> If that battery can put out 50 A peak to start the bike then remember that number.
Well, no, it is worse than that. But let me shift to a car-battery just because I know the numbers.
A car battery will supply 200A-400A while staying above 10V-11V. Just for round numbers, say 200A at 2V drop, 10.6V still available to light the sparks and keep the starter relay pulled-in.
So the internal resistance is 2V/200A= 0.01Ω, 1/100 ohms. If you drop a perfect short-circuit on it (even a yard of battery cable, as it is used in a car), you can pull 1,200 Amps!
Going back to your "50A" motorcycle battery, say 300 Amps into a short.
And when that happens, you have 12.6V*300A= 3,700 Watts mostly dumped into a small bucket of strong acid. 3,700W is bigger than any two burners on my stove, and a cycle battery is a very small pot. How long does it take to boil and spew? The bursting may not be wildly spectacular, but the hot acid eats things fast, and skin very-fast.
And as you say: near the end of charge, a lead battery makes Hydrogen gas. It may not be as dangerous (IMHO) as gasoline, but is invisible and odorless so you don't know when you have too much hanging around.
Between electrical, acid, and fire issues, building codes confine large battery racks to specially prepared rooms. "Sealed" cells get a little slack, because they are less likely to spill or make gas, but I don't think they are allowed in nice places without proper fusing to reduce electrical heating (which would burst any "sealed" cell).
Common household fuse-box fuses (which are no longer common) can easily stand the peak current, and are not seriously overvoltage (a 120VAC line with motor load can kick to 300V when broken at high current). They don't come smaller than 15A, which is still too big for what is only a 0.25A load. 3AG fuses like we put in boxes are not rated for interrupting huge (10,000A) peak current, because they are nominally protected from feeder currents by the basement fusebox and house wiring. A 3AG will "probably" fail cleanly at 300A, though it may shatter; I don't think it is rated for such work. Fusing a 90KVA source is indeed tricky.
A 12V 5A battery supply for heating is more reasonable. A 12V 10A car fuse is ample protection. A single cycle/car battery would last a session. Modern semi-sealed batteries on gentle charge don't make much Hydrogen. Keep it in a cat-litter box in case it seeps acid (stuff happens). I still think that heater-hum is curable without such heavy-weight measures. (Maybe I just don't run enough compression....)