Two 12 volt batteries:
Pro:
1. If one goes dead you still have one.
Con:
1. Not a 'true' deep cycle battery.
2. Harder to charge equally.
3. Inefficient charging if you do one at a time (slower overall to charge this way). Also it is hard to charge 2 batteries equally using 1-2-Both switch in the 'both mode' as then they are not connected in the proper way for an equal charge to both batteries. To have both charged equally you need to have them permanently tied together and wired properly. Of course if you do this you then take the chance they could be discharged accidentally together. If you are out mulitple days and recharging with solar you want your charging to be as efficient as possible. If you go in and have shore power this isn't as big a deal.
4. Not as robust as 6 volt deep cycle batteries.
5. Cost more for the same amp/hr capacity.
6. Normally have a shorter life span.
Two 6 volt batteries:
Con:
1. If for some reason one fails you have no elect. until it is replaced. For our boats if you have elect. start on the outboard you will have to pull start it and go get a battery.
Pro:
1. A 'true' deep cycle battery that won't be hurt as much by deeper discharges (still for the best life don't discharge below 50%)
2. Charge quicker than charging two 12 volt batteries one at a time.
3. Charge more efficiently if you are on solar than 2 12 volt batteries wired to a 1-2-Both switch.
4. More robust construction
5. Longer life span.
6. More amp hours for the dollar. (Note: the amp hour capacity for 2 six volt batteries is not the two added together but the capacity of one. Still most single 6 volt batteries have a higher capacity than 2 12 volt batteries do.)
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'My' conclusions: If you day sail chances are the one 12 volt battery that came with your boat is more than good enough. If you stay out longer periods you will have more capacity with 2 six volt batteries vs. 2 twelve volt batteries and more importantly if you are using solar panels to charge while out the charging will be more efficient with the 6 volt batteries which can be very important unless you have a lot of solar.
So take your pick. For us we have had 2 twelve volts batteries tied permanently together (charged via solar and discharged together) but will switch to 2 six volt when they are no longer good,
Sum
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