2002 A8L D2 No Power to ignition after Dead Battery
 

Left key in ignition with the ignition on. Drained battery, recharged the resulting dead battery. No power to anything after the charge. Fuse? Relay? ECU? Already disconnected negative in the trunk and reconnected, no luck, still no power to anything when key is turned. Battery charges shows relatively new battery is fully charged.

Can you load test the battery? Connect a 12V lamp or inverter to it? Test it with a multimeter?

It's not clear how you read that the battery is fully charged so I am guessing a newer "smart" charger is actually not charging the battery. In that case you need a dumb charger or another battery to boost it so it'll take a charge.
If you do not have a dumb charger just connect a running car to it via jumper cables for a few minutes, check to see if you now have power, then try your charger again. I am not a fan of jump starting if you have the time to charge the battery.

Hi - Thanks for your response. We used a smart charger with voltmeter to charge the battery with a trickle charge. The charger shows that it's done its' job (and the battery is only one month old). When the key is switched on, the dash is dead. I've see a Battery positive fuse for sale on ebay for this vintage of the A8 and I'm hoping it's this or some other fuse/relay.

https://www.ebay.com/itm/2000-2001-2...gAAOSwlS1chMy9

Hoping to avoid pulling the ECU out for service. Not exactly sure where the Battery positive fuse is located (trunk?, engine compartment?)

Thanks again!

The ECU would not be in play if you are not getting a dome light, lighter sockets, etc.
I suggest checking with a multimeter, these can be had for free at Harbor Freight. Check the battery, the lighter sockets, the power junction at the RF wheel well, etc. It's also good to check that you can actually pull current and not just get a voltage, a DMM will sometimes read voltage on a circuit that cannot actually take a load. A test lamp is great for this.

I do not know where that fuse is located, have not noticed it on my cars. Based on the part number it's in the relay box:
https://www.jimellisaudiparts.com/sh...ey_model=15631

Ok, helpful, Battery is fine, No power to anything else including dome lights, etc. Somewhere there must be a blown fuse or relay. Need to find that junction box.

Pull the RF wheel and under the fender liner find the big power junction for the front of the car. If you have a big problem and it's not at the battery connectors in the trunk it's probably here. Check for voltage, try jumping the car here or hooking up your battery charger here. Look for corrosion, etc.

The smart charger that I have (Schumacher) is not reliable in discerning if it's done its job. I've had 2 batteries show 99 or 100% charged when they were completely and absolutely toast. The % battery display only seems to work OK on batteries that are in good shape. If you can get to the electrolyte and measure specific gravity, that'll tell you the state of charge for sure. My buddy's battery that showed 100% on the charger had spG so low that none of the electrolyte tester balls floated. If it's sealed you'll need a load tester.

I also let a fairly new big "advanced glass mat" battery for my A8 draw down to 3 volts open circuit (no load on it, measuring with a voltmeter) by leaving parking lights on overnight (while working around the relays), and it wouldn't take a charge after that. But I don't remember if I tried to charge it with a smart or dumb charger. Took it back to Advance for a new one, and have been fine ever since.

You were correct!
 

Battery was toast although the charger indicated otherwise:. Took the battery back and after some discussion about neglect they did agree to replace and problem solved.

I have seen new batteries go bad before and still test ok with the low load/electronic testers. Probably cracked circuit inside the cell structure that the low load smart chargers/testers cant read.