Hacker News Reader

Comment by cwillu | Hacker News Reader

If you can identify the fuse for the unit, it might be less annoying to pull it when parking rather than hooking up a charger.

View on HN

I wired in an aftermarket DVD player in an old car and tapped into a fuse that was supposed to not draw power when off. Of course, it did draw power so the DVD player never shut off. It wasn't too much of a draw but I didn't want to end up with a dead battery situation because of it so my solution was to just plug the fuse tap in when the kids were in the car and wanted to watch something.

I'm sure there was a more elegant solution (find a different fuse, rig up a button, etc.) but this was the path of least resistance and wasn't really annoying to do.

“path of least resistance” is very fitting.

Definitely a cool, new term! Thanks to the GP for introducing it to my lexicon.

You also might enjoy this joke then:

https://i.pinimg.com/originals/a4/c5/bb/a4c5bbd867b819a687ea...

Aftermarket head units should have two power inputs - one for standby, and one that only gets power when "accessory power" is on. Every car I've owned I've been able to wire it up so that works, then it only turns fully on when the car is on.

I avoid very new cars like the plague though, so not sure how different they are.

It makes me anxious thinking about all the bullshit in new cars - touch screens, coded parts, 4G connections with GPS tracking, etc etc.

This was an rear-seat overhead DVD player, not a console one. Although, I probably would have been better off wiring to the head unit instead. At the time, I still had the factory one in there so it's not quite as expandable and more complicated to add stuff to.

That's a great idea. I can't believe I didn't think of that. Thanks!

Pulling fuses is a pretty reliable way to find the relevant fuse in the first place. If the battery doesn't run down while a particular group of fuses is pulled, the one you want to pull is in that group. Start with half the fuses, then narrow down.

You might have to reprogram your radio channels...

I can recommend searching for "South Main Auto parasitic draw" on YouTube. The guy is a genius at electrical troubleshooting.

His strategy isn't to pull fuses, it's to set the car sit for at least 30 minutes or so (with key off) and then check each fuse with a multimeter to see which has current on it, and then check everything on that circuit. Arm yourself with schematics and wiring diagrams, otherwise it'll end up being something of a wild goose chase.

(The idea being if you start pulling fuses, you can "reset" various computers in the car, which may show up as a false positive. It can take up to 30 minutes or so for all the various computers in a car to all go to sleep, although it's usually only a few minutes for most cars.)

It would take someone more skilled than I, or with better tools, to check current in a fuse box without removing fuses. Maybe I should watch that guy and get better...

A multimeter is cheap and really easy to use. This is the safest way also

You measure the voltage across the fuse. Then you look up the fuse's resistance in a datasheet. Current is Voltage / Resistance.

AFAIK the fuses have contact points you can use to measure the current.

You can check continuity across the contact points, but measuring a 75mA current via the voltage drop across a 0.0034ohm resistance (give or take) means reading 0.2mV, which is… borderline for a consumer multimeter.

I wonder if one couldn't identify the time it takes for the battery to get to some low point (50%?) and then determine the order of magnitude draw they would be looking for. 75mA would take a long time (I think) to drain the battery to where starter can't turn over (like a week?).

"Pine Hollow Auto Diagnostics" is also a very good source.

If it were me, I would pull the fuse, then jumper the block over to a different fuse on a circuit that is off when the ignition is off. Then you never have to think about it again.

They also make switches for fuses, so that you can kill the circuit without pulling the fuse.

If you are going with a hack then _this_ is the way to go.

WiFi arduino + disconnect relay on the battery.

Completely disconnect battery when not in use.

Make sure your relay is rated for enough current.

Yeah, I had an old car where the useless tape deck developed a short (I think) that would drain the battery after about a day. I didn't know it was the stereo until I played around with removing fuses one by one though.

Reply on news.ycombinator.com