So I've been researching this for days and I ended up here. There were a couple old threads on the subject, but it's like the replies just stop but the problem never resolved.
So, I've had my 09, currently at 89k miles, for 3 yrs. I had a new battery when I bought it, but I've replaced it 2 times since then. Without going into all the details of how I got here, here's where I am. There's a parasitic drain coming from the radio fuse. It took me a while to figure this out and to find a thread that explained exactly what's happening.

Here's the procedure:
Check the ohms, several hours after the battery disconnect.. Shows 2.3, I hear something running around the master cylinder for about 10 seconds, then it stops and drain drops to 2.0. BUT, if you stay on it, after about 20 seconds it starts dropping and ends flickering between .1 and .09. Pull the radio fuse and it drops to .01 (none of the other fuses made a change) Plug it back in and it stays at .09 The only way to get it to start super high again is to disconnect the multimeter leads and start all over again. Otherwise it'll just stay at .09-.10 SOOO.......I read about the ONSTAR fuse thats on the read wire coming off the positive wire, behing the plastic loom. That fuse MADE NO DIFFFERENCE. Only pulling the RADIO fuse makes a difference.

Now, I found a thread where someone mentioned the exact same scenario, said he checked this on 3 or 4 H3's and they all did the same thing. So he said it was "normal". But a battery every year is not normal.
So, it's NOT OnStar, it's the rafio, or some component of it. I need to figure this out it's drving me nuts to keep this thing on a trickle charger. Keep in mind, it hasn't not started, but a year into the battery, they fail the test and I replace it. I got a "hard start" once....
I considered riggin a swiitch to the fuse only, so I don't disconnect all the power. But that's a band aid fix, not the solution.

Any input? I don't want to change the Radio because It works and I use the XM Radio.

 

So we (not the Royal we, I'm speaking for the collective HF group) are going to need some clarification.

First, a current draw would be measured in Amps, not Ohms. Connecting a meter set to ohms either in series or parallel would destroy some older meters, and not do anything good to even newer ones.

You an disconnect the battery, and then measure resistance looking back into the rest of the car, but that would not produce meaningful results either.

Also, I am not clear if you are referring to a factory radio or an aftermarket radio/head unit.



My 08 has an aftermarket Android Head Unit. Aftermarket radios -- especially Chinese head units are notoriously bad for not caring about their standby current.

Modern cars -- in general -- are also very demanding on batteries. My '08 also has the same noise near the firewall with everything turned off. The vehicle I owned before the H3 ran fans and electronics for at least 20 minutes after shutting down.

I also notice weak cranking on my H3. I purchased mine recently, so I am kind of assuming that the PO just didn't give me a real fresh battery.

The H3 also has GM's "smart" charging system. I had the same thing in an 05 Yukon. "Smart" definitely in quotes. On a long trip, it will simply stop charging the battery after a while. The noble goal is to provide good charging at all engine speeds, electrical loads, and even during short trips, without overcharging the battery on a long day trip with low loads. The problem is that it tends to be a little tough on mediocre batteries.

Based on your post, my experience with GM products of the vintage is that nothing is wrong. Try to get a good quality battery, and monitor the charging voltage. My Android head unit displays the voltage at the top of the screen (this is an option that can be turned on in settings.) Another really easy option is a USB charger that displays the voltage. Voltage over 14.2 is tough on any battery, and will tend to kill gel batteries even more quickly.

Speaking of USB chargers -- note also that GM vehicles (of our H3's vintage and earlier) leave the 12V outlets hot all of the time. This is a leftover feature from when you could actually light a cigarette from a cigarette lighter outlet. I have an USB charger in the front, and in the back area, but I try to remember to disconnect them when I am not using them.

 

Thank you for the reply. I've inserted my info below.

Yeah, I found this out with my dash cam as it was still on with the car off...

I'm looking into a remote fuse kill switch setup.. I have to make it. It would work with a key fob and just switch that fuse off and on. The only downside is that my radios clock will always be wrong. I don't listen to FM or AM, just XM and it's channel presets are broadcast through the service, not saved on the head unit, so they never get reset.... This seems, for now, the only/best solution.

 

As mentioned in my previous post, my fairly recently purchased '08 H3 Base had somewhat anemic cranking since I bought it. Somehow, reading this thread made it get even worse. Smartly (If I do say so myself), I had put my little jump starter box in the truck last night because this morning, I was greeted my only about a half turn, and the chattering relay sound.

I hooked up the little jump box, fired it up, and went on my way. I added a stop to buy a new battery. So right now, I have much better cranking. Another thing to cross off my H3 "Issues List". We will see how it goes.

 

Did not take much to kill the OEM battery that came with the older H3. Possible to some of newer models as well.

Suggest when replacing to consider removing the "box" or cover that comes OEM. Upgrade to a larger battery..

The tray in the H3 will support much larger footprints.

 

I purchased the battery for my H3 at WALMART 3 years ago...It's still cranking strong