Testing Solar panels

[Close hauled]

I've been given an old solar panel which is showing a bit of de-lamination so I decided to 'test' it by connecting a voltmeter across the o/p. Today wasn't very sunny but I was getting about 23V.

My question is this - 'Is this (open circuit voltage of 23V) an indicator that the panel is OK, or do I have to connect it to a battery? If I do have to connect it why is this necessary?

Thanks.

 

[Anonymous]

It's good news that you are getting a good voltage and in all probability you have got a perfectly good panel but to prove that you need to see what current it can deliver into a load. You could use a resistor but that gets very messy and complicated - if you use a battery you can measure the current that you are actually getting into the correct load.

 

[pappaecho]

23 volts is very good - most panels deliver 17-18 volts open circuit. You need to fit a "blocking diode" which prevent reverse discharge from the battery to the panel at night... the panel might have one built in, so connect to a battery and monitor the voltage. If the voltage declines overnight then you may need a diode. Maplin sell them for pence each

 

[mhouse]

The quick way I use to test is to try various bulbs, that way you know the output depending on the light conditions.Start with the largest first.

 

[William_H]

Yes it is possible to measure a voltage even though there is a very high resistance in the wiring or connections because your voltmeter needs very little current. A quick though brutal test is to connect your multimeter on highest current range across the terminals in the sun. I am guessing you are talking a 10 watt panel which should give around 200 to 500 milliamps. .2 to .5 amp. However the best test is to connect it to your car battery and you should get similar sorts of current. Don't panic if you only get 20 milliamps when connected direct to the meter or to the battery that could be through lack of sun. Even inside under a flourescent light you should get a few ma. regards will

 

[tsmyth]

I presume that some solar panels have a diode alredy built in somewhere. I bought a 5 watt to trickle charge the battery, but nowhere on the packaging doe it mention a diode. Silly question, but how can I test to see if there is already a diode in the circuit?

I believe that some diodes have a lower voltage drop than others.

I presume that with a 5 watt panel I do not need to use a regulator.

Ray (of sunshine)

 

[Anonymous]

[ QUOTE ]
I presume that some solar panels have a diode alredy built in somewhere. I bought a 5 watt to trickle charge the battery, but nowhere on the packaging doe it mention a diode. Silly question, but how can I test to see if there is already a diode in the circuit?

[/ QUOTE ]Easiest is to phone the supplier but if not, put a multimeter in series on the amps range and see if the current flows backwards (i.e. negative) when there is no light (put a blanket over the panel?).[ QUOTE ]
I believe that some diodes have a lower voltage drop than others.

[/ QUOTE ]Yes, that's true. Schottky diodes have the lowest volt drop but most solar panels have plenty of open circuit volts and in a simple installation any silicon diode rated for the current would be fine. [ QUOTE ]
I presume that with a 5 watt panel I do not need to use a regulator.

[/ QUOTE ]Depends. If you are in the Med in summer with no load, then that's half an amp which would make your cells fizz (if flooded lead acid) or could damage them (if sealed). On the other hand, in the UK in winter, you should be fine. The problem with answering this is that people will say "I've been doing that with a 5, 10, 20, or whatever panel for the last n years with no problems". There are circumstances where there would be no damage and others where there could be so the safe answer has to be 'use a regulator'. For example, some people have a standing leakage current that they are not even aware of (e.g. radio-cassette), or they have very tolerant batteries, or are in a shady area. Solar panels can have quite high open circuit voltages - 18V or higher - so there is a potential to do damage.

 

[William_H]

I am one who doesn't believe in regulators for solar panels. This is based on a lot of sunshine here. it is based on the premise that a little boat will only have room for na small solar panel and small battery while a big boat with a big panel will have big batteries.
As for diodes I have tested several panels by using a milliamp meter in line from battery to panel when in darkmess with no discernable current and no discernable diode. It is very easy to test however. regards will