it was indicated to make a separate post for this Q, so here it is.
Particular interest would be a tester compatible with wide variety of batteries which includes deep cycle. I see Scotty recommends the Tondon, but it seems this is not necessary specifically set up for deep cycle compatibility (even though it might provide test access). Anyone have a recommendation ?
Well no cars use deep cycle batteries so I have no experience with that that's a boat guy thing. The top contest all the other batteries AGM whatever.
The Top Don BT200 (the big brother of the popular BT100 model) can do deep cycle batteries (acc. to its product description). Good luck with finding one available.
https://www.amazon.com/TOPDON-BT200-100-2000CCA-Automotive-Alternator/dp/B08CMPY86M
What exactly are you trying to find assess? Just the overall health of the battery?
One way is to use a battery hydrometer:
The only way to find the true capacity of a deep cycle battery is to test/measure it under load (real world conditions).
Lets say a battery is rated for 100 Amp-hours (Ah). The rating is typically given for a "20 hour rate" of discharge. That means a fully charged battery should maintain at least 10.5V for a 20 hour period, with a 5A load, for a given temperature. If it only provides 80Ah during that 20 hours, then it has 80% of its original capacity.
I've never needed any gizmos to recognize this. You just know when a battery is at the end of its life because the voltage sags too low, or it doesn't last as long as it used to.
Great educational website: https://batteryuniversity.com/
Do not ever buy used batteries. Get a brand new one, and make sure it's been kept topped-up. It should have a date sticker on it.
I already have a 2nd battery setup on this auto using a relay approach (connected with key on, disconnected otherwise). The thought process here was to put in a deep cycle as 2nd battery for use with 'camping accessories' to give greater capacity and as backup to primary (with low to no susceptibility to parasitic drain - with key off). Am I off base here in concept?
@mmj I dont have the 2nd battery yet..was going to look around for a used one, but dont want to go into it blind, hence the tester. My current 2nd battery is shot (only 9v after charge attempt)...so was thinking of moving over to a deep cycle at this point for camping (and the impending civil war)