Hydrometer kaput?

[Sunking]

You are NOT helpfull! Instead you are full of it. The batteries are new and fine. The junk from AUTOZone cost me more than 5 bucks in my area. And the question remains: How to tell a faulty hydrometer

I told you exactly what is wrong. Get rid of the cheap Auto Battery Tester and get a real hydrometer.

Secondly if you can get a reading on your car battery like you said, and not on you deep cycle battery can only mean one thing. It is in desperate need of a full charge. You battery voltage is screaming out loud telling you that and verified with your hydrometer.

[Mike90250]

.....And the question remains: How to tell a faulty hydrometer

it reads a good battery as bad OR a bad battery as good. If your working car battery tests as Good, the calibration is likely OK

How about mixing up a batch of calibration fluid for a 1 time test ?
Both home brewers (beer) and aquarists (salt water fish keepers) have simple methods to calibrate their gear.
You mix up a reference solution of pure water and a weighed amount of salt to produce a fluid with the properties you want to calibrate with
And keep the temperature correct, fluid density changes with temperature,
Rinse extra well to not contaminate your batteries,

[DeeZee]

You are NOT helpfull! Instead you are full of it. The batteries are new and fine. The junk from AUTOZone cost me more than 5 bucks in my area. And the question remains: How to tell a faulty hydrometer

[Sunking]

Just caught the rest of your post. If your car battery reads fine and your solar battery does not, guess what that means?

[Sunking]

Quit buying hydrometeors from Auto Parts dealers, they are junk. Instead buy a real one for $5. One of the best you can buy short of a Laboratory Master Hydrometer.

[DeeZee]

Just installed new 50a Mptt Fangpusun charge controller. Before, in the morning I read 12.45... Now, without inverter usage, I read 12.05
This is my second Auto Zone hydrometer. The first was definitely faulty. and it is too early in the day to verify with a multimeter.
Do I have another lemon? How can you tell? It reads my car battery correctly!