Accessory power drops while engine starts - how to fix?

[threadlock]

I've piggy-backed a 12v supply off an ignition-switched fuse in my 2007 Volvo XC90 that I use to power my dashcam and my Bluetooth-AUX hands-free phone adapter.
Whenever I start the engine the power is cut while the engine cranks, which means both the dashcam and the Bluetooth adapter restart within a few seconds of first being powered on each time. The dashcam seems unfazed by this, but the (cheapo) Bluetooth adapter has a benny 30% of the time and won't play phone media/calls until the power is cycled again.

I want to keep the 12v supply live for the second that the engine is cranking. I can't find an ignition-switched, fused circuit in the XC90 that stays live in the ign 'Start' position, so two solutions occur to me:
1. I wire a capacitor into the 12v feed that has enough farads to maintain the 12v for a couple of seconds.
2. I have a push-button 'on' switch that sets a relay allowing me to power-up the feed after the engine has started.

I can't find a straightforward off-the-shelf solution for 1, which makes me think it must be a Bad Idea for some reason.
Solution 2 doesn't appeal so much because although I'd remember to press the power button each time I drive the car my wife probably wouldn't, which would mean the dashcam wasn't running.

How do other people solve this issue?

[Penelope Stopit]

If you can't find a permanent ignition supply at the fusebox there will be one at the ignition switch

[richs2891]

As above
or wire directly to the battery (with a fuse of course)

[anonymous-user]

Probably cheaper and more effective to buy a decent bluetooth adapter than start mucking about with the car power. There's a reason cars disconnect accessories during start, it's to ensure you've got enough ampage to crank the starter and make sure there isnt a power surge that destroys your accessories

[threadlock]

Sorry if I wasn't clear; I want the supply to be switched with the ignition, not live permanently. It's just while the engine is cranking that the power drops.

[NugentS]

Install a second battery and a split charge relay so the car doesn't use the second much smaller battery to start.

Power the devices from the second battery.

Sean

[Dash-Cam Man]

What you are describing with the the ignition fuse being culled for a split second is normal in some vehicles. Some vehicles do this to give additional power during crank to the starter motor, but dash cameras aren't affected due the fact they usually have a super capacitor built in, or the usual internal 3.7V 200mAh battery.

The cheap bluetooth device obviously has no backup battery supply, for a situation power is culled for a split second, so is unhappy with power on, off and on again so quick. Being a cheap device, it probably requires a few seconds reset time.

Leave your ignition feed in place, wire it to a 3 pin switch. You don't require a diode protected relay for this option, just a cheap £0.85p switch. The 30% of time it doesn't work, you can just cycle the power through the switch, but that aside, leave the switch in the 'On' position so that the 70% of the time it does work, you don't require to actually do anything.

[Penelope Stopit]

If you can't find a permanent ignition supply at the fusebox there will be one at the ignition switch

[threadlock]

Thanks Dash-Cam Man. The idea of installing a reset switch is interesting - I hadn't considered that. It might be the simplest solution.

I did some Googling last night, and this is the sort of cheap capacitor-based, delay-off circuit I was referring to in my OP:
https://www.ebay.co.uk/itm/CAR-DOME-INTERIOR-LIGHT...
I fitted something similar to my classic car when I first bought it and it kept the interior light on (i.e., maintained power to the circuit) for ten seconds after I closed the door. I wondered whether this would work to solve my current problem, in that the circuit would be powered at ignition-on but this delay timer would hold the power on while the engine started. The one in that ebay listing is for switched earth rather than switched live, but you see the principle, I hope.

[Jakg]

threadlock said:

I did some Googling last night, and this is the sort of cheap capacitor-based, delay-off circuit I was referring to in my OP:
https://www.ebay.co.uk/itm/CAR-DOME-INTERIOR-LIGHT...
I fitted something similar to my classic car when I first bought it and it kept the interior light on (i.e., maintained power to the circuit) for ten seconds after I closed the door. I wondered whether this would work to solve my current problem, in that the circuit would be powered at ignition-on but this delay timer would hold the power on while the engine started. The one in that ebay listing is for switched earth rather than switched live, but you see the principle, I hope.

