# Need help with designing large solar "garden lamp"

#### mourits

Joined Jul 2, 2014
1
Hi Guys, I am looking for someone who can help me to design a large garden solar lamp.

I have 8 panels I want to put together (can only use these because of dimensions), each panel has a peak power of 18W, Voltage 19.4V, Current 0.92A and an Open Circuit Voltage of 23.7 Volt.

I want to build a larger lamp by using 25-30 high bright LEDs, each approx 120 lm.
I need a simple circuit with integrated controller and the light should go on when starting to get dark and go off after 6 hours.

I need the calculation of the system, leds and battery plus the circuit/board.

Who can help me? Willing to make a small contribution

Michael

#### wayneh

Joined Sep 9, 2010
17,189
What sort of batteries do you plan to use? Also, what are the LED specs for Vf, current, and wattage?

Last edited:

#### ErnieM

Joined Apr 24, 2011
8,346
Everyone seems to work these problems backwards. You start at the load, use that to determine what battery is required, then spec the solar array to give sufficient power to recharge the batteries in a timely mannor, alloting for coouldy days, or summer or winter. Summer is longer andf brighter, buit also hotter so the cells are less efficient. Winter is shorter and not as bright but also cooler so the cells work better.

At a peak power of 18W per each of 8 panels you have an input power of 144 watts. Say you are charging a 12V lead acid cell, that's a current of about 144/13.6 = 10.5 amps. You are good for charging a a00 AH battery at a C/10 rate, quite safe, or even a 200AH at a C/20 rate, still good. Extra capacity is best for extra long life.

I work on a solar system where we use marine deep discharge batteries, similar size to a car battery. We use two 100AH batteries in parallel for a total of 200AH.

Wanna guess how long you get that rate based on your latitude? You can look that up, and then toss in a factor for cloudy days. Say you are good for 6 hours. Then you are charging 10.5A for 6 hours, or 63 AH (Amp-Hours). You don't want to use all of that every day to allow for cloudy days. Let's guess 75%

That gives you 63 AH * .75 = 47.25AH. For your 6 hour run time you get 47.25AH / 6H = 7.8A. With the battery terminal voltage at 12.5 volts (discharge is less) you have 12.5*7.8A = 97.5W of useful power for your LED lights.

Drawing 47AH out of a 200AH battery is acceptable as it's about 25% of capacity. Less is better; the extreme is the US Coast Guard which uses no more then 1/30th the battery capacity as the daily load.

#### THE_RB

Joined Feb 11, 2008
5,438
...
At a peak power of 18W per each of 8 panels you have an input power of 144 watts. Say you are charging a 12V lead acid cell, that's a current of about 144/13.6 = 10.5 amps. ...
...
Minor nitpick, your calc assumes MPP (144W) which won't be the case for direct connection to the battery.

He said each panel makes "19.4v 0.92A" so at direct connection to a 13.6v battery will not be much more than 0.92A, maybe 5% more, so will be 0.92A*1.05 = 0.966A.

Times 8 panels will be 7.73A into the 13.6v battery.

#### NorthGuy

Joined Jun 28, 2014
611
Design is simple.

You have panels, batteries, a charge controller that limits the voltage into the batteries. Then you connect your LEDs to the battery with some kindo of LED driver which would limit current into LEDs.

You can use a timer to turn your lights on and off. Or you can use your panels to detect light level, which may not work well when it's gloomy and cloudy.

IMHO, this all is best organized with MCU and few discrete parts.

Since it's outside, it's important to know your weather. In cold weather batteries may not work well, some may even freeze. High temperatures are not good for most batteries neither. So, if you have extreme, the design might be challenging.

#### ErnieM

Joined Apr 24, 2011
8,346
Minor nitpick...
I was just looking for some order of magnitude for him, and yeah I assumed MPP (100% efficient too).

Thanks for checking my work.

#### THE_RB

Joined Feb 11, 2008
5,438
I live to serve.