On our most recent camping trip I decided to connect both batteries in parallel (using recommendations that I'll discuss shortly) and keep them permanently connected to the solar panels / charger and load (lights, fridge/freezer and so on).
Fortunately for us the weather was awesome - no shortage of sun for the full week we were away from civilization. I didn't measure the power usage on this trip but from past experience I assumed I would use around 2.5A/hr, which equates to 60A / 720W per day.
Based on a general guideline of 80% efficiency of the panels over a 6 hour period of sunlight our 240W panels would produce (240 x 0.8 x 6) 1152W - that's 1.6 times our estimated usage. Unbelievably, we had much more than 6 hours of sunlight each day and as such the batteries never fell below 13V, even when under load. In a way it's a shame because I didn't get an opportunity to assess the wiring configuration I used, as discussed below.
THIS IS BAD because it places a greater demand on battery #1 while the load is active. Battery #2 doesn't work as hard because of the extra resistance in the cables joining to battery #1.
Similarly, if the charger was connected in the same way then the battery immediately connected to the charger will receive a higher charging voltage.
As you can probably imagine all of this leads to inefficiencies. The preferred approach is shown in the following image:
Without going into all of the science behind it, electrons within a battery flow from positive to negative when being charged and when it is under load they flow from negative to positive. To help the electrons flow more evenly through both batteries this latter wiring configuration is highly recommended.
The configuration in the latter image is exactly how I wired our two batteries but there was one annoying issue - connecting them upon arrival and disconnecting to come home was time consuming and risky due to the possibility of an accidental short circuit. Our camping makes use of a camper van (trailer) rather than a caravan so there is no possibility of permanently wiring these batteries in.
Since arriving back home I've come up with an alternative approach that means I'll be able to keep an Anderson plug permanently connected to the batteries and the only extra work required when we arrive at the campsite will involve connecting those batteries to a harness. This harness has two plugs to connect to the batteries and two others to connect to the solar charger and load. The following image shows this diagrammatically:
I've also included a 10A fuse on the left-most lead and a 30A fuse on the right-most lead. This provides a level of protection for the load and charger respectively.
I used 6mm cable (rated to 51A) to connect the Anderson plugs to the battery terminals. The idea here is that since my leads are no longer connected directly to the battery terminals I want to make sure there is as little loss as possible due to resistance or heat.
As for the harness itself I used cable rated to 30A on the basis the maximum output of the charger is 20A and the connected load will typically draw 5A at most (but allowing for up to 10A).
The photographs further below show the final result of putting the harness together. The two unconnected plugs in the last image will be connected to the load (fridge/freezer etc) and the charger.
I can't wait to try it out on my next camping adventure.
I can't wait to try it out on my next camping adventure.
