Diode help.

[Jagalag]

Hello, new to this forum.

I'm trying to fabricate an automotive OBDII memory settings retainer for my car radio/ECM using an OBDII male plug and a basic 9V battery, during replacement of the main 12V car battery. I need to wire in a diode to prevent damaging the 9V cell during replacement of the 12v car battery. Which grade of diode should I use? I'm new to non-LED diodes and their parameters. Thanks for any help.

 

[Minder]

One of the 1n4003/7 range should suffice for a reverse blocking diode.
M.

 

[dave9]

Do you have evidence this will work on your specific vehicles?

If by "basic 9V" you mean the most common PP3 form factor/size, you might find that not only starting with too low a voltage but that voltage dragged down even further because it struggles to supply enough current. While most vehicles in lowest power state only use (approximately) a couple dozen mA, anything made after the carburetor era tends to use hundreds of mA in the woken, yet still engine off, state. Hundreds of mA is beyond the design intentions of a typical 9V battery.

Considering this factor, if you have very little voltage margin as it is, I would choose a diode with a low forward voltage drop like a schottky instead of standard silicon. Cost for a single diode is low so no need to try to get specs that are barely adequate, so I'd get something capable of at least a couple amps (but possibly significantly more, see below comment about lights) and > 15V, but ultimately, I would want a power supply capable of 12V instead of 9V and that can handle far more current to handle situations like the interior lights coming on when the door is open. I could try to guess how much current that is but it will be far more accurate for you to just measure it on the target vehicle(s).

You can probably pull certain fuses to restrict what is drawing current to allow a low current, possibly 9V power source, but it seems more versatile to just build to handle any normal situation with no need to determine which fuses to pull and doing that.

If you were to use, say 9 x NiMH or 3 x Li-Ion cells in series, you wouldn't even need a blocking diode. That seems like the best solution to me, I'd choose 3 x 18650 cells for their cost:capacity ratio and being less fiddly than having to juggle as many as 9, NiMH cells, since 4-bay Li-Ion battery chargers are pretty common and affordable these days, and handy to have around for other Li-Ion charging needs anyway.

That 3 or 9 cell voltage is assuming you either recharge them before they reach their low charge voltage or the vehicle is tolerant of 9V or lower, which brings me back to the initial concern about how low the voltage can be.

 

[Frankchie]

I don't think you should try and power through the OBDII connector as you are likely going to backfeed into the car's entire 12v system. I am not an expert on car electrical systems so unless you have specific knowledge otherwise I would not do it.

I suggest to backup the car battery directly at the car battery terminals. Of course, you need a 12v backup battery capable of supplying a few amps. Using one of the small jump start battery packs is probably the simplest solution and would not need any diodes.

Of course even simple jump starting has risks. So plan carefully and only proceed if you know what you are doing.

 

[dave9]

^ Yes many do backfeed, though I couldn't tell you if all do. Some people choose to use the lighter outlet instead, which also backfeeds (and may need the ignition turned on or at least to accessory position to complete the circuit, meaning that other things like the HVAC blower and radio should be turned off first to reduce current), but if it's really a lighter outlet, and most aux outlets (same socket but not tolerant of heat like a lighter outlet would be), are capable of approx 15A so when it backfeeds, you have some current margin as long as nothing using a lot of current is turned on.

Even so, it wouldn't be a bad idea to put a fuse on the power supply, or at least check whether you have spare fuses for whichever circuit is tapped into.

I'm not sure how the newer vehicles with the center console touchscreen display and computer factor in, whether they are going to need a lot of current because they'd be on too (with ignition in accessory or on position, or how to reach these states with keyless ignition instead of a switch connected to the lock cylinder), which I suppose is one reason to use the OBDII port instead of the lighter outlet or again to just measure the current draw ahead of time to budget what will and won't work.