Additional supercapacitors

mbocek

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I have the A119, recording 1 minute videos in a loop. Having a small capacitor, the dash cam switches off almost instantly after disconnecting from the power supply.

What I'd like to achieve is to keep the cam working for at least one minute after disconnecting from the main power supply (hard wired in a car) so that it finishes recording the 1 minute video in case of an accident during which the electrical power gets cut off. Ideally more minutes, let's say five, to capture what's going on after the accident as well.

What I've found so far is this - simply using additional capacitors added between the main power and the cam:
https://www.instructables.com/id/Simple-USB-UPS/
Does it make sense? Is it safe?
I'm trying to avoid the use of the lithium based power banks due to the fire hazard.
 
What would the timer help in this case? Sorry, I don't know much about designing circuitry.

I don't care if the capacitors would power the cam for a specific time or until they discharge completely. Unless you tell me that it would damage them. Basically what I need is to add 1+ minute to the runtime of the cam when the power is switched off.

I don't even know how to calculate what rating the additional capacitors should have, so I would go with those mentioned in the article and expect that the RPi draws more power than the dash cam.

The question also is whether this approach of using capacitors for this purpose makes sense or whether there's a better or easier solution at hand.
 
Adding capacitors to the USB cable will not work, the camera will switch off when the capacitors are still 90% full, so you would need extremely large capacitors.

Adding them to the 12 volts could work, but they would need to be super capacitors and then you would need at least 6 to cope with the voltage, it would get expensive and large.

A timer like jokiin suggests is the best solution, but you may already have a timer on one or more of your car circuits, some turn the accessories off after 20 minutes, many cars keep the engine control unit active for a while.

I have my dashcams connected to the engine fuse, which gives 4 minutes after stopping the engine. Only issue is that the fuse is in the engine compartment, so I had to install some extra cable through to the cabin.
 
Yeah, I meant supercapacitors, as mentioned in the linked article where they use three 2.7V 50F supercapacitors in series that is said to behave as a 6.9V 16.33F capacitor. I didn't know that the cam would switch off at 90% of the capacity. Even though I don't know how fast it would take to discharge the capacitors to the point of 90%. I guess in less then 1 minute based on what you say :)

The timer would work if I need the power for a few more minutes after the engine stops and I take the keys out. But what I'm mainly concerned about is the case of an accident where the power gets cut off completely. It's actually just my assumption that the car main control unit stops feeding power to all circuits upon an impact to prevent short circuits and fire hazard. Even though I guess that vary from car model to model.
 
If the accident is bad enough to cut off the power, then there is a good chance the camera will no longer be able to see the view, either because of broken glass or because it is pointing towards the floor, or because the car bonnet (hood) is now blocking the view!

You could get it to work if you put a voltage converter between the capacitors and the camera so that the camera sees a constant 5 volts until the capacitor is empty. Probably easiest to use 2 supercapacitors in series to give 2.7 x 2 = 5.4 volts, then charge that with a normal USB charger/hardwire kit, and then use a step up/down converter to maintain the 5 volts to the camera. If you use a single capacitor then you will have difficulty finding a regulator that runs at low enough voltage. If you use 6 capacitors in series then you can charge it straight from the car supply (via a diode so that it doesn't go back into the car when power is lost), and then just connect the hardwire kit to the super capacitors, but that will probably get a bit expensive on the capacitors and probably a bit big, although then you wont need to worry about getting enough amps to the camera.

If you want to calculate the size of the capacitors required:
http://www.circuits.dk/calculator_capacitor_discharge.htm
And remember that your voltage regulator will stop working when its input gets down to 3.5?, maybe 6? volts, and if you power an HK3 from your supercapacitors then it will stop working at 12.4? volts.
 
Thanks for the hints. I will dig deeper and try that.

I understand that when the accident happens, the view of the camera may get blocked. What is more important to me is to see what has happened right before the impact. Because as I have it now, when the power is cut off, the last 1 minute sequence is not saved on the card, which may be actually the most important piece.
 
Thanks for the hints. I will dig deeper and try that.

I understand that when the accident happens, the view of the camera may get blocked. What is more important to me is to see what has happened right before the impact. Because as I have it now, when the power is cut off, the last 1 minute sequence is not saved on the card, which may be actually the most important piece.
The last segment should be saved, that is why the camera has super capacitors.

If the super capacitors are not working then they can be replaced, a lot easier than adding some more. There have been a few instances of super capacitors failing.

You could also set the "Video Format" to ".ts" in the menu. That should ensure that the last segment does not get corrupted if it is not fully saved, and so you can then only lose a few seconds.

What model A119 is it, and how old is it?
 
I see, I'll try the .ts format. I have the V2 version. It takes like 1 second until the cam turns itself off when I unplug the USB cable, so yeah, maybe the capacitors in the cam are somehow faulty. For how long the cam normally runs after unplugging the power?
 
I see, I'll try the .ts format. I have the V2 version. It takes like 1 second until the cam turns itself off when I unplug the USB cable, so yeah, maybe the capacitors in the cam are somehow faulty. For how long the cam normally runs after unplugging the power?
I don't know about the V2, but the V3 is less than a second before the screen turns off, it then takes a few seconds flashing the record LED before it appears to be off. If you take the back off and look at the super capacitors, if they are OK then the ends will be flat, if they have failed then one, or maybe both will have burst at the ends so the ends will be rounded and leaking.
 
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