Do I have this right?

goingtoghana

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System Purpose: To keep already present battery bank topped off and (during daylight hours) supplement the battery bank as it is used to keep essential items going during frequent blackouts.

Location: Ghana, Africa (Pretty much on the equator) - Close to 8 hours of full sun per day

Distance from panels to Charge Controller: approx 15m

Questions:

1) Series versus parallel on the panels. If I go series, I need smaller guage (cheaper) wire. If I go parallel, I have to buy larger guage (more expensive)wire. Either way, I have to buy a charge controller that can handle either maximum (voltage or amperage). So, does it really matter as long as I size the controller for the current and potential exapnsion of the system?

2) Load Dumps - It seems some controllers have dumps and some don't. I am not sure why. Are load dumps simply the power generated by the panels that is not being used by the batteries? And, if so, can I put an inverter off this dump to power, say a fan for the room?

3) Anything else I am missing in my diagram?

BB.

Re: Do I have this right?

goingtoghana wrote: »

Questions:

1) Series versus parallel on the panels. If I go series, I need smaller guage (cheaper) wire. If I go parallel, I have to buy larger guage (more expensive)wire. Either way, I have to buy a charge controller that can handle either maximum (voltage or amperage). So, does it really matter as long as I size the controller for the current and potential exapnsion of the system?

First, normally, most folks would earth ground the negative battery bank terminal. That means the Breaker in the solar wiring should be in the positive lead.

And, if you have a breaker, you really do not need the disconnect switch too.

If you have Vmp~96 volts (a ~140-150 Volt max input controller typically) and a 12 volt battery bank, you have to use an MPPT charge controller for efficient energy transfer from the solar array to the battery bank.

A PWM controller would be only 20% or so efficient (vs the ~95% efficient MPPT controller.

If you can only get/justify a PWM controller, then all solar panels would have to be in parallel (Vmp-array ~ 17.5 volts to recharge a 12 volt battery bank).

And note, placing a lot of batteries in parallel is not the ideal method of configuring a battery bank--But if you are limited in your sources of batteries (cannot find large capacity / high AH 12 volt battery instead), then you should wire them per this website. Otherwise, the batteries near the +/- bus connection will get charged/discharged more than the batteries at the other end of the string (current flow will favor the batteries with the lowest wiring resistance.

Choosing a charge controller... A ~60-90 Amp output MPPT charge controller is around $500 to $600 (in US). A PWM controller will be 1/3rd the cost.

So, you have a trade off of a lot of copper to parallel connected array with a "cheap" PWM controller, or using much less copper and an "expensive" MPPT charge controller.

In general, for long wiring runs (like your 15 meter run), you probably would be better off with a MPPT charge controller and your Vmp~96 volt array and smaller diameter copper wire needed.

You need to figure out the voltage drop of 15 meter (remember, round trip distance) wire at the Imp-array current--typically, we recommend around 1% to 3% maximum voltage drop for the array wiring.

I can do the voltage drop calculations for you in US Wire Gauge--Don't have a metric calculator handy (and I don't really know standard metric wire sizes--although I am sure I can find a list somewhere).

2) Load Dumps - It seems some controllers have dumps and some don't. I am not sure why. Are load dumps simply the power generated by the panels that is not being used by the batteries? And, if so, can I put an inverter off this dump to power, say a fan for the room?

There are two reasons for Dump Loads.

First, for wind turbines/water turbines, they always need to be attached the battery bank/load to keep them from overspeeding (unloaded turbines can quickly exceed safe RPM unless a brake or other means is used to prevent run-away). So, the "dump controller" + "dump load" is someplace to send excess charging current once the battery bank is full (i.e., turn on dump load at 14.8 volts, turn off dump load at 13.6 volts--repeat).

Dump loads are not needed for solar panels. The charge controller simply "turns off" the current from the solar array when the batteries are full.

The other use for a "dump load/controller" is if you have "excess charging current". You can, for example, once the battery bank is "full" from your solar panels, turn on a relay and start a well pump for irrigation and/or turn on an electric heater for hot water.

The issue with solar electric to heat hot water is that, for many systems, there is simply not enough excess electricity to heat enough hot water to be of much use.

And if you have enough excess power--Then you could over heat the water and have other issues.

3) Anything else I am missing in my diagram?

For every + wire/cable that leaves the + battery bus, there should be a fuse or circuit breaker to protect the wire leaving the battery bus against short circuits.

For example, in the US, typically a 14 AWG wire would have a ~15 amp fuse to protect the wire from overheating if shorted.

