colin55 Sorry for the confusion! I hope I am giving you all the necessary information now. It is hard to know what to share.
ronv Thought I was losing it. I must have installed a half dozen of this model. As I started to read your post I was thinking to myself, man if there is a way to adjust the current I sure don't know how.
The batteries I am operating on right now are deep cycle lead acid batteries, nothing special.
I have always used a rule of thumb for charging this type of battery as ≈5 to 13% of the C20 AH rating. My current bank is rather small with only about 200 AH so I think the charging rate of 25 amps is about right.
I have a DMM with the following accuracy:
0 to 600Ω Scale Resolution 0.1Ω ±(1.0% of rdg+2 digits)
Is the accurate enough for the measurement you are requesting? I am assuming you need the resistance of the secondary winding. As I remember, it had no measurable resistance. I will double check that tomorrow morning.