Solar Charger

[flyingsparks]

I have a 20W Solar panel system which charges a 7AH Lead Acid battery. This works fine when no load is attached to the battery.
We are then connecting a electric fencer unit, which is 12V 200mA. This load is running 24hrs / day
The solar panels are not able to keep the battery charged up, and is going flat after a couple of days.

My maths tells me that the electric fencer should be able to run with that battery for 35hrs (ish) without charging it. The solar panels are 20W so input current to the battery of about 1.6A

During the day shouldn't the solar panels re-charge the battery, and then the fencer only really draining the battery at night? It should have topped up the battery again by afternoon?

 

[JohnD]

The solar panels are 20W

electric fencer unit, which is 12V 200mA. ?

Nominally.

Have you checked with a tester what it really is, and how it varies through the day?

I have panels facing due South, and only get worthwhile output between about 10am and 4pm

It rarely reaches the nominal output for a whole hour.

Allowing for clouds coming and going, I see I got 50%-60% for the four best hours on Wednesday, which was quite a sunny day.

Average daily generation (metered) from May to October varies between 10 and 16kWh/day. The panels are nominally 3.4kW peak. So they generate an amount per day equivalent to 3 to 5 hours of peak. So at that rate your nominal 20W panel might generate an average of 60Wh to 100Wh per summer day, and next to nothing in winter or cloudy days. The further North you are, the less you will get.

The actual load, and the actual generation, may be very different to the nominal figures.

 

[bernardgreen]

The solar panels are 20W so input current to the battery of about 1.6A

( John got here before me )

How bright does the sun have to be for the panels to produce 20 Watts ? Probably only for a few hours a day will the panels produce their "rated" power.

If the voltage from the panel ( in less than bright sun light ) drops below the battery voltage then there will no charge going into the battery unless there is a DC to DC invertor to raise the voltage above the battery voltage.

 

[JohnW2]

... The solar panels are 20W so input current to the battery of about 1.6A

How bright does the sun have to be for the panels to produce 20 Watts ? If the voltage from the panel ( in less than bright sun light ) drops below the battery voltage then there will no charge going into the battery unless there is a DC to DC invertor to raise the voltage above the battery voltage.

I agree with all that has been said, but there seems to be another relevant point (hinted at by bernard)...

... even though the solar panel may be capable of supplying a maximum of 20W into an appropriate load, and even when there is enough sun to achieve that potential maximum, it is far from certain that it will deliver anything like 1.6A into a battery it is connected to. What current flows depends upon Ohm's Law (voltage of the battery, voltage of solar supply and internal resistance of battery), not on the maximum capability of the solar supply. By analogy, if a domestic electrical installation has a 'maximum supply capacity' of, say, 80A, that does not mean that 80A will flow into a light bulb one connects to it!

Is your solar panel one that is specifically designed to charge 12V batteries (in which case one would expect it to produce a voltage appreciably in excess of 12V), rather than one which just 'supplies 12V'?

Kind Regards, John

 

[flyingsparks]

I agree with all that has been said, but there seems to be another relevant point (hinted at by bernard)...

... even though the solar panel may be capable of supplying a maximum of 20W into an appropriate load, and even when there is enough sun to achieve that potential maximum, it is far from certain that it will deliver anything like 1.6A into a battery it is connected to. What current flows depends upon Ohm's Law (voltage of the battery, voltage of solar supply and internal resistance of battery), not on the maximum capability of the solar supply. By analogy, if a domestic electrical installation has a 'maximum supply capacity' of, say, 80A, that does not mean that 80A will flow into a light bulb one connects to it!

Is your solar panel one that is specifically designed to charge 12V batteries (in which case one would expect it to produce a voltage appreciably in excess of 12V), rather than one which just 'supplies 12V'?

Kind Regards, John

I checked the voltage from the panel on an afternoon last week - it was sunny but around 5pm - I was getting 18v.

 

[DetlefSchmitz]

Were I in your position, I believe I would measure the current into the battery, using a suitable meter.

 

[JohnW2]

I checked the voltage from the panel on an afternoon last week - it was sunny but around 5pm - I was getting 18v.

Do you mean open circuit? If so, what was the voltage when connected to the battery?

As Detlef has said, you need to measure the current going into the battery - and I strongly suspect that you will find that its a lot less than 1.6A, even in the brightest of sun.

Kind Regards, John

 

[ericmark]

I bought a small one with the idea of maintaining the caravan battery, the unit was useless, it only charges when output voltage is above battery voltage, if connected to a simple resistive load yes 20W, but unless you have an inverter which sets up the voltage at dusk and dawn most of the day it does nothing, the regulators use some power so at 20W count each solar panel as 6 volt and use a simple zenor to stop over charging.