Bridging 1 second power gap for Android Auto device, calculating size of capacitor

Detritus70 · 19-10-2023 9:24am #1

I have fitted a wireless Android Auto screen to my car which hangs on the ignition. Very handy thing to have, it works as a reverse cam, dash cam, hands free and of course Android Auto device, it has vastly improved the user experience of my absolute bog standard car (Dacia Lodgy poverty spec, it hasn't even got central locking and it has keep fit windows all round)

The issue I have, if I turn off the engine and start it again, the power to the Android unit and the radio gets cut for a split second. The Android unit then has to go through the entire startup sequence again and connect to my phone. Also, it absolutely doesn't like short power interruptions and so it takes 10-20 minutes to reconnect my phone which is very annoying. Connecting it directly to the battery is also a no-go, since this would drain the power when parked up, so whatever solution I come up with has to cut the power to the Android device when the ignition is off.

My question, the plan is to fit a capacitor in parallel in the power line to the android unit, but I have no idea how to calculate the size of that capacitor.

12 Volt, 400 mA, 6W and the capacitor should be able to bridge a power-gap of about 5-10 seconds, which is already overkill.

I tried and had a look at the formulas, but I am a mathematical moron and I struggle with calculating fuel consumption in l/100 km on my car.

10-10-20 · 19-10-2023 10:02am

Hi.

Just to be clear, a capacitor isn't a battery, so its discharge curve differs significantly. A battery will maintain it's output voltage while a capacitor will drop the voltage very rapidly in line with its charge. This becomes a problem when devices such as the android tablet require a stable input supply of 12v, it only gives you a very small voltage input window to work with.

But this is what I worked out:

I took the resistance of the tablet to be 24ohms (12*0.5A).

So a 1F capacitor would do what you need as the Vdrop in 5 seconds would be only to 9.7v. Except that's a very large capacitance to have on a 12v circuit - you would need a super-capacitor and some protection on the circuit to prevent it pulling too much current from empty.

Have a google for supercapactor power supplies or equivalent 12v UPS's such as these (only cheaper):

https://www.bicker.de/en/company/news-events/dc-ups-with-supercaps-for-uninterruptible-12v-24v-power-supply?nbnet=0#news-produkt

Detritus70 · 20-10-2023 6:25am

https://www.boards.ie/discussion/comment/121258163#Comment_121258163

Thank you for that reply!

According to the specs the device has a voltage range of 9-36 volt and realistically the gap in power is 1 second at most.

A 1 F capacitor is quite hefty, I'm wondering if I might get away with half that, because bridging a gap of 5-10 seconds was wildly overestimated.

10-10-20 · 20-10-2023 7:23am

If the rating of 12v @ 400mA was taken off the power supply for the device then the device probably isn't pulling 400mA most of the time anyway, It could be pulling half of that. So the current-draw on the capacitor would be even less again.

Four of these 3.5v supercaps in series might do it:

https://ie.farnell.com/kemet/fgh0v474zf/cap-0-47f-3-5v-super-radial/dp/2362059

You'd probably need a 12 ohm resistor on the input to limit the in-rush current to one amp. That would be up to 12w instantaneous on the resistor, so a beefy-ish resistor would be needed. Plus add in a 2A fuse.

Bruthal. · 20-10-2023 7:36pm

May be able to use a relay switched on by ignition switch. Find a circuit which stays on while starting engine. Use it to switch a relay. Relay contacts from battery to device to be powered. If I was doing it myself I'd possibly use a microcontroller and mosfet setup to keep the device powered for a minute or any desired time, after ignition switched completely off. But the relay and an ignition circuit that's maintained during starting, should work.

lostinsuperfunk · 20-10-2023 8:29pm

Would an off-the-shelf power bank work? You can get models that supply 12V. You could power the Android from the power bank, and keep the bank topped up by charging it when the ignition is on .

This one for example: https://www.thomann.de/ie/ansmann_powerbank_20ah_pb320pd.htm?glp=1&gclid=EAIaIQobChMIxJCXi7qFggMVAeXtCh1tAgKeEAQYAiABEgJE7fD_BwE

Detritus70 · 20-10-2023 8:47pm

https://www.boards.ie/discussion/comment/121264860#Comment_121264860

Power bank may be massive overkill.

