# Using capacitors to keep an Apple TV powered on after supply is disconnected

#### Gonebytim

Joined Oct 15, 2020
26
Hello,

please forgive me if I’ve posted this in the wrong section. I’m wondering if someone would be able to help me with what might be a simple problem but is not simple to me with my limited knowledge.

I have a latest generation Apple TV 4K which has had the power supply board removed so that it can run on 12v (you guessed it -it’s installed in a car!)

it’s worked perfectly fine - however it reboots itself while I’m cranking the engine - as the battery voltage drops down to 9.5v or so while the engine is being started.

I’m looking at adding some capacitors and a diode to discharge power to the Apple TV and keep it powered on for about 5 seconds or so while the engine is cranking.

I already know the basic circuit needed to be wired up to do this using a capacitor(s) and a diode - however - I don’t know how to calculate what value capacitor I would need to get the Apple TV powered for about 5 seconds or so.

I have found out that the Apple TV draws 1.083 amps of current While it is running. I’m unclear of exactly how low the voltage is able to drop before the Apple TV cuts out - however - once when my battery went a little flat - I found that the Apple TV would not boot up when the battery was putting out about 11v

I’ve already got a 1N5408 diode laying around that is capable of handle 3amps and 1000v - so I believe this would be suitable - I’m just not sure what spec of capacitor(s) I need to keep the Apple TV running for about 5 seconds without dropping to too far below 12v

thanks in advance for any help I get

- Tim

#### AlbertHall

Joined Jun 4, 2014
11,131
So, to a first approximation based on the limited information, you want to supply 1A for 5 seconds while the capacitor discharges from 12V down to 11V.
The relevant formula is Q = CV, or it = CV, where i is the current, t is the time, C is the capacitance, and V is the voltage.
Rearranging: C = it/V = 1A * 5s / 1V = 5F so you would need a 5F capacitor.

#### Gonebytim

Joined Oct 15, 2020
26
So, to a first approximation based on the limited information, you want to supply 1A for 5 seconds while the capacitor discharges from 12V down to 11V.
The relevant formula is Q = CV, or it = CV, where i is the current, t is the time, C is the capacitance, and V is the voltage.
Rearranging: C = it/V = 1A * 5s / 1V = 5F so you would need a 5F capacitor.

Thankyou so much! That’s exactly what I was after.

So obviously - a 16v 5f capacitor is pretty extreme (and would be incredibly expensive even if I could find one available). I’m assuming my best bet is to either buy or build a bank of 16v capacitors that equal 5F in total.

Does this kind of thing look suitable?

https://www.ebay.com.au/itm/273123387954

Also - are their any concerns about operating temperatures or anything? Would you think something like this would require a heat sink or cooling of any kind?

#### AlbertHall

Joined Jun 4, 2014
11,131
Maybe:
1. A small lead acid battery could feed the TV and be kept charged by the car.
2. Some other 12V battery which supplies the TV while the car battery voltage is too low.

#### Gonebytim

Joined Oct 15, 2020
26
Maybe:
1. A small lead acid battery could feed the TV and be kept charged by the car.
2. Some other 12V battery which supplies the TV while the car battery voltage is too low.
From doing some googling just now - I found some Automotive UPS systems which feature a backup battery . These are designed for taxis or police vehicles who also don’t want their computers and other after market equipment to lose voltage when the car is cranked.
I could possibly use something like this / the only downside is that they have enough power to get a 1amp devices powered on for 44 minutes!

I don’t want to install some kind of manual switch to turn the device or the circuit off every time I get out of the car - so I would rather something that only keeps about 5 seconds of charge - so that it keeps the Apple TV powered for long enough while I’m starting the engine - but still powers off very quickly when I’ve finished using the car.

I actually just noticed that “Car Audio Capacitors” (which I’ve always ignored as being “snake oil” due to claims about “better sound” and “smoother bass” etc) - might be exactly something that would work as they are all designed for 12v (with maximum of about 20v) and come in sizes such as 2f, 5f, 6f etc!!

the only downside is that they are EXPENSIVE because they are marketed at car audio enthusiasts chasing audio file sound quality , and they are often covered In LED lights with a digital display on the front which I would imagine steal a lot of the power..

#### Ramussons

Joined May 3, 2013
920
So obviously - a 16v 5f capacitor is pretty extreme (and would be incredibly expensive even if I could find one available). I’m assuming my best bet is to either buy or build a bank of 16v capacitors that equal 5F in total.
You will also have to get a trailer to carry that capacitor Bank.

#### Kjeldgaard

Joined Apr 7, 2016
449
How about a relatively simple Boost Switch Mode Power supply circuit?

#### Ian0

Joined Aug 7, 2020
1,620
You will also have to get a trailer to carry that capacitor Bank.
NiMH batteries. 10 AAA cells would do the job, with a resistor to trickle charge.

#### Gonebytim

Joined Oct 15, 2020
26
This would still have the problem that you would need to turn the TV off when leaving the car.
I’m thinking I could use something like this and have it controlled with a relay that’s triggered by the accessory circuit in my car (so power is cut off to the Apple TV after I take the keys out and get out of the car).

I believe that the accessory circuit shuts off for a few seconds while the engine is being started-but I could use a fairly small capacitor and a diode to keep the relay switched on while the car is starting.

#### crutschow

Joined Mar 14, 2008
26,786
So who is watching this TV while you are driving?

#### Gonebytim

Joined Oct 15, 2020
26
So who is watching this TV while you are driving?
No one - the Apple TV is running Spotify and Apple Music apps

#### crutschow

Joined Mar 14, 2008
26,786
Like this?

https://au.rs-online.com/mobile/p/rechargeable-battery-packs/7770371/

What is the important of the resistor and what value resistor would I need?
Yes that battery should work.
But you could probably put one together for less using ten AA NIMH cells.

