You can do it, but it will hurt the battery charge and longevity a bit. Get as close as you can if you are set on just adding a second one.
If you do opt to replace both and you can’t afford or don’t have the hardware for lithium, we have had great success with using a pair of 6v golf cart batteries wired together for 12v. They withstand being drained below the 50% mark better than a typical 12v lead battery would.
I own a battery company called lightning lithium. (Check us out if you want). We deal with RV upgrades all the time. Even though my spiel would be go lithium. Just buy another 12v battery. If your using it for non cranking situations all your doing is adding capacity to your total battery bank. The bigger deal would be in wire size, location of wires (don’t just line up batteries and daisy chain them with terminals at one end) we make sure they are sharing the load properly. But again if your not using a big inverter or anything have at it. Anyone telling you that it will be weaker is technically correct but not in a practical real world scenario. The smaller battery, once at a lower voltage will just balance with the higher voltage one. This may shorten life but again not enough that your going to care. Just keep them trickle charged in storage and be wired for correct load. But if you want lithium www.lightninglithium.com – we mostly sell on our website ice-fishing products. Good luck!
Is it ideal? No. Will it increase your capacity, yes.
If you have a charger that will do lithium I’d save up and switch. The 2nd lead acid helps but a single lithium will more than double your usable capacity and save weight.
Yes, it matters, and so does the age of the battery and it’s wear level. At least it matters if longevity is your goal.
I would be more worried at the being unbalanced. The new battery is going to be hurt by the older battery (you will only ever get the capacity of the oldest battery, so if the existing battery has about 80ah of life in it and the new battery is fresh and has 100ah, your only going to be able to pull the 160ah out of both instead of 200ah (or really half of that since lead acid you really only want to run down 50% to expand the life of the battery and prevent damage).
In the future I would totally go with a lithium battery, you can run them down 100% which means on a 100Ah battery you get 100Ah instead of 50% of what ever Ah rating a lead acid has. You also get a battery that can easily last 8 – 10 years vs 3 – 5 years on a lead acid. You pay double the price for Lithium right now but its coming down. I have seen 100ah for $249 on Amazon and Chins are $299 all the time. That is double what you pay for a 100Ah deep cycle but you get twice the capacity and twice the longevity.
It is highly recommended to replace both batteries . Your new one will only be as strong as your old one .
Yes, just connect it correctly.