Two solar questions: wind breaks and parallel/series

[dzl_]

Originally Posted by Iceni John

the panels' voltage needs to be comfortably greater than the batteries' to allow the MPPT magic to happen, but not by a huge amount.

I think we are on the same page here. Actually I think we are on the same page in general.

Several folk on the Northern Arizona Wind and Sun forum, whose advice I greatly respect and trust, suggest a 2:1 ratio between input and output voltages as a good rule of thumb for efficient and effective MPPT charge controller function. If they support that notion, then it's good enough for me!

2:1 sounds reasonable enough, I got hung up on the first part of your earlier comment ("MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible.") and wasn't considering "as small as possible" in the context of the latter half of your comment (2:1 ratio), now it makes more sense.

I hadn't heard that rule of thumb before, if you have links, either to the forum posts or the morningstar advice, I'd like to read up on it, MPPT efficiency is a subject I want to learn more about.

Too many folk get hung up with the perceived cost of wiring their bus PV systems, especially if using a 12V house system. Compared to a remote array for a house, even the largest bus array will need only a small fraction of its wiring, so in the big scheme of things who cares if one spends a few more dollars for heavier cables?

Agreed. Most solar wisdom is derived from on and off grid stationary systems. A lot of that wisdom doesn't fully apply to small vehicle based systems. Wiring losses do matter, even in small systems, but they are not nearly as big a factor as they are in large systems with long wire runs.

[somewhereinusa]

Originally Posted by Iceni John

One will be (or should be) using honking-great 4/0 cables for the starter motor and the start batteries anyway,

John

John, are the batteries in a Crown at the opposite end of the bus like an MCI? I just tried to do some quick checking on starter specs. My 3116 Cat uses a Delco type 37MT which has a current draw of about 375A with a lockup draw of 1000A.

My start batteries are less than 5 feet from the starter. Using a worst case 750A draw and 10 feet distance I still get 2 gauge as being more than enough. I haven't actually paid that much attention to what I actually have but, from casual observation I don't think they are larger than 2 I'm thinking 4.

I don't think the cables on my semi, N14 Cummins at 12V, were bigger than 2 if that. The batteries were about 5 or 6 feet from the starter.

Just wondering why you think battery cables should be 4/0? I've never seen starter cables anywhere near that large except on really big heavy equipment.

[dzl_]

Originally Posted by somewhereinusa

John, are the batteries in a Crown at the opposite end of the bus like an MCI? I just tried to do some quick checking on starter specs. My 3116 Cat uses a Delco type 37MT which has a current draw of about 375A with a lockup draw of 1000A.

My start batteries are less than 5 feet from the starter. Using a worst case 750A draw and 10 feet distance I still get 2 gauge as being more than enough. I haven't actually paid that much attention to what I actually have but, from casual observation I don't think they are larger than 2 I'm thinking 4.

I don't think the cables on my semi, N14 Cummins at 12V, were bigger than 2 if that. The batteries were about 5 or 6 feet from the starter.

Just wondering why you think battery cables should be 4/0? I've never seen starter cables anywhere near that large except on really big heavy equipment.

SomewhereinUSA, there are two factors to consider when sizing wire.Voltage Drop and Ampacity (current carrying capacity of the wire). If the calculator you used recommends 2AWG it is almost certainly not considering Ampacity (which is the maximum safe current carrying capacity of the wire + wire insulation).

I don't know how to properly size a cable for a starter, ampacity ratings are based on continuous duty, but it is something that should be part of your decision making. maybe the manufacturer gives some guidance on wire size. Maybe there is a good rule of thumb for starter motor wire size.

For all other circuits, its important to consider both voltage drop and ampacity, and ampacity should be your primary concern since its a safety concern whereas voltage drop is an efficiency/proper functioning of electronics issue.

For reference here is a Marine Ampacity table:

[Native]

As a point of reference in this conversation, our Freightliner FS-65 chassis (CAT 3126 engine) comes stock with 2/0 battery cables. The positive is 158" (13' 2") and the negative which runs to the frame is 4'. That is 17' of wires.

