Help with a basic solar lighting and water pump system

justjonesin

Hi,
I am hoping to wire a very simple 12 volt dc system for a tiny one room off grid guest cabin I just built.  I have some questions I hoped could be answered.  I’ want to run 4-5 small 5 watt led lights and a small dc water pump for teeth brushing and water bottle filling, etc. I have a single 75 watt solar panel of which I was planning on connecting to a charge controller then to a battery (battery to controller first) and then a fuse block and from there to the lights and pump.  I also want to be able to completely unplug the battery to take it to my main house (on grid) where I can trickle charge the battery with a charger  if the solar isn’t keeping up with the demand in the guest cabin.  
My question are:
1) where in the circuit should I put fuses,  what kind of fuses, and what size?
2) what size wire is appropriate from battery to charger,  charger to fuse block, fuse block to lights and pump?
3) can I install a circuit breaker or shutoff switch to easily remove battery if it needs charged?
4) how do I ground the system if that is needed?

mike95490

We are pretty short of information.

Copper wire is expensive, so you don't buy larger than needed.
Wire size is related to length of run, and size of load  ( how far will the pump be from the battery, and how much power does it consume)
Same for lights
same for charging circuit

Fuses are sized to protect the wire.   Need wire calculations first

There are some automotive quick connects for batteries, I've not used any of them

Grounding is a touchy subject, someone else can address it.

justjonesin

The water pump is 1.7 amps and will not be used often.  The pump will be roughly 3.5 feet from the battery.  The lights are led and are going to be 4-6 watts each wired with a switch so there will likely only be a couple on at any given time and they will be between 5 and 15 feet from the battery.  I figured I would probably use 12/2 wire for the lights and pump and then size 4 or 6 from the battery to the fuse block. I’m guessing I will need a fuse between the battery and fuse block, just not sure how big and if I need a small dc breaker or battery shutoff switch anywhere.  

BB.

The "right way" to design your system is to know your loads (AH @ XX volts or Watt*Hours per day), location, and usage (sunny weather, weekends, full time off grid, etc.).

To take a stab at your system...

• 5 watt lights * 5 hours per night = 25 Watt*Hours
• 1.7 amp pump * 12 volts * 1/4 hour per day (15 minutes) = 5.1 WH per day
• 3 AH * 5 volts (12 volt cell phone charger) = 15 WH per day
• total = 25+5+5= 35 wH per day

Typically, a nice system has 2 days of storage (works out well for solar charging and supplying load currents) and 50% max planned discharge (better battery life) for a battery bank:

• 35 WH per day * 1/12 volts * 2 days storage * 1/0.50 max discharge = 15 AH @ 12 volt battery bank

To charge such a battery--5% rate of charge for weekend/summer/sunny weather usage... 10%-13% for full time off grid/year round use:

• 15 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 14 Watt array minimum
• 15 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 28 Watt array nominal
• 15 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 37 Watt array "typical" cost effective maximum

And then there is sizing the array for your daily loads and amount of sun (and season of usage). Guess around Helena Montana, fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Helena
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 43° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.73	3.67	4.48	4.98	5.21	5.48
Jul	Aug	Sep	Oct	Nov	Dec
5.87	5.71	5.04	3.95	2.94	2.34

Lets say not many folks visiting in deep winter... Use 2.94 hours of sun per day (Feb thru Nov):

• 35 WH per day * 1/0.61 off grid DC solar system eff * 1/2.94 hours of sun per day = 20 Watt array for "November break even"

Your 75 Watt panel (assuming good quality, glass panel for permanent outdoor mounting) could easily support 2-3x larger AH battery bank than 15 AH @ 12 volts.

You could also save powering the water pump with a "foot pump" system (how is water getting to the cabin? A 5 gallon container, cistern, etc.?):

https://www.amazon.com/Whale-Marine-BABYFOOT-OPERATED-GP4618/dp/B003ELUZHK

Before we get into the details of sizing the battery/charge controller/wiring--Are my guesses for your location and usage correct?

