Firstly you can't compare a an Interstate SRM 4D "Deep Cycle"/Cranking battery to a Trojan DEEP CYCLE. This is akin to comparing the reliability of a Honda or Toyota to a Renault Le-Car IMHO. Those brands are not even in the same class. The Interstate SRM-4D is made by Johnson Controls/JCI for Interstate.
I actually called interstate to try and find out the Ah rating and plate thickness. What a debacle. Even their tech support had no clue. I got three different answers when I asked about the 20 hour Ah rating.
If you have a battery monitor you NEED to know the precise 20 hour Ah rating or it will NOT work correctly. Buying this battery is a crap shoot without knowing the accurate specs.Here's a good general rule of thumb. If the manufacturer does not publish a 20 hour Ah rating the battery is often not a true deep cycle battery but then again Interstate does not market this as a true deep cycle battery they market it as a deep cycle/cranking battery.
There are however exceptions. NAPA sells the SAME deep cycle 12V batteries that West Marine does but NAPA does not publish the Ah rating in their specs for the battery. A simple call to the manufacturer, Deka, and you know this.
I can tell by looking at the CCA of that battery that it is very likely not a true deep cycle wet cell. A thick plate battery would have a lower CCA value in a wet cell. Interstate labels it as a dual purpose battery or "deep cycle/cranking" these are hybrid batteries that attempt to do both but almost always miss the mark when put into a true deep cycling application like a boat.
One of the only true 4D deep cycle batteries I know if is the Dyno 4Dd (the small d designated deep cycle) not to be confused with the Dyno 4D which is for starting. This battery has an Ah rating of 180 Ah's and cost $268.00 and like many true deep cycle, thick plate batteries, they do not publish a CCA value. My guess is that the SRM 4D is going to be under 200 Ah's at the 20 hour rate.
If you want to compare a more similar product compare to the Sam's Club/Energizer 6V GC2's which are about $72.00 each and 225 Ah's or the Interstate U2200 6V at 232Ah's. All of these batteries are made by JCI. Interestingly enough the old U2200 6V used to be made by US Battery and was an EXCELLENT top quality battery. Now that it is made by JCI it had better cost the same as the Sam's Club JCI battery...
I guess paying for the quality of the US Battery, to put the Interstate name on, left them with little margin so they outsourced it to a less expensive vendor.That being said any 6V battery, good quality or lower end, will almost always have thicker plates than a dual purpose flooded 4D and thus last longer in most deep cycle applications. I've yet to see a floor machine or golf cart with 4D's or 8D's..
6V batteries are also taller by design so sulfation can be shed off the plates and fall to the bottom of the case during equalization. A shorter battery fills up with shed plate junk faster than a shorter one. As you get into even thicker plate deep cycles like the Trojan L-116 they are even taller. If you have the room for 4 6V batteries they are almost always the better choice and in most cases, unless comparing to a very low end 4D and a high end Trojan, as you are, will cost less on an Ah basis.
As has been said the number of cells is the same in a 4 battery 6V bank as in a 2 battery 12V bank. The plastic case around the individual batteries makes little difference as all batteries are made up of individual 2V cells. A 12V battery has to pack 6 2V cells into the case thus they have thinner plates. A 6V only needs to pack 3 2V cells and the plates can be thicker as can the cell separators.
My Synopsis of Wet Cell Batteries by Grade/Quality:
Top Tier:
Rolls/Surrette
I intentionally left a space here because the Rolls batteries are significantly more top tier than US or Trojan..
Trojan
US Battery
Second Tier:
Dyno
Crown
Superior
Deka/East Penn
Third Tier:
Johnson Controls / Private Label
Exide
Interstate (most of these are made by Johnson Controls)
Various Private labeled batteries
Here's some quick math comparing apples to apples or manufacturer to manufacturer:
Cost:
4 Sam's Club / JCI GC2 6V Batts = $288.00
2 Interstate / JCI 4D Batteries = $434.00
Ah's
4 Sam's Club / JCI GC2 6V Batts = 450 Ah's
2 Interstate / JCI 4D Batteries = ????? (under 400 Ah's anyway)
Cost per Ah:
Sam's Club / JCI @ 450Ah = $0.64
Interstate / JCI @ 400Ah = $1.09
NOTE: Please keep in mind that 200 Ah's is likely not what the 4D's are and they are likely less in Ah capacity so your Ah per dollar will likely be higher.
When comparing apples to apple or JCI batts to JCI batts the 6V batteries are nearly HALF the cost per amp hour....
