Diode or not diode?

[la7qz]

Hi

First of all, a little background information:

I am using three 55W PV panels to feed four Trojan T-105's connected as a 12V bank. I live aboard my boat full time and rarely have too much power, so I do not use a regulator. If my battery voltage is high one evening, that makes it an excellent opportunity to watch a movie. As there are so many times when part of the rigging may shade one or the other of the panels, they are wired in parallel to avoid reduced output from one panel affecting the whole array. The two panels that are permanently installed are mounted on the guard rail and if I leave the boat for several days, I'll simply flip them down so they are hanging vertical. That way, only one panel can get full sun at any time, and only in the late evening or early morning IF the boat happens to swing so that one side is facing the sun, so I reckon all I'm getting then is a maintenance charge.

In boating magazines and books, there is sometimes mention of PV panels and the writers usually state that there should be a diode in series with the panel to avoid the panel draining the battery at night. However, I have yet to own (or see) a PV panel of a size suitable for a boat that does not come with built-in diodes installed at both the positive and negative junction box. My three panels all have built-in diodes, so it is clear that another diode in series will do nothing other than cause voltage drop.

My question is how much real life performance drop am I going to see from an extra diode in series? The reason I ask is that on my last boat, I used a charge splitter (two hefty diodes in a box with big heat sinks) to feed both the house batteries and the engine starter battery from two 50W panels. I would like to do the same again to make sure the starter battery is always topped up, but preferably without losing too much power through the diode. The unit in this photo is similar to what I was using on the old boat. Overkill for the PV array of course, but I happened to have it available:

I also have another reason for wanting the charge-splitting diodes. On the old boat, I wired a relay in parallel with the panels between the panels and the diodes and used it to control my anchor light. A very simple and elegant solution which I would have liked to have this evening when I spent 45 minutes rowing about in the dark before I could find my boat...

On the same topic, is there any good reason I can't bypass the diodes in the negative junction box on each panel? I would think one diode in each panel would be sufficient? I can't really understand why the panels have diodes on both poles? Or maybe I could bypass all the built-in diodes and just use external diodes to feed the two battery banks? If there is no reason not to do this, it would solve all my problems including controlling the anchor light and there would be less voltage drop than I have at present with the built-in diodes on both poles.

Regards,

Owen Morgan

Yacht Magic

Falmouth Harbour, Antigua

[willib]

hows the wind down there

A small wind turbine would help ya get the power you need ?

[Opera House]

If there are two diodes at the panel, these are usually in parallel with 6V sections of the panel.  Thy do not prevent reverse current from the battery, but prevent forward current from going through the cells if a section of the panel is blocked from light.  Adding a diode in a minimal system would probably not add any advantage.  Manually bypassing this diode during the day with a switch would add slightly to charge.

[wdyasq]

Greetings Owen,

One of the 'tricks' used on RVs is to use a relay to charge what you know as 'house batteries' when the engine or generator is running. This eliminates the ~0.7V drop caused by the diode. This could be used instead of a set of 'battery isolation diodes'.

I wonder if a transistor could be used with a Zener diode and avoid voltage drop and be 'automatic' also.

Ron

[la7qz]

Hi

I already have the PV panels in commission, so adding a wind turbine is not going to answer my question on connecting those. Anway, a wind turbine is an ongoing project. Look here:

http://www.fieldlines.com/story/2007/4/3/22128/99260

I have all the bits for that except for new bearings and rectifiers. I'll wait until I get to St. Maarten as that is a dutyfree port and has a good selection of stuff. A Volvo marine engine repairshop in Martinique wanted the equivalent of 45 US dollars EACH for the bearings... I didn't even try to look for rectifiers.

Regards,

Owen Morgan

Yacht Magic

Falmout harbour, Antigua

[la7qz]

Hi

"If there are two diodes at the panel, these are usually in parallel with 6V sections of the panel"

Nope, the ones in the junction boxes are definitely in series with the whole panel. There are two possible places to connect the outgoing cable in each junction box on my panels. One each side of the diode. Connecting the cable to the other side of the diode removes it from the circuit, so my question remains, is there any good reason to have a diode in series on both the positive and negative side?