-Bill

justjonesin

Thanks for the thorough and thoughtful response Bill, I really appreciate your time and help.  You almost nailed my location, within 60 miles.   The foot pedal pump is a good idea and something I wouldn’t have thought of.  I imagine I could install both a dc pump and the foot pump and then if I’m out of power I’d still have water.  I figured on a 5-10 gallon container under the counter to supply the water.  I do have some trees in the way that I will eventually remove but it will lower my direct sunlight to roughly 4 hours a day.  I haven’t looked into how far away I can put the panel from the cabin but if I move it ~200 feet away I would get full sun, I’d guess I’d lose a lot of power that far away.  The panel itself is in good shape, I purchased it a few years back for my camper and used it very little, recently sold the camper so the panel has just been laying in the garage getting dusty.  I recently noticed a few panels on Craigslist that are new, 250 watts for $100 that could be purchased if I needed more power.

Cheers,
Jason

BB.

You are very welcome Jason.

Lead Acid batteries really need a lot of time "on charge" to properly and fully charge. 1/2 of day blocked sun--It does make it much more difficult.

Li Ion (LiFePO4) batteries do well in those conditions--But do not like cold--Anything below (roughly) 40F and they have lots of restrictions on charging and discharging (below freezing, the batteries are basically "in storage"). Lead Acid batteries work pretty well in subfreezing conditions (yes, temporary loss of capacity in 0F weather, but they do not freeze as long as they are kept relatively well charged). Also just cycling an FLA battery (flooded cell lead acid) in an insulated box keeps them warmer.

You really need to nail down your power/energy needs. The solutions for a small system vs a large(r) system are different. Like what kind of vehicle do you need--Motorcycle, small car, pickup, or Mac Truck. 200 feet of a small 75 watt array worth of cable--May not be a good idea (voltage drop, lots of copper, $$$). However a few 1,000 Watts of array and a 100 Vmp or even 400 Vmp array (there are high voltage solar charge controllers--They just are expensive and usually larger capacity)--And 200 feet can be done reliably.

Using a Kill-a-Watt type meter (if you end up using an AC inverter--Highly recommended for anything more than just a small 12 volt system) to measure your AC loads... And there are DC AH/WH meters too that are pretty cheap these days:

https://www.amazon.com/s?k=kill+a+watt+meter&ref=nb_sb_noss_2
https://www.amazon.com/s?k=dc+AH+meter&ref=nb_sb_noss_2

You do have to keep track of costs and complexity... Solar power is not cheap--So getting high efficiency appliances, LED lighting, Small Laptop computer, "solar friendly" water pumping, etc. all help to save energy and costs.

-Bill

justjonesin

Great thoughts and advice Bill!  I have a better idea now about my setup.  Your calculations for the lights and water pump are exactly right as the pump I found on Amazon matches your numbers.  I forgot to include the composting toilet fan which is less than a watt, .07amps and in 24 hours is 1.7ah or 20.16 wh.  The usb cell charger numbers you used are also the same.  
So...per day...this should be my total consumption roughly:
Lights - 25wh
water pump ~5 wh
cell charger ~15wh
composting fan 20.16wh
Roughly 65 watt hours/day when the cabin is used.  And 20.16 wh per day when not in use.  

