I finally got around to getting my dual battery installed and noticed a few things I want to understand further / confirm:

- it seems that the battery that i added for stereo usage may be on the downside of life and only showing 12.2 volts (via my little digital meter) where my starter is newer and in better shape at 13.2 volts.......the day after the install I noticed that the batteries are now both showing around the 12.2-12.6 range so my question: could the stereo battery be pulling down the performance of my starting battery??

- if my stereo battery is indeed on its last legs and needs to be replaced, what would be my best low-cost option to replace it?

- i'm using these voltage gauges (https://www.amazon.com/dp/B00YALV0NG...v_ov_lig_dp_it) on each of the batteries and hooked DIRECTLY to each battery and always on.......is this is a bad idea that may cause the batteries to drain too quickly?