# I want to design a single led to be powered by 4 garden-sized solar cells

#### bejingie

Dec 11, 2021
12
I want an all night solar led to be powered by 4 solar cells with 4 batteries in succession. That means when the first solar cell/battery gets near too low to power led, the circuitry switches to the second solar cell/battery and so on for all 4 batteries. The purpose is to be very sure I can get a full night's led light on even though the solar charge on some days may be weak, and only strong enough for 3 hours on a single battery. The alternative to this plan is to use the single led with a larger solar cell and larger storage battery. Maybe I am saying too much here and should just say I am unhappy with my little garden type led going out before dawn.

#### bejingie

Dec 11, 2021
12
I should say I am new to this forum and want to give back. I will try to offer comments in other threads. I am an EE with interest in hobby electronic circuitry.

#### WHONOES

May 20, 2017
1,215
You could just diode "OR" them then the battery with the highest voltage will take over.

#### AnalogKid

Jun 10, 2015
2,688
OR-ing the batteries gets you a longer, slower fade-out, but that still is not optimal. The issue here is holding the LED at a near-constant brightness while the battery voltage decreases. For best battery life overnight, this should be a switching circuit, maybe like a joule thief circuit, although you could start with a more simple linear circuit just to prove the concept.

What is the desired LED current, and what is the LED Vf at that current?

ak

#### Audioguru

Sep 24, 2016
3,650
A cheap solar garden light has a small solar panel that produces a low current and a poor quality battery that holds only a small amount of current. Then the LED lights dimly only for a few hours.
Instead use a solar garden light with a larger solar panel and with a higher quality battery.

Solar garden lights use one 2V solar panel and a voltage stepup IC. Then the solar panel produces 2V and the single battery is 1.2V.

#### bancroft

Apr 23, 2021
76
I have the same idea recently, and I am practicing it recently

#### bancroft

Apr 23, 2021
76
I think you can choose TFT. I've used it before. It's really good

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,884
I think you can choose TFT. I've used it before. It's really good
TFT? Quite a bit off-topic, don't you think?

#### bejingie

Dec 11, 2021
12
I am still on my quest for a better garden light but I have lots of other projects also. In this topic, last week I decided to go to a single Lithium 3.7 volt battery with 3 leds at 20 ma. I put it all on a cds solar control to only come on dusk to dawn. I do not have any solar cell to charge it, I wanted to find out how many days the single battery would last.
I just finished my fourth night at no perceived diminished brightness. The device is enclosed in a large pickle jar on my patio. The light produced casts light into my dark kitchen at night, it is so strong. I will report how many nights it lasts before needing re-charging. Then I need to design a solar panel to charge a 3.7 lithium 18000 mah battery.
bejingie

#### Martaine2005

May 12, 2015
4,399
Has the battery got a cut off circuit?.
If not, you risk damaging the battery if you discharge it below approximately 3V.
So don’t wait until the LEDs start dimming.

Martin

#### bejingie

Dec 11, 2021
12
wow. Thanks. I didn't know that. I'll investigate right away.

#### bejingie

Dec 11, 2021
12
I brought the LED battery-powered pickle jar inside. Here are my readings:
• 3.7 lithium battery nominal 18000 maH, (old battery, probably less.)
• 4 nights outside on the patio as of now, maybe 4 x 11 hours of light, 44 hours.
• CDS cell circuit keeps LEDs off during the day
• The voltage at beginning of test was not measured, but was freshly charged to 4.2 v before being removed from charger.
• Readings now, using Fluke DVM, 83III:
• Across battery terminals, LEDs on bright, 3.685 v
• " LEDs off, 3.689 v
• Battery removed: 3.689 v
• Current with LEDs on: 24.11 ma
• Current with LEDs off: 00.15 ma
• I will put the jar back outside now for more nights' testing. I wonder how many days this can work. I wonder why there does not seem to be any 3.7 v garden lights that could go a week all night long without the sun charging them. Maybe I have not looked hard enough. ha.. The next step is to design the drop-out circuit that Martin said I need and then design a solar LI battery charger.

#### AnalogKid

Jun 10, 2015
2,688
I wonder why there does not seem to be any 3.7 v garden lights that could go a week all night long without the sun charging them.

