Basic Solar Battery Charger by a Noob

[TotalMadness]

Hi, I’m just starting with electronics so have a very basic skill level. In order to learn this I decided to build a solar battery charger as my first project.

I have found with solar chargers that if the weather is bad, the voltage coming from the solar cells is half what it would be on a sunny day. So I want to use the cells that come with solar garden lamps in series to generate around 20v on a sunny day. I want to also be able to charge only one 1.2v battery if required. I assume the batteries will have to be lined up in parallel while the solar cells will be in series. In order to make sure the voltage does not exceed the upper charging limit, something will be needed to regulate this based on a setting. Any advice on what I will need in the circuit?

Thanks

 

[audioguru]

I have found with solar chargers that if the weather is bad, the voltage coming from the solar cells is half what it would be on a sunny day. So I want to use the cells that come with solar garden lamps in series to generate around 20v on a sunny day. I want to also be able to charge only one 1.2v battery if required. I assume the batteries will have to be lined up in parallel while the solar cells will be in series. In order to make sure the voltage does not exceed the upper charging limit, something will be needed to regulate this based on a setting. Any advice on what I will need in the circuit?

I have many solar garden lights so I know about them.
The cheap Chinese old fashioned Ni-Cad battery cells last only about 9 months.
The cheap Chinese solar cells last only about 2 years with the output power dropping all the time.

A solar garden light has a small solar cell that produces almost no power when it is cloudy. You talked about its no-load voltage but I talk about its power. The small charge in the battery lasts for only 1 hour driving one dim LED. When sunny in summer, the charge lights one LED fairly bright all night long.

The solar panel has four 0.5V cells so its max voltage is 2V. It charges one 1.2V Ni-Cad battery cell until its voltage is about 1.4V when fully charged. The solar garden light uses a voltage step-up circuit to drive a white or multi-coloured LED at a fairly low current.

A solar panel from a solar garden light is small so it can charge one 400mAh Ni-Cad cell all day long when it is sunny in summer. Its current is too low to charge cells in parallel. Adding solar panels in series does not increase the current. The current is so low that a battery cell is not overcharged much.

 

[TotalMadness]

Thanks for the info.

I think I will need to keep this simpler for the moment and build from there. Going by the information you have given me, I think I will charge two 1.2v batteries in series to start with. I will use two sets of two panels in parallel.

The circuit I will use will be as in Circuit 1 attached.

As I understand it, the diode is used to prevent the batteries discharging to the solar cells. However, at this time I have no idea what the value for it should be.

 

[audioguru]

Two 1.2V Ni-Cad or Ni-MH cells in series are about 2.8V when fully charged. The parallel sets of solar panels will probably produce too much current at noon in summer on a sunny day so the battery cells will be destroyed.

If you use more voltage then you could regulate the current from the solar panels then the battery will not over-charge on a sunny day but will charge on a cloudy day.

The diode used in my solar garden lights is a 1N5817 Schottky diode. It has a low voltage drop and its max allowed current is 1A.

 

[TotalMadness]

I finally got round to building the circuit and took some voltage measurements. What I don’t understand is why there is a drop in voltage once the batteries are connected:

Battery: 2 X 1.2v Ni-MH 2000 mAh. Written on the battery: Charge before use 7 hours – 480mA

The solar panels appear to be able to produce 2.35v when held next to a 100w room light bulb.

The testing was done on a desk at night with the room lights on, no sun.

Battery disconnected:

A – B 1.83V
E – F 2.98v
G – H 2.98v
G – B 2.95v
D – B 2.95v
C – B 2.91v

Battery connected:

A – B 1.83V
E – F 1.93v
G – H 1.93v
G – B 1.93v
D – B 1.93v
C – B 1.83v
D – C 0.1v

My questions are:

1. Why the drop in voltage when the battery is connected?
2. How do I measure the amperage?

I tried measuring the amperage across the diode (which came from a garden solar lamp) and got the reading of 0.03 with the multimeter set to 20m. I assume the reading would be 0.03mA.

 

[audioguru]

Solar cells use powerful sunlight not the small amount of light from a desk lamp.
That is why the power (voltage times current) from your solar cells is too low to charge your battery cells.

You do not measure the amperage "across" something. You measure the voltage across something (in parallel) and measure the current in series with the circuit.

