I would like to know what are the benefits of using an onboard charger
instead of a portable charger.

I have been using rechargers for many type of household electronic
equipments (such as digital carmera, toy cars...etc). Therefore, I am
very familiar with portable recharger. But I notice that there are
many onboard rechargers available in marine supply store when I was
trying to shop for a recharger. I can imagine one benefit of having an
onboard recharger: It is neatly mounted inside the battery compartment,
instead of being sliding around on the floor of the battery
compartment. What are the other benefits? Can I connect the recharger
with the various batteries (I have 2 batteries, will be 3 in the
future) in a permanent fashion instead of using alligator clips? Then
I only need to run a power cord from an outdoor power outlet to the
receptacle of the recharger. Then I don't need to remember which
alligator clip goes to which terminal on the battery. That will be a
very nice feature if I understand this correctly.

A side-question:
Should I use a 10-amp or a 15-amp battery charger? I believe a
regular 110-volt household power outlet should provide at least 15-amp.
This means either 10-amp or 15-amp charger should work if I understand
this correctly. Then why would one person choose 10-amp over the
15-amp version or the other way around? Does a 15-amp version simply
charge the batteries faster? If I want to charge two 12-volt batteries
plus one 24-volt battery overnight (12-hours), does choosing the 15-amp
version makes any difference to me?

I urgently need to know this info because I need to buy a battery
recharger pretty soon.

Thanks in advance for any info.

Jay Chan