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.
