I use a sealed battery in three of my 5 bikes and I have had no problems with them. However, all of those bikes have a solid state regulator, which is very important with a sealed type battery or any of the more exotic batteries that are currently available.
These more modern batteries do not take kindly to overcharging as some of them, like the sealed type, have plates that are spaced very close together, so the heat from overcharging can warp the plates to the point where the plates can short out and kill a cell.
The mechanical (relay) type regulator that was stock on the '78 is not a good choice to use with a modern battery, for this reason. That regulator has the tendency to increase the battery voltage as it warms up at higher RPM's even when properly adjusted.
The other common issue with our vintage bikes is poor contacts in the ignition switch which causes the battery voltage to also go high because the regulator is on the load side of the switch (brown wire) but the output from the rectifier is on the source side of the switch (red wire) where the battery is, so if there is a 1 Volt drop across the ignition switch, the regulator will still maintain the voltage on the brown wire but the red wire and battery voltage will be 1 Volt higher.
So, bottom line is, do not install a sealed battery or other exotic battery in your bike with a stock relay type regulator.
Now, some will chime in here and say that they have been running a sealed battery with their stock relay type regulator for years with no problem, and of course that can happen, if you are lucky. Like my friend who I haven't heard from for a while who used to play Russian roulette.
