Indeed, it is a bit more complicated than that....for at least three reasons:
1. Already mentioned above, is the inefficiency of the charging
process itself; there are losses which depend on a host of things...battery type, condition, temperature, wiring
, etc.....which usually amount to somewhere between 10 and 25 or even 30% loss; and
2. Your 60A alternator isn't going to put out 60A continuously. As it heats up, it will put out less current
. How much current
, again, depends on a bunch of things, but especially on the state of charge of the batteries (SOC), their condition and type, and whether your alternator is internally or externally regulated; and
3. As a battery
reaches a SOC above about 80%, its acceptance of charging
current will go way down, and it will take quite a long time for it to reach full charge.
In practice, most house batteries on cruising boats tend to live somewhere between 50% and 80% SOC since it takes so long to get that last 20% or so back into the batteries, unless you have solar
charging power or unless you happen to be motoring for several hours.