I'm right there with ya on owning more cars with this type of terminal connection than I can remember. And this is the only one that has had this problem, but I seldom kept a car for as long as I've had this one.
And yes, the battery was replaced at the very end of last April, and that makes the third battery in the car, in less than 4 years. (well at the time the battery was installed in April I had owned the car less than 4 years by 5 months)
I just thought of this, I didn't think to check the date on the battery that was in the car when I bought it, so I have no idea of how old it was when it gave out, but it gave out 6 months after I bought the car.

And now that I've thought about this a little more, I don't recall having had a corrosion problem with the battery that came with the car. Hmmm.

Anyway, I replaced the battery that was in the car when i bought it with a Walmart MAX, with the 3 year free replacement. Good thing, the MAX gave out 3 days before the warranty expired. Trust me, I usually never end up on the good side of a deal like that one. But give credit when due, Walmart handed me a new battery with no questions asked. Even though its supposed to be a free replacement, they nicked me for a few bucks by refunding what I paid for the original battery, and then charging me the currant price. So It cost me a few bucks, so what, it beat paying full boat.

But now this has me thinking, could it be a problem with the battery? I've used the same model battery in many cars, for many years now, and I never had any problems. Granted, the battery doesn't last the full 3 years, but who cares, a new one is almost free. lol

ACK, how could a battery cause corrosion? Ok, I can see maybe one may of had a bad seal, or something like that, but 2 in a row...3 years apart? That'* pretty hard to believe.

Oh well, another day shot, and I didn't get the voltage output checked, maybe tomorrow.

if the connection is no good the battery will not charge right and they dont like that.

ive had a few walmart batteries and didnt have much issue, didnt go before the warrantee. about the same as az or advance about 5-6 yrs. if im going to keep the car though i get interstate batteries, those will last 10yrs or more and serve you better the whole time.

A bad connection or an electical system that is over charging the battery. Both will cause corrosion on the positive terminal, and both will cause batteries to fail prematurely. I got 9 years out of the original battery on my Bonneville, I believe because it is located in the passenger compartment (under the rear seat) away from the environment of the engine compartment. You should expect to get at least 5 years out of a battery.

Is your battery securely tied down?

Yes, the battery is mounted as it should be.

I wasn't aware that Bonneville had the battery inside the car, what year, or years was that done? Any sister models have the battery inside?

I think that how long the OEM battery will last is kind of a crap shoot, in that, I've had some OEM batteries last as long as 10 years, and some as little as 2 years. For instance, I had a '74 Blazer that was used as a work truck, including a snowplow. And the original battery lasted like 7 years. Now in my opinion, that'* pretty impressive. Back then, the highest output alternator I could find was a 100 amp, or I guess I should say the highest output alternator that would fit in the same location as the original. And the 100 amps had a hard time keeping up, so the battery was constantly being run down.

Funny how one tends to remember such meaningless things as a battery that lasted a long time, but I doubt that I'll ever forget the service I received from that battery, as I'll never forget another battery that flat out died while cranking the engine. I mean the battery when from cranking like it was new, to dead as a rock, like right now. There wasn't as much as a dim light. Think that didn't take a little time to find.lol
Who would think of the battery being dead when the engine just stopped turning over while cranking perfectly? Of course from that point on, I'd think of it.lol

It was 2000+ that has the battery under the rear seat.

On average, batteries last about 5 years. It comes down to several factors. Such as temp. Around here it can get to -20f and even lower. Prolonged starting at those temps is hard on a battery. Also, a weak charging system, or corroded cables will kill battery life. Batteries do not like to discharge and recharge. This kills them fast. They are meant to be fully charged at all times.