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

jarod347

Hi all,

I've been trying to figure out what battery charger I need to use to charge my battery, but all the posts and answers are not helping me select a specific charger. What I would really like are specific suggestions on what product to buy. I'm totally new to this stuff, so having someone make a specific suggestion based on my system would be very helpful!

So here is what I have:

Cabin with lots of shade during the winter
1 100w Renogy solar panel
40A Rover MPPT Charge Controller
1200watt 12v DC to 110v/120v Giandel Pure Sine Wave Inverter
 WKDC12-100P Duracell Ultra 12V 100AH Deep Cycle AGM SLA Battery

Honda EM5000sx generator

 

I want to be able to charge the battery with the generator when the solar panel isn’t getting it done.

Any suggestions?

Thanks!

Jarod

mike95490

For the occasional bulk charging for 12V batteries, i like using an automotive 40A charger. They are not a full 3-stage charger, but they can pack a bunch of amps in for a couple hours from the generator, and then you can go enjoy silence again.
  That might be a bit of a load for small generators, but for one your size, shouldn't be an issue.

However, your inverter is a pretty large one, and while it can power large loads. the battery will not last long.  And a large inverter consumes a lot just sitting idle

MichaelK

I think one reason you have to focus on keeping the battery charged is because your solar panel is too small.  Assuming you want to charge it at up to 1/8C, you need 100Ah X 0.125C X 13V charging X 1.25 fudge factor = 203W.  You could add a second 100W panel in series to the first one, or replace the single 12V panel with a 240W grid-tie panel.  Your controller will transform the raw 30V solar voltage down to 12.5-13.0V for bulk charging.  I'd bet you, that looking on Craigslist, you'd get a single 240-250W grid-tie panel for half of what you'd pay for a single 12V panel.

jarod347

MichaelK said:

I think one reason you have to focus on keeping the battery charged is because your solar panel is too small.  Assuming you want to charge it at up to 1/8C, you need 100Ah X 0.125C X 13V charging X 1.25 fudge factor = 203W.  You could add a second 100W panel in series to the first one, or replace the single 12V panel with a 240W grid-tie panel.  Your controller will transform the raw 30V solar voltage down to 12.5-13.0V for bulk charging.  I'd bet you, that looking on Craigslist, you'd get a single 240-250W grid-tie panel for half of what you'd pay for a single 12V panel.

Hi MichaelK, Thanks for the response! This may be a ridiculous question, but I really have no idea what I'm doing  

Are you saying that the single 100W panel I have won't charge the battery I have at all or that it will just charge it very slowly?

It seems to me that it is not charging it at all or at least not much at all. The charge controller shows that the panel is pushing in energy, but not much. At the same time, the battery never seems to gain charge. I've assumed this is because it is super cloudy out most of the time and the sun doesn't break through the trees much this time of year.

But maybe you are saying I need at least 203W to get any charge at all? I've always planned to get more panels, but thought 100w would do for now. Maybe I'm wrong?

jarod347

Mike95490 - Thanks for your reply!

I'll look into car charges.

You mention my inverter. I got a large one with the plan to increase the size of my battery bank in the future, but for now we are not running any appliances on the inverter, just computer charges and the such. You said that large inverters consume a lot while sitting idle - we keep it turned off while not in use assuming it wouldn't pull any electricity; is that a correct assumption?

Thanks again!

BB.

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