Adding Second Battery to My Existing Solar System :::

But two used telecom agm jars with surplus acid design. They weigh 105 lbs each.

Buy 2 new batteries, 12v AGM, wire in parallel,
Or
But 1 new, and use the old for the core charge

Do not connect an old weak battery to the new one

If the old one was still good? Maybe?
But not a weak bad battery

Surviver,

As long as you really are stuck buying two new batteries, you might as well save a lot of grief and buy a pair of 6V golf cart batteries. These will give you better than any pair of 12V jars and the likelihood of a battery explosion is greatly reduced.

Matt

I agree with SoundGuy, the “best” option a new battery... then “double-up” on batteries!

Surviver76 wrote:
I need to add a second battery but remember something about that the bad batt will rob the good new one and to wire them a certain way. I know using the old batt is probably a bad idea but it seems that two batts are better than one in all cases.. Any enlightenment appreciated..

Conventional wisdom says to never parallel an older battery to a newer one. However, if that first battery still has some life left in it and you'd like to continue to use it for awhile as well as a second new battery I'd wire in a Blue Sea m-Series 6007 battery disconnect switch that will allow you to run the old battery on it's own, then switch to the new one alone, or vice versa. If you insist on running both batteries together then you'd want to ditch the old one and install two new batts ... but I'd still run them through a Blue Sea switch.

RV batteries should be matched by manufacturer, AHs, age etc. For instance see: https://www.rvrepairclub.com/article/rv-battery-basics-beginners-guide/