Charging batteries

To my understanding ideally connect to the positive of one and the negative of the other. As most parallel battery setups have both batteries sited close together and the cables joining them are short and are of sufficient diameter in practical terms the advantage is minimal.

Cheers
Pop

Rarely do you get the opportunity, let alone the need to charge multiple batteries in this way.

Consider your average dual battery installation in a vehicle.
The alternator is connected to the primary battery and the auxiliary battery is connected to the primary battery terminals, + to +, - to -.
Therefore all charging is applied to the primary battery and usually an isolator manages the charging connection to the auxiliary, once the primary battery has reached a minimum voltage level.

Similarly, in a camper with multiple batteries, the batteries are connected in parallel and it doesn't matter if a charger, (whatever it may be) is connected to one battery, the other in the circuit will benefit the same, as any charging current will be applied equally to both batteries. It is always good practice to ensure an intelligent isolator is included in the circuit with any auxiliary batteries to protect to primary (starting) battery from accidental discharge.

In my setup, the auxiliary battery in the rear tub of the vehicle is charged by the alternator, managed by an isolator between the two batteries.
When the camper is connected up, the two batteries in it receives their charging current via connection to the auxiliary battery. To all intents and purposes, I then have three auxiliary batteries connected in parallel, which all receive input from the alternator equally.

To ensure an adequate charging voltage and current, the two batteries in the camper are managed by a dc-dc charger in the circuit, to ensure any voltage drop in the long cable run is eliminated.
This dc-dc charger also includes a solar terminal input, to apply a charging source when the vehicle is not connected. This dc-dc
The camper also includes a separate "built-in" AC charger, which is automatically applied when the camper is connected to a 240AC supply.
This charger is also connected to the left side battery and only the two linking cables connect the right side battery to the "battery bank".
Generally, I don't set up camp in caravan parks, so I rarely connect 240v to the camper, except on occasions at home to maintain a "maintenance" charge to the camper batteries.

When two (or more) batteries are connected in parallel, the main thing to consider, is that a fault in one battery (eg dropped cell) will drag the other battery down. This is why is is often mentioned that both batteries be of similar age and construction to limit such a situation from occurring.