Hypothethical Battery Question

[Lin]

I call this question hypothetical since I am not changing start batteries at this time and considering future options.  I currently have 2 8d start batteries.  Obviously, they are heavy and therefore inconvenient to deal with.  I know that some have changed to 2 group 31's and are happy with that set up.  However, I only have a 45amp/24v alternator and prefer to have extra battery capacity.  From the dimensions of the group 31's, it does not seem that I would be able to fit two per tray, so I was thinking of going with something like 4 group 35's.  They are reasonably cheap at Costco and would easily fit 2 to each tray.

It appears that 8d's, depending on the brand, etc., are around 1200 CCA, 400 RC, and 1500 CA.  Group 35 are 600+ CCA, around 100 RA, and 800 CA.  It would seem that the group 35 option (with 4 batteries so double those numbers) only falls short of the 8d in RA.  However, they still would beat out the group 31's.  Is there a hole in this plan that I am missing?

[Tenor]

Lin,
Do you have anything where the original AC compressor was?  You can drop 2 gp31's inside the old AC compressor supports. I put a shutoff right there between the batts and the starter. Left the trays for house batts.

Glenn

[Lin]

I guess whatever rack was there is long gone.  I keep an electric compressor on the tray there though.  Although what you are doing sounds like an efficient use of space, I am not much interested in moving the start batteries at this point.  Also, as mentioned, I want to stay with a bit higher capacity.

[gus]

Lin,

There is one other alternative, 4Ds. I installed two when my 8Ds failed.

However, when these fail I plan to go to 31s or 29s or 27s, whatever is available at WM because my engine fires on the first turn and I just don't need large batts.

If I need a boost I can always connect the house batts.

[Ed Hackenbruch]

Lin,  Besides the starter what else are you running off of them? I also have 24v running/marker lites and my headlights are 24v reduced down to 12, and i still have my bus heater fans. No problems running all at once but i do have the 50dn alternator, don't know if i could get by with a smaller 24v or not.  Why don't you try 2 group 31s to start with and see how they do. If those work for you, you save money and have less batteries to deal with. If you need more you could always add some later.

[Lin]

Gus- Aside from not wanting to need to one system from the other, my house system in 12v and coach is 24v. Certainly 4d is a lighter option, but half that weight with each lift would be better.

Ed- All of the coach systems are 24v.  That is why I would feel safer having the extra capacity of a larger battery or two pairs.  The only trouble with trying two 31's is that if I wanted more capacity, they would not fit in the trays.  31's are about 13 x7.  The tray is probably about 22" long and 11" wide.  That is why I was asking about using pairs of batteries like 35's that are around 10" x 7", since they would easily fit one behind the other.

[TomC]

At least on my battery tray, instead of 2-8D, I had enough room for 3-4D.  But-I run 2-31's since they are 950cca each and I also have a jumper solenoid that can jump my 2-8D AGM deep cycles for help.  Good Luck, TomC

[fortyniner]

The PO on my 4106 replaced two8ds with three group 31s (12v) leaving the other tray open.  I could fit six group 31s in the two group 31 trays if needed. BTW, on my old 24v 4905 the battery cranking demand is a fraction of the 12volt 4106. I would be totally comfortable with a couple of group 31s for 24volt crank duty.
TomP.

[gus]

Tom,

Amps is not the whole story.

Cranking power demand is in watts (volts x amps).

All 24v does is allow reducing the wire size since the amps are halved from 12v. Higher amps heat the wires more!

However, the power demand is the same in either voltage.

If the proper large size batt cables are used there is little difference between the two starting voltages. My 12v 8V71 starts on the first crank.