Please critique my system

[thane]

I am getting ready to build a new home and will be living in a travel trailer onsite and off grid while building.

I put together an introductory system with what I could patch together for my travel trailer power... partly because I need power and mostly to get myself acquainted with off grid systems for my future home.

I got the PV panels and inverter VERY cheap, so my system revolved around what I could do with 12 volts and the use of mppt.

It has been operational for a week and it looks like I will have plenty of power, but winter will test it.

How did I do?  What did I do that should have been different?

[thirteen]

Is this the voltage your going to use in your house? Can your inverter charge the batteries? I have my genset wired into my inverter using the 220 v plug in 30 a breaker.It charges my batteries on long no sun days and helps when using power tools. for a temp system it looks fine. Just be careful with the batteries . I would suggest a cover if they are outside and kids can play with them and keep the battery bank down wind from the trailer so the fumes don't get inside. Keep records of the batteries temp and voltage. Best of luck. 13

[thane]

This probably won't resemble my full house system, but who knows... I will learn more along the way.

Yes, the inverter is wired to the generator and can start it when low volt cutoff or high load is triggered.

The whole system will be inside what I am calling a "Power Shed"... with sound proof vented area for the generator and separate area for batteries, etc.  The AGM batteries don't off gas much if at all.

One of my questions is... did I take the disparity of PV string volts to battery bank volts too far...? It averages 100V --> 13V.  I figured that's where mppt gains the advantage, but how much difference is too much?   The system is actually "charging" until it's almost dark outside.

[SparWeb]

Damn good start, if you ask me.
Not a lot jumps out at me as design problems.  Some "what-if" stuff is all.  It's a schematic, not a photo, so I have to ask: Have you actually built it yet?

Eight batteries in parallel is probably the biggest weakness.  If one battery begins to fail, then its inability to reach the same voltage as the others will cause it to rob current from the others.  One weak battery can kill the others in the parallel bank.  It will be necessary to watch the individual cell voltages, especially as they age, by disconnecting them all from the common power supply to the inverter, letting them rest, and looking for differences in resting voltage.  I wouldn't rely on specific gravity of the electrolyte alone.

There's another potential problem, but it depends on how you hooked up the common bus wires among the batteries - I won't know without a photo.

There are a thousand other details in every system that could be OK or potential disasters, from connectors, to wire gauges, to grounding, to the condition of the used equipment you have.

[thane]

It's all on my garage floor right now.  Panels are racked, but just sitting on saw horses. It's all connected and operating.  Just need a few more things... safety switches, conduits, etc.

Here are some photos prior to hooking up the Xantrex XW-MPPT60

 

[thane]

Other rationale for the configuration:

- The line conditioners became necessary while testing... the generator put out 131 volts and the inverter faulted... plus computers will be used, so I put one between the inverter and the Avion trailer.  Not sure of the losses in doing this?

- Plugging the inverter AC directly into the Avion trailers AC/DC converter makes it very convenient for using any loads.  I may have some losses here since the converter is on all the time... looks like a 1 amp load at all times.

- I'm keeping all wire runs as short as possible.  PV-to-MPPT 10AWG / MPPT-to-Batteries 6AWG / Batteries-to-Inverter 2AWG


I will take your advice on checking the resting batteries... maybe do 2 banks of 4.  The AGM 110Ahr batteries came from a big power backup system and saw little or no cycling... just floating.  They are 2 years old.

[dbcollen]

battery to inverter cables should be 4/0 with a 250a breaker.

[thane]

Typical sunny input / output:

[thane]

battery to inverter cables should be 4/0 with a 250a breaker.

Point taken.  I have limited the generator charging to 12 amps to keep from loading the generator too hard.  Plus, the inverter is only 2500 watts, so there won't much load on that end either.

[thane]

I live in the Puget Sound area outside Seattle and there is almost zero risk of lightning strikes around here.  I will be the only one messing with connections, so how much need is there for grounding this system?

[dbcollen]

battery to inverter cables should be 4/0 with a 250a breaker.

