Today's vehicles start so easily, the starting battery has a very easy job, it really only runs into difficulty when discharged lower than 85% and the owner expects the alternator to top it off which it will never do unless driven for hundreds of miles as it takes time to fully charge any lead acid battery. Even in this treatment it could be a good 2 years before it cannot start the engine.
A previous battery of mine spent 23 months being shallowly cycled but chronically undercharged, then was relegated to engine starting duties only, and it lasted another 5 years, and it was still quite capable of starting my engine.
But my engine cranks for a second or less before catching. But disconnect the coil wire and crank for 3 seconds and it slowed considerably.
My friend has a yellowtop Optima battery from 1999 in an Astro DD. The thing reads 10.6 volts, never any higher. I told him it had a shorted cell about a year ago and to replace it before it strands him. He pointed out it starts his engine just fine and he will replace it when it no longer can. He never gets tired of pointing this out to me, and I point out that it simply has a shorted cell and can fail any time and to at least carry jumper cables, but he refuses.
The point is that a starting battery, when it does fail to start the engine when the lights have not been left on overnight, is very seriously degraded. For most it is just fine, until the day they hear the dreaded starter click. There is no inbetween for such people. One day just fine, then the next it is not and most all reports of starter battery longevity, good or bad, are by people with absolutely no Clue as to the health of their battery. One or Zero, and no shades of grey between the two.
A deep cycle/marine battery actually deeply cycled, that is similarly compromised, would be much more noticeable to the user with a simple voltmeter, but the starting battery has to seriously degrade, or be subjected to very cold temperatures before it reveals to the average user that its time is nearly done.
One can certainly wait until it is about ready to fail, but when it actually does fail is an unknown. My weak starting battery was never a concern, as i had a house bank that could always contribute enough CCA to get me going. Right now I do not have a house bank, I have a single 90AH AGM for both house and engine starting duties. I cycle it to 50% or more 4 to 5 nights a week. It has no issue starting my engine even when depleted.
I have no bad weather buffer, unless I just never use my laptop.
I've gone full circle. in the beginning I only had a 85AH starting battery. I went upto 345AH of capacity at one point, and now am back down to 90AH, and never had more confidence in my system.
But back in the day, I simply had NO IDEA the state of charge of the battery, how much of it I was using, and how much I could use. Now I do.
And I have multiple very capable charging sources and the ability to actually attain a 100% recharge which makes lead acid batteries so dang happy .
Maintenance free flooded starting batteries are strange ducks.
They are marketed toward people who when asked when they last changed their oil reply:
"Changed it to What?"
I have 2 digital voltmeters on my Dashboard. One is disconnected as I only have one battery, but I can gauge the state of charge of my AGM battery by how low voltage falls when cranking my cold engine. Fully charged and it does not drop below 12.2v. 50% and it drops to 11v, 30% and it falls to 10.2v.
My flooded battery's numbers were much much lower, and it cranked the starter much slower too.
While voltage is not a great indicator of battery health or state of charge, it is a very useful tool for guessing at it. One on the dashboard that actually says 12.7 or 14.3 is quite useful and informative. The stock ones which have an analog gauge with 11 on one side and 18v on the other, one cannot discern the difference between 12.8 and 13.6v, and this is the difference between a functioning charging system and one that is not, so many stock voltmeters are basically worthless.
I recommend people interested in their batteries put a digital voltmeter on their dash whose voltage sense lead is right on the battery terminal.
Get a 3 wire version, not a two wire version.
This one is able to be calibrated:
http://www.amazon.com/SMAKN®-Digita...28918&sr=8-24&keywords=3+wire+voltmeter+smakn
The 2 wire version does not have a separate voltage sense wire, so unless you wire it directly to the battery(+), it cannot read accurately.
The voltmeters one plugs into a ciggy lighter can read much different than one whose voltage sense lead is right on the battery terminal as there might be other loads on the circuit feeding the ciggy receptacle.
My only complaint with my dashboard voltmeters is they are only one decimal place, and are green, and too bright at night. I put two layers of window 35% tint in front of them to tame them.
I have a red one to swap in.
They only come on with my ignition, and they seem to have a 3 to 4 times a second refresh rate, though I see a ~500Millisecond refresh rate listed on similar products. Mine is definitely more than 2x a second.
Anyway a digital voltmeter right on the dashboard whose sense lead is right on the battery(+), will allow the driver to see when their starting battery is weakening to the point replacement is wise. It also allows one to see the wacky behavior of their vehicle's voltage regulator, and if one has a voltmeter for their house battery, one can see the difference between depleted house battery and fully charged engine battery. When the two voltmeters agree, usually the house bank is nearly fully charged, but this also depends on the size and length of the circuit.
While there is certainly bliss in ignorance, ignorance can come back and 5 finger prostate exam you.
