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Solar panel blocking diode

Solar panel blocking diode

(OP)


In a solar system during day power is obtained, but during night the same panel which acts as a power source act as load and will drain the battery. This is what most people say. And to avoid the power drain at night it is suggested to use a diode in series (Blocking Diode).

Now if i have a 12 V/30 Amp charger that charges a 120 AH @ 20 Amps and a diode in series with a drop of 0.5V the power dissipated across the diode will be 0.5x20 = 10 Watt.

Now my question is how much current will discharge during the night if i do not use the diode, because if it is very low this current can be neglected as the power drain at night is low compared to the power dissipated across a diode. This point is argued by some people on the internet.

Are there any values on the maximum amp that a solar panel will drain at night so that i can decide if i need to go for a diode or not.

Thanks
Regards
Gokkul

RE: Solar panel blocking diode

Are such blocking diodes not already included in the panels you have?

Leaving that aside, since the panels are a current source (and typically with excess voltage compared to the battery float voltage) then a series diode would have no real world impact under most circumstances. Yes it'll drop voltage, but the current (which is the actual limiting factor) would be essentially unchanged.

RE: Solar panel blocking diode

(OP)
Panels have Bypass diodes not Blocking diodes.
Bypass diodes are to reduce the impact of shading on the panel.

But the whole question here is what will be the current that will be drained by the panel at night, assuming that the charge controller does not have a blocking diode in series.

Thanks
Regards
Gokkul

RE: Solar panel blocking diode

Oftentimes there are questions that can be investigated, or simply bypassed.

You might as well add a series blocking diode. It's a standard feature of many panels. If your panels lack it, then you should simply correct this omission.

As explained above, it's unlikely to have any real world power loss at the battery.

RE: Solar panel blocking diode

In general, unless you have a mind-numbingly simple panel, there will be no discharge, simply because the charger can't be operated backwards. Note in


the charging circuit already has diodes that would block current, and this is a relatively simple charger. A regulated charger will have even more stuff between the panel and the battery being charged.

Blocking diodes are used for other reasons, usually, such as to prevent damage to the charger if the panel is hooked up incorrectly.

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RE: Solar panel blocking diode

(OP)
I have uploaded a simple charger concept for reference. Here the battery and the solar panel negative is connected through a N-mosfet as a low side switch.

This charger will monitor the battery state of charge and will cut the connection between the battery minus and solar minus by controlling the mosfet.

But in all mosfets there is a body diode and during night time when the panel voltage goes below battery voltage it gets forward biased and it conducts. Now here as per this circuit there is no blocking diode so the solar panel will act as load and will discharge current at night.

To confirm how much current a panel would draw at night i connected my 150 Watt panel to this circuit and kept the mosfet in off condition and the body diode did conduct about 5.5mA (milliAmpre) and i also switch the moset on and the current was 6.5mA. I did this experiment at 10PM and the panel open voltage was in mV range.

So this seems to confirm my doubt that if i add a series diode i will be loosing more power across the diode than the battery current being discharged at night.

A series diode to protect reverse polarity connection is a topic for another day :). Please let me know if there is any difference that you see in your panel if you do the same experiment :).

Thanks
Regards
Gokkul

RE: Solar panel blocking diode

(OP)

Quote (VE1BLL )

Yes it'll drop voltage, but the current (which is the actual limiting factor) would be essentially unchanged.

Yes as you said the current will remain constant and this power loss may play a small role in a MPPT charger efficiency but in pwm charger this can be negleted.

But adding a 20 Amp diode will increase the system cost as it will need some heat sink as the power dissipated is approx 10 Watt.

So system cost is also a small factor for me raising this question :)

Thanks
Regards
Gokkul

RE: Solar panel blocking diode

Good point about the cost for production.

If the 20A panel is actually an array of smaller panels, then perhaps the blocking diode function could be distributed across multiple blocking diodes, one on each sub-panel. That might bring the cost of the diodes down to negligible, and not require heat sinking.

Also, if this is for production, then perhaps you approach the panel vendor and ask them to include the blocking diode.

5.5 or 6.5 mA is pretty small leakage compared to the daytime current of 20A and battery capacity of 120 AH. So perhaps it's not worth worrying about.

But you might want to think about various failure modes, e.g. panel damage (being installed outside) causing a short circuit and massive failure. Perhaps a fuse would address that possibility.

RE: Solar panel blocking diode

I have mixed feelings about it. I currently don't use any blocking diodes in my camp system which operates at power point. Solar panels are really just crappy diodes in series. If you have enough diodes the possibility of leakage diminishes. Just ten to fifteen years ago the panels still had a lot of leakage. You could actually see burn through holes in the cells. They seem to be better now. That said, if you have multiple panels in parallel one of those will eventually go bad. I like to say there are two types of people in this world, those with bad panels and those that don't know they have bad panels. If you have 36V panels, none in parallel and a 12V battery, the diode may not be needed. How long do you want your product to last and what is the cost of failure?

RE: Solar panel blocking diode

The batteries will self discharge at probably 10 times the leakage you're seeing, making the diode pointless.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Solar panel blocking diode

(OP)

Quote (VE1BLL)

But you might want to think about various failure modes, e.g. panel damage (being installed outside) causing a short circuit and massive failure. Perhaps a fuse would address that possibility.

Now that the current leakage is taken care of, my next issue is failure modes :), mostly connecting the panel in reverse.

Quote (itsmoked)

The batteries will self discharge at probably 10 times the leakage you're seeing, making the diode pointless.

Yes another point to consider :).

Thanks for all your feedback. If possible please confirm my experiment with the panel that you have just to make sure that the leakage current is low on all panels and post your result :).

Thanks
Regards
Gokkul

RE: Solar panel blocking diode

You still seem fixated on 'loosing 10 watts' adding the series diode. Do keep in mind you are loosing 0 watts - as fellow ham stated, the solar panels voltage is much higher than the batteries so your 1/2v drop looses no power that otherwise would have gone into the batteries as a charge current.

It also is a question of scale... I have added small solar cells to keep my wildlife cameras 4.8v rechargeable batteries topped off; I can charge at about 50ma for a few hours each day or each other day (cloudy); no series diode discharged about 5ma all the other time. Net gain = minus. I used at these low currents germanium diodes to keep Vdrop down to about .2v

www.KilroyWasHere<dot>com

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