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Oops! Correction!
Sorry Oomaroo - I think I've been at the pc too long today! The 2nd paragraph of my first reply should read thus:
If you series connect two 6volt batteries as I described to create a 12volt battery as stated the capacity remains the same as for one: ie, if, as you state, they are each 70Ah jobs the storage capacity remains 70Ah. (As stated rather facetiously) I am amazed at them being only 70Ah for submarine batteries - their physical size would be similar to the old 6volt VW Beetle car batteries (if you're old enough to know what I'm talking about!)
However, presuming they are 70Ah units : connecting two together will give you 12volts and if you "construct" all 18 batteries into this configuration to give you nine 12 volt 70Ah batteries and you can then parallel all these individual 12volt sets together to give you the 12volt 630Ah unit you desire.
Assuming they are rated at the 10 hour rate that means the maximum charge rate should be below 63amps - preferably around 30-40amps: though your 15amp charger should do the job. With a 15amp RMS maximum charge rate charger it would take approx 80-100 hours to fully charge a flat 630Ah battery bank (taking into account battery efficiency etc which is best presumed at about 60%)
As stated before, if the charger you use will only "trickle" at about 2amps or so the batteries will be fine - and whilst you could just charge them up and leave them fully charged as someone suggested it is definitely not the best to then leave them such for any extended period. The other option to my suggestion of leaving them on trickle at about 2amps continuously is to fully charge them and then every month trickle them for a day or two.
Make sure all your "straps" (and all the terminals on the batteries) that you use to series and parallel batteries together are cleaned/polished before tightening and greased afterwards - there are lead connecting "straps" that you can obtain for the purpose if you don't already have said.
Regards, Kokatha man.
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