Hello, I need assistance with a circuit diagram. I'm looking to charge a battery using a solar panel. My setup includes a 300-watt solar panel with an open-circuit voltage (Voc) of 42V and a maximum power voltage (Vmp) of 36V, paired with a 12V battery. I'm employing a non-isolated asynchronous buck converter and wish to incorporate a relay between the converter and the battery. The relay should disconnect when solar power is insufficient.
I've observed that the panel doesn’t deliver adequate power when the voltage drops to about 20V under load. Therefore, I've set the relay to disconnect the buck converter at this voltage. I've designed a basic circuit using a voltage divider, NPN and PNP transistors to control the relay. However, I'm encountering an issue with the relay flickering. When the voltage dips below 20V, the relay disconnects, causing the load to disconnect as well, which makes the panel voltage jump to 25V, leading to continuous switching on and off of the relay.
I need a circuit that disconnects the relay at 20V and reconnects it only when the voltage exceeds 25V, without using a microcontroller. Could you please help me resolve this issue?

 

A simple Diode should work just fine to prevent Battery discharge.

It will possibly require readjusting the Charging-Voltage to
make up for the lost ~0.7-Volt Diode-Forward-Voltage.
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You can buy an MPPT controller designed specifically to do exactly that. Except, it would probably continue charging at a lower rate when the power was insufficient. Why would you not want to do that? Why cut it off if it could only charge at say 9A instead if 10?

 

I've observed that the panel doesn’t deliver adequate power when the voltage drops to about 20V under load.

So what occurs when that happens that you want to prevent?
Does it discharge the battery?

 

A simple Diode should work just fine to prevent Battery discharge.

It will possibly require readjusting the Charging-Voltage to
make up for the lost ~0.7-Volt Diode-Forward-Voltage.
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Yes i can use that. but the problem is i am using it for 20 amp and diode waste a lot of power.

 

You can buy an MPPT controller designed specifically to do exactly that. Except, it would probably continue charging at a lower rate when the power was insufficient. Why would you not want to do that? Why cut it off if it could only charge at say 9A instead if 10?

I don't want to buy an MPPT charger; I'm just curious about how to solve this issue.

 

So what occurs when that happens that you want to prevent?
Does it discharge the battery?

Yes, I want to prevent the battery current from flowing back to the solar panel. This is necessary because, with a non-isolated buck converter, when there is no solar power being generated, the power from the battery flows towards the solar side. I could use a diode to prevent this, but there is a concern about power losses, especially since it's rated for 20 amps.

 

OK, so to simply isolate the solar panel from the battery, use a diode rated for enough current. Yes, some power will be wasted, how much do you pay for the sunshine on the solar panel? And if you use a relay, they also consume power. You will not get a simple scheme than an adequate rated diode.

 

when there is no solar power being generated, the power from the battery flows towards the solar side.

Yes, but I think a 42V solar panel will not absorb significant current from the 12v battery when it is not illuminated.

Otherwise, a Schottky diode will waste significantly less power than a standard diode.

 

Yes, but I think a 42V solar panel will not absorb significant current from the 12v battery when it is not illuminated.

Otherwise, a Schottky diode will waste significantly less power than a standard diode.

I would not bet money on that being the case. At lease not in every instance.

 

I would not bet money on that being the case. At lease not in every instance.

Okay.
But since a solar panel consists of many photoelectric diodes in series, I would think that applying 12V to a 42V unlit panel would not allow much back current through all those series diodes.

Certainly that's something that could easily be tested on the panel.

 

I believe there is no need at all to install a ´blocking' diode in series for battery discharge prevention.
When dark, the panel still behaves as a diode itself and does not allow battery discharge trough it.
When insolated, generates more than the battery voltage and charges it.

I made garden lights with 18650 cells and 8 solar cells little 1W panels, been working perfectly well 2 years untouched with no blocking diode.
For a 40VDC panel as a have a bunch on my roof, a ´charger controller' that can handle that input and outputs 14VDC is the recommended way.
48 VDC solar input. :
https://www.temu.com/1pc-100a-solar-charge-controller-solar-panel-controller-12v-24v-adjustable-lcd-display-solar-panel-battery-regulator-with-usb-port-10a-20a-30a-40a-50a-60a-70a-80a-90a-100a-solar-panel-controller-g-601099525407372.html?_oak_mp_inf=EIyFv52m1ogBGiBkZWYyNmViYjA4N2M0MjdlOWVkOWM0OWI5YmY0NDhlYSDx9NP37TE=&top_gallery_url=https://img.kwcdn.com/product/Fancyalgo/VirtualModelMatting/d2ae58b1b2d931045f93cc364fe831a0.jpg&spec_gallery_id=2027470032&refer_page_sn=10009&refer_source=0&freesia_scene=2&_oak_freesia_scene=2&_oak_rec_ext_1=NjQ5&_oak_gallery_order=1535053303,1160976057,554776041,527463951,1721563449&search_key=solar charge controller&refer_page_el_sn=200049&_x_sessn_id=hngq81qjix&refer_page_name=search_result&refer_page_id=10009_1713137581912_xq7665x6cz

You can buy the expensive ones if preferred.

 

Add some hysteresis to your relay control circuit. That will stop it from turning on again until the voltage has increased. We need to see a circuit diagram of it to figure out how to do that.

 

The first step is to determine if You actually have a problem that needs to be solved,
and only then, start working-out how to solve it.

If your Buck-Converter has adjustable Current-Limiting, and Voltage-Regulation,
You should not need anything more than that.
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The first step is to determine if You actually have a problem that needs to be solved,
and only then, start working-out how to solve it.

If your Buck-Converter has adjustable Current-Limiting, and Voltage-Regulation,
You should not need anything more than that.
.

This. You could introduce the panel voltage into the converter feed back.

 

Yes, I want to prevent the battery current from flowing back to the solar panel. This is necessary because, with a non-isolated buck converter, when there is no solar power being generated, the power from the battery flows towards the solar side. I could use a diode to prevent this, but there is a concern about power losses, especially since it's rated for 20 amps.

Be more specific about your power loss concerns. Wasted energy? Thermal management?

You can get diodes with very low forward voltage drops. A quick check of DigiKey turned up 16 A diodes with forward voltage drop of just 140 mV. I also recall that you could get transistor arrays that were designed to mimic diode rectifiers having extremely low forward drops (tens of mV) while conducting hundreds of amps. I these were for high-power DC-DC conversion applications, among other things. I don't recall the typical price points, but I don't recall them being outrageously expensive.

 

If you are using a switcher regulator, or any other control type, between the solar panel and the battery, you will be using less than all of the panel output. So the amount of power lost in an efficient diode installation will not be a big deal. Posts #15 and #16 make very good points.

 

Add some hysteresis to your relay control circuit. That will stop it from turning on again until the voltage has increased. We need to see a circuit diagram of it to figure out how to do that.

Help me how can i do that

 

Help me how can i do that

Show me your relay control circuit first.

 

Show me your relay control circuit first.

Thanks for your Suggetion
I found my solution here
https://www.homemade-circuits.com/opamp-hysteresis-explained/