If you have a 20 amp load, you should have a x1.25 safety factor for wiring and breaker/fuses:

20 amp * 1.25 NEC safety factor = 25 amp minimum rated wiring + breaker/fuse

If you try to run 20 amps through a 20 amp fuse/breaker, they typically will "open" in minutes or hour (or so) of loading.... That is why a 20 amp circuit should have a 25+ amp fuse/breaker.

You do not show your loads at all--Each load should have its own fuse/breaker. And for some loads (like an AC inverter), you may need a DC disconnect switch too so you can turn off the DC power if the inverter is not needed.

And--Ideally, you should have a fuse/breaker on each parallel battery connection--But not many people do that. With lots of parallel batteries (where you really need the fuse per string), it is expensive and a lot of extra wiring work.

Blue see makes a really nice fuse assembly that can bolt to a battery terminal--But not sure you can easily/cost effectively get them:

http://bluesea.com/products/5191
Attachment not found.
-Bill

goingtoghana

Re: Do I have this right?

Thanks so much. With the exception of the fuses on each battery, I have updated my diagram based upon your input. A couple of questions remain:

1) How do I know the amperage from the battery bank to the inverter? I know if is 12 Volts, but is the amperage the accumulation of the AmpHours of the batteries? I'm trying to understand what size fuse I need between batteries and inverter.

2) Can the breaker not act as a disconnect between the batteries and inverter just like the breaker between the panels and the charge controller?

Attachment not found.

BB.

Re: Do I have this right?

goingtoghana wrote: »

Thanks so much. With the exception of the fuses on each battery, I have updated my diagram based upon your input.

In reality, for safety you only need a On/Off switch from the solar array to the input of the charge controller. A breaker/fuse is redundant and does not provide any additional protection.

Note: even an On/Off switch from the array to the controller is of questionable use... The handy-ness of being able to turn on/off the solar array (if ever needed) vs the possibility of somebody turning off the array and you not knowing that (and killing the battery bank if nobody is paying attention).

A couple of questions remain:

1) How do I know the amperage from the battery bank to the inverter? I know if is 12 Volts, but is the amperage the accumulation of the AmpHours of the batteries? I'm trying to understand what size fuse I need between batteries and inverter.

First, the "common" point for your battery connection -- The +/- connection points have very high current potential (1,000 amps or more into a dead short from the battery bank). So any connection from the + terminal shall have a fuse or breaker sized to the wire rating (i.e., a 12 AWG wire should be 20 amps fusing or less, if using the very conservative NEC requirements).

Wire Current Ampacities NEC Table 310-16

Other organizations have "their standards" which tend to allow higher current limits:

West Marine

For "low voltage" solar, we generally have very heavy wire vs current flow--This is to keep voltage drops "low" (i.e., a 12 volt system, about 0.5 volts drop max I recommend for loads, 1.0 volts for 24 volt, and 2.0 volts max for 48 volt system).

For battery charging, voltage drop can even be more important... Every 0.1 volt wire drop reduce battery charging voltage/current... For a 12 volt system, I would recommend a charge controller have no more than ~0.05 to 0.1 volt drop (maximum current flow). If you have 0.5 volt or more drop, it will take much longer to recharge your battery bank and you will waste many hours of "full sun" because of this.

Wiring and fusing batteries... This can be done several ways. One is to look at the maximum current from the battery bank that they can supply. A Flooded Cell Lead Acid battery can supply ~C/8 current continuously, and C/2.5 for short surges... You would want wire heavy enough not to overheat and/or too much voltage drop for this current range.

When you have 5 batteries in parallel--The will not evenly share current (not each carrying 1/5th of the current). In general, I would assume that when you have parallel batteries--2 batteries, design such that each battery will carry 100% of the load.

Three or more batteries, assume each battery will carry a maximum of 50% of the load... This will help reduce problems with wiring/overheated connections/etc. down the road.

So, say you have a 1,200 watt 12 volt AC Inverter (1,200 watts is the maximum I would recommend for a 12 volt system--~100 amp nominal current flow).

The branch circuit for such an inverter would need to be rated to carry:

• 1,200 Watt * 1/0.85 inverter eff * 1/10.5 battery cutoff * 1.25 NEC derating fuse/breaker/wiring = 168 amp minimum branch circuit.

So, if you have:

• 1 battery : 175 Amp or larger fuse/wiring rating
• 2 batteries : 175 Amp per parallel battery string
• 3 or more batteries: 168/2=84 amp ~100 amp breaker/fuse/wiring for each parallel battery

Yes, my examples above are conservative--And fusing/breakering each parallel battery string for 3 or more batteries is a common engineering requirement for commerial equipment safety--But not many people do this on home power systems as it is expensive to do.

