Hrmm....a few thoughts.
For me, the point of a cheap aux battery is that I know ahead of time that I will be draining it below 50% and thus shortening its useful life. Since I know I'm going to abuse it and plan to replace it every couple of years, I don't spend a lot on a high quality aux battery. (I do spend money for a high quality engine start battery however, that's mission critical.)
Putting the battery in the rear brings up the spectre of voltage drop. If your alternator is putting out 14.4v but only 13.4v is getting to the battery, then your nice expensive AGM will not be getting a full charge and you'll be doing a thing called "deficit charging" - which will shorten the battery's life.
Wire size is not only about the amperage, it's also about getting the voltage from Point A to Point B with the least amount of drop. This becomes more important the lower the voltage you start out with, and 14.4v ain't all that much voltage.
Diode type isolators generally have -at least- a .5v drop to begin with, and then add whatever additional voltage drop due to insufficiently sized wire. There are a few diode type isolators that are designed to not have that problem - you probably won't find one at the local auto parts store. Solenoid isolators are the good stuff.
With a 100w flat mounted solar panel, your power budget will be roughly:
100w x 5 hours of good sun = 500 watt*hours / 12v = 41.5 amp*hours
It's actually worse than that, since most of the time the panel will be hot and only putting out about 80%, so:
80w x 5h = 400wh / 12v = 33.3ah
But actually, your charge controller won't be putting out 12v, it'll usually be putting out more like 15v or 16v (unless you spend extra for an MPPT charge controller - I wouldn't) so:
80w x 5h = 400wh / 16v = 25ah
Is what you can realistically expect to harvest on average.
But...the battery isn't 100% efficient either, nor is the charge controller, so figure you'll only get 25ah * .8 (80% efficiency) = 20ah actually stored in the battery.
Okay, so you can replenish 20ah per day into the battery. If your fridge actually does consume 25ah/day as mentioned in a previous post - then your 100w panel won't keep up. Deficit charging again, and shortening battery life.
(I have no idea how much the fridge will draw. On another thread someone posted actual usage figures for their fridge, but since I don't have one of those fridges, I never looked at them.)
The other thing to consider is battery capacity vs. solar harvest. With a 20ah/day harvest, you could take a 40ah battery down to 50% and recharge it in one day. If you use any battery capacity larger than that - then you can't recharge it in one day.
If you had a 100ah battery, and you drain it 25ah/day, then it'll last 2 days before it'll hit 50% and need recharging, but will take more than two days to recharge.
That's fine if you only use it on weekends - you can leave it sit in the sun all week to recharge, and then drain it on the weekends. It's not fine however for long-term daily use - because once again, you'll be into deficit and reducing the life of that nice expensive battery.
For portable solar systems - RV, boat, whatever - it basically comes down to "how many watts of solar do you have (or can fit)?" That dictates your average harvest. Your average harvest dictates how much battery capacity you need to store that harvest.
Half of that battery capacity is your daily power budget if your only source of power is the solar.
Last edited by dwh; 03-22-2011 at 04:03 AM.
Reason: typo gremlins
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
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