Small Off Grid PV Schematic

davemac · September 2015

Hi All

This is my first post and I wanted to share my humble off-grid design schematic that I have designed but am keen to explore feedback.

I'm not one who easily deciphers technical circuit diagrams so I went for a schematic approach for a system that requires around 1 kw/h per day to operate a small pump, small modified chest/fridge, phone & laptop charging & some LED lights.

If anyone is keen to offer advice I would be grateful, my main concern is in the sequencing of components and earthing issues.

Thanks for a great forum! DM

BB. · September 2015

The 350 amp DC switch should probably be replaced with a mechanical circuit breaker (both protecting the wiring and the ability to turn off power to the inverter).

I would probably run a 6 awg ground wire directly from the surge protector to the local ground rod--I don't like bringing surge/lightning current into the home/out building.

I would run the battery negative ground to the local ground rod directly. And I would run a separate green wire ground from the inverter AC panel to the same local ground rod.

If you have lightning in the area--You probably would want to put a surge suppressor at the AC output of the inverter (or inverter panel). Also, a 3rd surge suppressor where the DC solar panel wiring enters the home/out building (depending on where the DC panel combiner is installed, etc.).

Also, think about backup power (generator, AC battery charger, AC mains charging, etc.). Lead acid batteries do not like to sit uncharged (below ~75% state of charge)--So it is usually a good idea to have alternatives to keep your battery bank healthy.

-Bill

davemac · September 2015

Bill
Thank you so much for your thoughts and advice here, greatly appreciate you taking the time.

A fused disconnect for battery to inverter path to protect the circuit makes sense, I would assume the amperage of that fuse would match the 150 A fuse rating for the inverter (the battery leads are factory installed to the Victron so assumed current matched).

Your mention of a generator is welcomed, I have one with a 12 volt DC out which of course is no good. I am looking at building a direct charging unit that uses a truck alternator running of a generator. Jury is still ou on the final method t, but yes, that is a requirement even in sunny Australia.

With your suggestion of running 6 gauge green earth from multiple earth chassis directly to the grounding rod I had a concern.
My preliminary read on lightening pulse induced energizing of conducting elements stressed the need to reduce 'multiple paths to ground ' . My feel was to create a continuous path to ground by chaing all the chassis and system grounds to a system ground bar and then one run to the grounding rod.
That way, in the event of lightening pulse there is one run to ground and less chance of a variety of stray voltages between earthing wiring?

Is there a benefit to multiple earth runs? I am keen to learn more, thank you.

Taking this all in board for this system, than you Bill!

vtmaps · September 2015

davemac said:
With your suggestion of running 6 gauge green earth from multiple earth chassis directly to the grounding rod I had a concern.
My preliminary read on lightening pulse induced energizing of conducting elements stressed the need to reduce 'multiple paths to ground ' . My feel was to create a continuous path to ground by chaining all the chassis and system grounds to a system ground bar and then one run to the grounding rod.
That way, in the event of lightening pulse there is one run to ground and less chance of a variety of stray voltages between earthing wiring?

Is there a benefit to multiple earth runs? I am keen to learn more, thank you.

I think that what is important is that they all go to the same ground rod. My system uses a Midnite ePanel, which has a ground bus, and then just one green wire from that bus to the ground rod.

--vtMaps

davemac · September 2015

Thank you vtMaps

Those ePanels look mighty handy in wrapping up all this in a tidy manner. They have circuit diagrams on the website which I am finding really insightful, thanks for the fresh lead.

Dave Mac

BB. · September 2015

Grounding has two reasons to exist.

The first is pretty straight forward. If there is a short from "hot" to a piece of sheet metal/electrical box or conduit, plumbing pipe, stove, etc... The current eventually goes to the "return lead of the power source" so it pops the fuse/breaker.

You don't want to mix AC and DC grounding (ideally a single bonding of AC and DC grounds in one place) because the DC currents are generally a lot larger than the AC currents (your inverter uses a 150 Amp DC breaker and a 15 amp AC fuse). The "wrong" type of short to the wrong spot could dump >150 amps through your small green wire AC ground (i.e., you "fuse" the AC green wire ground system).

The second reason for grounding is lightning. More or less, lightning acts like radio frequency energy. The path of the flow of lightning current does not follow wiring like 50/60Hz/DC power does. For example, if your lightning ground wire makes a sharp 90 degree bend, the lightning energy may simple find a different path (in the house AC wiring behind the wall, into rebar in concrete walls, etc.). A very different animal. For RF/Lightning grounding, multiple short paths to ground rods (or plates), that are straight (or have large diameter bend radius) wire runs, move/keep the lightning from going into the house (run lightning wires on roof, down outside walls, do not bring into building and out to ground rod at foundation). (note links have been bolded to make it clearer)

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's FAQ:

Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

Remember that ground rods have relatively high resistance to earth (in the US, roughly 25 ohms from rod to earth is "OK". Good enough for lightning grounding but not good enough to short/pop a breaker for 120/230 VAC.

