Dual battery questions for a simple solar setup (again, sorry...)

[jbeer]

After reading every awesome thread on the forum and attending DiploStrat's awesome presentation at Overland Expo East, I think I'm ready to get this system up and running but I'd love one last sanity check before I go.

Requirements:
Very simple setup for me... Basically the dual battery setup is almost entirely dedicated to running a 40L Dometic fridge and some scene lighting.

Parts I have now:
Overland Solar 120W Solar setup. This is a briefcase style portable unit but I will most likely mount it on the roof so it can run/charge full-time.

Dometic 40L fridge with slide and blanket thing.

Parts I am attempting to run together for a functional system:
Blue Sea ML-ACR with switch

1/0 Gauge wiring - Diplo said that most battery systems struggle from undersized wiring... I believe him but I am not sure how big I really need to go. The main battery will be under the hood, the house battery will be in the bed of my truck. Is 1/0 required here or will something smaller / easier to route be okay?

And a switch panel...

Here is a crude diagram of what I think this will look like:

Does this make sense or am I missing something?

THANK YOU!

 

[DiploStrat]

How Big is Big?

On my old truck I ran a pair of 1/0 AWG wires, about 100mm2 of copper. This was great, but, after two years of real world use I realized that it was, in fact, a bit of overkill with lead acid batteries.

(Insert plug for my three part electrical presentation, which I should be giving at OEXPO West in 2018.)

Now, I think I would argue for a single run of 1/0 AWG or even 1 AWG. Why? Lead acid batteries are perverse; they start to take a charge very fast and, as they do, the internal voltage climbs rapidly. As the internal voltage rises, the batteries ability to take a charge drops. This means that no matter how big your wires, your lead acid battery is only going to charge so fast. This of course, is why it takes so long.

The rate of charge of a lead acid battery is largely a function of the difference between the voltage of the battery and the voltage of the charge source. When you first start charging, the battery voltage is at its lowest so you don't need a high a charge voltage to achieve a large difference. So as long as your wires are not so small as to cause a massive voltage drop (which they are on most RV's wired with 10 or 8 AWG) there is no benefit to going to wires that are too big.

So, whipping out our voltage drop calculator, we get this:

Charge voltage: 14.4v
Charge Amps: 75
Wire: 1 AWG

This gets you about 13.9v at the battery. Assuming a 50% discharge, the battery is at, say, 12v. So that is pretty good.

(http://www.calculator.net/voltage-d...nce=25&distanceunit=feet&amperes=75&x=68&y=11)

But, but, but - you keep insisting that I gotta have a charge voltage of 14.4v (or some such number at 70F).

True, but as dwh keeps trying to explain, voltage drop is a function of voltage and amperage. As the number of amps you are trying to pass drops, so too does the voltage drop. (which is really a function of resistance, but we digress) The dirty secret is that even with my fire hose size cables, I could only get over 100A of charge for about 30 minutes, then, as the battery voltage rose, so did the amp flow.

So, we'll start with a guesstimated 75A and accept that the actual charge rate is going to be under 50A within an hour. So let's look at what happens at 25A, which is where you are going to spend a lot of time charging.

Again, everything as before, except that amperage is reduced to 25A. Now we get a voltage of 14.2v - which is more than enough to complete the charge.

(http://www.calculator.net/voltage-d...nce=25&distanceunit=feet&amperes=25&x=84&y=16)

This is why the folks who sell battery to battery chargers (CTEK, REDARC, Sterling, et al.) can get away with such small units - they know that, even with the biggest wires, most of the time, the charge rate is going to be under 50A.

So, do your own math, but I suspect that you will find that the sweet spot for cost, ease of construction, and fast, complete charging is between 1/0 and 2 AWG. Remembering, of course, the old rule: When in doubt, go bigger.

N.B. All of this applies to lead acid batteries (e.g. FLA, AGM, etc.) Lithium poses different challenges and the rules are a bit different.

 

[jbeer]

The man himself - thanks again DiploStrat!

Okay, so I'm gonna go big on the wires - does the rest of the diagram/plan make sense?

 

[DiploStrat]

Right ! Go big or go home. Or stay hell off 'Expo...:wings: Your plan looks great.
Obviously its missing overcurrent protection & Solar controller, But as concept & block diagram, Its great.

Agree. Fuse it or lose it!

As a practical matter, I would connect the solar to the positive terminal of the camper battery, or an actual positive bus.