A maintainer which seeks to bring battery to and then hold it at 14.7v for a period of time, and then drops to a temperature compensated float voltage is obviously much better than a single voltage max 13.6v maintainer.
But its foolish to believe 'ALL' maintainers out there, are 2 or 3 stage 1.5+amp capable chargers seeking to hold 14.4 to 14.7v for a period of time before reverting to float.
How many amps the maintainer has available in order to seek 14.4 to 14.7v, is a big issue.
Both batteries I am now using both deep and shallow cycling regularly, now require way more than 1.5 amps to reach 14.7v. The one starting my engine now, for the last year, has required more than 1.5 amps to reach and maintain 14.7v when fully charged.
so a 1.5 amp maintainer, even if it is seeking 14.7v, will never get either of these batteries to 14.7v, as the one battery requires 4.7 amps to get to and be held at 14.7v and the other requires 5.4 amps when they are very near or at full charge.
Both these batteries are end of life, and have many hundreds of deep cycles on them and thousands and thousands of engine starts and are both a month shy of 6 years. They have delivered many hundreds of KWH over their lifetime, whereas the average starter battery will have delivered a tiny fraction of that when they are too weak to do their job.
They could indeed have 'high impedance shorts'
As I've watched these batteries age I've seen that amps stop tapering at higher and higher levels when fully charged, then starts rising, along with battery temperature.
They are still being cycled and charged.
One of them is still easily able to start my engine, the other, I'll not bother trying. It hasn't since June 2015.
A 1.5 amp maintainer placed on either of these two batteries would not be able to get them near full charge, even if it was a maintainer capable of seeking 14.4 to 14.7, which not all do.
ALL lead acid batteries, as they age, will require more and more amperage to be held at the same voltage when they are fully charged. These 'tail' amps at absorption voltage, are indicative of age/ condition and when the battery requires more than 1.5 amps to reach 14.5v, and the maintainer only has 1.5 amps available, then the battery will not reach 14.5v and as such it will likely not be able to achieve full charge.
Some say a failing battery will take out an alternator. I don't know how much a shorted cell sucks up. But the extra 4 to 5 amps my alternator now has to produce to maintain my aged battery at 14.7v is not going to push my alternator or external voltage regulator over the edge.
I got plenty of alternator temperature data( and voltage regulator temperature data) at different output and speeds. 18 more amps of output ( blower motor on high) at 65mph makes the alternator stator 2.5 degrees hotter. Under 25mph is a different story.
So trying to fully charge a battery with a maintainer is unwise.
Can some maintainers achieve full charge given enough time?
Sure, on a newer healthy and generally smaller battery.
Can every 'maintenance' charger sold since they came out, achieve full charge of a depleted battery when given enough time on a larger mid life battery, NO.
Easily Provable with a hydrometer, but few to none bother. I have, I do, Its obvious. When one wants to see 1.280 on all cells and sees 1.255 instead, the battery is NOT fully charged, no matter what voltage it might hold. If 14.7 or higher voltages applied and held, never raises specific gravity over 1.255, then the battery is sulfated, and is fully charged to its maximum remaining capacity.
I've not used the impedance testers.
I've not used carbon pile load testers, but few can actually load half the CCA figure of the battery and hold it for 30 seconds.
According to the SAE J537 CCA test, the fully charged battery is to maintain more than 7.2v after 30 seconds when loaded to half the CCA rating of the battery, not 10.5v as stated in another thread....https://batteryuniversity.com/learn/article/how_to_measure_cca_cold_cranking_amp
Half the CCA figure of a battery, is a big amperage number, asking the battery to deliver this, for 30 seconds, is very hard on the battery.
A true load test is damaging to the battery, to some degree.
The voltage the battery can maintain under load, ANY load, is a load test.
Obviously bigger loads are more revealing.
My starter draws nearly 1800 watts when i last measured it.
Its rated to output 1400watts.
1800 watts at 10 volts is 180 amps.
1800 watts at 11 volts is ~163 amps
1800 watts at 12 volts is 150 amps.
The voltage as the battery ages keeps dropping when starting the overnight cold engine.
Higher amperage causes more voltage drop on the cabling leading from battery to starter.
Each time I start my engine therefor is a 150 to 180 amp load test.
Seeing how low voltage falls each overnight cold engine start, as the battery ages, at different temperatures and state of charge, is very enlightening, and I have been doing this for many years now.
This equals Thousands of observations of voltage maintained during engine cranking. The voltage falling lower and lower during engine starting, has been a steady decline over the nearly 6 years I've been using this battery.
One does not need to purchase an impedance tester, or a carbon pile load tester, or take their battery to an autoparts store to have it tested.
They can simply watch a voltmeter every time they crank their engine instead. An analog needle would be better than a digital voltmeter which samples voltage 2 times a second or has a delay.
When voltage drops to a certain level while cranking the engine, it is obvious the battery is weak, or perhaps just undercharged, and one should recharge and try again hoping to see a higher voltage held while the starter is spinning, or begin shopping for a battery, and carrying their fully charged jumperbox, or cables, or just replace the battery. I am carrying a fully charged jumper battery, and have been, for nearly a year. Glad I did not buy a new full size battery a year ago, as this one can obviously still do its job.
The money saved by not buying a carbon pile or impedance tester can be put toward a good quality maintenance charger and the hardware to easily/conveniently plug it in, or a bigger battery.
That said if I owned either, I'd definitely use them, as another data point, but neither are going to tell me what my other observations of performance, already have.
But I'd rather spend that money elsewhere.