I know, a full battery reads 12.8 open circuit but ...

That's all well and good ... but, but, ... if one was to read the Megger article in the link I provided, then it would become obvious that if one could only figure out how to measure battery impedance over time so as to watch it increase with battery age, then one should be able to see the high impedance target point that essentially means "replace me".

I guess it must be that measuring lead acid battery impedance while the battery is still in the RV is too difficult to pull off ... but with lithium batteries in a vehicle there's other ways to estimate their end of life ahead of time?

I guess I'll have to read the owner's manual for my daughter's new Honda hybrid to learn what dash light comes on 10 years from now indicating it's battery funeral time.

P.S. Another goal of mine is to not ever have to use a hydrometer cuz my RV batteries don't have a hole for one.

I've been saying the same thing about greed and Uncle Sam. He's gotten too fat and lazy... Time for a new Uncle Sam, but how and where do we get one?

Modern lithium CPU's and sensors (and control mechanisms) cost more than my rig and livingroom furniture together.

When to replace in my book is when a kWh meter and hydrometer both agree that the critter is being fed properly but thumbs its nose at the chores.

It's darned tough to beat a kWh monitoring system, and a brand new battery baseline amp hour capacity test for future reference.

By establishing CEF through trial and calibration to a hydrometer, somewhere down the road you might want to compare and determine percentage loss of capacity.

Old batteries eat more and work less. It would be bad enough if they worked less and ate the same, but the things are regular delinquents.

CEF charge efficiency factor is the percentage difference in ampere hours between what goes in and fills the battery, and what comes out to do work. They are worse than a Swiss Bank, even when brand new. An aged LA battery can get utterly ridiculous about its minimum wage. "Pay me 130% or the lights go out" Energy has to go somewhere or do something, mechanical motion, electromagnetic field, heat, electrolysis, Chinese fire drill electron swapping, something. The last is what you are looking for but all those things listed before it seem to get in the way. A battery that becomes a soup kitchen patron does everything worse, including electrolyze more water into hydrogen and oxygen. This is why old batteries suck water like a Turbocharged V-12 camel. Energy never "disappears". Even Clouseau could eventually yank off its disguise. Where it's gone.

Trends and Tendencies 101. The magic way. Comparison of capacity between new and "now". Couple that with CEF and you have your diagnostics. The hydrometer plays the part of Lie Detector. It catches the Amp Hour Meter playing Pinocchio not infrequently.

"The Populace" is ready for Lithium - now. The batteries are costly, the monitoring system is pricey but it's the closest thing to Plug N Play on the market. The price has to dramatically fall, similar to what solar voltaic panels have done. China is breathing fire about lithium. As soon as Tesla gets up and running, there will be 100,000 Chinese engineers waiting in line. They aren't there for the scenery (desert). Australia has sold damned near the entire continent to the Chinese. Lots and lots of lithium included. The lithium isn't for pharmaceutical manic depressive Chinese.

I like kWh meters. Ones with data ports that can be downloaded into a PC and charge and discharge trends and tendencies graphed. This isn't a Frankenspark nerd's dreamscape. The monitor and user are gazing at trends and tendencies, the history. Looking for idiosyncrasies, deviation, or degradation in battery performance.

But in the end it boils down to (Agh a pun) how much storage capacity an accumulator has lost. It is up to the user to determine when a battery has gotten too lazy and too greedy for its own good.

The "fuel" bar indicator in the Nissan Leaf also shows battery life. There have been a few instances where there's been the loss of only one bar. When you don't have to deal with Peukert and temperatures are regulated, you can get pretty accurate on cycle life estimates for Lithium Ion batteries. Probably not possible on lead acid without a built in hydrometer and something to monitor the plate shedding and thickness (and probably some other stuff).

Mex, do any/all the components of your testing method(s):

1) If done when a battery is new and then repeated periodically as it's used - indicate about when the battery is ready for replacement?

AND

2) Lend themselves to portability for carrying along with you on an RV trip?

AND

3) Lend themselves to being done without removing/disconnecting the battery(ies) from the RV's 12V system?

Remember the/my goal here ... monitor an RV battery in real time using a built-in system quickly and inexpensively on RV trips so as to indicate BOTH -> when it needs recharging and when it needs replacing.

Perhaps the "when to replace" goal is an impossible one ... but I can't help wondering if modern hybrid vehicles don't contain some built-in way of notifying their owners when their lithium battery banks should be replaced. I'll bet they do and if so ... why can't something be done to provide the same capability for the users of good old lead acid RV batteries.

Thanks for the elaboration!

I "test" batteries in the following manner...

Hydrometer dip. If any samples come up smoky colored or red, end of test regardless of what a carbon pile load tester or impedance tester indicates

Next is a 1/2 CCA adjustable carbon pile load test. If the battery is supposedly fully charged (hydrometer) and fails this test. Awaaaaay......

The impedance test. This is like creating a fine finish with 800 grit sandpaper. I use this test mostly on single cell entities.

If the battery passes all three tests then it is a guaranteed healthy specimen.

But clarity of electrolyte is essential. It may argue with the other three tests (gravity, load, and impedance), but it over-rides, trumps, the other tests.

Please note, there is a difference between "Good" and "Good Enough For Me". Some customers used to nursemaid basket cases - which would have driven other customers up a wall. Like William Jefferson Clinton, once quoted "It depends on the word 'IS'"

By the way, a modern "real" electronic battery analyzer costs in excess of thirty thousand dollars. IMHO they absolutely do not do as good a job as the hydrometer, carbon pile loader, and impedance testers when all three are employed.

A battery is a very large capacitor. A very large capacitor does not respond well or much at all to high frequency voltages - hence battery testers use low frequency AC voltages to measure the AC component of battery impedance.

A square wave contains only very high frequencies and zero-HZ flat DC portions. The very high frequencies are what's forming the nearly vertical leading and trailing edges of the square waves that show on an oscilloscope and ... the large capacitor/battery can't respond to, or "follow", these edges.

I'm not sure what an old lab tester is measuring. It's probably just providing a highly accurate value for a battery's DC resistance portion of it's total impedance via the form of Ohm's Law where DC resistance (R) = equals delta-voltage (change in voltage) divided by delta-current (resulting change in current). The applied voltage "delta" part comes from over and over application of a square wave's high portion and low portion.

The above is my best quess ... which is dangerous without drinking enough coffee before making a quess.

My handheld and old lab tester uses square wave. Or my Tektronix is lying to me :)

Wet cell lead acid battery impedance testing does require measurements using low frequency AC (sine waves ... not square waves) voltages due to the fact that a lead acid battery is basically a very large capacitor storage device and hence their impedance value has an AC component to it. Currents drawn from batteries during testing must be set properly high enough (not micro-currents) by the testing loads used.

Here's a link to probably way too much information on lead acid battery testing as published by the manufacturer of Megger testing equipment - which includes battery testing products. Reading through their document confirmed my suspicions - lead acid battery internal impedance readings do indeed increase as the batteries age. Now ... if only we could easily monitor this increase as our RV battery(ies) age so we can know when to get rid of them before learning the hard way on a camping trip:

http://www.surgetek.co.za/items/btg100.pdf

RoyB - Good posts.

Before I had a battery monitor I used voltage under load to determine when to charge batteries. Maybe this doesn't sound right to all of us "experts" but it to was also real world experience.

How? By paying attention one can determine with experience a reasonably idea of the amount of discharge. I initially made several no load voltage and hydrometer checks in the morning after a typical evening for example and from there it was seat of the pants knowledge about battery state.

OP You can do something similar.