Easy: Waaaaayyyyy too expensive.

Also, excellent data collection and reporting. Please post your circuit. There probably is a way to reduce the 150 uA daytime current.

ak

#### Audioguru

Sep 24, 2016
3,650
Didn't you do the simple math?
Three LEDs at 20mA each use a total of 60mA.
A huge dangerous (explosion and fire if not charged correctly) 18000mAh Lithium battery.
18000/60= 300 hours which is 25 nights of 12 hours each.

Looking for huge 3.7V rechargeable batteries, I see that most AA size are 800mAh and cost about $10 each. For 12 hours each night for one week you need 84 hours and three LEDs at 20mA each use 60mA. Then the battery must be 60mA for 84 hours which is 5040mAh. Your guess of 18000mAh is too high. 5040mAh needs 5040/800= seven of the 800mAh AA batteries at a cost of$63.

#### bejingie

Dec 11, 2021
12
Audioguru,
Thank you and please allow me to get back to my workbench later today. I had a terrible typo above, battery is 1800 maH and not 18000!

#### Audioguru

Sep 24, 2016
3,650
Your 1800mAh battery is probably a 18650 size which is a little larger than an AA.
It is not new but if it is new then it will power three LEDs at 20mA each for 30 hours (2.5 12 hour nights).

It needs a circuit to disconnect it when its voltage drops to about 3V and it needs a charger that is made for it.

#### bejingie

Dec 11, 2021
12
Audioguru, AK, and Martin,
Here is my circuit as described earlier, powered by a Lithium Ion battery at 3.7v nominal and dropping volts over time.

To sum up - I am trying to design a long-lasting solar light for my patio that will last in full brightness from dusk to dawn, and a bonus if it goes on for days. I will add a low voltage drop-out later and add then a solar charger circuit. I don't like the cheap dusk-to-2am solar lights that are in the stores.
This is the exact circuit drawn with the EveryCircuit app. I pulled the three parallel LEDs from a flashlight. Their little led holder board had no electronics but did have a 10 ohm resistor that I kept. The upper resistor in this simulation represents the CdS light sensor. This particular image represents dark conditions and shows the circuit at 24.3 ma, 8.07 ma for each led. I have the LEDs set 120 degrees apart on a horizontal plane in my test pickle jar and the light is fine for my purposes. The CsD sensor changes all the way down to 400 ohms in high sun and up to 400k ohms in the full dark.

The above image resents the ohms in normal daylight. The "sleeping" circuit draws about 0.15 ma (0.16 in simulation).

This image is a mystery to me. It represents the simulator's math in that I should get 92 ma draw above 400k ohms. On the bench, and not connected to anything, the CsD sensor easily reaches 400k ohms when I cover it with a dark, dense, black towel. In fact, it can reach 1 meg. But, if I cover my actual circuit's CsD sensor, the Fluke measurement does not exceed 29 ma, in extreme darkness. A head-scratcher. But, for me, that is a relief for I don't want more than 29 ma at the battery. So, the extreme simulated condition shown above with LEDs at 30.8 ma can not be duplicated in the actual circuit.

Now, for your comments about my battery. I did not give the details and I confess I was not too educated on Li-Ion cells. The Li-Ion cell I am using was salvaged out of a Asus Chromebook. It is one-half of what the specs say is a 7.4 volt, 4400 maH battery. I separated the two halves and have now two Li-Ion flat batteries. I assume the longevity of operation I am experiencing is due to that massive maH spec of 4400. Previously, above in this thread, I thought all such cells were 1800 maH. (I checked on eBay, and very similar flat cells are going for about \$15 each.) It is about 3 1/3" square and 3/16" thick. It is bendable slightly and indicates by bend lines that there are 4 cells in each half, obviously wired in parallel and brought out to two wires providing 3.7 volts for each half. Each cell seems to be about the size of a stick of gum.

on edit - the transistor is a 2N2907 PNP.

Last edited:

#### bejingie

Dec 11, 2021
12
fyi: I separated this into two flat batteries.

#### AnalogKid

Jun 10, 2015
2,688
With nothing connected to it except an ohmmeter, measure the CdS cell resistance out on the porch at the ambient brightness level that you want to turn on the LEDs.

ak

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