Mixing old and new in parallel without some sort of BMS is a pretty bad idea. Mixing a new one into a series bank is not quite so bad. If you have a 12v system and use 12v batteries
in parallel, then I recommend you don't use them in parallel. Connect them as two separate banks with an A/B switch is what I would do.
I have a 48v bank for propulsion
, made up of 6v golf cart batteries in series. A year after installing, a "helper" drilled through the cover we were making and into the case of one battery. I tried plugging but the battery went bad and I replaced it with not only a different month, but a different brand. Careful monitoring has detected no problem. So far. SG matches, no load voltage matches. and even individually charging
them with a 6v charger
shows similar charge current
and times. After discharging to 60% or 50% the readings are very consistent among the batteries. YMMV but careful matching can be okay in a series string if they aren't TOO dissimilar in age. 3 years later, still no problems.
Batteries of unequal performance in parallel can be a problem because all current
can pass through just one battery. This can overheat the battery, leading to premature failure of the "good" battery. With FLA it is usually not a catastrophe. With some other types things can get noisy.
With FLA, one big bank is somewhat more efficient than two separate banks, due to Peukert effect. Peukert gives you more discharge capacity than the 20 hour rate capacity, at current below the 20 hour discharge rate. Less, at higher discharge current. So you get more bang for the buck by having a higher capacity bank, and one way this is done is to wire batteries in parallel. I'm not saying don't do this, but I am saying don't do this with a mix of batteries. For more capacity use two or more separate banks if you have differently aged or rated or branded batteries. The extra Peukert losses are not a total dealbreaker except at extremely high discharge currents.