And this is why I like to recommend getting larger AH batteries and getting back to 1 or 2 strings (maximum) instead.

But, not everyone can get large AH batteries for such an installation (minimizing parallel battery connections).

2) Can the breaker not act as a disconnect between the batteries and inverter just like the breaker between the panels and the charge controller?

Yep--Just make sure you get DC rated breakers... Many/most home AC main breakers are not rated to disconnect DC current (DC power is very good at sustaining Arcs and need heavy contacts/special designs to prevent the breaker/fuse from acting like a DC arc welder).

-Bill

goingtoghana

Re: Do I have this right?

"So, say you have a 1,200 watt 12 volt AC Inverter (1,200 watts is the maximum I would recommend for a 12 volt system--~100 amp nominal current flow)."

Holy Crap.... Since the guy's batteries are paralled 12V/990AHR into a 8000W inverter today are you saying that the amperage would be ~666 (8000W/12V) amp nominal current flow?

inetdog

Re: Do I have this right?

goingtoghana wrote: »

Holy Crap.... Since the guy's batteries are paralled 12V/990AHR into a 8000W inverter today are you saying that the amperage would be ~666 (8000W/12V) amp nominal current flow?

Sad but true, yes. A good candidate for a 48 volt system instead.
At that amperage the wires are going to jump apart from magnetic forces when the inverter fires up. And be careful what magnetic-sensitive objects you get near the wires.

Cariboocoot

Re: Do I have this right?

The reason why I created this reference thread: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

goingtoghana

Re: Do I have this right?

So, now I ask the question.... What is the big deal of having a 12V versus 24V versus 48V - 8000 Watt inverter. I realize the higher the volatage, the lower the current thus requiring smaller guage wire.

Is it inherently more dangerous to have the higher amperage assuming you size the wires correctly?

Does higher amperage have a negative affect on battery performance? Based upon the link earlier, it seems like you can string parallel in an efficient manner to spread drain evenly.

It just seems like you can buy any piece of equipment to match any formation of batteries. Why is one better then the other? Safety... cost?

inetdog

Re: Do I have this right?

goingtoghana wrote: »

So, now I ask the question.... What is the big deal of having a 12V versus 24V versus 48V - 8000 Watt inverter. I realize the higher the volatage, the lower the current thus requiring smaller guage wire.

Is it inherently more dangerous to have the higher amperage assuming you size the wires correctly?

Does higher amperage have a negative affect on battery performance? Based upon the link earlier, it seems like you can string parallel in an efficient manner to spread drain evenly.

It just seems like you can buy any piece of equipment to match any formation of batteries. Why is one better then the other? Safety... cost?

Well, consider a few factors:
1. In addition to the wire size, there will be a certain amount of resistance at each wire termination and connection.
2. Using parts that can handle that much current probably makes the inverter both larger and more expensive (and maybe less efficient) than one with a higher input voltage. A .7 volt diode drop out of 48 volts is a lot lower percentage power loss than .7 volts out of 12 volts.
3. Wire that large, in addition to being expensive is much harder to work with, especially in short lengths.
4. Do you have a hydraulic press to attach the crimp connectors to the cables? There is a limit to what bolt-type connectors can reliably handle.
5. Fuses and circuit breakers become hard to get and switches almost impossible. (For a POCO high voltage system, a 1 volt drop across a number of connections is not a big deal, and the switchgear and breakers have to be large anyway to handle the voltage.)
6. High amperage, it is not in more dangerous than high voltage, but the set of dangers is different and you need to be aware of that. Also, 48 volts is not really high voltage, but 600 amps is high current! ]
7. Putting batteries in parallel to get high current does have an inherent set of problems, and at some point you should have fuses in each of the parallel strings. More than 4 in parallel is very hard to manage properly.

At the battery end, it is not harder on a properly sized high current battery to handle high current than it is for the corresponding low AH battery to handle the lower current, but the design details will be different.

Photowhit

Re: Do I have this right?

You forgot;

#8 Copper is expensive, From my faulty memory prosine 1800 (peak only 2800 watts) recomends 0000 wire for connections longer than 4 feet. 8000 watts must need 500kcmil? 0000 weights almost 1lb a foot, copper is north of $2 a lb (down from $3+)

BB.

Re: Do I have this right?

In the US, every ~3 gauge increase in wire "number", is a 2x increase in cross sectional area (i.e., 2x the amount of copper and about 2x the cost per foot).

Remember that heating is Power=I²R.

