pianotuna wrote:
Gdetrailer,
All you have said is correct--but the REAL difference is that two six volts in series have 6 cells and two twelve volts in parallel have 12 cells. So the load per cell is 50% less with a pair of 12 volt cells. There are other factors too.
Piano, what you state has "some" merit, however with 12V batts to fit 6 cells into the case, you now have to REDUCE the size and amount of grids along with the amount of electrolyte within each cell. Parallel operation also means your interconnects can be lighter since they are only needing to handle half the current.
The big downside to parallel operation is not "everything" is "shared", due to manufacturing and material tolerances the batteries will never "share" the current load 100% equally, resulting in one battery discharging or recharging at a different rate. You can get them close but in the end one battery will be slightly weaker and that weaker one will ultimately kill the other battery by over discharging and undercharging.
The 12V batteries due to thinner and lighter plates are also NOT designed for very deep draw downs, typically rated at only 50% DOD but for max LIFE only 20% of their capacity should be used..
On edit, forgot to mention 12V batteries with smaller and less plates per cell, the internal resistance of each cell goes up.. Yep, in reality the 12V battery jars actually will have a higher internal resistance! You are only getting away with it working since your are paralleling and reducing the OVERALL internal resistance slightly but in doing so you have now limited the max draw down depth of discharge you can use..
GC2 batteries on the other hand with larger and thicker grids are designed from the ground up to handle extremely deep discharges, their life ratings are based on up to 80% capacity being able to be used..
20% usable capacity (12V marine) vs 80% usable capacity (6V GC2 pair) per pound and dollar 6V GC2s basically net you much more usable capacity for the money..
Obviously your situation is a bit different with whatever you are doing with a 3KW load (AC unit?) but in your case, if the internal resistance is bothersome to you, then the fix would be AGM GC2s (extremely low internal resistance)or if you have the room 4 FLA GC2s setup in two series pairs that are paralleled. That combination would give you far more usable capacity than 8 12V group 27 Marine batteries..
For my situation, so far, have never had my inverter shut down or go into alarm during my home fridge startup and that is at a min of 100A surge and by morning the batteries have been drawn down to 50% of capacity.. I don't see any "advantage" to swap to 12V batteries as that would mean I would need to add at least 4 if not 6 more 12V batteries to the mix. That is a lot of money and weight that I am not needing to add to the trailer..
The big key is USABLE capacity for the cost!
