I'm trying to find the source of a battery drain when the vehicle is off. It has been discharging overnight. Ammeter shows 300mA draw after sitting for 30+mins.

New battery Dec 2019
New alternator Sept 2020
Both tested good at dealership.

New start/stop button (OE Honda part).
(Draw tested <10mA after replacing this, but came back)

I've pulled every fuse under the hood, but couldn't find the discharging circuit.

Before I spend a day pulling fuses under the dash and waiting 30mins (40+ fuses) each time. Does anyone have a suggestion?

I've got an accord for my daughter and an odyssey for my wife and they both have been awful on batteries.

Everything I've read speaks to constant drain due to mainly the key fobs. Also, these newer cars have tons of more electronics while driving and they have not increased the strength/size of the alternators. So if you don't drive for a significant amount of time each day, they are almost always running with a battery that is on low charge.

The trick is not just pulling the fuses, but putting them back one by one and seeing what each draws

So... pull 'em all and then replace one by one?

I've read about some pcm update that helps.

That's how I found my battery drain (aftermarke door locks). Replaced a fuse,check the drain. Pull that one, put in #2, check the drain

On my car I hooked up the meter and measured the drain and pulled a fuse at a time until I figured out what was pulling the 5A. Turned out to be my stock radio - total wtf for me. Thanks to covid I don't drive much anymore so I had to hook up a trickle charger for it when I'm confined to this house. Still have no idea how it changed since it didn't do that for many years before it started.

Today I'm using an inline ammeter instead of the loop I used last time. It shows the same 160mA draw I was seeing.

Removing engine compartment fuses #21 (Backup Main) and #22 (Audio) kick the draw down to normal levels.

Which model of ammeter?

On top of a fuse you can see where the tips of an ammeter, or whatever electronic measuring device you're using, can be inserted into the fuse... to check to voltage across the fuse... and/or amps.

You don't have to pull the fuses... just check them each one by one across the top.

Have you considered a wonky relay?

Good luck. Electrical gremlins are challenging.

Can't really check amps through a fuse like that unless you know the resistance of the fuse.

The two ammeters at my disposal are a Fluke 324+ and one of those yellow cheapy digital multimeters.

At this low amperage i trust the inline more than the loop.

New issue though, the battery was charged and disconnected on Saturday, but too drained to turn the engine this morning. Replacing that again today.

As for the 160mA drain, that's more than 3x the 50mA spec, but still only just more than 7Ah in 2 days.

I'm hoping I can get this battery to test bad and the drain is irrelevant. Though seems to be common on these HRVs. I'm getting a schematic to figure out what those two fuses do. "Backup Main" isn't very descriptive. It's weird that part of the pre-delivery inspection process is to install those two because of their parasitic drain.