It should still be fine.
You already own it, so I would run it. The yellow tops are deep cycle batteries so normally have lower CCA‘s and sometimes higher RC‘s than similar starting batteries.
But the optima’s are sized well enough to where even the lower numbers should still work fine for most.
Is it best? Maybe, maybe not. But it should be fine under most conditions.

Might lose something if you were running super high compression, a particularly radical cam, or very high initial ignition timing.

 

Installed battery today . I do not know how it fit in tray I think it is a size 27 . According to Measurements it is too wide for my tray but somehow it barely slipped in .
Length wise it has 1/2 or so to spare . 640 CCA & 810 CA @ 32 degrees , & no reserve rating I know of these 2 ratings seem to be backwards ??? Confusing ...
Seems to me CCA should be @ 32 degrees
I have looked at several factory manuals & do not see any amp ratings other than amp hours....... & lots of info on alternator ratings , but not much on battery ratings , just a size 24 listed at many parts store is about all I came away with
Does anyone know how many CCA @ 32 degrees is a minimum for a moderate Winter , 0 degrees few times a year maybe ,

 

Went back and read some of the threads I missed or did not completely understand & I learned the alternator supplies the juice when running a stereo , and smoke machines , vibrating bed etc ...etc . I did not know exactly what did what , I thought it was part battery & alternator . So I got a discounted 27 instead of a higher priced 24F . I wanted a strong Winter time battery also is why I went with the 27 , & I got tired of looking for a battery , they have gotten so expense , that is another long story.............. IT NEVER ENDS........ ha..........

 

Cold cranking amps are rated at 0°.
Cranking amps, also called “marine cranking amps“ when referring to marine rated batteries, are rated at 32 degrees.
Cranking amps are higher, because the output of the battery is reduced as the temperature goes down.

The alternator’s primary function I suppose would be considered to be recharging and maintaining the charge of the battery. But to do that, it puts out more voltage than the battery is rated at.
For example, 12.6 for a fully charged battery, versus the 14.5 volts when the engine is running and alternator is working.
By doing so it is also by default powering everything else on the vehicle anytime the engine is running.
This is true whether a vehicle is equipped with a an old school ammeter, or on one where the output wire goes almost directly to the battery.
So while the engine is running and the alternator putting out 14 V literally everything is working off the alternator.
In our trucks, the alternator is connected to a common point near the battery. In our case, the starter relay.
Since power for the truck is taken directly from there as well, the alternator is also powering the accessories when it’s putting out a charge.

The most common obvious example of this is your headlights, getting brighter when you fire up the engine. Or even in some cases, especially with older alternators, the headlights get brighter as you rev the engine up.
it’s simply the lights using the lower battery voltage, versus the higher alternator voltage.

 

Thank you. Yeah, that is what I concluded as well. The motor was built somewhat conservatively, so nothing radical, just a big, strong engine.
I guess I could go larger in a few years when this one is older. For now, since I only drive on weekends, it's doing fine with the 35 yellow...

 

Another well stated explanation that simplifies my wondering confusion .....ha
Your last 2 sentences I question slightly , but cannot disagree with .
I conclude from everyone's appreciated responses is that the batteries only function is to START THE VEHICLE !!!
The reason for large CAs is IF you have all the accessories ON when you start the vehicle , A/C , radio/monster stereo , DVD player , water bed heater , refrigerator , & video screens..........etc...etc...etc.
Also if your alternator is going bad you have time to diagnosis it , or use accessories to much with the vehicle turned off !!!
Thank-You all.......

 

At one time Optima made Group-27 size batteries too I think? Maybe they were only in Blue-Top variety, but I remember some big honkin' Optimas on the shelf now and then.
Might even have been larger than 27's, which are generally easy to fit in most of our truck's factory sized battery trays.
Even when Ford installed a modest battery (minimum needed) they at least sized the trays for later upgrades.

I bought some monster thing for my '79 on the last go-round. Don't remember what size it is, but it's a big 'un. I'll get some numbers, but I remember deciding on it primarily because it had a really big reserve-capacity number. Something like 180. Maybe even more, but 180 is still pretty high.

Did we discuss RC in detail already? I think I mentioned a 10amp drain as a test. But that was incorrect.
Reserve capacity is how long (in minutes) it takes a battery to go from it's nominal 12v down to 10.5v with a constant 25a load. I think I said 10a, but it's 25.
So a battery with a 120 minute RC can go for 2hours while it's being drained at 25amps which is a pretty high load. Regular headlights only use about 10a if you include all the other running lights along with the main headlights. Less for LED's, more for halogen high-beams.
Maybe a big stereo might do that 25a, but a regular radio, at just a few amps, would probably take all day before it made the battery too low to start the engine.
Maybe exaggerating that, but "sitting and listening to the radio" or "leaving the lights on" are what I use to gauge how long a battery might last before being drained.
A good battery might last all night with the dome light left on.
All of which is why more is better sometimes.

Paul

 

Thanks!

In what way? Happy to concede, or even to argue a point!

Sort of...
If you want to get nit-picky, a battery is a storage device, so while it's primarily important function is to start the vehicle, it's also there to run any needed(?) accessories while the engine is not running.
So yes to starting, but with another duty that most of us take advantage of at one time or another.

Exactly!
Especially that last bit about the alternator going bad. Same thing for the battery in fact.
Getting a bigger/badder/better battery up front, gives you more leeway when the battery starts to get old. As it loses it's capacity, it has farther to go before it won't start the engine anymore.
Which usually happens in cold weather, but can happen any time.

I feel the same way about battery cables. Which we may have discussed already. But "headroom" is why I like larger battery cables.
Sure, a 6ga cable can do the job well for years. But if it becomes compromised from wear, age, or damage, a bigger cable might still start the engine. A smaller cable might not...

Paul

 

Oh no, cables!
Yeah, one thing I have to say I am surprised by are lots of pretty cool vintage vehicles with lots of mods and you see these ratty, thin, 'the clamp is hosed' setups and think why didn't they spend some money to fix that important part up?
I mean, for the price of some other less needed items, you can get a good battery, tray (if needed) and cables for not too much money.
Even in high school auto shop, I didn't paint my valve covers before I had good spark plugs

 

You’re right. Even some total custom vehicles that probably cost the builder way more than $100,000, still have crappy cables on them!
Sometimes they’re nice and new, but they were not sized properly. I hate seeing muscle cars with a big *** 4 foot long, red cable coming off the battery!

 

This morning battery had 12.4v . Night before it was 12.6 At the end of today it has 12.1v
Will be interesting to see what voltage will be tomorrow
Battery has a 7/23 sticker on it so it sat on the shelf over a month

 

Is that with the battery cables connected? Or disconnected?

 

Sitting for a whole month? JK...it's fine. It will never be the same any time you check it - esp if you're using a cheap tool to measure current...
I'd say not to worry and drive it. Batteries are pretty much works/fails. If it's working, yer good.

 

Connected . Late this afternoon 12.07v.

 

If it starts and runs, dial it in and run it. When it broke/quit you fixed it. Perfection is not going to be achieved with this era of vehicle. Make it go and put the miles on it. Put mine together 8yrs ago and drive it every day. I wash it once a month and look under the hood just about as often.