Point taken.  I have limited the generator charging to 12 amps to keep from loading the generator too hard.  Plus, the inverter is only 2500 watts, so there won't much load on that end either.

2500w at 12v is ONLY 208.33 amps. 1440w from the generator is 120a dc....... Your cables to the inverter are way too small. 2AWG is rated to 130a if it is 90c wire. only 95a if 60c rated. 0000 is rated to 260a @ 90c. You should also take pos from one end of the battery bank, and neg from the other end. That way all the batts see the same resistance in the cables. the batts farthest from your present connections to the inverter will receive less charge.

[thane]

Thank you for clarifying that in my head dbcollen!

I had my head in the AC side of things.  The short pig tails that I have on it now (see photo above) are 4/0.

I will plan accordingly and get some 4/0 and a 250VDC Breaker.

[thane]

"250VDC Breaker"...  I meant 250A DC

[madlabs]

I have the same inverter. The thing is a beast! It will put out far more than it's rated for. It even runs my welder OK.

To hook it to a genny and use it to charge, you are supposed to have a relay to switch grounds. I made one so that when you start the genny it flips the relay. The reason for this is because the neutral is bonded to ground at the inverter I believe and so are most genny's. I don't claim to be a grounding guru, this is what Trace told me to do.

Jonathan

[SparWeb]

Just going by the way the inverter is set up for the test, let me throw in that the "chain" of cables that make up the entire circuit from + to - of the inverter's DC input should be equal, from the perspective of each battery cell.  What I mean is that, the inverter's (-neg) cable should start at the negative of Battery 1 and the (+pos) cable should be connected to the positive of Battery 8.  This allows every battery to see the same resistance relative to the inverter.

Put it another way, if you connect the inverter to the + and - terminals of Battery 8, like the picture, then the resistance of that circuit is just the resistance of the 2AWG cables (should be 4/0 as pointed out before) cables to the inverter, but the resistance of the circuit to Battery 1 is the total of the 2 AWG cables plus all 14 of the other cables jumping from battery to battery.  A much higher resistance.  Even if you can't measure it with a typical multimeter, the resistance is there, and the inverter definitely can see it.  Move one of them, either pos or neg, from Battery 8 to the corresponding terminal on Battery 1.  The run has to be longer, but it looks like you need to change the inverter supply cables anyway.

[thane]

There are a thousand other details in every system that could be OK or potential disasters, from connectors, to wire gauges, to grounding, to the condition of the used equipment you have.

You were right about that!  Thank you for this valuable input.

I will be getting the correct 4/0 cables and connecting as you advise (along with dbcollen).

[joestue]

regarding these battery cable debates i would solder an 8 awg wire for each battery into the bolted terminal on the inverter.. yes, use a propane torch.

[Frank S]

I won't debate the 8 AWG  Idea that joestrue mentioned but to me that sounds a little too light on the wire size.
 I have a tendency to use way over sized cables for most everything that I do
 My 8 panel 1100 watt total array, has the leads of each individual panel brought to a single connection point. From there I have 2AWG leads running 45 ft to my PWM
 My Inverters (1 3000w xantrex & 2 1800w AIMs are connected via 1 ft long 000 & 1 ft long 00 leads respectively to a pair of 5 ft long 500 MCM cables.
 The CM6024Z  PWM and the Shumacher SE6030 both have 2AWG cables about 6ft long leading to the point where the 500 MCM cables connect to the bank
   MY bank currently is 6 220 AH golf cart & 2 4D 145 AH batteries
here is a shot of how the cables all come together Admittedly the connection point is double UGLY but it works for now and has done so for 15 months
 each pair of golf cart batteries connect direct to the 500 MCM via 1 ft 000 cables
 I've never bothered with trying to calculate resistance I just know that my inverters stay happy most of the time we use only 1 of the 1800w inverters to power everything the 2nd 1800w inverter is only there incase the 1st one goes bad, the 3000w is only used to power the things like my lathe & band saw or drill press

 Eventually I may up grade to a bank of 6 L16s then replace my inverters with 24 volt units