Hooking up solar cell to relay, battery, and light

Thread Starter

StaticVarg

Joined Mar 7, 2012
2
Hello

I am trying to construct a circuit for some outside lighting.

The idea is to have a set of LEDs light up at night, using solar panels to recharge a battery during the day and then power the lights at night.

I apologize for the childish circuit diagram, using the real symbols just confused me and I wanted to keep this as simple as possible.

The problem I am currently having is that once the solar panel activates the relay and closes the NO circuit (to charge the battery), the battery feeds into the solar panel's connections and locks the relay into the "on" position, so that even when it's night and the solar panels no longer supply current the relay is still on.

How do I connect the solar panels to the battery to keep this from happening? I've been trying to find an answer for several weeks now but I always run into the problem of the battery locking the relay into the "on" position.
 

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wayneh

Joined Sep 9, 2010
16,769
You need a blocking diode between the solar panel + terminal and that of the battery. This will prevent the battery from driving current through the panel while it's dark, and is a standard part of virtually every solar battery system.

I'm not sure this solves every problem with your design, but it's a minimum requirement.

Personally, I'd look to use a MOSFET circuit instead of a relay, since the MOSFET would not require any current to operate. Your relay may be using as much current as your LED string.
 

Thread Starter

StaticVarg

Joined Mar 7, 2012
2
You need a blocking diode between the solar panel + terminal and that of the battery. This will prevent the battery from driving current through the panel while it's dark, and is a standard part of virtually every solar battery system.

I'm not sure this solves every problem with your design, but it's a minimum requirement.

Personally, I'd look to use a MOSFET circuit instead of a relay, since the MOSFET would not require any current to operate. Your relay may be using as much current as your LED string.
Thank you! I will try the diode and MOSFET as soon as I can get a hold of the parts. :)
 

wayneh

Joined Sep 9, 2010
16,769
The diode is a single-part solution. You just need to be sure the diode can handle the charging current. I'd just use the max current for the panel as the minimum current spec for the diode. In other words, size the diode assuming the battery is a short on the panel.

A proper MOSFET circuit will require a bit of thought and design, so I'd suggest you post your plans here for folks to help you get it right before you build something. It's not too complicated, but easy to get wrong.
 
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