Charger Diode

[Joetech]

I have a solar charger maxing out at 70mA. I want to put a diode in line to prevent battery discharge. A Diode should be at lease 120% of max output and more is OK. Do I need one on both Pos and Neg sides? Also, I assume I position them to flow out charge to the batt and out of the Batt to the charger on the Neg side. Or is it the other way? I know a diode is like a check valve with the line showing the direction of flow. I want to prevent the discharge of the battery when the sun goes down. I am using a 12v U-1 battery on a C-22. This is mostly for Nav lights(used rarely) and to suppliment the power to the GPS and sometimes a stereo head and speakers.
I know this is only a trickle charge. I have a altenator on the outboard that can give it the extra boost when needed.

 

[GuyT]

Diode polarity

Diodes have two ends called anode and cathode.
The cathode has a stripe at its end.
Put the diode between the panel and the battery on the + side of the battery.
The cathode goes to the battery, the anode goes to the solar panel.
Use a 1 amp diode since these are so readily available. They are available in an axial style that can easily be soldered to or you can even use a wire nut since the 1 amp diode has a large enough lead that will withstand some twisting.
A schottky type diode would be best since it has a lower voltage drop and will make your panel a wee bit more efficient.
Good luck.
Guy

 

[tsmwebb]

There is no problem adding a diode. It will be relatively cheap and easy. But, it will reduce the power that you get out of your panel all day long. Particularly during the long days of summer I think it is possible that you'll lose more than you gain...

--Tom.

 

[Don12364]

Most solar panels used for charging 12 v batteries have an open circuit output voltage of about 22 volts. So when supplying a battery on charge at 14.5 v the 0.7 volt additional drop across the diode will cause not more than a 10% reduction in charge rate, and then only when the battery is nearly fully charged. Follow GuyT's advice and you will be okay. Radio Shack keeps suitable diodes 99c for two:-
http://www.radioshack.com/product/index.jsp?productId=2036269.

BTW many solar panels already have built in diodes for the exact reason you need so it would be an idea to check if yours has too. Either check after dark or cover the panel completely and use a meter to check if there is any reverse current flow.

 

[tsmwebb]

Most solar panels used for charging 12 v batteries have an open circuit output voltage of about 22 volts. ....

Okay, I must be missing something here. I'm not sure if a diode is a good thing or not in the OP's case. It doesn't seem to me that we even have enough information to take much of a stab at the question of which is better, diode or not.

First, we don't don't know the voltage of the panel. The OP could probably read the specs off the back of the panel or count the cells and tell us the open circuit voltage. But, we're talking about a very small and, I presume, inexpensive panel and, as a worst case, it could be a 30 cell device... Add a diode and a hot summer day a little shading from the rig and put the panel flat on the deck and you've got yourself a door mat. At best, 22 volts is just nominal and even in ideal conditions will be much larger than the average voltage over a day. In a perfect world mean open voltage over the daylight hours might be 15.5 with a nominal 22 volt panel, less than 13 with an 18 volt panel and in the real world the averages and peaks are going to be a good deal less yet.

Second, the question is: is it better to put in a diode or not. And to answer that we need to know what the current draw is of the dark panel. To know that we need to know the construction type of his panel and the environmental conditions. It is going to be a small number, probably on the same order as the loss from the diode. But more or less? I don't know and I don't think it is a given. For example, my Kyocera panels specifically say that they don't have a blocking diode and that they don't need one. For the OP it might not be too difficult to set up a test to measure the draw from the cell in the dark.

In short, it seems to me that we don't know the input voltage and don't know what the power loss over a typical day would be with a diode and we don't know the night time draw for the panel... GIGO for the numbers. The literature that I've read on this comes to mixed conclusions -- you may or may not loose more power to the diode than you will to the night. Yes, a diode is cheap, but it might make a marginal system not work at all and there is no guarantee that you will gain anything. You'd need to test or simulate the circuit to know. Even if you do gain something it might not be enough to be worth the drive to Radio Shack...

I do agree that if the panel already has a diode putting anther one into the circuit would be all bad.

--Tom.