Using a portable battery charger onboard
I got this portable battery charger from West Marine to charge the deep cycle / starting batteries (a pair of Walmart-brand 12V cells, model no. "24-DC", 75 Ah each) on my boat. I got it instead of the $10-more-expensive "onboard charger", not knowing that the instructions said that a marine battery must be removed from the boat and charged onshore.
What's the reason for this requirement? I figure if the 6A alternator on my outboard is okay to use on the boat, why not a 6A charger? Is it because there's a power cable running from shore or something?
So the first time I used it, I lugged the batteries to shore. Egads, were they heavier than I expected. Also they are not stowed in super-accessible locations (like, under the v-berth). I really really want to just attach the alligator clamps to the terminals in the engine compartment. Will Ι create any micro black holes if I do this?