We have a 2008 260 DA with twin...

[Dino__DA]

We have a 2008 260 DA with twin batteries and a selector switch. When on the hook, we'll use batt#1 as our house battery for stereo, fridge, lights, etc. and save batt#2 for starting the gen or engine.  I'm considering adding another battery in parallel with batt#1 to extend the time that we can remain on batteries (w/o having to start the generator to charge them).  I will wire the new battery in parallel with the existing battery and use the same diameter cables; connect the positive (red) cable to the battery post and the negative (black) cable to the ground post on the back of the engine block.  I'm pretty sure the battery charger can handle charging both if they get drained down.  My question, has anyone completed this or know of a better way to do this?   

[DeepEnd]

Several companies make Automatic Charging Relays (ACR). When one battery has a charging voltage and the other does not, they get connected together. When the charging source goes away, they get separated. It is automatic and avoids the voltage losses and battery draining problems of a traditional "Battery Isolator". I'm not whether all of them do, but the Blue Seas version provides an extra connection to the ignition switch that deactivates the ACR during starting.

I solved this problem on my boat in an entirely different manner. I added 2 100 watt solar panels and a charge controller and connected it to my starboard side. It's like having a 15 amp charger connected whenever the sun is shining. I also replaced the starboard starting / house battery with 2 heavy duty 6 volt golf car batteries (wired in series of course). Nearly 4 seasons - no need to jump the starboard engine.

[wingless]

Ensure that the available charging current is sufficient and appropriate for the battery type, flooded cell or AGM.

If additional batteries are added to the system, ensure that batteries that are cabled together in-parallel are of the same manufacturer, model number and date code.

Ensure that all batteries are mounted and connected per the applicable ABYC standards.

The battery lugs need to be crimped or bolted onto the cable end, not soldered.