 

[TotalMadness]

So are you saying that the voltage drop I measured happened because when the load was connected (the batteries), the power was not enough, even though the voltage I measured when the load was disconnected, looked high enough. In other words, creating the illusion that there was enough power?

 

[audioguru]

You never measure the voltage of a weak power source with the load disconnected.

I have a cheap AC-DC power supply (wall-wart) that is rated at 9V/100mA. It measures 9.2VDC when its load is 100mA.
But its voltage is 17.4V without a load.

 

[TotalMadness]

Thanks, I understand.

I took the charger out into the sun yesterday and managed to charge the batteries. Before I blew the multimeter fuse, I got a reading of 187mA. The voltage was 3v which charged the batteries to 2.72v at which point I decided to stop.

Later today I will go out and see what the readings are because it is an overcast day, so that I know how many solar panels I will need to charge with that amount of light.

I will also now start to see how I can regulate the it so that the batteries don't blow.

 

[audioguru]

A rechargeable battery gets hot when it is overcharging.

Just think about the battery in a solar garden light that is in the hot sunshine all day and has a very simple charging circuit that does not limit the current. The current is high enough so that the battery charges on an overcast day so the over-current is very high on a sunny day.

My solar garden lights have a battery that lasts about 9 months. It is not destroyed by over-charging, it is destroyed by rust because the battery has a cheap steel case instead of expensive stainless steel. The name-brand rechargeable battery cells I use as replacements cost as much as an entire solar garden light (thee bucks each) but they last nearly forever and are rated for the high charging current.

 

[TotalMadness]

Why are rechargeable AA and AAA batteries 1.2v instead of 1.5v?

 

[audioguru]

Their chemistry determines their voltage. They are 1.4V to 1.5V when fully charged when they are are becoming over-charged.
Most Lithium rechargeable battery cells are 3.7V. They are 4.2V when fully charged.

 

[TotalMadness]

I have come across a circuit diagram that shows a simple way to hold the voltage at a certain level. I now use two rows of four solar cells in parallel to produce the current. An image of the circuit is attached below.

With the circuit drawing came an explanation on how to calculate the values of the components. At this point, I am not sure whether I would still need a diode to stop the battery discharging back to the solar cells. Any thoughts on what I might have missed would be appreciated.

In future, when I have the knowledge of how to, I plan to add an LED that lights when the batteries are charging, and an LED that lights when the batteries are fully charged,

 

[audioguru]

Your "battery charger circuit" limits the current because the solar cells do not produce much current. It does not measure if the cells are fully charged so it will overcharge cells on a sunny day in summer. Overcharged Ni-MH cells do not last long.

But maybe you are lucky that cheap Chinese solar garden lights use a cheap 400mAh to 600mAh Ni-Cad battery cell and a cheap little solar panel that produces only 60mA at noon on a sunny day in summer and less current when the battery is charged. Then a 2400mAh Ni-Mh cell can tolerate the overcharging because normally it is charged at 240mA.

 

[TotalMadness]

I have now been able to test the circuit posted previously. I did this in the sun and with batteries as the power supply. It was quite interesting. When using the batteries to charge, there was much less current. When using the the solar panel, the current was around 50mA in direct sunlight around 2pm. The max voltage under load appears to be 5v from the solar panel. It was interesting watching a cloud come over and seeing the voltage very little, but the current going right down to around 3mA.

This project has become R&D for me. I am learning a lot from this. I would now like to add an LED to show that the circuit is charging (On/Off light); an LED to light when the battery pair are fully charged (2.7v); and somehow cut the supply to the battery pair when they are charged. I still need to figure this out. I am guessing that an SCR may appear in the circuit.

 

[TotalMadness]

Ok, this is the final design for this build. The circuit is not perfect, but I have a much better understanding now how this all works and how important current is in all this. I've decided that I will buy some solar cells off eBay for the next project that I will start in a new thread for.

Thanks audioguru for all the help.

 

[audioguru]

A 3.6V zener diode changes its voltage when the temperature changes and the same for the emitter-follower transistor.
The "regulated" output voltage will be about 2.9V which will fully charge two Ni-Cad or Ni-MH cells in series if the sun never sets at night.
It might take a few days to fully charge two low capacity AA cells or might take a couple of weeks to fully charge high capacity AA cells.