Battery charger

NinGar

Hi all, trying to diy my first system for small offgrid cabin, what are my best options for a battery charger for FLA or Gel? I will use a generator to charge them in case of extender bad weather. I am planning for 2 12V 100ah to form a 24V system, I prefer to charge them while on load.

Thanks

BB.

Welcome to the forum NinGar,

First... Sizing the components of your system using our rules of thumbs (for quick/workable system design). And to do that, need to look at how much energy (Watt*Hours or Amp*Hours @ 24 volts) you need...

First the battery bank... Suggest Flooded Cell deep cycle storage batteries, or AGM. GEL typically are not as good for solar applications (many, at least USA brands of GEL batteries do not like >5% rate of charge... For solar 10% to 20% Rate of charge is usually needed).

You may also want to look at using 6 volt @ 200 AH "golf cart" batteries. Usually cheap(er) and easy to get. Same size/weight as your 12 volt @ 100 AH batteries (and same Watt*Hours of energy storage)--And if you get 4x "golf cart" batteries, you will have 2x more storage than your 2x 12 volt @ 100 AH battery bank.

Energy usage... For a cabin that will be used for many days/weeks/months at a time, for Lead Acid batteries suggest that you plan on using 25% of capacity per day (overnight, 1 day of bad weather, etc.), for 2 days of storage. And 50% maximum planned discharge (for longer life)... For the two bank designs we are looking at:

• 2x12 volt bank: 24 Volts * 100 AH * 0.85 AC inverter eff * 1/2 days of storage * 0.50 max planned discharge = 510 Watt*Hours per day of 230 VAC power
• 4x6 volt bank: 24 volts * 200 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max planned discharge = 1,020 Watt*Hours per day of 230 VAC power

Looking at the solar array... 5% minimum rate of charge suggested (weekend use, sunny weather usage), 10%-13% or so for full time off grid:

• 24 volt @ 100 AH bank:
• 100 AH * 29.0 volts charging * 1/0.77 solar panel deratings * 0.05 rate of charge = 188 Watt array minimum
• 100 AH * 29.0 volts charging * 1/0.77 solar panel deratings * 0.10 rate of charge = 377 Watt array nominal
• 100 AH * 29.0 volts charging * 1/0.77 solar panel deratings * 0.13 rate of charge = 489 Watt array "typical" cost effective maximum
  
• 24 volt @ 200 AH bank:
• 200 AH * 29.0 volts charging * 1/0.77 solar panel deratings * 0.05 rate of charge = 377 Watt array minimum
• 200 AH * 29.0 volts charging * 1/0.77 solar panel deratings * 0.10 rate of charge = 753 Watt array nominal
• 200 AH * 29.0 volts charging * 1/0.77 solar panel deratings * 0.13 rate of charge = 979 Watt array "typical" cost effective maximum

And sizing the array based on the amount of sun you receive. Fixed array, facing south for Nicosia, Cyprus:

Nicosia
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 55° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.68	4.44	5.64	6.05	6.59	6.99
Jul	Aug	Sep	Oct	Nov	Dec
6.99	6.88	6.59	5.61	4.16	3.37

Lots of sun, using genset during bad weather/winter(?).. Pick December at 3.37 Hours of sun per day for "break even" design:

• 24v @ 100 amp bank: 510 WH per day * 1/0.52 off grid AC system eff * 1/3.37 hour of sun per day (Dec) = 291 Watt December "break even" array
• 24v @ 200 amp bank: 1,020 WH per day * 1/0.52 off grid AC system eff * 1/3.37 hour of sun per day (Dec) = 582 Watt December "break even" array

Maximum suggested AC inverter sizing:

• 24 volts * 100 AH * 1/8 hour discharge rate * 0.85 AC inverter losses = ~255 Watt AC inverter suggested maximum
• 24 volts * 200 AH * 1/8 hour discharge rate * 0.85 AC inverter losses = ~510 Watt AC inverter suggested maximum

Anyway... Lots of guesses (with possibly a few typos) and suggestions on my part... Your thoughts?

-Bill

PS: Generator/AC chargers...

For a 100 AH battery bank suggest 10% to 20% rate of charge (10-20 Amps)
For a 200 AH battery bank suggest 10% to 20% rate of charge (20-40 Amps)