The popular wisdom is that an alternator can only bring a battery up to about 80% of charge.
This is not true. An alternator certainly can fully charge a battery to 100%. However on a sailboat we are very often UNWILLING to run the motor for the 4-5+ hours it would take to go from 50% to 100% so the alternator often gets blamed.. Getting from 50% to 80% is easy and relatively fast because you are in bulk mode. Once you hit the absorption voltage, usually around 80% SOC, less and less and less current flows into the battery and the last 20% takes a lot longer than the 30% you just replaced.
The Allmond 30 I've been messing with the last couple of weeks has a diesel with alternator and two new batteries.
While the engine is running I get a pretty good charging voltage for battery 2 but only 12.2 for battery 1.
Because you are likely not charging battery 2. If the batteries were in parallel they would be at the same voltage. They are not in parallel so they are at differing voltages. Your battery switch is like being used as a "charge directing switch"...
I'm assuming that for some reason battery 1 is not being charged.
The puzzle for me is that battery two shows a voltage of 12.6 after it has sat for 20 hours even though there has been only engine charging, no shore side cable since the boat went in the water a month and a half ago.
This is pretty good when you consider most flooded LA batteries will be at 12.72V+ when fully charged.
The engine runs maybe an hour a week.
There are almost no other demands on the battery other than engine starting. Once we ran nav lights for an hour, most of the gauges don't work.
The engine is hard to start and needs to be cranked a bit before it starts, maybe 30 to 60 seconds total over three tries.
An hour a week, with only light loads, should be adequate to push you into the mid 90's if you only started at 90% SOC to begin with...
He will want to address these starting issues. 30-60 seconds of cranking is a LOT. It is bordering on hydrolocking the engine if the sea **** is not closed when cranking for that long.
So my question is why isn't the battery being depleted more than it is.
I would think that just from sitting losses and engine starting the battery would run down a bit.
Even 30 seconds of cranking (which is WAY more than an engine should eve crank) removes very little capacity. Lets assume the average cranking load is 150A and then see how many Ah's 30 seconds at 150A consumes....
150A X 0.0083 Hours (30 seconds) = 1.245Ah's consumed.
Not a heck of a lot of capacity. The average starting duration for the engines and boats I work on from "loaded" to "unloaded is under 3 seconds with most being under 2 seconds.. This is how a properly running engine starts. Ours starts loaded starter to unloaded starter in .75 seconds.
Lets figure a 3 second start at an average of 400A, just for grins:
400A X 0.00083 Hours (3 seconds) = 0.332 Ah's consumed during starting.
Starting an aux diesel engine uses very little Ah capacity due to the short duration. IF we factor in for Peukert it uses a little more but still very little overall capacity in Ah's...