That seems interesting but I don't think it'll do what you want - it appears to ramp the current down to dim, hence "Can be used with incandescent lights only".

Have a similar problem with my Z4 I'd like to address and hadn't realised something so close could be so cheap.

[Penelope Stopit]

threadlock said:

Sorry if I wasn't clear; I want the supply to be switched with the ignition, not live permanently. It's just while the engine is cranking that the power drops.

You will find an ignition supply that remains on during cranking at the ignition switch
Since when have people wired in gadgets to save themselves from finding the correct supply at the ignition switch?

[threadlock]

Penelope Stopit said:

You will find an ignition supply that remains on during cranking at the ignition switch
Since when have people wired in gadgets to save themselves from finding the correct supply at the ignition switch?

Ah, I see what you mean. I misunderstood originally. Thanks for the suggestion.
As well as not wishing to dismantle too much of the car to find a feed within a loom, I don't feel as though arbitrarily patching my own wiring into a CANBUS-controlled car is a recipe for reliability. Hence me trying to find plug-in "gadgets" that'll do what I want from the simple fused accessory socket circuit that I know is low-risk.

[threadlock]

Wow - thank you
Are these XC90-specific diagrams? (I guess they must be, but I can't tell from your profile whether you've got any experience with the XC90.)

If you have access to this sort of technical info, can you tell where I would find the loom plug please? The first diagram of the terminals 15, S, 15l etc. suggest they're at the switch itself which (in my limited) experience won't be easy to access. Is the loom plug further down the steering column, or otherwise accessible in some way?

And when you say "fit a terminal with cable", do you mean a connector pin/socket that fits that specific loom plug? As you may be able to tell we're getting close to the limit of my skills! However I like the idea of a neatly-connected, reliable solution that doesn't need a permanent mod to any wiring.

Edit: hold on - I've a feeling you do mean the plug/socket at the ignition switch itself don't you? I thought originally that you meant there was a plug/socket further down the loom away from the ignition switch.
If it's at the switch itself, do you know how accessible it would be?

Edited by threadlock on Thursday 14th March 18:47

[Penelope Stopit]

You did visit back
I don't know how easy/difficult it is to get to the plug but past experience tells me it's not a bad job
You could buy a terminal to fit the plug from Volvo or a good auto electrician
Send me a PM and I'll email you back about something

Edited by Penelope Stopit on Thursday 14th March 22:31

[slybunda]

What voltage does the adapter run at internally?

[threadlock]

slybunda said:

What voltage does the adapter run at internally?

Both the Bluetooth adapter and the dashcam run off a USB socket, so 5V.

[gmaz]

Sounds link you need something like this

https://www.amazon.co.uk/BlackVue-B-112-Power-Magi...

or this

https://www.amazon.co.uk/d/Car-Electronics/Cellink...

But they are expensive

[Charlie Croker mk2]

wire just one of these capacitors across the 12 volt feed to your usb socket 680 micro farads should be quite sufficiant as the current draw will be quite small - or find a local shop if you dont want to wait for China but these look good .
https://www.ebay.co.uk/itm/35V-680uF-35Volt-680MFD...

[threadlock]

Charlie Croker mk2 said:

wire just one of these capacitors across the 12 volt feed to your usb socket 680 micro farads should be quite sufficiant as the current draw will be quite small - or find a local shop if you dont want to wait for China but these look good .
https://www.ebay.co.uk/itm/35V-680uF-35Volt-680MFD...

I wondered about a capacitor. But wouldn't it need diodes etc too to prevent the capacitor discharging into all the other circuits around the car?

[Charlie Croker mk2]

No the capacitor will in effect charge up as soon as the ignition feed goes live and will provide a small ( very small ) reserve power as you start the car , as the BT adaptor will only take a small current draw the capacitor will hold the voltage up for this short time . When you switch off at the end of your drive the capacitor will discharge very quickly and it will not effect any other ignition controlled circuits , if the time taken to start the car still causes the BT to disconnect simply add a 2nd capacitor in parallel 2 x 680mfd = 1360mfd that's got to work LOL

Edited by Charlie Croker mk2 on Thursday 21st March 20:36