Since I have partial shade I was thinking about using the victron mppt 75/15 and a Dakota Lithium 23ah battery.  And the 6 circuit blue sea fuse block.  I used a wire size calculator for the wiring coming out of the fuse block and it spit out 16/2 wire for the pump, lights, and usb charger when I entered the wire length and current of each appliance.  
What I’m still confused about is
-what guage wire should I use between the charge controller and the battery?
-do I need a fuse between the panel and the charger, or between the charger and the fuse block, if so what kind and what size?
-since I have a fair amount of shade and may not be able to get the battery recharged, what kind of shut off can I install (and where) for safely unhooking the battery to swap a discharged battery with a freshly charged (charged on the grid at my home with a charger) battery?
-will the bms actually stop the battery from charging if it gets below freezing or is that something I manually need to control?  
Thanks again for all the help!

mike95490

You have to be very careful about disconnecting batteries.   The expensive charge controller requires the battery be present to prevent it from being cooked, it's internals work on battery voltage, not the variable PV voltage.  Remove the battery and your controller is now running at 97 DC ( or whatever you panels produce )  and smoked.

>  Since I have partial shade I was thinking...
   NO.   STOP .  Panels do not produce power in partial shade.   They are solar panels.   You need to go the the store and get shade panels.  Those will be made in 2086, if I recall correctly.......

Do not fall for the lies sales droids tell you.   A panel with shade actually produces little usable power.

justjonesin

Thanks for the advice on disconnecting the charge controller from the battery, that was why I was inquiring about a possible safe way to do this with a disconnect switch between battery and charger.  Being as the cabin won’t be used much and my power consumption was so little, I thought it might be possible that a solar set up, even with partial shade, could power at least a small fan when the cabin wasn’t in use.  I could just for go the panel all together and still use the  2 batteries and just swap the charged and discharged battery every week or so, I figured the panel I have that is collecting dust in the garage could help extend the time between charges and possibly run the composting fan while the cabin was not in use.

BB.

justjonesin said:

Since I have partial shade I was thinking about using the victron mppt 75/15 and a Dakota Lithium 23ah battery.  And the 6 circuit blue sea fuse block.  I used a wire size calculator for the wiring coming out of the fuse block and it spit out 16/2 wire for the pump, lights, and usb charger when I entered the wire length and current of each appliance.  
What I’m still confused about is
-what gauge wire should I use between the charge controller and the battery?
-do I need a fuse between the panel and the charger, or between the charger and the fuse block, if so what kind and what size?
-since I have a fair amount of shade and may not be able to get the battery recharged, what kind of shut off can I install (and where) for safely unhooking the battery to swap a discharged battery with a freshly charged (charged on the grid at my home with a charger) battery?
-will the bms actually stop the battery from charging if it gets below freezing or is that something I manually need to control?  
Thanks again for all the help!

You might want to use 14 AWG (or heavier when needed) house wiring. That is easy to get and you can use ROMEX, J-Boxes, Wire Nuts, etc...

You fuse/breaker circuits based on the wiring ratings (example, 14 AWG is typically 15 Amps max)--You can use smaller fuses/breakers if you wish (especially if the load/fixtures at the end of the run are smaller AWG wiring). Fuses and breakers are there to prevent wiring from overheating/starting a fire.

The other thing to check, especially for low voltage 12 VDC wiring... You (typically) would want no more than ~0.5 volt drop from the battery bank to the load... Say you run 4 amps for 12 feet @ 12 volts and want a max 0.5 volt drop. Using a simple voltage drop calculator:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=12&phase=dc&noofconductor=1&distance=12&distanceunit=feet&amperes=4&x=0&y=0

Result for 14 AWG

Voltage drop: 0.24
Voltage drop percentage: 2.02%
Voltage at the end: 11.76

If you did the same calculation with 16 AWG wiring:

Result for 16 AWG

Voltage drop: 0.39
Voltage drop percentage: 3.21%
Voltage at the end: 11.61

As you can see--It does not take much current/length to get "excessive" voltage drop on 12 volt circuits.