-Bill

davemac · September 2015

Thanks Bill

I can see your points here and thanks for the links.

From this I can see that the best way to satisfy the two grounding scenarios available to the Victron inverter is to run the inverter chassis ground direct to the grounding rod.

Thank you Bill !

davemac · September 2015

BB. said:

Also, think about backup power (generator, AC battery charger, AC mains charging, etc.). Lead acid batteries do not like to sit uncharged (below ~75% state of charge)--So it is usually a good idea to have alternatives to keep your battery bank healthy.

-Bill

Hi Bill, I have been looking into building a direct charging battery generator for my proposed offgrid PV and have a local lead on a reliable setup (a 5.5 HP Honda stationary engine connected to a single 55 amp truck alternator providing backup power to a 24 volt battery bank. This one has been in place working trouble free for over 10 years. It punches out 50 amps at over 30 volts when required, but I am unable to personally see it as it is on an island

).

My question;
When it comes to connecting this generator to the batteries I figure that this should be done at the PV combiner box point (as a 2nd DC input similar to a 2nd PV string) with the required disconnect switch and comb box fusing which then routes the generator DC current (50 amps & 30 volts) into the Morningstar Trimetric (150 V, 60 A) to manage the battery charging steps. My system is 24V / 260 aH FLA Trojan.

Is that common practice or are there more specific or cruder ways to service a low battery with a direct charging generator?

Cheers

vtmaps · September 2015

davemac said:

Hi Bill, I have been looking into building a direct charging battery generator for my proposed offgrid PV and have a local lead on a reliable setup (a 5.5 HP Honda stationary engine connected to a single 55 amp truck alternator providing backup power to a 24 volt battery bank. This one has been in place working trouble free for over 10 years. It punches out 50 amps at over 30 volts when required, but I am unable to personally see it as it is on an island ).

My question;
When it comes to connecting this generator to the batteries I figure that this should be done at the PV combiner box point (as a 2nd DC input similar to a 2nd PV string) with the required disconnect switch and comb box fusing which then routes the generator DC current (50 amps & 30 volts) into the Morningstar Trimetric (150 V, 60 A) to manage the battery charging steps. My system is 24V / 260 aH FLA Trojan.

Morningstar Trimetric?!? Maybe a Morningstar MPPT Controller?

A couple of issues...

Just how much generator voltage are you starting with? Your battery may need almost 32 volts for a temp compensated absorb (let alone an EQ). An MPPT controller has some voltage drop and it needs a bit of headroom above that voltage in which to sweep and find a maximum power point. If your generator voltage isn't high enough for a full charge, you can probably use it for bulk and then let solar finish it off.

Next issue is protecting the controller. Some controllers let out the smoke when they try to regulate too "stiff" a voltage source. The controller will try to load the generator and drop the generator's voltage so it can find the maximum power point. If the generator is too stiff, its voltage won't drop, and the controller may hurt itself trying. You may want to put a power resister in series with the generator output. Of course, this introduces further voltage drop.

--vtMaps

davemac · September 2015

Yes, a Morningstar MPPT Tristar 150V 60A controller with GFPD is the controller that I have purchased.

My suspicion with the direct charging generator is that it probably prides itself on being a stiff voltage source as it is an alternator belt connected to a petrol powered drive wheel ...I can't see how the MPPT controller could 'govern' that side of the input ( from my naive understanding).

I figure I would be okay with using the direct charging genset to do the bulk charging , with the 750w x 100v of panel addressing the later stages of battery charging.
Given that concession, with the charging plant max'ing out at ~30v & 50 A, does the issue of regulation still raise its ugly head?
Cheers vtMAPS

vtmaps · September 2015

davemac said:

Given that concession, with the charging plant max'ing out at ~30v & 50 A, does the issue of regulation still raise its ugly head?

You must regulate. At the very least you want temperature compensation so that as the battery heats up the voltage applied to it drops. Otherwise you can have thermal runaway. But even 30 volts without thermal runaway can boil those batteries dry pretty quick.

You might consider an inexpensive PWM dump controller to protect the battery.

--vtMaps

davemac · September 2015

Thanks vtMaps

I am considering the following now given your thoughts here.

