Help Designing Schematic

Lead Acid batteries are not a bad place to start... And simply using the same numbers for a Li Ion pack will get you started.
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Li Ion tend to be known for high surge current support--But many of the Li Ion batteries do have hard limits on surge current that limits them to Lead Acid or less of similar AH. Need to know the details when designing your system.

And another reason why FLA rules of thumbs work for other batteries (AGMs are >90 efficient, even upwards of 98% or so--Similar to Li Ion).

The big differences include that Li Ion do not have to be recharged to near 100% capacity within a day or so (especially if to sit unused). And Li Ion typically last longer if stored at 50% SoC (or at least, do not exceed 90-95% SoC for longer life).

And it really does go back to your loads and energy needs... Typically we size a Lead Acid bank to use nominally 50% to 100% State of Charge (with occasional discharge down to 20% SoC). And a Li Ion battery around 20%-90% state of charge--Which gives you a useful capacity of around 70% of battery capacity--Whereas lead acid we suggest a typical cycle of 25% to 50% of battery capacity...

The issues become--If you use your Li Ion battery to cycle 70% every day--They are certainly capable of doing that. With Solar, a Lead Acid battery should be cycled something like 25% per day (it is difficult to fill a Lead Acid battery from 50% or less SoC to 100% in one solar day--Simply not enough hours of sun in the sky to fully recharge a Lead Acid bank deeply cycled in one day.

So we have the 2 days of "dark" plus 50% max discharge or ~25% depth of discharge for 2 days of dark. This helps to avoid generator use if the next day or 2 is stormy weather.

If you size your system to use 70% of Li Ion capacity (small/light weight battery bank, and save some $$$ too where as Lead Acid tends to be big, heavy, and cheap per AH/kWH of storage)--You are left with "going dark" waiting for the next day of sun, or using a genset.

The sizing of 5%-10%-20% rate of charge is a good rule of thumb for sizing a Li Ion system too... While the technical reason is that FLA and other lead acid batteries typically have minimum rates of charge and equalization power requirements that simply are not required for Li Ion type banks.

However, the 5% and 10% minimum are also good numbers to get a battery bank recharged quickly... You don't want to take 2+ days to recharge after one "dark day" of energy usage (i.e., 5% rate of charge * 5 hours of good sun = 25% charge cycling--Good minimim for summer solar). More panels, "less day to day power management when off grid".

And this comes back to balancing loads against Battery Bank Capacity. Flooded Cell Lead Acid battery banks tend to be "oversized" to handle large loads (like starting a well pump)--But when you match with 2 days of dark, and 50% max discharge--The typical FLA Bank will generally have enough surge current to support the typical "matching" AC inverter capacity and surge loads.

Li Ion generally support surge loads better--Allowing you to reduce bank AH/WH capacity vs FLA... However, you still have the "raw capacity" question... How many AH (at XX Volts) or Watt*Hours per day, and how many "dark days" do you want your system to support before starting the genset. If you size a Li Ion bank to do 70% cycling or even 35% cycling per day, after you "hit" the 20% bottom (or whatever you choose to use), you have effectively "zero" battery energy left... Where as our 2 day/50% limit, still gives you another 30% capacity between 50% and 20% for occasional use during bad weather/etc. So our FLA rules of thumb give the owner a bit more room for energy management/genset use.

For Li Ion battery banks with "minimum AH/WH" capacity... You have to know and manage your loads with more care. Or else you end up with more genset time/or more "darkness" than you would have liked. If only Li Ion Batteries were not so expensive--Then could throw some more reserve capacity at the issue and be a bit more loose with system planning.

And only you know your loads... Typically for off grid use, I suggest maximum conservation and the most efficient appliances/inverters/etc. you can find. The typical cabin/RV may have many hours of small loads (5 hours of lights, charging laptop and cell phones, a small fan for summer use, and a small RV Water pump for tens of minutes per day).

If your bus was also your job vehicle--Perhaps running a skill saw, air compressor, etc. or relatively short bursts of high power/current--Not hours of high power...

Just to give you an example, a Microwave is a tough load--But for other reasons a typical desktop computer can be worse:

• 1,500 Watts microwave * 1/3rd hour per day usage (20 minutes per day) = 500 Watt*Hours
• 300 Watt desktop computer * 8 hours per day = 2,400 WH per day
• 30 Watt laptop * 10 hours per day = 300 WH per day
• 12 volt * 8 amp RV water * 1/2 Hour per day (30 minutes) = 48 WH per day
• 8 Watt LED * 5 lights * 5 hours per night = 200 WH per night
• 40 Watt AC inverter Tare Load (larger AC inverter have larger Tare) * 24 hours per day = 960 WH per day (if running 24x7)
• 120 Watt refrigerator * 1/5 hour duty cycle * 24 hours per day = 1,440 WH per day

As you can see, small loads * lots of hours of use per day = lots of WH/AH of battery bank usage.

-Bill