Seeking help with off grid cabin

znate wrote: »

I like that idea. Could I set it up so that when the 24v batteries are full the extra power would feed another charge controller that would charge the 12 volt bank?

Is there a dump load type circuit built into charge controllers that could be used to feed the second controller?

Cariboocoot wrote: »

I am not talking about a 3X overbuild or a 4X overbuild: I'm talking about sizing a battery bank so that it doesn't go dead quickly. For 90% of the batteries used this means you automatically lose 1/2 the calculated capacity because regularly discharging to less than 50% capacity is murder on batteries. That gives you 50% capacity to work with, and if you divide the average consumption over two days you get 25% DOD. The remaining 25% capacity provides one "no sun day" of energy and on the third day of clouds you start the generator. This also allows the most efficient use of generator capacity.

PV array has to be sized to charge the whole battery, regardless of SOC, and the shortcut of basing peak current on 10% capacity works most of the time. In some areas you'd want even more, due to less-than-perfect insolation. Cutting it down to the minimum battery manufacturer's recommended 5% is asking for trouble as it doesn't account for concurrent loads and will not charge the batteries sufficiently in the hours of daylight available (which should be based on minimum sun, not maximum).

My advice is based on my experience and includes a lot of repairing of systems designed by people who didn't do the calculation right in the first place.

I have seen people (including Bill, who does not have an off-grid system) advise on the basis of "three days with no sun" which is impractical because you end up with more panel and battery than you need.
I have seen people advise on the basis of panels producing 100% of their rated power, which they don't.
I have seen people advise on the basis of 100% system efficiency, which doesn't exist in the real world.
I have in fact seen people give out a lot of really bad advise which results in system with no margin for error or variation in operating conditions.

And then I have to fix their messes. Some of these are "professional installers" who in fact have only a vague understanding of what they are doing based on being able to connect wires together and have it work momentarily. I wouldn't trust them to change a light bulb. Whereas my success rate is 100%, even with seemingly hopeless situations.

You can improve over-all efficiency with certain management techniques, but you must be able to adapt your usage to them. Until you know whether or not this can be done on an individual system it is better to err on the side of caution.

The two biggest mistakes repeated over and over are insufficient panels for charging the amount of batteries and insufficient batteries for supplying power for the loads.

The sun does not shine brightly all the time.

P.S.: I'm getting sick of people derailing threads of newcomers with bad advise and off-topic discussions.