And that leads us to what AWG for the Charge controller to battery bank... Say you have a 75 Watt panel with Vmp~17.5 volts:

• 75 Watts / 17.5 volts Vmp = 4.3 amps Imp
• Use a 1.25x "safety factor" for solar wiring: 4.3 amps * 1.25 NEC derating = 5.4 amps

Using a simplified NEC wiring capacity chart:
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Using 14 AWG is more than large enough... You can use smaller wiring--But heavier AWG will not hurt. Fusing/breaker for 14 AWG wiring (controller to battery bank) 15 amps or less, but at least 5.4 amps * 1.25 NEC derate = 6.75a ~ 7 amp minimum (to prevent false trips during normal operation of solar charging).

The other thing to look at is the voltage drop from controller to battery bank... Suggest a max of 0.05 to 0.10 volts drop for a 12 volt charge controller (you want the controller to have "accurate" battery voltage"). Using a voltage drop calculator for 14 AWG / 4.3 amps / 3 feet one way wire run for this calculator:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=12&phase=dc&noofconductor=1&distance=3&distanceunit=feet&amperes=4.3&x=45&y=25

Result

Voltage drop: 0.065
Voltage drop percentage: 0.54%
Voltage at the end: 11.935

Again, keeping wiring length short and "heavy" from battery charger to battery bank is critical for good operation (quick/accurate charging).

You typically only need fuses/breakers (combiner box) for solar arrays when you have 3 or more parallel panel connections. The fuse protects a shorted solar panel/wiring from the other 2 or more solar panels feeding the short (check panel specs for series protection fuse rating).

To "swap" the battery--You would want two switches (at least)... One for the solar array, and the other for the solar charger to the battery bus. Always turn the solar array off first, and the controller->battery bank second. And reconnecting, controller->battery on first, and solar array second. The Charger->Battery bank you can use a switched circuit breaker to both "protect" and switch with the same single device (the array side could be a breaker or a simple switch--Your choice if you don't need a combiner box).

There are lots of different BMS designs out there--Some shutdown the battery connection if the voltage/current/temperature is out of spec... Others do less.

Li Ion batteries are relatively new to the solar world--And older controllers may not shutdown charging (or loads if they have LVD--low voltage disconnect--connection).

Foir example, the newer Morningstar MPPT controllers will shutdown charging in cold weather:
https://www.solar-electric.com/morningstar-prostar-mppt-controller-ps-mppt-25.html
https://www.solar-electric.com/lib/wind-sun/MSC_Data_Sheet_ProStar_MPPT.pdf

• Detailed battery programming options allow for advanced battery support for the latest Lithium, Nickel Cadmium, and Lead Acid battery types.

I am certainly no expert in what is out there that supports Li Ion batteries and their BMS modules... You can call our host (NAWS--Link above for controller) or look around for other controllers/suppliers and they can probably give you some options (that controller I found for Morningstar is not a small/cheap unit).

We did not talk about mounting an array farther from the cabin (200 feet away) for better sun--We can go into those details if you want--But basically similar calcustions. Usually an MPPT controller (can take "higher voltage" array) and send the power longer distances with something like 1% to 3% max drop... Just as an example... The above MorningStar controller can take 120 VDC max input voltage. Say we pick 4x 75 watt panels in series with Vmp~17.5 volts and 4.3 amps Imp:

• 4 * 75 Watts = 300 Watt array
• 4 * 17.5 volts Vmp = 70 Volts Vmp-array
• 70 volts * 0.03 max voltage drop = 2.1 volts max "ideal" (can allow larger drop to save copper costs at expense of more wiring losses
• 200 Foot one way wire run... Play with voltage drop calculator:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=70&phase=dc&noofconductor=1&distance=200&distanceunit=feet&amperes=4.3&x=61&y=14

Result @ 10 AWG / 200 feet / 4.3 amps (3x 75 watt panels in series)

Voltage drop: 1.72
Voltage drop percentage: 2.45%
Voltage at the end: 68.28

Result @ 12 AWG

Voltage drop: 2.73
Voltage drop percentage: 3.90%
Voltage at the end: 67.27

So, 10-12 AWG cable and 4x 75 watt panels in series with a (not cheap) MPPT controller...

-Bill