Working on a small 20 W all DC Solar Panel system

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nickless15

2 posts

Wannabe Geek

#195150 8-Apr-2016 05:33

First off, I know this is probably covered in the "Small DIY, Camping/Power Outage Solar System" somewhere, however I just have a few specific questions and I have not been able to find the answers in there or anywhere else for that matter.

Simply put, I a designing a small self-sustainable, off-grid power system in my back yard. I have mostly everything I need to complete my project and I have done the calculations to the best of my knowledge and ability, but I am not experienced or educated in the field of circuitry very well so I would like a second or more educated, final opinion.

What I have:

- 20 W Polycrystaline solar panel (w/ mount and proper positioning data)

- Charge Controller (small and cheap, but should get the job done with this size of system)

- One 10 W LED floodlight

- Programmable DC 12V Time Switch Relay 10A

- Simple 18AH 12V battery (Lead Based)

With adequate sun, I am calculating more than enough power generation to run one 10 W light for 4-5 hours a night so I am confident the system should hold up just fine. The main thing I really am not sure about is whether or not I need a junction box or not? I know the box helps serve a safety purpose, especially when you are joining together multiple panels, but I am not sure if I still need one for this system. I have toyed around with the system a little bit inside and have got everything to run correctly, but i am not sure if I am required to have a junction box outside for projects like this or if I am missing anything else vital in this system. Please let me know if you have any opinions on the system or whatI am doing in general; anything helps!

Thanks to everyone in advance

Aredwood

3885 posts

Uber Geek

#1528152 8-Apr-2016 09:15

Is this intended to be entirely automated? If so then you might want to add low voltage detection to your setup. To prevent the battery from being damaged due to being discharged too much. As solar panels output alot less power on overcast days.

nickless15

2 posts

Wannabe Geek

#1528180 8-Apr-2016 09:47

Sorry, but would you mind elaborating. How would I incorporate it into the system? Am I looking at something like a $10 addition?

spearsniper

133 posts

Master Geek

#1528194 8-Apr-2016 10:02

Aredwood: Is this intended to be entirely automated? If so then you might want to add low voltage detection to your setup. To prevent the battery from being damaged due to being discharged too much. As solar panels output alot less power on overcast days.

Or use a deep cycle battery which is designed for this type of duty cycle.

Car batteries are designed to be almost fully charged all of the time. Their expected life if used in a solar system like this could be only a few years, vs 10-15 for a good quality deep cycle battery.

RileyB

247 posts

Master Geek

#1528271 8-Apr-2016 11:23

nickless15: Sorry, but would you mind elaborating. How would I incorporate it into the system? Am I looking at something like a $10 addition?

The fancier charge controllers have this built in, it will turn off the load when the battery level gets too low. There a lots of cheap "voltmeter" style devices you can add to monitor the voltage yourself, maybe see if there is something that will give an an audible alarm. From memory, you don't want your SLA to get below about 11.8V

I think junction boxes are only needed for joining multiple panels together.I think the main safety thing it to make sure water can't run along the cabling back to the battery/charger controller, by making sure there is a low point in the cabling below where it is connected to the batteries/charge controller. I would also get a inline fuse between the battery and the load/charge controller (assuming you have these both attached directly to the battery), but that's up to you.

Aredwood

3885 posts

Uber Geek

#1528782 8-Apr-2016 21:46

spearsniper:
Aredwood: Is this intended to be entirely automated? If so then you might want to add low voltage detection to your setup. To prevent the battery from being damaged due to being discharged too much. As solar panels output alot less power on overcast days.

Or use a deep cycle battery which is designed for this type of duty cycle.

Car batteries are designed to be almost fully charged all of the time. Their expected life if used in a solar system like this could be only a few years, vs 10-15 for a good quality deep cycle battery.

My understanding is that even if you use deep cycle batteries. They should only be discharged to 50% for them to retain capacity when used in a cyclic situation. Especially as the OP wants to charge them each day via the solar then use the power each night.

elpenguino

3421 posts

Uber Geek

#1529675 10-Apr-2016 22:13

i have a feeling your panel is way too small for your load. Remember, in winter you will only get a few peak hours of sun per day, if the weather is clear. Since your load will run every night, you run the chance of over-discharging your battery.

I will run the data through a spreadsheet i use at work and come back with some information for you.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

elpenguino

3421 posts

Uber Geek

#1532810 13-Apr-2016 23:44

sorry to say , it looks marginal at best for your system as specified. I ran the number based on 5 hours per night : an average of 175mA load each hour for 24 hours.

Remember , when the sun is shining the panel has to run the load and provide charge for battery.

The calcs show that in winter you will not reach a high level of charge so you will be vulnerable to patches of bad weather - this reliability information is based on actual records.

Calcs suggest you want 50W panel , 32 AHr batt for year round reliability.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

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tripper1000

1617 posts

Uber Geek

#1532816 14-Apr-2016 01:31

Not sure by what you are meaning when you say "junction box". Being 12 volts and low voltage, there are virtually no legal requirements to meet, so you don't "have" to have anything.

That said, it would be prudent to have a fuse at the battery (less than 10 amps but more than 2 amps) so that if something shorts out, it doesn't burn the place down. (without a fuse, your battery when new will be capable of putting several hundred amps into a short circuit).

You will want to keep things dry so that your project doesn't die an early death from corrosion.

As someone alluded to above, scrimping on the battery size can be false economy. Out of a deep cycle lead acid battery, you can typically expect to get 100 cycles if you discharge it completely flat every night and 400 cycles if you discharge it to 50%. (read that in a Victron Energy design book, and it is just a rule of thumb which will vary slightly with the quality of your batteries and other factors). This is no reason to ditch the battery you already have, but if doesn't last as long as you wanted, you know to upsize when replacing it.