[Test-Equipment] "Conductance" battery checker

[Raymond LaRue]

Looking for information on designing and building a battery
"conductance" tester.  In an extended web search, I see a couple
companies that sell small 6 and 12 volt conductance testers.  

They apparently inject a small AC voltage into a battery on or off line,
in a non-intrusive way.  Calibration data is developed by profiles from
testing good batteries.  The resultant voltage and current phase
differentials are processed and utilized to give an indication of
remaining battery capacity.  

The commercial testers available are not very expensive, but those found
on line give little detail about how they operate and they appear not to
quite meet my needs, as I would like to try this technique by composite
testing of a series of SLA batteries.  

I am prone to try building one if helpful info were available.

Does anyone have any experience with such, or have design info for
building a battery conductance tester?

Thank you for your attention.
73,
Ray, W4BYG
Cleveland, TN