What battery charger for my off-grid cabin solar system?

I keep "harping" on designing "balanced" systems... In your case a (relatively) small battery bank and solar array, and a relatively large AC inverter.

A 1,200 Watt AC inverter running at full power would take:

• 1,200 Watts * 1/0.85 AC inverter eff * 1/10.5 volts battery cutoff = 134 Amps (at full 120 VAC load)

A 100 AH battery bank would be sucked "dead" in less than 1 hour.

So it comes to your expectations regarding your loads... If 120 VAC @ 1,200 Watts for ~30 minutes per day, then great (AGM batteries can surge high amounts of current for 1 hour or less--Typically used for UPS--Uninterruptible Power Supplies for computers--Give enough time to save and shutdown and/or start a backup genset).

However, for a cabin--Lets say you charge during the day, and run lights/radio/cell phone charger/tablet for 5 hours per night.. 2 nights of energy storage, and 50% max planned discharge (basically 1/4 of battery storage per night)--That is a good place to start.

Your 12 volt @ 100 AH battery:

• 12 volts * 100 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 255 Watt*Hours of battery stored energy "per night"
• 255 WH of nightly energy / 5 hours of usage = 51 Watt average 120 VAC load

As you can see, that is a far cry from 1,200 Watt AC inverter for running a cabin in the evening. Apparently (one Amazon user reports) that this inverter idles at 5.6 Watts

• 5.6 Watts * 5 hours = 28 WH of energy just to run the AC inverter for 5 hours per night

28 WH is over 10% of the total electric load--So you can see that if you left the inverter "on" 24 hours per day, that would be around 134 WH per day--Or over 1/2 of your planned battery storage. If the inverter is truly "off"--They should take no or very little amount of power/current ("off" in electronics is becoming a very "iffy" term).

Regarding how much energy a 100 Watt panel can harvest for you. Say near Renton WA, fixed panel:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Renton
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 42° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.70	2.81	3.53	4.17	4.57	4.77
Jul	Aug	Sep	Oct	Nov	Dec
5.23	5.20	4.71	3.15	1.97	1.53

Lets say you use the cabin from May through September--Or 4.57 hours of sun per day (average May):

• 100 Watt panel * 0.52 off grid AC system eff * 4.57 hours of sun per day = 238 WH per "average" May day

And there is sizing the array for the battery bank. For a typical system, suggest 5% (summer/weekend system) to 13% rate of charge for solar--And 10%+ for full time off grid.

• 100 AH * 14.5 volts charging * 1/0.77 * 0.05 rate of charge = 188 Watt array minimum
• 100 AH * 14.5 volts charging * 1/0.77 * 0.05 rate of charge = 377 Watt array nominal
  
• 100 AH * 14.5 volts charging * 1/0.77 * 0.05 rate of charge = 490 Watt array "typical" cost effective maximum
  

So--You would be good to get (at least) a second 100 Watt panel for your battery system.

And there is sizing the AC inverter for the typical flooded cell lead acid battery... Basically 250 Watts per 100 AH:

• 100 AH (at 12 volts) * 250 Watt AC inverter * 1/100 AH (@ 12 volts) = 250 rough maximum suggested AC inverter

Anyway--Kind of working backwards of how much energy your existing system can harvest... Vs design a system to support your daily power/energy needs.

Regarding 10%-20 Rate of charge for a genset with a "typical" AC battery charger:

• 100 AH * 14.5 volt battery bank * 1/0.80 AC inverter "efficiency" * 1/0.80 Genset derating (for continuous loads) * 0.10 rate of charge = 266 Watt minimum rated genset (10% rate of charge)
• 100 AH * 14.5 volt battery bank * 1/0.80 AC inverter "efficiency" * 1/0.80 Genset derating (for continuous loads) * 0.20 rate of charge = 453 Watt minimum rated genset (20% rate of charge)

10% of 100 AH = 10 amp battery charge. 20% of 100 AH battery bank is a 20 amp battery charger.

Your EM5000sx genset--Nice generator, but a whole lot larger than you need for charging just the battery bank... Charging the battery bank + other loads (heater, cooking, power tools, etc.) can work out OK.

In general, standard (non-inverter) gasoline/propane gensets are pretty fuel efficient from 50% to 100% loading... When you run them at less than ~50% loading, their fuel flow remains the same (basically around 50% of fuel flow).

Inverter-Generators however, are relatively fuel efficient down to around 25% electrical loading (the inverter-generators slow down their RPM with light loads--So they use less fuel with smaller loads.

For just charging the 100 AH 12 volt battery bank, you want a genst that will run at 25% to 50% loading at least (up to ~80% of rated load suggested). So for a 20% / 453 Watt battery charger--A smaller genset in the ~900 Watt to 1,800 Watt (continuous) rating would be nicer. Also, if you get an inverter-genset, they are usually a lot quieter too.

Keep the 5,000 Watt genset for when you need more electricity (power tools, running a vacuum, washer, electric chainsaw, etc.), or as a backup if the small genset stops working.

Your thoughts,
-Bill