[somewhereinusa]

Originally Posted by dzl_

SomewhereinUSA, there are two factors to consider when sizing wire.Voltage Drop and Ampacity (current carrying capacity of the wire). If the calculator you used recommends 2AWG it is almost certainly not considering Ampacity (which is the maximum safe current carrying capacity of the wire + wire insulation).

I don't know how to properly size a cable for a starter, ampacity ratings are based on continuous duty, but it is something that should be part of your decision making. maybe the manufacturer gives some guidance on wire size. Maybe there is a good rule of thumb for starter motor wire size.

For all other circuits, its important to consider both voltage drop and ampacity, and ampacity should be your primary concern since its a safety concern whereas voltage drop is an efficiency/proper functioning of electronics issue.

For reference here is a Marine Ampacity table:

I'm aware of all of that, was basically already engineered by someone that is supposed to know what they are doing. Even if replacing I see no reason to go any larger than maybe one size larger. 0000 just seems way too large for the amps in question.

[kidharris]

Originally Posted by Iceni John

I was just reiterating Morningstar's info for my two TS-MPPT-60 charge controllers! Yes, obviously the panels' voltage needs to be comfortably greater than the batteries' to allow the MPPT magic to happen, but not by a huge amount. Several folk on the Northern Arizona Wind and Sun forum, whose advice I greatly respect and trust, suggest a 2:1 ratio between input and output voltages as a good rule of thumb for efficient and effective MPPT charge controller function. If they support that notion, then it's good enough for me!

Too many folk get hung up with the perceived cost of wiring their bus PV systems, especially if using a 12V house system. Compared to a remote array for a house, even the largest bus array will need only a small fraction of its wiring, so in the big scheme of things who cares if one spends a few more dollars for heavier cables? One will be (or should be) using honking-great 4/0 cables for the starter motor and the start batteries anyway, so another few more feet of 4/0 for the house system is no big deal. And compared to the overall cost of a typical bus conversion, it's just a drop in the ocean, hardly even quantifiable. Just spend what it takes to do it right!

John

" 2:1 ratio between input and output voltages as a good rule of thumb for efficient and effective MPPT charge controller function." Does this mean that if I have a 12v battery voltage that my panels should be no more than 24v "for efficient and effective MPPT charge controller function."? In other words, a solar array voltage higher than 24v would be bad for MPPT effeciency if I had a 12v battery bank (or 48v array & 24v battery)?

[bus-bro]

With a bigger step down you lose efficiency in the conversion. From recall, it's not a huge drop.

I've seen some complex MC4 connector setups in order to parallel wire the panels. But every connection is a failure point.

My system has a 12 volt battery, and a series string of panels topping out at a 100 volts. If I was doing it over I'd probably do a 48 volt battery bank.

[Iceni John]

Originally Posted by kidharris

" 2:1 ratio between input and output voltages as a good rule of thumb for efficient and effective MPPT charge controller function." Does this mean that if I have a 12v battery voltage that my panels should be no more than 24v "for efficient and effective MPPT charge controller function."? In other words, a solar array voltage higher than 24v would be bad for MPPT effeciency if I had a 12v battery bank (or 48v array & 24v battery)?

Remember that a "12 volt" house system is charging at about 14V most of the time (depending on what stage of charging the CC is at), and most typical grid-tie 60-cell panels like my Sharp 255W have about 30V output. That's just a smidge over a 2:1 stepdown ratio. Any MPPT CC will step down whatever you want within its rated limits, but it will get hotter if it steps down a lot of voltage, and heat is the sworn enemy of electronics.

John

[Iceni John]

Originally Posted by somewhereinusa

John, are the batteries in a Crown at the opposite end of the bus like an MCI? I just tried to do some quick checking on starter specs. My 3116 Cat uses a Delco type 37MT which has a current draw of about 375A with a lockup draw of 1000A.

My start batteries are less than 5 feet from the starter. Using a worst case 750A draw and 10 feet distance I still get 2 gauge as being more than enough. I haven't actually paid that much attention to what I actually have but, from casual observation I don't think they are larger than 2 I'm thinking 4.

