So, my understanding of Amperage is that it is affected by how much a device attached to the charger actually demands. So, if my device demands 1 Amp, a charger rated for 30 Amps would be okay to use since the device will still only draw 1 Amp.
Not exactly. Current depends on voltage and resistance. If you have a 12-volt power supply and a load that has a resistance of 12Ω, Ohm's law (Current = Voltage / Resistance) says the current will be 1A.
Batteries aren't loads. They have very low internal resistance, enough so that for all intents and purposes, a large battery is a dead short when seen from the outside. If you connect a charger that's capable of delivering 30A, it will have no idea whether the battery can handle that kind of current and will deliver all of it because there's not enough resistance to slow it down. The battery will have lots of current flowing through it, making heat, boiling the electrolyte and doing other damage to the internal parts. (It'll also burp out a lot of very flammable hydrogen gas in the process.)
This is why you can't use a 12V power supply to charge a battery. Chargers have to be
current-limiting, which means they have circuitry in them that adjusts the voltage to keep the amount of current flowing down to something that isn't going to destroy the battery. The rule of thumb for batteries is that charge current should be limited to about 10% of capacity, so a 14 AH battery should be charged at no more than 1.4A.
So, what would be the difference in using a .75Amp tender versus a 5Amp tender?
If you're using an honest-to-god Deltran Battery Tender or something that understands how to safely charge a battery, nothing other than the .75A model taking longer to bring the battery up to a full charge. If you're using something else, see the comment above about boiling electrolyte, frogs, locusts and death of first-born batteries.
I want to plug a battery tender directly into my Powerlet jack in lieu of pulling the battery every Winter. The last time I hooked my battery tender up this way, it blew the in-line 30A fuse to my Powerlet.
First things first:
You should not be fusing your Powerlets at 30A. Pull that fuse out of there and don't install another one until you've figured out the right size. (I'll be more than happy to help you figure out what that is.) Powerlets are only rated to carry 16A, and unless you've put in very large wire, that fuse isn't going to prevent the socket or the wiring from melting down and possibly starting a fire in the event of a short.
Whatever you connected was either a dead short from the battery's perspective or was able to deliver loads of current and did so until the fuse blew. Sounds like the wrong thing in either case, and you were fortunate that the current was enough to blow the fuse. If it wasn't, things would have become very messy very quickly.
As long as your Powerlet is on a properly-fused, un-switched circuit and connect it to the right battery charger for the job, charging through it should be no problem. My ST has charged that way for at least a decade.
--Mark