I had the Rayovac batteries in my hub for more than a year. Immediately after the last update, I pulled the batteries to do a hard reboot. They were fine then. I checked them again yesterday and they were all leaking.
Add me to the list of leaking batteries, mine were very bad, lucky for me the terminals were not bad at all so cleanup was very easy
Rayovac Batteries
Purchased 12/2015
Model STH-ETH-200
They were fine in late Nov early Dec
Support ticket submitted to ST
Rick
More of the same here:
I too issued a support ticket, but Iām not expecting much ā they have the following official position posted: https://support.smartthings.com/hc/en-us/articles/115000177703?input_string=leaking
It would be nice if they provided a battery replacement reminder within the app, even if itās strictly a generic time-based reminder (e.g. 6 months after user indicates new batteries have been installed) at a bare minimum if this is going to be their stance.
Interesting. Iād expect the following to be included in the set up instructions for the hub itself then, because I donāt think most people expect backup batteries in a mains-powered device to need replacing every six months. 
Replace the backup batteries for the SmartThings Hub once every 6 months or so.
I think the 6 months reminder is not going g to help. @JDRoberts mentioned earlier about hardware or software issue and this needs to be addressed first. In my opinion. I think itās a software issue because when the hub first came out. I have the batteries in there for almost 7 months without issue. Sounded like the recent firmwares caused the issue.
Add me to the list. Just opened Mine up and they were all coroded.My hub was just replaced a couple months ago due to other reasons so the batteries arenāt very old.
Add me to the list
Rayovac Batteries
Purchased 12/2015
Model STH-ETH-200
I got lucky, while all 4 batteries we very corroded, the ends/terminals had minimal corrosion. I had the batteries out about 4-6 weeks ago, in hand, laid them on the counterā¦ and never saw this, it seems to have come on pretty quickly
Hope someone finds the cause here soon
1/15 Update: Support ticket submitted
Rick
@slagle&@Tyler , there needs to be some sort of official communications from ST asap on this matter! Whether itās the batteries or a hardware/firmware issue. It seems to me that EVERYBODY should check their hub batteries out and at least take out the batteries if not replace them if leakage/corrosion is present.
Update: I put in a support ticket regarding this issue.
I hope all the people with batteries issues put in a support ticket and not just posting it on the forum.
Now now children.
Play nicely. 
Mine were fine, but replaced them with Duracell Ultra.
Iām pretty much convinced itās related to a hub update. People are saying theyāve found batteries like this with just a couple of monthās use. I had the original batteries in there working fine for nearly a year. I pulled them out recently in November to carry out a āzigbee healā so know they were perfectly fine then. So for 11 months Iāve had no issues leaving them in there.
My hub is a little over 2 months old. While the batteries hadnāt leaked, they were dead flat, and thatās while attached to a UPS so no power cuts could be blamed. I have a feeling the āparasitic drawā is more than it should be and itās discharging the batteries. Then, combined with the heat in the unit, batteries are leaking
I have the first version of the V2 hub (STH-ETH200), which I never installed but I pulled it out of the box and measured the voltage across the hub battery terminals and when I first plug it in it reads 3v but then begins to drop until it reaches 0.5v. When I unplug it the voltage drops down to mV. Plugging it back in again jumps to 3v and tapers off, taking about a minute, before leveling at 0.5v
That could be a problem. 3v of reverse voltage leakage, even dropping down to .5V. Alkaline batteries have a nasty habit of leaking when voltage is applied to them. This is why you cannot recharge alkaline batteries (you can, but a low current pulse-type charge but itās not very effective) We could be looking at a serious design or manufacturing flaw with the V2 hubs.
Please, anyone with leaking batteries please measure for voltage present without batteries installed. I havenāt noticed batteries leaking on my hub but I have already replaced them after a 2 hour power failure earlier this year. Iām going to meter mine tomorrow.
Crazy, my hub just died this past weekend after having started smoking and losing all power. Smelled like burning and I ended up opening it up to check if a capacitor blew or something. Noticed 2 of the batteries had leaked and the white crud had made itās way onto the circuit board. Re-assembled it after cleaning and replaced batteries. Started working again for 24 hours then died for good. Had to replace the hub and start over.
Model: STH-ETH-200
Purchased in April or May.
Device was powered from a UPS
I was still using the original batteries that came with the hub.
Random date point. I removed all of the batteries to do a zigbee heal as well after the recent hub firmware update with zigbee OTA took out my zigbee devices. Did not see evidence of corrosion at that time, so it started happening within the last several weeks.
Just checked mine, no leaking thankfully but the batteries were quite warm, much warmer than I would expect them to be if current isnāt being drawn from them. So Iāve taken them out because theyāre kinda pointless anyway, ST needs the internet to work, and my router needs powerā¦
Itās clearly a feature they got excited about implementing because it sounds like a good idea, and just chucked it in without really thinking about it. Or engineering it properly apparently.
My v2 hub is only a month old, and while I was hard resetting some of my network, checked the batteries. One terminal was corroded and leaked across the battery compartment. Didnāt think to check for voltage, Iāll check again in about 2 weeks. New Kirkland batteries installed.