NEED HELP-Question About LED and Solar Panel

Thread Starter


Joined Dec 9, 2009
guys im doing some project with solar panel,led and rechargable battery

situation-i want make a light box using 40 white led with solar panel and rechargeable daylight the solar panel will charge the battery and by night it will on the led.

all 40 led is in series with 10 of then per row ( mean got 4 row x10 led each) inside 2feet x 2feet box.

im trying to use 12v rechargeable battery to light up 40 led at night.

my problem is i dont know if 12v battery can light 40 led for estimation i need higher rechargeable battery voltage or something??? or do i need to reduce LED amount by using super bright white LED because i read that super bright LED using higher voltage...

please give me some idea...i know its simple project but for newbie like me im totally out of idea..


Joined Oct 7, 2009
please check the current requirement for the LED to glow and you will have to use the battery of that current capacity.
simple example if your LED needs lets say 10mah
so 40*10=400mah so you require 400mah battery to light all the leds per 1 hour
so capacity of the battery will be based on your requirement of how many hours you want to glow the LEDS.

Thread Starter


Joined Dec 9, 2009
wow ok ok thanks alot big guy..its really help me....

hurmm so how bout the solar panel that need to charge up the thre any calculation for what kind of solar panel need to use???

thanks thanks alot


Joined Dec 2, 2009
19 long as you are not overcharging, you should be good....does the battery have a charger? what output is it? we need more info.

Duane P Wetick

Joined Apr 23, 2009
My reading lamp uses 42 white LED's; 14 strings of 3 + 150 ohm resistor in each string, total current 242ma @ 12 VDC., ~3 watts. So, to keep this lamp on for 12 hours, 12 x .242 = 3 AH (Amp-Hours) is required from the battery. Your solar panel must provide this each day to re-charge your battery. Now assuming 20% sun over each day, 3 AH / .20 = 15 AH will be required daily to keep your battery charged.
Can your solar array deliver this amount of power in a regulated form that will charge the battery?

Regards, DPW [ Spent years making heaters out of op-amps.]
Last edited:


Joined Dec 20, 2007
First you had 40 LEDs in series.
Then you had four rows with 10 LEDs in each row.
Current-limiting resistors were not mentioned.

White LEDs are about 3.3V each so 10 in series need 33V plus maybe 3V for the current-limiting resistor for each row. They won't glow with a supply of only 12V.

The battery has a capacity printed on it. It is in mAh or in Ah. What is it?
The solar panel has a rated voltage and a rated current. What are they?

Like Duane says, connect three 3.3V white LEDs in series and in series with a 150 ohm current-limiting resistor to make one row that will light with 18mA from a 12.6V rechargeable lead-acid battery. Since you want 40 LEDs then make 13 rows from 39 LEDs. Each row needs its own 150 ohm resistor. The 13 rows draw 18mA x 13= 234mA.
The battery must be rated for at least 234mA for 10 hours which is 2.34Ah.

If you have winter then the solar panel must be huge because in winter the sun is low in the sky, is there for only a few hours each day and doesn't work when cloudy.
You need a charger controller to prevent the solar panel from over-charging the battery in summer.

Of course the solar panel must be outdoors (not behind glass) pointing at where the sun might be at noon. It is best to have the solar panel follow the sun across the sky and change its elevation for each season.


Joined Jul 17, 2007
More on battery capacity:
If you discharge a lead-acid battery completely, it will only last for about 250 charge/discharge cycles.
If you discharge a lead-acid battery 50%, it will last about 550 cycles.
If you only discharge it 30%, it will last around 1,200 cycles.

Basically, the less deeply a battery is discharged, the longer it will last.

So, if you need 2.34AH to keep your LEDs lit for 12 hours, divide that requirement by .3 to get 7.8AH. A 7.8AH battery would have much longer life than a 2.34AH battery.