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oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: February 20, 2014 at 9:07 PM / IP Logged  
The voltage drop whilst driving shows that the alternator is not outputting enough to cover total demand.
A bigger or more batteries does not change that, it just means the battery has more reserve to supply so it takes longer to drop from (say) 12.6V to 12.5V etc.
If the battery is above (say) 12.7V whilst driving, then the alternator is supplying enough current - or rather, the battery is not discharging (excluding the surface charge that a battery has after being charged - ie, it will typically be ~13.5V reducing to ~12.7V as the surface charge depletes).
Keep in mind that a battery showing (say) 11.5V under load might be fully charged. Load current times the battery's internal resistance is the voltage drop between the battery's "internal" voltage and its terminals (V = IR).
Remove the load and the terminals will return to the internal voltage, and may then slowly increase if partly discharged as the battery undergoes some self recovery.   
For the sake of spending the same on a Deka AGM whilst having nothing wrong with your existing battery, I'd consider going dual battery (with isolator) even if the AGM is in the engine bay.
You then have cranking independence if the amp is on when not charging, and an emergency cranking battery if your main fails.
Tho the heat may not be that good, you extend the life of both batteries by halving the load - ie, halving the discharge rate and depth).
Furthermore, with a dual battery setup you should recover more total charge during the times the alternator does exceed other loads.
IE - if other loads total 150A and the alternator is revved enough for 200A output, your main might only accept 20A so the extra potential 30A is not output. But a second battery might absorb that available 30A - especially if it's an AGM.
(AGMs having ~half the internal resistance of equivalent wet cell accept ~twice the charge current of an equivalent discharged wet cell).   
Of course that is another reason why an AGM might be used instead of a wet cell as the main cranker.
It's all so very simple isn't it? (LOL! not?)
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