He needs a diode between the two batteries in parallel. Otherwise slight differences in voltage between them causes them to cyclically charge and discharge each other, wearing them out and wasting energy.
No one uses diodes on their battery banks, lead acid can tolerate some overcharge which is how they are equalized and lithium use a BMS for balancing cells.
The only diodes I have seen are used for Chassis/House bank isolation but usually its a relay system instead.
Remember a 12v lead acid battery is 6 cells in a single package no different than two 6v batteries in series (which is what my RV has).
That's incorrect. At no point or state of charge can you connect two lead battery packs in parallel and have them oscillate charging/discharging no matter how long connected. Either one will charge the other or they will sit without changing.
That's only true if they were ideal devices, but they are not.
I have not tried it myself, but from what I read if you leave two batteries connected in parallel for a while they will have less charge than if you had left them not connected to each other.
Batteries also don't discharge at identical rates, so each time you use them you will end up cross charging them, which wastes energy.
You are partially right but seem to lack some actual understanding of the underlying reasons why this is the case.
These are both new batteries that have been installed at the same time (without an O2 vent by the way, that is a serious mistake imo) and for all intents and purposes will simply function as a larger battery.
Even if they were not identical and not the same age they would still not do anything cyclical, but would function as a larger battery with the charge/discharge characteristic much like that of the older/weaker battery.
This is why it is not recommended to add a new battery to an existing pack, it will age very rapidly.
You do need a diode separator if you intend to charge your RV batteries from the regular alternator as well so that you don't end up trying to run your starter motor off the (usually) gell household batteries (they can't provide that kind of current), and so that you won't run your fridge/monitor/whatever off the starter battery which you will need badly to turn the engine over once you decide to move.
there would be a 0.7 V drop across the diode so then the system voltage would be 12-9.8V instead of 12.7 to 10.5 (taking 10.5 as the voltage at which your batteries are well and truly dead)
I've used diodes [0] to isolate redundant power supplies, but we turn up the voltage on the power supply to compensate.
However other than redundant applications I've never isolated batteries. I've just googled this and mostly found posts recommending not to use diodes and one that didn't comment on whether or not cross-discharging was actually a problem, but said not to use a diode but to use a mosfet[1]. It generally doesn't appear to be a problem for most people.
The most interesting part of the blog post for me was the eye opener for the author; 200 W of solar and 250 Ah of battery isn't enough to run a computer day in and day out. I would have gone for something like 1200 A-h of battery but not lead-acid, as well as added an AC powered battery charger so I could plug in.
