I have a battery analyzer that I find invaluable. It tells you the condition of the battery internally. Mine is a HFT model that I have had for a couple of years and it has paid for itself many times. When a battery gets down to 50-60% I bring it back under warranty and they test it and give me a new one.
The tester electronically test the cranking amps of the battery, and also tests the internal resistance and gives you a % of capacity. I could not do without it.
This is the HFT one:
Digital Automotive Battery Analyzer
I also notice Amazon has several of them on offer as well. I have no experience with any of these but they appear to be similar to what I have and the prices are very attractive:
https://www.amazon.com/s/ref=nb_sb_ss_c_1_11?url=search-alias%3Daps&field-keywords=battery+analyzer&sprefix=battery+ana%2Caps%2C232&crid=36IUTJ28TY60J