# Simple Solar Powered LED Circuit

#### Zaper

Joined Aug 17, 2010
43
I'm trying to work on a small project for on of my relative's. Bear with me please because I really new at this.
Basically I'm trying to copy to solar night lights that people have around their gardens or sidewalks. So far I've come up with using a 3V 45mA solar cell to power 2 3.8V 20mA LEDs. Because the main purpose is that this will light up at night I thought that I would use the cell to charge 3 3.2V 400mAh batteries during the day and then use the batteries to power the LEDs at night. Since the 3 batteries will be 9.6V total and the 2 LEDs will take 7.6V I believe that I'll need a 100Ω resistor in there as well.

My question is this: Is any of this even close to being correct? And if so will solar cell be able to power the batteries sufficiently and will the batteries be able to keep LEDs going through at least most of the night?
Thanks to anyone that can lend any advice.

#### iONic

Joined Nov 16, 2007
1,650
A single 3V, 45mA solar cell can not charge a 3.2V battery unless the battery voltage is less than 3V. And once you get to 3V you will no longer be charging it. Thus, the maximum voltage for the three batteries in series would be 9V Max. But to add to your troubles, the solar cell is likely not to produce a constant 3V and 45mA so your 3.2V batteries will always be under charged, even less than 3.0V/cell. Also, your gonna need more than one of those solar cells to charge three batteries. I'll try to find a couple of solar lawn light schematics for you to reference so that you can get a better idea of what is required to build one. You may find it will be much quicker and even cheaper to buy one.

SolarLight

#### Zaper

Joined Aug 17, 2010
43
Thanks for the help. Some schematics would be awesome. This was kind of just me looking for somethin to do, but I'm finding that I'll be lucky to make an OK light at the same price as buying a regular one.

#### Audioguru

Joined Dec 20, 2007
11,249
Nobody makes a 3.2V rechargeable battery.

A solar garden light is powered from ONE 1.25V Ni-Cad or Ni-MH rechargeable cell. It is charged from a 2V/80mA solar panel that limits the charging current and the battery cell limits the charging voltage.

The LED is powered from a voltage stepup circuit (so it can give 3.5V to a color-changing, blue or white LED). The voltage stepup circuit limits the LED current to about 10mA (not very bright). In the middle of summer the LED glows for about 6 to 8 hours following a sunny day.

There are many Solar Garden Light circuits in Google.
My electrical utility company gave away solar garden lights for free. I got 10.
You can buy a solar garden light for $2.00 or make one for about$10.00. The ones you buy usually rust away within one year. The cheap solar panels get sunburned.

#### wayneh

Joined Sep 9, 2010
16,390
Thanks for the help. Some schematics would be awesome.
You might like to visit this thread.

...I'm finding that I'll be lucky to make an OK light at the same price as buying a regular one.
Lucky? Yes, like winning the lottery. Unless you already have the parts and value your time at zero, you have no chance against the commercial options. Around where I live, if I watch the sales, I can pick these lights up for ~$0.50 apiece. That's less than the battery inside would cost on its own. And just FYI, the battery is usually the problem when these lights fail. Bad connection, too old, whatever. Ant infestation is my second leading cause of failure - their nests rot the circuit boards. The leads on the LED are often the first thing to go. #### nickelflipper Joined Jun 2, 2010 280 Here is the 3.2V lifepo4 rechargeable battery. Walmart sells them too. I had to take back two 4-packs of the Walmart cells, because only two of the eight were good.The lifepo4 cells have very low self discharge, and a flat V vs. I load curve. A mini solar panel (5.5V 100ma) works well for charging the 3.2V cells, if the battery voltage is monitored (lm393 or micro), and a high side pfet switch/shutoff employed. #### wayneh Joined Sep 9, 2010 16,390 Here's another simple way to limit charge current once the battery reaches a given voltage. The target voltage is set by D2 (zener) and R2+R3. At the target voltage, the transistor turns on and starts dumping current through itself and R1. #### Audioguru Joined Dec 20, 2007 11,249 Solar garden lights do not use an expensive and dangerous Lithium battery cell that needs a charging and discharging circuit to prevent a fire. Instead they use a cheap and safe Ni-Cad or Ni-MH cell that simply charges from the solar panel without a circuit. Solar garden lights do not throw away battery power with a shunt voltage regulator. They do not have and do not need a voltage regulator and do not have and do not need a current regulator. Instead their circuit is simple and cheap. #### nickelflipper Joined Jun 2, 2010 280 At around$2-2.50 a cell the 3.2V LifePo4 cells are on par with nimh for cost. LifePO4 is safe, and have run a cell unregulated for over a year, (up to around 5.5V) no flames yet. LifePo4 can't cause a fire as I understand it, because of the chemistry. Westinghouse uses them in their higher end solar landscape lights. The negative to unregulated voltage is reduced capacity and cycle life. My unregulated cell has a very short run time now.

Agreed, using a zener in a low voltage, low capacity battery charger is not a good idea. Something like 5-10ma standby current. Lm393 standby current can be around 1-2ma, and a micro is less than that.

#### Pipes59

Joined Nov 7, 2011
4
Here's another simple way to limit charge current once the battery reaches a given voltage. The target voltage is set by D2 (zener) and R2+R3. At the target voltage, the transistor turns on and starts dumping current through itself and R1.
View attachment 36188
Not sure here but just looking at the circuit. Seems like it would indeed keep the battery voltage from receiving more then its nonimal voltage, however, would it not also impair the charging ability from giving most available power charge while solar cell has ample sun?

#### wayneh

Joined Sep 9, 2010
16,390
Solar garden lights do not throw away battery power with a shunt voltage regulator. They do not have and do not need a voltage regulator and do not have and do not need a current regulator. Instead their circuit is simple and cheap.
I generally agree. All the simple solar lights I have looked at have no active control over charging. The circuit I posted was from a solar light SET (separate panel and LED string) that used a 4v SLA.

#### wayneh

Joined Sep 9, 2010
16,390
Not sure here but just looking at the circuit. Seems like it would indeed keep the battery voltage from receiving more then its nonimal voltage, however, would it not also impair the charging ability from giving most available power charge while solar cell has ample sun?
As long as the battery is below it's target voltage, there's virtually no drain. The panel does have a higher open circuit voltage, but the battery makes sure that is not seen in the system. The transistor is open (not shunting power) until something like a milliamp is flowing through the zener, which is plenty to bring it well into range, probably 4 or 5v in that circuit.