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Davo99
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Joined: 03/06/2019
Location: Australia
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Posted: 01:59am 25 Oct 2020

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Having a discussion with a friend.

He asserts that if a panel ( normal household / commercial NOT camping type) is direct connected to a battery, the battery can discharge back into the panel.

My belief is that the panels have diodes at least between the groups of cells and therefore this can't happen.

Anyone know what the facts of the situation are?

noneyabussiness
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Joined: 31/07/2017
Location: Australia
Posts: 506

Posted: 07:07am 25 Oct 2020

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I have panels hooked directly to batteries, no back feeding here... normal household panels ( "cms" by memory)

I think it works !!

mab1
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Joined: 10/02/2015
Location: United Kingdom
Posts: 151

Posted: 06:40pm 25 Oct 2020

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Strictly speaking you should have a blocking diode; the diodes in the connection boxes of household panels are usually 'bypass' diodes to allow part of the panel to work when another part is shaded.

It's less important nowadays with modern panels as the back leakage is much less than on panels from the last century, but it is still there: I've just popped out to put a meter across the blocking diode for a 2 panel array of 30v 60cell 235w panels i've got feeding into my 24v battery: the backfeed current was 27mA.

noneyabussiness
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Posted: 10:03pm 25 Oct 2020

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Mine is zero, tested it early on in system as i was " told " the same thing ....

As the panels are silicone junctions just like a diode, current will generally only flow in one direction, less " leakage " again like any diode. Obviously the voltage tolerance is much lower than a standard silicone diode ,where i personally believe this " rule " came from, people putting too much voltage on smaller panels and they overcoming the voltage ceiling and " backfeeding". Of course there are exceptions and differences.

Attached is a pic of the makeup of a modern solar cell.

I have been wrong before, however from practical experience it doesn't matter. If you want to prove it to yourself, a 12v battery (or any battery with sufficient voltage) a multimeter on current setting and some wire, in the shade of course.

I think it works !!

mab1
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Joined: 10/02/2015
Location: United Kingdom
Posts: 151

Posted: 06:35pm 26 Oct 2020

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interesting that yours is zero - I was a bit surprised that mine's so high TBH - it is possible that the damp has gotten into one of the junctions as i live in a very wet climate - i'll check that out.

Silicone cells always were basically a diode of course, but I assumed the advice of a need for a diode came from the early days on solar when panels did have significant internal leakage - which is partly why those old panels were poor performers - particularly in low light as the output was generated-current minus leakage-current. One of the main improvements in modern cells is a reduction in internal leakage which means they keep generating at voltage down to low light levels.

Whatever the reason, the simple way to check is to connect the panels to the battery it's going to be charging anyway, and when it's dark, open the circuit (at a fuse/ circuit breaker if you don't have a diode like me) and put an ammeter across the break: if the current's negligible, no worries; if it's significant, get yourself a diode.

Georgen
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Joined: 13/09/2011
Location: Australia
Posts: 462

Posted: 01:20am 27 Oct 2020

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How do you choose diode?
(Are there rules to follow for: voltage, amperage?)

Does diode affect performance?
(if added unnecessarily)

George

noneyabussiness
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Posted: 08:11am 27 Oct 2020

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Georgen, depends on what voltage your talking about... if you can get away with a schottky diode it can be quite low...

I think it works !!

Georgen
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Posted: 09:32am 27 Oct 2020

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I heard that most panels have diode fitted.
Then I suppose it has to suit voltage that panel can produce plus margin of safety.

However panels often come with information that they can be in string up to say 700V or even 1000V

Does it mean that another diode is installed at the end of string?

I asked question because I have next to zero practical knowledge with solar systems installation.

George

Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 582

Posted: 09:43am 27 Oct 2020

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I have several 2 & 4 panel strings feeding my 60A outback MPPT regulator. Battery voltage is 48v nominal.
As the panels are only matched within a string but there are several different type of panel (many second hand) I noticed there was current flowing from one string to another when the MPPT went to sleep due battery being full. Each string has its own analog Amp meter BTW.
So I fitted 10A Schottky diodes at each string output to the MPPT. Voltage rating of the diodes is a bit more than the string open circuit voltage.
The TO 220 type diodes are on a heat sink. They get quite hot when a full charge of maybe 50A+ flows into the battery.

No more funny currents between the strings.

mab1
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Joined: 10/02/2015
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Posted: 06:30pm 27 Oct 2020

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if you're connecting your panels to the battery via a controller then the controller should have it's own diode I think (excepting Murphy's Friends' multiple assorted strings situation).

If connecting direct to battery like me, then the diode should be rated for > max battery volts, and I'd recommend at least 1.5 times Imp of the panel(s) - although that's probably overkil.

I actually use a Mosfet wired so the 'body diode' of the mosfet is conducting when the solar panels are generating; i built a simple op-amp circuit to turn the mosfet on when the voltage across it is >0.1v so I don't lose significant voltage in the Vf diode drop. If the circuit should fail the mosfet would just work as a regular diode but get warm (it's on a small heatsink).

It's a simple enough circuit, but as you're running the mosfet 'backwards' it can be a bit confusing to setup, especially as to use an n-channel mosfet you either have to put it in the -ve line or run the op-amp off a supply 'above' the battery +ve terminal voltage (probably easier to use a P-channel if you want to put it in the B+ line, but they're not as good).

davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 07:09pm 27 Oct 2020

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If the power loss across blocking diodes is of concern that maybe look at something like the LTC4359.

I use a product based on Fig 6 in the datasheet.
Edited 2020-10-28 05:37 by davef

pollenface

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Joined: 01/09/2020
Location: Australia
Posts: 41

Posted: 06:31am 28 Oct 2020

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The diode under the black box on the back of the panel is the "bypass" diode. It allows power to flow if you have a few connected in series and one getting a bit of shade. The shaded panel creates resistance so the bypass diode offers a path of less resistance.

Off grid man caver

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