What diode for my 20w solar panel

[BlueChip]

I'm fitting a 20w solar panel for trickle charging a 12v battery in Spain.
It's just a bare panel with no diode, what spec diode should I fit, which line should it go in, and how do I tell which way round to fit it?

 

[VicS]

Your regulator should contain the necessary diode.

Otherwise a silicon rectifier diode rated at 2amps or more eg 1N5400 ( 3 amps and 50 volts PIV)

The cathode faces the battery positive.

A Schottky rectifier diode will introduce a smaller voltage loss.

Don't forget a fuse close to the battery connection!

 

[BlueChip]

I dont have a regulator

 

[philip_stevens]

As VicS says, it should already have a diode in circuit with the positive lead.

If not, have a look at this Wiki, and it will show the way to connect one.

For a 20watt panel, a five amp Schottky would be ample, fitted in the positive lead with the anode/cathode in line to the battery.

Even with a 20watt panel, it may be best to fit a small regulator - especially with Spanish sun.

 

[VicS]

I dont have a regulator

A regulator is generally advised if the solar panel exceeds 1 watt per 10Ah battery capacity or for panels in excess of 10 watts .

Your panel exceeds 10 watts it is therefore advisable to fit a regulator.

But see my edited original reply

 

[BlueChip]

Thanks for those useful replies. I'll only be leaving the panel connected for a week or so at a time so I doubt I will overcharge the battery.

 

[ghostlymoron]

Thanks for those useful replies. I'll only be leaving the panel connected for a week or so at a time so I doubt I will overcharge the battery.

In Spain that will be quite enough to fry your battery. A simple regulator only costs about £10 and will protect your £100 battery. Plus, the indicator lights will show you what the panel is doing.

 

[William_H]

Solar

As said any silicon diode of a few amps capacity will suffice. The panel will give about 1 amp for about 9 hrs a day or 9AH.(Without a regulator)
So as said with a smallish battery you might need a regulator. If it is a battery with filling caps you need to check fluid level as first sign of overcharging is loss of water.
However if you use the panel only when you are on board and are regularly using more than 5AH per day then a regulator is not required. Indeed you may get more charge from the panel without a regulator but of course you must monitor the charge yourself.
A typical regulator reduces the voltage from the panel from 20v open circuit to 14volts which is presented to the battery. The current that goes into the battery depends on the state of charge of the battery and the difference between the regulated supply 14v and the inherent voltage of the battery. So a regulated panel will give full current into a flat battery but as the battery charges the inherent voltage rises so the charge current falls. Ultimately a battery never reaches 100% charge. Hence the stepped voltage regulators used in chargers and alternator controllers which raises the voltage over 14 until the batteries sensed to be charged when it reduces the voltage.
Now a solar panel without regulator can produce 20v no load so voltage it is trying to supply is always way more than the battery inherent voltage so the panel pushes in all its power (albeit quite small) regardless of battery charge state. Great for max charge but it can if the panel is big enough or battery small it can cook the battery.
Just to explain a solar panel has an internal resistance. This means that the more current that is taken from the panel the lower the voltage measured at the panel output is. So at not load you measure 20v if you short the panel out (not connected to a battery) the terminal voltage goes to zero and the current flowing is the max the panel can supply. ie 1 amp for 20w panel. No harm is done because the panel dissipates the heat in itself. Small compared to suns heating. When you connect the panel to a battery the voltagee becomes that of the terminal voltage of the battery the current flows depending on difference between what the panel wants to produce and the battery voltage. Hence you can connect a 12v panel to a 6v battery without much harm. Except for overcharging the 6v battery eventually.The current will still be 1 amp for 20w. This all explains how MPPT (max power point tracking) regulators determine the voltage and current that the panel can best produce most power in the circumstances (sun level) and use the power at that rate so often producing 15% more power than straight connection.
Put another way a 20w panel will produce 1 amp max. at 20v. If you force it to 13v of a battery you are only getting 1amp times 13v or 13watts. (7 watts is lost in the panel itself) If you take 20v and transform it down to 13v by making it AC then back to DC you can calculate 13 v into 20w gives 1.5 amps. But of course the transforming loses some power.
For all that just connect the panel to the battery with a diode and fuse. Just be aware that you may cook the batteries. good luck olewill