Changing from 12 volt to 48 volt will allow you to size the wire:

• 4x less current, means 1/4 or about 6 awg smaller diameter copper wire (difference between #4 wire vs #10 wire or 1/4 the cost of copper wire).
• Also, very roughly you have about 4x more voltage drop allowed (0.5 volt drop for 12 volts, 2 volt wiring drop for 48 volts)--Allows you to now send that same power ~4x farther on the same diameter wire, or use even smaller awg wire with more voltage drop and same losses for same distance (4x the votlage, 1/4 the current, 1/4 the voltage drop per unit length of wire).

And, that is why I like to recommend a 120 VAC (or 230/240 VAC) inverter for even smaller systems... You want to send 120 watts any distance--that is 10 amps @ 0.5 volt for a 12 volt battery bank. But that is about 1 amp @ 5 volt drop at 120 VAC--Much less money spent on copper wire and much easier to send 50-100+ feet (or farther).

-Bill

goingtoghana

Re: Do I have this right?

Did find out something interesting about the 8000W inverter. Since it is drawing such heavy current from the 12 V battery, they designed it with 4 negative and 4 positive input terminals so you actually use 4 - 1 AWG positive and negative wires fromthe battery bank instead of 2 huge cables.

Not sure if that matters much in the 12 V versus 48 V battery setup as suggested. I really have no option to get him another 8000 W 48V inverter.

ggunn

Re: Do I have this right?

BB. wrote: »

In the US, every ~3 gauge increase in wire "number", is a 2x increase in cross sectional area (i.e., 2x the amount of copper and about 2x the cost per foot).

Remember that heating is Power=I²R.

Right. Doubling the cross sectional area halves the resistance and the current, which lowers the power loss in the cable by a factor of eight. Is that right? Something smells funny...

BB.

Re: Do I have this right?

ggunn wrote: »

Right. Doubling the cross sectional area halves the resistance and the current, which lowers the power loss in the cable by a factor of eight. Is that right? Something smells funny...

OK ggunn... My snark detector is not always accurate...

But Power=V*I=I²R=V²/R

So--1/2 the resistance only reduces the power loss by 1/2

However, if you go from 12 volts to 48 volts, this reduces current by a factor of 4.

And the I²R loss (with same cable) is reduce by a factor of 16x (1/16th the loss).

Or, you can use a much smaller diameter cable to carry the power with "normal" losses (of ~3% maximum power loss, and ~2 volt (to 4 volt total) maximum wire drop at 48 volts.

-Bill :D

ggunn

Re: Do I have this right?

BB. wrote: »

OK ggunn... My snark detector is not always accurate...

But Power=V*I=I²R=V²/R

So--1/2 the resistance only reduces the power loss by 1/2

Never mind; it was a brain fart. Too much coffee this morning or not enough. Didn't I say I smelt something funny?

inetdog

Re: Do I have this right?

ggunn wrote: »

Never mind; it was a brain fart. Too much coffee this morning or not enough. Didn't I say I smelt something funny?

One problem with knowing that P= both I**2R and V**2/R is that we tend to forget that in some cases I is constant and in other places it is V that is constant as we change the value of R. :-)

Photowhit

Re: Do I have this right?

goingtoghana wrote: »

...they designed it with 4 negative and 4 positive input terminals so you actually use 4 - 1 AWG positive and negative wires fromthe battery bank...

If this is for a 6" cable run maybe, but do they have a recomended distance for this size wire run?

Who makes this? maybe we can look into their specs.

Cariboocoot

Re: Do I have this right?

8kW 12 Volt inverter using four parallel wire runs for the DC?
Let me guess: AIMS.

Best use for this is as a door stop.

ggunn

Re: Do I have this right?

inetdog wrote: »

One problem with knowing that P= both I**2R and V**2/R is that we tend to forget that in some cases I is constant and in other places it is V that is constant as we change the value of R. :-)

Or in my case I was halving both I and R for constant V due to temporary brain death. Ohm was spinning in his grave. Hey, if Ohm is spinning in his grave, is he a generator?

Cariboocoot

Re: Do I have this right?

ggunn wrote: »

Or in my case I was halving both I and R for constant V due to temporary brain death. Ohm was spinning in his grave. Hey, if Ohm is spinning in his grave, is he a generator?

That would be Faraday.

niel

Re: Do I have this right?

ggunn wrote: »

Or in my case I was halving both I and R for constant V due to temporary brain death. Ohm was spinning in his grave. Hey, if Ohm is spinning in his grave, is he a generator?

as far as i know if he did spin that the coffin (wooden faraday cage?) would keep the waves of death confined. i know, poor taste.