Yes I am a bit of a Philistine asking this here

[merl]

All batteries then should be of the same output age and condition.

Although that's a rule that's commonly touted it's actually not true and there's no science behind it. The only factors that MUST match are battery chemistry and cell count.
So, don't mix AGM with Gel, flooded etc and don't try and parallel a 12v battery with a 6v battery etc or a knackered battery with a cell short.
The idea behind matching the age/ condition/ capacity/ brand etc is to keep the Ah capacities the same but that actually doesn't matter. Ultimately each battery attains full charge simultaneously and each battery attains discharge simultaneously, therefore the capacity of each battery is realised no matter if their Ah capacities are mismatched or not.
NOW all of the above is correct when connecting batteries in PARALLEL, BUT the batteries MUST match perfectly if you want to connect them in SERIES and this confusion between series and parallel, along with safety bias and information from biased sales literature etc perpetuates the myth.

 

[Glynno]

Always connect your batteries in parallel for motor home /caravan. This way you will keep 12volt system. You could end up with 48 volts if in series and blow the lot. All batteries then should be of the same output age and condition. It sounds as though yours are all new and the same anyway.

Sooo! The reason of my previous suggestion is that
Connecting an old battery in parallel with a new one is generally a bad idea because
the new battery will try to charge the old one, causing uneven discharge, reduced performance, and potentially damaging both batteries (or even causing fires, especially with lithium) due to different internal resistances and states of charge, leading to premature failure and safety risks. The weaker, older battery will drag down the newer one, shortening its life, while the new battery will get overworked trying to balance the system, leading to faster overall degradation.

 

[merl]

Sooo! The reason of my previous suggestion is that
Connecting an old battery in parallel with a new one is generally a bad idea because
the new battery will try to charge the old one, causing uneven discharge, reduced performance, and potentially damaging both batteries (or even causing fires, especially with lithium) due to different internal resistances and states of charge, leading to premature failure and safety risks. The weaker, older battery will drag down the newer one, shortening its life, while the new battery will get overworked trying to balance the system, leading to faster overall degradation.

Nope, wont happen. Think about it...
The batteries are ultimately connected together so they MUST be at identical voltages, therefore they reach full charge simultaneously and full discharge simultaneously They share the charge and discharge process pretty much perfectly and nothing gets "Dragged down" and the batteries stay in a state of equilibrium due to natural atomic/chemical forces and benefit from this balance rather than suffer because of it. The battery (newer?) with the higher capacity will obviously do more work than the smaller battery but that's what you'd want because it doesn't have the same capacity. Both batteries contribute EXCATLY their respective capacities and the smaller battery will therefore contribute and assist the larger battery proportionally according to size and therefore extends the larger batterie's life. No risk of fire, explosion or whatever.
What IS very important is matching chemistries. Different LA battery types like flooded, gel, AGM etc all require different voltages for their bulk and float stages, because the batteries are connected together they both 'see' the same voltage so the chemistries should be the same.
Cell count should be obvious.
Take a look at you tube, there's loads of actual tests with unequal battery capacities showing the results I mention above, mostly if not all LFP based but the laws of physics apply to lead acid too.
Enjoy

 

[Pudsey Bear]

My thoughts (SORRY) about old dragging the new down were just that, surely that would happen, once, until they were both full, or if one was so badly shagged it needed to binning.

 

[Pudsey Bear]

Sorry mate, I didn't see your post when I started mine.

 

[merl]

So shagged it needs binning.

Is that a scientific quantity of charge level Kev?

 

[maingate]

Is that a scientific quantity of charge level Kev?

If he had been using technical terms, he would have said it was 'Donald Ducked'.

 

[Pudsey Bear]

Is that a scientific quantity of charge level Kev?

Indeed