"Adding a diode in a minimal system would probably not add any advantage."

My question was: "How much DISadvantage does it add, as I want to use diodes to charge two totally independent battery banks at the same time. I know there are PV regulators available that have outputs to charge more than one bank, but they cost money and I assume they use diodes to split the charge anyway?

I do not want these banks connected in parallel under any circumstances, not even through a relay, but I would like to have some of the energy from my PV panels diverted to the starter battery to make sure that is always topped up, even in long periods of not using the engine. And, I don't want to spend any more money than I have to. Unlike an RV, a yacht can not pull over to the side of the road when something happens at sea, and there is no towtruck to call. The starter battery is used only for cranking the engine and nothing else.

For the engine alternator, I use a charge splitter like the one in the photo, a smart regulator and sense the battery voltage direct from the batteries which compensates for the voltage drop through the diode. However, I try to avoid using infernal combustions to charge my batteries at all cost. My engine alternator may be off grid, but it is not renewable energy, and... it's noisy and smelly and the engine warms the boat up, which I don't need in the tropics.

Regards,

Owen Morgan

Yacht Magic

Falmouth Harbour, Antigua

[ghurd]

Opera House is correct.

The 2 built in diodes are bypass diodes.  They serve no purpose in a 12V system.  

I cut them out so they can't fail to short. Not common, but it happens.

The panels are designed to operate at a higher voltage than "12V" to make up for some losses.  

I would add a diode for each PV.  It won't hurt.

It may be possible to reuse the bypass diodes as blocking diodes.  They would be rated for the PV current.  

Might be possible to move one to different terminals right inside the box so it acts as a blocking diode instead of a bypass diode, if the cells are 4x9 instead of 3x12, or if there is a spare non-connected terminal in the box.

"One each side of the diode"

If the diode has 2 connections on both ends (a cable and a panel connection) then it is certainly a bypass diode.

A factory installed blocking diode in a decent quality PV over 10W is very rare.

G-

[Nando]

Yes,

I have circuits that eliminate the diodes completely and have very little losses,

I have switches that can carry from 30 to 150 amps, depending this in the number of MosFets that are in parallel.

I call them BATTERY ISOLATORS

Nando

[americanreman]

Living my dream!

You should really look to super bright led lighting for some of your needs, it would solve your anchor light problems real quick.

As simple as scrounging all parts from solar outdoor lights that can be purchased for as little as 5.00 or less (look for the dual nicd battery types), a daily charge can light a led for 12 to 18 or more hours.

Or design a system from scratch, I recommend nicd for lighting, you can cycle a bank of 1.2 volt batteries daily for years leaving your house bank alone.

Great pathway lights and night lights and can even light a whole room or cabin space well.

Just sold my boat after converting most of the non task lighting to led, I'm itching to build a 38 foot multihull, hopefully this year I start the project....I'm not a sailor though, never tickled me, I guess you have to get bitten by that.

nicds, led's and your panels if done right can change your life on the water.

[commanda]

Generally speaking, solar panels produce current first and foremost.

So long as the resistance of the cable run is low enough that the actual panel voltage is lower than the panels mpp voltage (which is typically about 17 volts for most 12 volt panels), adding a series diode should have neglible (sp? it's too early in the morning and not enough coffee) to zero effect on the charging current.

Amanda

[johnlm]

Since you asked specifically how much loss would there be...

Lets assume you actually get about 4 Amps of current out of each panel.  If you put another diode is series (assuming a silicon diode that has a forward drop of 0.7V) then you will be losing 0.7V X 4 amps = 2.8 watts of power that would have been going to help charge your battery.  This might vary some contingent on the maximum power point, the actual drop across the diode etc, but it gives you an idea of the loss. Using a Schottky diode would cut this loss in half.

Johnlm

[la7qz]

Hi Nando

Where did you get them battery isolators?