Here's the charge info for that battery:

Since the battery is nominally 12V you should use Schottky diodes (2A or better) for isolation so there's still sufficient voltage to keep the TV operating.

With that. the maximum charge voltage will be about 14V, so the resistor for a trickle charge (circuit below) should be about (14V-12V) / .03(1.3) = 51 ohms.
You can connect this circuit directly to the car Accessory power (no relay needed) since the dropout during the start will be bridged by the battery.

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#### Gonebytim

Joined Oct 15, 2020
26
Yes that battery should work.
But you could probably put one together for less using ten AA NIMH cells.

Here's the charge info for that battery:
View attachment 219725

Since the battery is nominally 12V you should use Schottky diodes (2A or better) for isolation so there's still sufficient voltage to keep the TV operating.

With that. the maximum charge voltage will be about 14V, so the resistor for a trickle charge (circuit below) should be about (14V-12V) / .03(1.3) = 51 ohms.
You can connect this circuit directly to the car Accessory power (no relay needed) since the dropout during the start will be bridged by the battery.

View attachment 219731
firstly - Thankyou very much for that detailed and extremely helpful post.

I purchased a 12v SLA battery (This One Here) and constructed the exact circuit you drew. I’ve already installed it in my car!

I assume it’s working as the car hasn’t caught fire and the battery hasn’t exploded. I am facing another issue though - as previously mentioned - between this new backup battery and the Apple TV - I have a relay in the circuit (it’s a standard automotive 4-pin normally open 12v mini relay).

the I’ve wired up a 2200uf 16v capacitor and a diode on the trigger input of the relay in an attempt to get the relay coil to stay energised for a while after power is lost on the control circuit. This either doesn’t appear to be working or it’s not working long enough because the Apple TV is still rebooting when I crank the car.

i wired in a second 2200uf capacitor to try and double the delay time on the relay switching off - in case one 2200uf capacitor wasn’t enough (i actually have no idea what is needed to keep a relay working - I just used 2200uf because lots of other people on forums were saying they used one to keep a relay energised for 5 seconds

I’m not sure what I’m doing wrong or what to try next?

#### Gonebytim

Joined Oct 15, 2020
26
Yes that battery should work.
But you could probably put one together for less using ten AA NIMH cells.

Here's the charge info for that battery:
View attachment 219725

Since the battery is nominally 12V you should use Schottky diodes (2A or better) for isolation so there's still sufficient voltage to keep the TV operating.

With that. the maximum charge voltage will be about 14V, so the resistor for a trickle charge (circuit below) should be about (14V-12V) / .03(1.3) = 51 ohms.
You can connect this circuit directly to the car Accessory power (no relay needed) since the dropout during the start will be bridged by the battery.

View attachment 219731
Sorry - a bit of an update since my previous post - I did some testing....

I used an LED light after the relay to check whether the relay is staying on while I’m cranking the car - and the relay stayed on while the car was cranking! It never turned off - So I may have gotten my relay setup right after all.

next - I left the engine off and turned on as much stuff as I could - the whole audio stereo system, air conditioner fans on full and I watched the digital battery voltage reading in my dash - and as soon as the voltage dropped to 11.9 volts - the Apple TV shut off and went into its error mode with a flashing light on the front. My LED test light was still working (which gives me a good indication of the fact that the Apple TV definitely won’t run on any less than 12v

so next - I bypassed my relay and allowed the Apple TV to be run directly from the new backup battery (with the car battery disconnected - and the Apple TV wouldn’t work - the LED test light was working and the AppleTV had its error light flashing on the front.
I reconnected the car battery and the Apple TV started working.

so I’m convinced that the new battery is working (meaning that its connected and installed properly) - it just doesn’t seem to be giving enough voltage to the Apple TV.

I’ve called it quits for the day because it’s getting dark outside here in Australia but I will do some more testing tomorrow (getting the multimeter out and measuring the voltage going to the Apple TV from the new battery, and perhaps checking the voltage of the new battery itself.)

do you think that the battery needs a proper charge before being used? Would the trickle charging be too slow to be giving it a good first charge? I have no idea if a battery like this comes fully charge or almost empty when you buy it?

#### crutschow

Joined Mar 14, 2008
26,786
Yes, the SLA battery likely needs to be charged with a battery charger or power supply (limit the current to less than 0.39A as stated on the battery).
Its open-circuit voltage, after letting it sit for a few hours should be about 12.6V.

Since the trickle charge voltage for the SLA is 13.5V, you need a smaller trickle charge resistor.
The Schottky will drop the charge voltage from the car below 14V, so the resistor needs to be no more than about 1 ohm.
You can check the voltage across the resistor when the car is running to determine the actual charge current.

What diodes are you using?

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#### Gonebytim

Joined Oct 15, 2020
26
Yes, the SLA battery likely needs to be charged with a battery charger or power supply (limit the current to less than 0.39A as stated on the battery).
Its open-circuit voltage, after letting it sit for a few hours should be about 12.6V.

Since the trickle charge voltage for the SLA is 13.5V, you need a smaller trickle charge resistor.
The Schottky will drop the charge voltage from the car below 14V, so the resistor needs to be no more than about 1 ohm.
You can check the voltage across the resistor when the car is running to determine the actual charge current.

What diodes are you using?
I’m currently using a 47R resistor and two of these diodes (they didn’t have the diode you recommended in your drawing).

I’m going back to the store this morning to get myself a battery charger - so I should get a 1ohm resistor? And are the diodes I’m using suitable?