A good article on why its important for us to replace our vehicle battery with the same Type it was delivered with.
Why upgrading the battery with a different Type, like AGM, or Gel, without also giving consideration to also changing the alternator charging profile to meet the battery needs, is likely to be a recipe for disappointment.
Why its important for us to use a "smart" battery charger to maintain our batteries & how our old manual chargers output could weaken, or fry our vehicles electronics, by the unfiltered AC ripple on its charging output.
So if we're going to use old sparky anyway, at least unhook the vehicle B- battery cable, so we only fry the battery!!!!! lol
Anyway a good article with some graphics to make the point here. Enjoy the read. A Practical Guide to Today?s Batteries and Charging Systems

 

Good article but has a couple misunderstandings that are inexplicable.

Manual or "taper" chargers, don't work at all the way he describes. While it is true that they continue to charge after the battery is recharged, the current starts out high, and the voltage low. As the battery absorbs a charge the voltage will rise, and current decrease. In fact the end point current is well under an ampere. The "dumb" chargers work very well for charging the typical "12 volt" automotive lead acid battery.

The battery "should" be disconnected when charging, though this isn't the way battery tenders or maintainers are usually employed. I've not considered AC ripple too much, but have wondered about the surge spikes or transients during nearby lightning strikes or thunderstorms. I usually unplug or disconnect entirely anything expensive when possible.