All the battery isolators I've seen sold for boats are like the one in the photo. Big diodes, one for each battery bank in a box with a big heatsink. The voltage drop is not a problem when the charging source is an engine alternator and one can either sense the voltage from the battery (best) or just crank up the voltage setting on the regulator.

On a PV system, the voltage drop may be more significant.

Regards,

Owen Morgan

Yacht Magic

Falmouth Harbour

Antigua

[la7qz]

Hi Ghurd

"The 2 built in diodes are bypass diodes.  They serve no purpose in a 12V system."

I'll have to take a closer look at this. At the moment it's dark out and the next few days I'm sailing a friend's boat in the Antigua Classic Yacht regatta. It's a hard life... (This crazy Australian built his own boat from wood he cut in his own forest.)

I'll get back to y'all in a couple of days with a photo of the inside of the junction box which will prove someone right.

PS. All the power to write and post this message as well as light my boat and fill her with music and voices came out of the PV panels we are discussing I don't have a fridge. Too much trouble, and they have cold beers in the bars on land with better company.

Owen

Yacht Magic

Falmouth Harbour

Antigua

[la7qz]

Hi Americanreman

Serious topic drift alert.

LED lighting would not solve my anchor light problem. I would still want it off in daylight and on at night rather than leaving it burning and consuming power through the day or leaving the boat unlit at night if I am late getting home. There are only two ways to do that. By being aboard the boat (and remembering the switch) at the appropriate times or by some automatic system (which consumes less power than leaving the light burning).

I do want a combined LED anchor light and masthead tricolor, but there are legal requirements and the model I want costs 250 dollars. It consumes around 500mA in anchor light mode. Unfortunately, it's out of my budget at the moment. The solar outdoor lights are not nearly bright enough for an anchor light. I'd like the skipper of the other vessel to see me before he hits me even when there is a town full of traffic lights, street lights and neon signs behind me from his point of view. Remember, the job of an anchor light is not only to make my vessel visible, it is to be able to prove, in a court of law that I was showing the legally required lights when the other vessel hit me.

One of the reasons I'm able to sit here now and write this to you from my new boat is that I was able to say that when my old boat was sunk by a freighter halfway across the Atlantic last May. A non-approved light might just provide some lawyer with a loophole to jump through even if it's actually brighter than the requirement...

As for LED interior lighting, I have found that several of the very expensive products that are commercially available for boats fail within weeks of installation. The symptoms are usually one or more groups of three LEDs starting to blink erratically and then fail.

I think the only way forwards for reliable LED lighting on a boat where the battery voltage can vary between 11 and 14.4V is to go for a lower voltage and use something like a 7809 in each lamp with the resistors sized and LEDs grouped accordingly.

I'm not sure how the Nicad batteries you suggest are going to help when I already have a 460 Ah 12V battery bank? I'd love to replace that whole bank with NiCads of course, but that is hardly realistic when I am broke and trying to figure out how to get the money to fix the gearbox that broke outside Guadeloupe the other day. A marine gearbox is very expensive to repair even if I do it myself.

... Cruising is repairing boats in exotic places ...

Regards,

Owen Morgan

Yacht Magic

Falmouth Harbour

Antigua

[commanda]

I think the only way forwards for reliable LED lighting on a boat where the battery voltage can vary between 11 and 14.4V is to go for a lower voltage and use something like a 7809 in each lamp with the resistors sized and LEDs grouped accordingly.

I'm working on putting together a short article on constant current sources for driving leds. Use a transistor to soak up the variable voltage difference between the string of leds and the battery voltage. So don't give up yet. Just waiting for my 100 off super bright leds to not get lost or stolen by the postal service (they've been bad round here lately).

Amanda

[Nando]

They are my own design.

I used N-MosFets in series with extremely low Rdson with several in parallel to attain the current.

One needs series topology to insure proper isolation between differential voltages.

Nando

[Bruce S]

Owen;

   Having beautiful scenes going through my head while reading of the description of the area and the sailboats:--)

I would like to also wade in here of you don't mind too much.

