Low Voltage Disconnect diode anyone please help.

Dear Fella's

I'm having problem with my Solar Panel charger, because I made a solar charger but in times of bad weather, my battery charge keeps losing out. I have known about the function of the LOW VOLTAGE DISCONNECT which requires a particular diode or something that has  to be connected to the panel in series towards the battery charger. Can anyone please help me find out what particular diode should I use in order for me to have the LVD function.

Guys your help would mean a lot for my experiment and hopefully I could post it here as soon as it is done.

Thanks in advance...

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iceng6 years ago
Solar panels are like a resistor across your battery when the sun is not generating enough Voltage to cause current to flow into the battery. In the dark the battery will push current back into the solar cells discharging the battery.

Two ways to stop battery discharge.
When solar cells were new, expensive and hard to get. There were pricey current relays that switched on very small differences in voltage. The relay would use a very tiny bit of energy and LVD Low Voltage Disconnect the solar panel whenever the battery would no longer be sun charged.

As solar cells became more available, semiconductor diodes
(Germanium and Schottky with a Low forward voltage drop) came into use. The diode is a low cost one way valve for current and is perfect for preventing reverse current flow in the dark and the ensuing battery discharge.
But the diode has one undesirable property that a relay does not. A diode has a forward voltage drop and this means a solar panel needs to generate the extra voltage to overcome the battery and diode. This meant an additional cell was necessary when making a solar panel or the battery would never fully charge.

A normal silicon diode has a voltage drop between 0.6–1.7 volts, while a germanium diode has a voltage drop of 0.3 volts and a Schottky diode voltage drop is between approximately 0.15–0.45 volts.

You need a 20V or more Schottky diode capable of handling the short circuit current of your solar panel. Place the diode in series with the battery so current flows into the battery. Your solar panel needs to be able to generate one extra volt more then the needed battery charging voltage required.

Hope this helps :-) AP.

+1. Though if you want to get fancy, there are some nice integrated synchronous rectifier solutions from Linear technology.

I looked it up. Linear products are always impressive. Using a MOSFET as switch is a brilliant solution and Linear has the " where-with-all " to make the IC draw only 7uA. Thanks,

Have a really good poke around in their switchers too Alex - look at the ones that can boost a few mV from a thermocouple to something usable !

They also do MPPT controllers, which saves a lot of trouble,

mitchiko (author)  iceng6 years ago

Yeah this means a lot, though I would like to further understand if I am about to wired it in series with my system where should it be properly placed should it be in the + or - sign of the system?
Also if my panel is capable of generating voltage of about 10.4 v at max and .223A and rated at 2watt peak, could you help me determine what would be the best diode that I would use considering that I intend to charge 4 NiMH battery with a rate capacity of 1000mAh and 1.5v each connected in series?
Will I have problem with the over charging also?

Thanks for your help friend.

Best Regards

iceng mitchiko6 years ago
You want a 20V 1A Schottky diode 1N5817FSTR (Digi-Key).
"Place the diode in series with the battery so current flows into the battery."
Means it can be in the + or - sign of the system.
Later it will depend on what other circuits you plan to add to your design.

Yes, you could have a problem with the over charging, you may need to regulate.
However, I don't think a 2watt peak solar panel can damage a 1000mAh battery.

I can't explain all the electronics of charging regulation in this simple text box.
But, you can study regulation solutions at Linear technology.
As steveastrouk said: "there are some nice integrated synchronous rectifier solutions from Linear technology."
mitchiko (author)  iceng6 years ago
Thank you for your wonderful help, it really helps me a lot. Good to have guys like you here.

Thanks again and Best Regards

Ah. That does make sense; I hadn't thought about backward flow through the solar cells.
I am not familiar with LVD but it sounds like step 5 of this instructable should solve your problem.