I am starting to see why it is unwise to push the direct charging unit through the MPPT as if it were a PV voltage (given that the MPPT can short circuit the PV array to divert excess energy, while I would hate to think what would happen if the MPPT did that while being driven by the direct charging unit?).

With that in mind I should leave the PV-Combiner Box-MPPT-Battery power chain as is and instead connect the direct charging alternator to the batteries via a PWM 60 A Charge Controller that would function purely as a 60 A diversion controller to a heating element to dump energy once battery voltage is at the desired diversion voltage.

Maybe there would be provision in the PWM 60 A diversion controller to send a 'kill' message to the alternator engine at the desired dump voltage eliminating the the need for a heat sink element?

cheers DMAC

mike95490 · September 2015

I would simply use a 24V truck alternator, wired to the battery via a disconnect breaker, and allow it's internal regulator to do the job of bulk and absorb charging. Since you are burning fuel, you want high amps (within the battery limits) and ignore needing to float the battery, that's what the sun is for!
And you don't ignore it, you manually monitor voltage so you can shut down ASAP

davemac · October 2015

mike95490 said:

I would simply use a 24V truck alternator, wired to the battery via a disconnect breaker, and allow it's internal regulator to do the job of bulk and absorb charging. Since you are burning fuel, you want high amps (within the battery limits) and ignore needing to float the battery, that's what the sun is for!
And you don't ignore it, you manually monitor voltage so you can shut down ASAP

Thanks Mike

Do you have a technical source for using an alternator for a direct charging plant (schematic / instructions?) that you know of? I have seen plenty of comments on forums and downloaded an e'book which described this approach but it required taking apart the alternator and rewiring and various modifications that resulted in a marginal result (in the authors opinion). I like the idea of doing this but when I look into the specifics, I find few replicable examples that I can emulate.

thanks again for the advice here, really appreciate it!

mike95490 · October 2015

I'm talking about using a native 24V alternator, not re-doing a 12V rig. That aside, most 12V alternators can be run up to 24V. The problem with them is heat, under full load, they heat up, HD Truck alternators handle the high load/heat better,

Photowhit · October 2015

Davemac, Most military vehicles are 24 volt, You ca likely find leads to a military vehicle grave yard at a army navy store.

davemac · October 2015

thanks Mike95490 & Photowhit ... i'm going to chase one down, spin it up and see what I can get ... hoping to hit 30 odd V & 50 A ... cheers!

MarkC · October 2015

Davemac - I may have missed something. Have you considered buying a small (cheap nowadays!) generator that you can use if your solar system is down for maintenance? You could then use it to power a golf cart style 24 volt charger - that has the proper bulk, absorption, float charge cycles. I use my "secure power supply" on my SMA Sunny Boy ON-GRID inverter to power a 15-18 amp - 48 volt golf cart charger and draws around 700-900 watts. In the US such a charger would be:

http://www.batterystuff.com/battery-chargers/brands/eagle/E2425.html

This might be on the small side of your 260 AH battery capacity and assuming FLA type, but something that fits your batteries.

davemac · October 2015

thanks MarkC ... on my build site I do have a 2500 W generator that should be able to output the required power to does some bulk charging. My original idea for building a direct charging plant from an 24 V truck alternator is the inherent efficiencies of that approach.
I am throwing this all into the mix and appreciate your ideas and input, many thanks!

davemac · October 2015

An idle thought on load management and running more than one inverter off a battery bank.

I wanted to ask if there were experiences out there in which multiple inverters were run off the same battery bank. I kind of like the idea of using my Victron 1200 W (in search mode) for intermittent loads (water pump, refrigerator etc) and a Morningstar Suresine 300 for lighting, laptop and small device recharging station. In effect I would have two independent AC outputs

I have read that you need certain loads for inverters to 'kick in' and wondered if there was merit in dividing & matching loads between (say) two inverters.

The battery bank is the major cost in the BoS and so I am interested in minimizing phantom and unnecessary loads and my reading is yet to discount a dual inverter concept.

vtmaps · October 2015

davemac said:

I wanted to ask if there were experiences out there in which multiple inverters were run off the same battery bank. I kind of like the idea of using my Victron 1200 W (in search mode) for intermittent loads (water pump, refrigerator etc) and a Morningstar Suresine 300 for lighting, laptop and small device recharging station. In effect I would have two independent AC outputs

It is a perfectly reasonable strategy. I wired my house for two inverters, but never got around to installing the small inverter. I just leave my big inverter on all the time. I figure that I could shave 200 watthours off my daily consumption with a smaller inverter, but I just haven't gotten around to it... there are bigger gains to be made elsewhere. Also, I have a 24 volt system and there are not many low power, quality inverters (like the Shuresine 300) at 24 volts.

