In-Car save your SD card solution theory

I had a brain storm the other day and was going to come here and hint to Uniden for a firmware idea that would be awesome -- but then I found in the manual it already exists.

We know to shut down an SD based scanner you need X seconds to write, so if you leave it wired to your ignition chances are you're going to corrupt your card. Short of Uniden adding a physical ignition pin to the scanner with a capacitor that lets it stay powered long enough to write ignition based operation is probably not coming any time soon.

But I have a charge guard in my car between my battery and all of my toys because sometimes I leave them on with the car off. This got me to thinking if you could tell the 536HP to "auto shut off" in a time frame that is no longer than your maximum Charge Guard time you'd be fine. (The Charge Guard can also shut off if the voltage drops below 11.5 or so to protect the next start)

So in theory if you tell the 536HP to shut off after say 1, 2 or 3 hours you could technically "leave it on" all the time wired to constant 12VDC through a charge guard. If you left your vehicle off for an extended period of time the 536 would auto off and presumably close the SD card writes cleanly, right?

Or am I missing something obvious with this idea?

* Obviously manually shutting it off remains an option, but this offers a CYA protection.

You could probably go to any car audio shop and ask them to wire up a "supercapacitor" to whatever power socket you plug the radio into. That's typically a "hockey puck" that acts like a short term rechargeable battery, storing electricity when the power is on, and trickling it back out when the power is turned off. Used to make "delayed off" domes lights and such.

Add one of those, and your radio should get the minute or two that it needs to complete memory writes after the ignition has been turned off. With no effect on anything else, unless it is on that same circuit.

The shop should be able to tell you what value of supercap to get, or you can look around online for one. Easy to hook up just a + and - wire to tie into the power at that power socket.

My issue is not from shutting the car off, but rather turning it back on. If I turn the car off the radio keeps running just fine, but the voltage drops enough on restart that the scanner will always power cycle though my old school 996T will not.

I've thought about putting a capacitor in line to stop this but haven't pursued it. Some folks suggested a power converter that will accept a lower voltage input and step it up to 12 volts. Since they're only about 25 bucks I bought one for a proof of concept. While it does work as advertised it draws so much current that even with no load by my calculations it would drain the car's battery in a matter of days. I only use my car 3-4 days a week. Even if it doesn't get completely discharged between trips deep cycling an automotive battery is not good for it in the long run.

SD cards are relatively cheap and I do not use the record or replay feature on the 536 except under very specific circumstances. The odds of an SD card write occurring when I start the car is extremely low. Rather than spend more money to futz around with the power line I'm just letting the radio do what it has to do. If I'm leaving the vehicle for some time I'll turn it off. If I'm going to come back soon I'll leave it on. It's been months now and I've not managed to corrupt a card yet.

Dutchster it sounds like you are connected to a continuous battery power source, unlike the usual ones that turn off with the ignition.

That can be a problem since the starter motor disengaging after a start can throw a 600 volt spike into the electrical system. Only microseconds long, and often swamped out by the battery itself, but still enough of a risk that a major LED supplier (to the auto industry) noted they will not warranty their components (among the nest out there) if they are installed in a way that allows them to be subjected to spike damage from that.

On a larger vehicle or light truck, that spike can hit 2000 volts.

Traditionally, as the ignition key was pushed back from "start" to "run", the short delay before the electrical system was switched to "run" was long enough to protect components from the spikes. With push-button start/run, there's a similar delay programmed into the system.

But for anything hardwired into battery power? Especially radio electronics? Just beware, the designers DO NOT do anything to protect you from those spikes. (For arguably good reasons.) A supercap, or 15v zener diode, or transzorb, would be a good cheap way to do that.

I sent them another alternative which I believe would be a firmware-only solution (no scanner mods required) that would allow ignition control with safe power-down. I'm not going to re-post the details, but it would solve many of these issues.