Can you mix and match batteries….yes but you’re going to cause all sorts of system problems.
First off NEVER mix batteries of different chemistry, capacities, or voltages. This can lead to various problems including catastrophic failure. Assuming all the batteries you’re thinking of joining together are the same (voltage, capacity, chemistry, brand) then realize that the weakest/poorest condition battery will drag all of the other ones down to that level. So you paid good money for shorter battery life from those new batteries. Virtually every battery vendor will advise their users to not mix or add additional batteries, even exactly the same type, after a six months of service. They recommend that you purchase and install all the batteries at one time. Ideally they should all be manufactured at the same time too. This keeps the batteries as identical as possible.
Short of selling off the old batteries and purchasing all new batteries, one possible remedy would be to create separate battery banks that can be switched into the load after the current battery bank powering things has been disconnected from the load. So as one bank is drained, needs servicing, etc. the alternate bank takes over the load.