With respect to the approved lighting? could you list those for us when you get the chance?

Along with Amanda and americanre I too have a whole bunch of knowledge in using both LEDs and NiCds>> see post by nothing to lose visa-ve "bruce packs".

Where americanre fell short was that unfortunatley he/we didn't see or know the power requirements for the anchor lights and thier associated mandated brightness levels.

I have a younger sister that works around the fresh water docks of the Ozarks and they of course are no where near the depth and therefore probably don't have the same requirements.

Inside task lighting can of course be dealth with easily enough with a photosensor as can the anchor ligting. The photosensor would answer the problem of turning off/on for the anchor. Along with the cool circuit in Amanda's first training lesson you can even have a dimmer...especially important when the Alky is working as promised in commercials:--)

I have these nice little buggers on outside lighting ( a small 100 LED string and of course those Christmas LEDs that finally came down last weekend, and they have survived -20F & +98F as well as snow/wind /rain and standing water.

These came free to me by way of trashed out lawn lights and their power needs are even less than the LEDs.

I like others here are intrigued by your boat,power and lighting needs not to mention that 3rd PV you have.

You ever get even close the the US coast? I could make sure you have some of those "packs" waiting for you if you would like to try them out.

Have fun sailing and may the wind be with you and not across your bow:-)

Cheers

Bruce S

[Bruce S]

Owen;

 Apologies to being serioulsy off topic with regards to the PV question, but Ghurd and OperaH pretty much have that covered.

Just think how long a string of LEDs would last using Amanda's constant current cicuit and your 460Ahr battery:--)

Cheers

Bruce S

[ghurd]

The daylight sensor is cheap and simple.

G-

[la7qz]

Thanks for this.

Makes perfect sense of course, and if I had employed my brain I would have been able to figure that out for myself.

Regards

Owen Morgan

Yacht Magic

Falmouth Harbour, Antigua

[ghurd]

Not quite right.

If the battery is charging at 13.5V, with 1V line drop, and 0.7V diode, then the PVs are at 15.2V.

The PV amps are almost the same at 15.2V or 14V. With the charging amps being the same, the power into the battery is the same.

That's why they are made for peak power at 17V, diode losses, etc.

It won't hurt.

G-

[la7qz]

Hi

> The daylight sensor is cheap and simple.

A relay connected to the output of the solar array before the diode is simpler and cheaper, does not need any extra wiring and does not need to be mounted somewhere it will not see any cockpit lights etc that I might want on when sitting out for sundowners. It also does not need to be weatherproofed (The sea is a seriously bad environment). And it does not need any more holes drilled in the boat to run cables through which will then need expensive deck glands to prevent leakage.

I can not remember that I've ever seen a commercially available 12v daylight sensor that is suitable for an ocean going boat.

Regards,

Owen Morgan

Yacht Magic

Falmouth Harbour, Antigua

[la7qz]

Hi

The requirements for navigation lights for a boat of my size are as far as I remember that they should be visible for at least three nautical miles. (<> 3.5 land miles)

However, the lights also need to be type approved. Legally, it doesn't matter if our light is visible for ten miles if it does not have the stamp of approval. There are some LED based products on the market that have been approved, but they are expensive.

On interior LED lighting, I will definitely be talking to you guys when I'm ready to start on that project. First I need to settle somewhere I'll stay long enough to receive mail (probably St. Maarten), and then I need to earn some money before I buy the bits.

Regards,

Owen Morgan

Yacht Magic

Falmouth Harbour, Antigua

[nothing to lose]

Hey, if you don't mind and have time how about e-mailng me.

I have some question about living on a boat like that and sailing around, but off topic type stuff, like how does a person make a living, expenses like boat insurance, etc..

Not forum related stuff really.

[ghurd]

Same idea.  

About $4 and solid state.

Uses about 3ma for controlling the circuit.

Tablespoon of epoxy to make it suitable for the current conditions the Titanic is seeing.