Small 12V solar backup system

[DavidH]

Hi all,

I've been doing a fair bit of reading but have no experience so wanted to run everything by you guys here, get some advice or some corrections. Bit of background first - I'm currently living in Puerto Rico and went through hurricane Maria. Power grid here is fragile and I don't want to have to rely on a generator for running a fan throughout the night for when there are power cuts. I realize that a solar backup system isn't cost effective but the lack of noise, cleaner air and the fact that I'm generating my own power has its appeal to me. So this is my process

• My power requirements are a 75Wh Lasko 16" fan to run for 10hrs (total of 750Wh). This was measured using a kill-a-watt meter.
• Puerto Rico sun hours are 5.53
• Power generated by single 100W panel = 100 x 5.53hrs = 553 watts. Factor in inefficiencies in system (75%) = 414.75Wh. Therefore 200W of panels = 829.5Wh which is sufficient.


Choosing a battery.

• I don't need any cloudy day backups. It's not worth the money and I believe there's a way to charge the battery bank with a generator? - I'd appreciate any advice on this generator option which I could do during the day.

• Battery bank discharge should only be 20-30% of capacity to get money's worth for longer life.

• Trojan T105 6V 225Ah (x2 in series) = 12V battery with 2,700Wh (225Ah x 12V). I need 750Wh so overnight I would discharge the battery by 27.8% ((750Wh/2700Wh)*100). I believe this is acceptable?
• Cost of these two batteries is something like $360. Is there any other battery type that would be cheaper or better?

Taking into account the C-Rate of the battery

• This is where I need some help. I have to take into account the charge rate of the battery, if PV array charges too slow or too fast then I shorten the life of my battery.
• I read that a recommended charge rate is between 1/10 to 1/4 of battery C-20 amp hr value (C20 of T105 is 225Ah). Maybe the range is specified somewhere for two T105 6V batteries in series?
• 1/10 of the 225Ah (12V) T105 battery bank = 22.5A. This is the lowest acceptable current. PV Watts needed = Charging Voltage x Amps (14V x 22.5A) = 315 Watts. This means that 200 watts of panels is insufficient to charge the T105 12V 224Ah battery at the correct rate and will shorten the battery life. Have I got this correct? 

• Question: Do I have to take into account (in this charge rate calculation) the fact that the panel will never produce 100W per hour due to inefficiencies? Meaning that an estimate of 75% efficiency would require me to have 400W of panels to give the lower end C-Rate?

Choosing a solar charge controller

• Amps = PV wattage/battery voltage = 200/12 = 16.7amps. So I'm thinking 20amp charge controller would be sufficient. Probably go 40amp MPPT especially if I actually need more than 200W of panels. Since we live in a hot tropical climate, I believe it would be worthwhile that the charge controller have a battery temperature sensor too. Would the EPEVER AN 40A negative ground be reasonable? Can't find the warranty info.

Choosing an inverter

• Pure sin wave so I have option of using system to charge/run electronic equipment, is I believe the recommendation I've read.

• I've read that the general guide for 12V system is 1:1 (PV watts:Inverter wattage) which would be a 200 watt inverter - 400 watt inverter (depending if I need to increase PV array to get correct C-Rate).

• What could I use for a voltage cut off? - say the battery reaches a discharge of 30% and the system could just stop to protect the battery

So that's where I'm at for the time being. I was able to purchase 2 100W HQ-100D panels which I miraculously didn't have to pay the shipping for, so it worked out at $1.2 per watt which I thought wasn't too bad. PV panels are hard to find locally as I am sure you can imagine.
The one big "IF" in my whole thinking process is that there may well be a more efficient fan out there, but for the life of me I couldn't find anything that would allow me to know if the system could be made smaller. Anyway, any input will be much appreciated.
Thanks!

[Mike90250]

It's unlikely you will overcharge the batteries enough to damage them, the typical user undercharges and the batteries die of sulfation..

Running a fan - it uses a motor, so you are now best off with a Pure Sine Wave inverter. I've used a 12V SureSine 300w for the last 7 years to run a string of LED lights for the privy and tool chargers. Too large of an inverter, you waste power with it idling or under loaded. The Morningstar Suresine has a very low self-comsumption

[DavidH]

So do you think that my calculations are correct and 200W PV array is too little to make the correct C-rate? I don't want to undercharge and sulfate them. Thanks for the advice on the inverter.

[Brian53713]

DC powered fans will save power loss through conversion.

[Guest]

[/LIST]Choosing a battery.

• I don't need any cloudy day backups. It's not worth the money and I believe there's a way to charge the battery bank with a generator? - I'd appreciate any advice on this generator option which I could do during the day.

• Battery bank discharge should only be 20-30% of capacity to get money's worth for longer life.

• Trojan T105 6V 225Ah (x2 in series) = 12V battery with 2,700Wh (225Ah x 12V). I need 750Wh so overnight I would discharge the battery by 27.8% ((750Wh/2700Wh)*100). I believe this is acceptable?
• Cost of these two batteries is something like $360. Is there any other battery type that would be cheaper or better?

Taking into account the C-Rate of the battery[LIST][*]This is where I need some help. I have to take into account the charge rate of the battery, if PV array charges too slow or too fast then I shorten the life of my battery.[*]I read that a recommended charge rate is between 1/10 to 1/4 of battery C-20 amp hr value (C20 of T105 is 225Ah). Maybe the range is specified somewhere for two T105 6V batteries in series?[*]1/10 of the 225Ah (12V) T105 battery bank = 22.5A. This is the lowest acceptable current. PV Watts needed = Charging Voltage x Amps (14V x 22.5A) = 315 Watts. This means that 200 watts of panels is insufficient to charge the T105 12V 224Ah battery at the correct rate and will shorten the battery life. Have I got this correct? 
[*]Question: Do I have to take into account (in this charge rate calculation) the fact that the panel will never produce 100W per hour due to inefficiencies? Meaning that an estimate of 75% efficiency would require me to have 400W of panels to give the lower end C-Rate?

I just know the LiFePO4 battery better, long-cycle life and more environmental.