But something along those lines may be an idea.

whizbang · 25-10-2023 9:31pm

Power-off Time Delay Relay Module

https://www.aliexpress.com/item/1005006020052830.html?spm=a2g0o.detail.0.0.6f49WPJbWPJbev&gps-id=pcDetailTopMoreOtherSeller&scm=1007.40000.327270.0&scm_id=1007.40000.327270.0&scm-url=1007.40000.327270.0&pvid=726100b6-a5ed-42aa-977d-529821ce94b3&_t=gps-id:pcDetailTopMoreOtherSeller,scm-url:1007.40000.327270.0,pvid:726100b6-a5ed-42aa-977d-529821ce94b3,tpp_buckets:668%232846%238107%231934&pdp_npi=4%40dis%21EUR%215.38%213.66%21%21%215.57%21%21%402103146f16982690460536270eabd7%2112000035351959688%21rec%21IE%21%21AB&search_p4p_id=202310251424060793464784038568908002_5

Detritus70 · 26-10-2023 7:10pm

https://www.boards.ie/discussion/comment/121284486#Comment_121284486

I'll have to check that out, because being an electronic dummy I might burn my car down if I start soldering stuff together myself..😁

Detritus70 · 26-10-2023 7:47pm

I might go for this because I don't know if I could bring myself to order something off AliExpress.

https://www.ebay.de/itm/272567126084?var=0&mkevt=1&mkcid=1&mkrid=707-53477-19255-0&campid=5338765332&toolid=20006&customid=DE_12576_272567126084.145383977414~1425950120591-g_CjwKCAjwnOipBhBQEiwACyGLum2CcHnuiW9u68lf5x6PAS2I-fAz2KOGdjejr5vkyT1FbryYrS2raBoC6zYQAvD_BwE

Detritus70 · 17-11-2023 8:57pm

https://www.boards.ie/discussion/comment/121284486#Comment_121284486

I'm such a fool, what I was looking at provides power for a preset amount of time when power is supplied to it and then switches off.

Your doohickey is the right one, I need to wire a permanent 12 volt from the battery into it (will need separate fuse) and I will take the existing power line that is turned on and off with the ignition as a trigger line. This setup I can then program to keep power going from a few seconds to one hour after I turn the car off.

I cannot make friends with Ali, he wants access to my Facebook or Google and he ain't getting that. I found the same module on eBay.

Detritus70 · 29-12-2023 3:14pm

OK, the thingy arrived some time ago and I managed to wire it all in over the Christmas.

The best permanent power sauce that runs without interruptions is the horn. I'm also stealing power for the central locking I installed in the car from there. Central locking and the Android screen do not use all that much power, so it "should" be ok.

When I ordered that thing I had the wrong idea about how it worked. It seemed logical to me that you have permanent power, signal power and ground on one side and a switched +/- power outlet on the other side. How wrong I was! Permanent power from the car goes to the + of the thing you want to run and the device switches the - side on and off.

Closer view

Yes, it's messy. But it works.

I can now start the engine and the Android screen won't do a restart. When I shut off the key, it goes off after 5 seconds. I could program a longer or shorter delay if I wanted to.

I have fitted a backup camera, the Android screen, central locking plus boot popper and four new speakers to my car. While I did solder the odd wire, 99% of it was done using crimps. It has made this job SO MUCH easier! Had I done this using just pliers and a soldering iron, I'd still be at it.

The description of the item on Ebay on how to wire it was not making sense to me. Thankfully I found a wiring diagram. As I was wiring it, I kept thinking "this can't be, this doesn't make sense, this is nuts!" but in the end it worked. And worked very well, this now does exactly what I want it to do

My only problem now is that audio from that screen is in mono! Aaaargh! It really hurts my soul to listen to music over that, so I mostly listen to scratchy, crackeldy FM radio. But then I can't hear the sat nav, which is annoying. Does anyone know how I can use Android auto but with the audio coming from the phone's headphone socket?

Detritus70 · 04-01-2024 3:48pm

https://www.boards.ie/discussion/comment/121264586#Comment_121264586

https://www.boards.ie/discussion/comment/121284486#Comment_121284486

And that's exactly what I ended up doing 😃

Detritus70 · 21-01-2024 10:48am

Just to give a final update.

The Android Auto screen I was using turned out to be mono only. I contacted the manufacturer just to ask them to confirm or deny that fact to me. The resulting conversation in Chenglish consisted of nothing but incomprehensible gibberish from Hodozzy.

They offered me €60 if I write a 5 star review, which I did and I promptly received €70 back to my account. Not Amazon vouchers or some other form of credit, actual, usable money. I did mention that the device is mono in my review.

I then ordered the device you see in the attached picture and it is stereo, works better and has good sound quality.

Funny thing, this second screen got lost in transit by Amazon and I got a full refund. And later on I received a phonecall from someone in our village and they actually had my package!

So I got a 50% refund on my original screen and a second one for free. So all in all that worked out pretty well for me. The new screen has exactly the same connections as the old one, so it dropped right in.

10-10-20 · 21-01-2024 9:08pm

https://www.boards.ie/discussion/comment/121647295#Comment_121647295

Hi, so what device is that? I'm interested as I have an older car, might work for me too.

Detritus70 · 22-01-2024 8:11pm

https://www.boards.ie/discussion/comment/121651257#Comment_121651257

This has been great so far, reverse cam, dash cam, hands free, Android Auto.

So far it's been good, I run an aux cable from the screen to the car stereo.

https://www.amazon.de/-/en/gp/aw/d/B0CFV743DM?psc=1&ref=ppx_pop_mob_b_asin_title

Bridging 1 second power gap for Android Auto device, calculating size of capacitor

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