- Voltage 14.4V~15.0V @ 20°C (68°F)
- Temp Coefficient: - 30mV/°C

Standby:
- Voltage 13.5V~13.8V @ 20°C (68°F)
- Temp Coefficient: - 20mV/°C

It seems to be sitting right between the two ranges.

do you think it will be alright - or is it important to lower the charging voltage a bit more?

 

Glad to know it's all working as desired.

Yes, you can check the battery voltage that way.

You can also measure the voltage across the 1 ohm resistor to determine the charge current (1V equals 1A).

do you think there would be any benefit to adding a second relay between the car battery and the SLA so that the battery isn’t charging unless it’s in use - and adding a buck boost module onto the SLA to regulate its charging voltage to the exact amount required.

also possibly adding a buck boost module just before the Apple TV to regulate the voltage it is getting too (as someone else suggested previously?)

 

do you think it will be alright - or is it important to lower the charging voltage a bit more?

I think it's OK.
You are operating below the normal cycle operating range (which is how it will operate in the car) so I see no reason to lower the voltage.

do you think there would be any benefit to adding a second relay between the car battery and the SLA so that the battery isn’t charging unless it’s in use - and adding a buck boost module onto the SLA to regulate its charging voltage to the exact amount required.

No, as it will not be charging significantly when the car is off.

also possibly adding a buck boost module just before the Apple TV to regulate the voltage it is getting too (as someone else suggested previously?)

The TV is likely fairly tolerant of over-voltage and, if it has operated OK up to this point, then I see no reason to add a regulator,
Of course, if you add that, then you possibly wouldn't need the added battery.

 

I think it's OK.
You are operating below the normal cycle operating range (which is how it will operate in the car) so I see no reason to lower the voltage.
No, as it will not be charging significantly when the car is off.
The TV is likely fairly tolerant of over-voltage and, if it has operated OK up to this point, then I see no reason to add a regulator,
Of course, if you add that, then you possibly wouldn't need the added battery.

the only thing I might change is to prevent the Apple TV ever being powered off the window circuit

I had a moment last night - when the key wasn’t in run or start mode - while the Apple TV was being directly powered from the window circuit - it lost power and rebooted itself when I used a window switch to lower a window.

I figure if I use a second relay I can always have the Apple TV running off the backup battery circuit by just creating two paths from the backup battery to the Apple TV - one path with a relay triggered by the window circuit - one path with a relay triggered by the ignition switch.

 

I’ve altered it to have two relays like I mentioned in my last post.

couldn’t be happier with it now.

Thankyou so much for all of the assistance . The internet really is an amazing place. I really appreciate it!