So with the gratitude to Phil who set up the 12v boating group, Brian off Harnser who has given me good advice in the past and Tom ex of Waiouru all the way from very hot Australia here is the advice.
I have deliberately put it into one post so I can refer to it when eventually getting onto my mini electrical adjustments.... that I'll blog about of course.
So Phil says....
The size charger will depend on the depth of charge you take the batteries to. The shallower you cycle them the less charge current they will take.
In terms of alternator size for that bank the general rule is 20% of capacity so 60A however this will not charge at this rate for long unless batteries are quite low. In terms of a charger I would probably be looking for around 50A.
You don't need to worry about great 'intelligence' on the charger, in fact you will be better off without any as these university educated chargers are really designed for long term landline operation and can easily undercharge in a sporadic mode of operation.
You will be wanting to turn off charging as soon as possible so a float setting isn't even needed. Your battery monitor will be able to tell you when the batteries are full by monitoring the charge current and when less than about 1.5% capacity, 4.5A in your case, they are full.
Don't ever rely on the monitor's battery percentage reading to stop charging as they are usually wrong and will often say full when they aren't.
If you have AGM batteries then they can take a higher charge. You could use more than one charger if 2x30A are cheaper than 1x60A
My suggestion would be to start with the size (or intended size) of your battery bank. Then establish how many amps (on average) you need to put back into your batteries. Remember to allow approximately 50% on top of that (you have to put more in than you took out!)
Let's assume you have a battery bank of 200ah and your average use is 100ah. You will need to put 150ah back into the batteries. Your recharging rate should be approximately not more than 30% of your total capacity. So the maximum recharging rate would be 60amps. The batteries will take up to 60amps until they are 80% charged (Bulk Stage) 80% of 150 is 120amps. The maximum charge rate is 60amps, so it will take 2 hours to Bulk charge the batteries. The remaining 20% charge is the Absorption Stage where you are attempting to squeeze the last of the charge into the batteries. This will take quite a long time and the batteries will only accept a low number of amps. Solar panels are very useful for the Absorption stage.
In the above example we needed to put 60amps back into the batteries for 2 hours.
Roughly convert that to Watts (V x A) 14.4V charging rate and 60 amps = 864 Watts. so a 1000W generator would probably Bulk recharge the batteries in two hours. After that you're burning petrol for not much recharge and would probably be better to use solar panels for the Absorption Stage.
We had a 450ah battery bank on Waiouru and when we weren't cruising sometimes used a 2000kva inverter generator to recharge the batteries. The generator would run for 2-3 hours on full power (Bulk Stage) and then you would hear the generator RPMs drop as the batteries went into the Absorption stage. The generator supplied 240V to a Victron 3KVA Inverter Charger.
...and Brian says
If you are using a genny you don't need a multi stage charger as once your batteries are charged you will turn the dam thing off, so it wont ever go into float. You just need a big basic one to get your batteries up to full charge so you can turn it off, unlike a shore supply where the volts are going to keep coming after the batteries are full.
Thanks also to Andy who confirmed my thinking about splitting the charging and inverting - his advice comes from experience, the best sort of advice in my book.
So I know more and will now need to think more seriously about batteries - I know I have waffled on about them a few times and I think I know what its between now but that'll wait.
Also some news about the donk's head and the need for a couple of parts, nothing critical and understandable - but thats for another post !