sizing blocking diode and creating trickle panel

[dellsworth]

I'm creating a solar panel to charge a pair of old 6V Trojan batteries in series that I use for a remote phone antennae.  The antennae uses about 2 watt of power full time and up to 9 watts when on hook.  They're remote so can't check but seem to remember on full charge they get up to about 14.5V.  I have a bunch of cells I picked up that generate about .52V and supposedly about 2000ma.

I don't want to put in an expensive charge controller just for this application so wanted to know four things.

First what size panel to I need to create to charge the batteries but not overcharge them?

Second what size blocking diode do I need so as not to discharge the batteries in the night back into the panel.

Thirdly what "formula" do I need to consider to extend the application.  In other words if I decide to put in a bank of 10 12V batteries linked as pairs to create a 24V system (the antennae supports 12-48v) using the same power consumption above what would I need for the blocking diode.  I know it's a ridiculous example but it'll help me understand.

Lastly - what am I forgetting?

Thanks very much.

[Opera House]

Diodes are generally rated for current at 25C.  If they get hotter than that the current capacity has to be decreased.  As a simple rule, use one that is rated at twice the current expected.  Diodes generally have about 0.65 voltage drop.  So at 2A it will generate 1.3 watts of heat.  Give it clearance for air flow and if stud mount a couple inches of heat sink.  Again, to cover all cases use a peak reverse voltage rating of at least double the battery voltage. This is only the minimum and you can use a diode of much higher voltage.  Almost any diode will have enough voltage since the general minimum reverse voltage rating for diodes is 50V. In breakdown tests I have conducted this voltage was gnerally several hundred volts on better than 90% of them.  It is only the minimum that is guaranteed.  Some caution has to be used with high speed switching diodes found in computer power supplies.  These are special and only rated at about 35V.  That could be a problem with a 48V battery.

[ghurd]

""First what size panel to I need to create to charge the batteries but not overcharge them?""

Alaska or Florida? Buffalo or Santa Fe? Summer or winter? How cloudy will it be NEXT year?

It's a balancing act that never works out well. But 2000ma would be close, probably not enought in the winter, too much in the summer. Depends on how much you have the 9 watts going too.

""I don't want to put in an expensive charge controller just for this application""

A 4.5 amp charge controller is only half the price of just one of the batteries.

SG-4, about $30. Just about the cheapest thing in the whole setup.

You should check a solar site for a 'Load Worksheet' to see where to start sizing.

G-

[dellsworth]

Thanks very much for your reply.  It was extremely helpful.

[dellsworth]

The $30 does put a nice perspective on it and thanks for the comment.  Still I'm in southern Mexico with lots of light year around, little access to charge controllers, a few left over batteries from rebuilt battery banks and an antennae that is not very complaining about power - it'll run with volts between 10 and 48.  It won't cost me much to play.

   

[ghurd]

For the money to play.

I would try to increase the solar.  Even a little will help.

Include a Morningstar SG-4.  Morningstar has suppliers in your region.

If you can not find a supplier, make a new post. I will try to help you.

G-