--vtMaps

davemac · October 2015

Also, I have a 24 volt system and there are not many low power, quality inverters (like the Shuresine 300) at 24 volts.

--vtMaps

Thanks vtMaps! Interesting that. I imagine what the demand would be for a fan-less 24 V version of a Suresine 300 or same such?

mike95490 · October 2015

YES. splitting loads up is fine. Well pump on a large inverter, with sleep mode enabled.

davemac · October 2015

vtmaps said:

davemac said:

I wanted to ask if there were experiences out there in which multiple inverters were run off the same battery bank. I kind of like the idea of using my Victron 1200 W (in search mode) for intermittent loads (water pump, refrigerator etc) and a Morningstar Suresine 300 for lighting, laptop and small device recharging station. In effect I would have two independent AC outputs

It is a perfectly reasonable strategy. I wired my house for two inverters, but never got around to installing the small inverter. I just leave my big inverter on all the time. I figure that I could shave 200 watthours off my daily consumption with a smaller inverter, but I just haven't gotten around to it... there are bigger gains to be made elsewhere. Also, I have a 24 volt system and there are not many low power, quality inverters (like the Shuresine 300) at 24 volts.

--vtMaps

Thanks vtMaps

I have searched for a 24 V low wattage (~300 - 500 W) inverter with idle/phantom draw in the sub 1 watt range as per the 12 v Suresine 300, but nothing is really out there as vtMaps noted.

Does it make any sense to run a highly efficient 12V inverter off a 24 V bank (via a converter or other some such rigging) in the above mentioned split system?

My guess is that the voltage drop conversion would possibly defeat any efficiency gains.

DaveMac

Photowhit · October 2015

I wouldn't want to try the 12 volt route. ExelTech's 250 watt inverter draw 6 watts and 600 watt draws 8 watts in 'no load'. They are quality inverters and can be hard wired.

The idle mode on my Prosine has played havoc with small LEDs on power strips, and if you have just enough to turn them on they will start the inverter then power down not sensing the minimal load, only to start back up moments later.

davemac · October 2015

Photowhit said:

I wouldn't want to try the 12 volt route. ExelTech's 250 watt inverter draw 6 watts and 600 watt draws 8 watts in 'no load'. They are quality inverters and can be hard wired.
The idle mode on my Prosine has played havoc with small LEDs on power strips, and if you have just enough to turn them on they will start the inverter then power down not sensing the minimal load, only to start back up moments later.

thanks Photowhit

How do you correct for the cycling of your inverter issue, do you have a smart fix to ensure a suitable load is maintained (wall wart on a powerbar or some such)?

The Morningstar Suresine 12 V 300W has an idle draw of less than 1/2 watt and a standby on/off threshold of ~ 10 watts (that's good IMHO).

If I could find a 24 V version of quality inverter with these efficiency specs then I would be keen!

vtmaps · October 2015

davemac said:

If I could find a 24 V version of quality inverter with these efficiency specs then I would be keen!

I too have been researching 24 volt inverters, and I have found a quality 1000 watt pure sine wave inverter with an incredibly low 5 watt tare. I am referring to some of the Victron Phoenix inverters. The reason I have not purchased one is that they are NOT approved for hard wiring. They do have marine certifications, but they have outlets built into the inverter.

--vtMaps

davemac · October 2015

vtmaps said:

davemac said:

If I could find a 24 V version of quality inverter with these efficiency specs then I would be keen!

I too have been researching 24 volt inverters, and I have found a quality 1000 watt pure sine wave inverter with an incredibly low 5 watt tare. I am referring to some of the Victron Phoenix inverters. The reason I have not purchased one is that they are NOT approved for hard wiring. They do have marine certifications, but they have outlets built into the inverter.

--vtMaps

Agreed!

I have bought a Victron Phoenix 24v/1200w which will be servicing my water pump / chest fridge and recharging station for my offgrid studio.
All still in its box waiting for me to mount panels and wire the whole thing together. I also got their battery monitor which has all the info I need including DOD (SOC), AmpHrs used, alarms, a programmable relay and communication port ... even a bluetooth add-on so that my phone has the info on hand. Geeky, but neat!

zoneblue · October 2015

Is that 0.5W idle or sleep? I thought they were higher.

davemac · October 2015

I've attached a grab of the spec sheet FYI

Self Consumption 450 mA with AC switched on / 25 mA with AC switched off

davemac · October 2015

The above spec sheet is for the Morningstar Suresine 300

Small Off Grid PV Schematic

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