I don't think the cables on my semi, N14 Cummins at 12V, were bigger than 2 if that. The batteries were about 5 or 6 feet from the starter.

Just wondering why you think battery cables should be 4/0? I've never seen starter cables anywhere near that large except on really big heavy equipment.

My bus originally had 4/0 starter cables, so I saw no reason to down-size them; Crown typically over-engineered most things anyway, and that's one reason I bought a Crown instead of something else. My original two 8D start batteries were originally just ahead of the left rear wheels, with about 24 feet of cable running to the main Cole-Hersee disconnect and then on to the starter motor. Because I needed the batteries' space for the poo tank, and because I had an empty space just behind the right rear wheels where an optional fire suppression system would have been mounted if specced, I moved them to their present location there. The first thing I noticed after doing that was the engine started quicker, using the same batteries but now with cables only a few feet long.

Another reason for me to not down-size the starter cables is because I have a big 42MT starter motor that is rated at a maximum of 10.5 HP. For a worst-case scenario that's about 7,800 watts, and with a 12V starting system and allowing for voltage droop under load one could be seeing 700 amps or more, albeit very briefly (one hopes). That's A LOT of current. Heck, if I could have found the bigger 250MCM cables and lugs I would have considered using them instead! One day, just for shits and giggles, I'll use my 800 amp clamp-on ammeter to see exactly how much current is needed to start 2100 lbs of Detroit's finest iron (it has a peak hold function to show the maximum current flow). I suspect I'll be seeing well over 400 amps even on a warm day. Yes, I know that brief peak currents are not the same as continuous loads, but I prefer to err on the side of caution.

One thing I always do after making any cables is to feel them and their lugs for warmth while in use - if I can detect any noticeable warmth at all I assume I need to use a heavier cable instead. So far so good with all my cables! Considering the small difference in cost between 2/0 and 4/0 cable and lugs, I prefer to just make them heavier in the first place so I don't have to worry about them later or have to replace them prematurely. I use 4/0 for all the starter's cables, for the house-to-starter interconnect (to boost the current two Group 31 batteries if it's cold weather), and between the main DC house busbar and the Magnum MS2000 inverter.

I hope this answers your question.
John

[dzl_]

Originally Posted by kidharris

Just to clarify, to ease my ignorance,

"2. MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible. Asking them to step down more than about a 2:1 ratio...." is referring to the voltage step down occurring in the MPPT from the solar panels to the battery bank. Correct?

Originally Posted by kidharris

In other words, a solar array voltage higher than 24v would be bad for MPPT effeciency if I had a 12v battery bank (or 48v array & 24v battery)?

If you just want the simple answer this is it:

Originally Posted by bus-bro

With a bigger step down you lose efficiency in the conversion. From recall, it's not a huge drop.

Based on the info I've found so far, what we are talking about here is a gradual decline in efficiency of about 3-4% from a PV to battery voltage ratio of ~1:1 to ~8:1.

I don't know the exact logic behind John's 2:1 rule of thumb, I'm still hoping he can dig up those resources, but my guess is its just a good compromise between conversion efficiency and MPPT efficiency in poor conditions. I think 2:1 is a rough target to shoot for, not a maximum, or a hard limit after which efficiency falls off.

Here are some rough efficiency numbers from Outback and Morningstar, at 500W power output:
Outback Flexmax:
1.4:1 = 95.5%
2.8:1 = 95%
5.7:1 = 94%
8.3:1 = 92%

Morningstar Tristar:
1.3:1 = 96.5%
5.2:1 = 94.5%
7.7:1 = 93%

(if you are more of a visual person, see here)

Victron doesn't provide a voltage efficiency curve but in their support forum they state efficiency between 90-94% across the full usable voltage range, and state in their manuals for maximum efficiency, a roughly 3:1 ratio (72 cell panel or 2 x 36v for a 12v battery).

[somewhereinusa]

[QUOTE=Iceni
I hope this answers your question.
John[/QUOTE]
I wouldn't downsize either, just asking. I make my own cables too and shoot with a heat gun under load. I once had a bad connection on a cable that LOOKED fine.