Adding a new battery to existing single house battery!

[Contest1]

Had a look through the miriad battery threads but don't see a definite answer.
I replaced both the house 110 ah and separate starter battery after purchase 3 years ago.
Now hoping to extend my cruising range I would like to double my domestic storage.
I often read its not a good idea to mix batteries of different ages but is a 3years old battery thars mainly on shower power and only generally runns the radio and lights likely to cause a deteriation in a new similar battery.
There's a fancy charging unit built in and a wind generator linked to the charger. Even a connection for a solar pannel that I plan on replacing when I instal the New battery.
Any thoughts or advice welcome.
There are two pair of unused battery leads in the battery area and i presume these are from the time a previous owner crossed the Atlantic a couple of time around 15 years ago.
Additional.
Just read over my post from 2015 about the electrical system that includes some observations at the time. My son thought the two spare leads were for the wind generator and solar pannel but since then Iv run the wind generator only a few times in reasonable winds and I reckon it is charging through the charging unit.

 

[lpdsn]

You could avoid mixing the old and new batteries by making a domestic bank out of the two 110Ah batteries and buying a new 50Ah high CCA battery as a starter. Quite cheap if you walk past the chandlers and find the local auto factor down on the industrial estate.

Don't skimp on the cables connecting the two batteries of the domestic bank.

 

[VicS]

Had a look through the miriad battery threads but don't see a definite answer.
......................................................

I dont think there is a definite answer. Ideally two new batteries. A definite "no" if the existing one was getting close to its end of life.


LPDSN's suggestion is a good one if the stater battery is also 110 Ah or similar but I assumed from the way you worded your post that it is probably not.

Cannot help with the unused battery leads but it should be easy enough to trace where they come from/ go to. You will need similar sized cables to those in use to connect the second battery.
Also remember it is preferred to connect the + feed from one and the - feed from the other in a two battery bank

 

[jakeroyd]

There is some useful info here:-
https://caravanchronicles.com/guides/how-to-connect-two-batteries-in-parallel/

I found this when considering the same question.