I am about to build out this system and need some advice.

Rv_Adventure_Couple

So this diagram is for a system I am putting into our RV. I would appreciate if anyone would chime in and tell me if the diagram is correct. From the shunt I would also have a chassis ground. The inverter has low voltage disconnect at 10.6v. This system will be separate from the dc side of my RV, purely to power ac loads when we need them while dry camping.

Estragon

Fuses/breakers are sized to protect wiring but wire size not shown.  In particular, the 300a inverter fuse would be for really big wire.

Rv_Adventure_Couple

Oh sorry, The 300 amp fuse should be 150 amp for 1/0 and the CC wiring is 6awg. I realize the battery protect is overkill for the battery monitor, and I may not end up using it. The power wire for the battery monitor will probably be 14awg with an inline 15a fuse.

Estragon

That sounds better.  You might want to consider a DC rated breaker instead for the inverter tho.  It could surge to 500adc or so.  Or carry lots of spare fuses.  

BB.

I highly suggest that you measure your loads (will a Kill-a-Watt type power meter) to figure out what you need for energy. And that you focus on conservation (almost always cheaper to conserve vs generate power).

Are you looking at a LiFePO4 battery bank? If so, the bank needs to be kept above freezing when charging/discharging--Or you will kill your cells (or worse).

For the battery bank, if not using LiFePO4, I would suggest the first bank use 6 volt @ ~200 AH "golf cart" batteries. 2x 6 volts in series for 12 volts, and 1-3 parallel strings (depending on your loads). Cheap, rugged, relatively forgiving. And most people probably end up "murdering" their first bank or two.

A 3,000 Watt AC inverter is (in my humble opinion) serverily oversized for your battery bank. For flooded cell lead acid, a 300 AH battery bank @ 12 volts would suggest a 750 Watt maximum AC inverter. And for an RV, I would suggest that a 300 Watt PSW inverter is a better fit.

For a 12 volt battery bank, suggest around 1,200 to 1,800 Watt AC inverter is the "practical limit" for 12 volts (cable size, voltage drop). A 3kW inverter at full load takes:

• 3,000 * 1/0.85 AC inverter eff * 1/10.5 battery cutoff = 366 Amps max draw

That would drain your 300 AH battery bank in less than an hour (even Li Ion batteries).

You don't list any backup genset--A 400 Watt solar array, mounted flat to an RV roof does not collect that much energy... For example, a flat mounted array in Austin Texas:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Austin
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.83	3.41	4.40	5.25	5.62	6.36
Jul	Aug	Sep	Oct	Nov	Dec
6.56	5.94	4.97	4.05	3.07	2.63

Say, 3 hours minimum sun (spring/fall) per day:

• 400 Watts * 0.52 off grid system eff * 3.0 hours of sun per day = 624 Watt*Hours per day

A large AC inverter can take 20-40 Watts "just turned on" (vs a 300 Watt inverter with 6 Watts of Tare losses)... Your solar array in fall would just run your AC inverter:

• 624 Watt*Hours per day / 30 Watts Tare loss = 21 hours (no other AC loads)

Your thoughts?

-Bill

Marc Kurth

Based on your diagram, it looks like you have (8) 150 AH, 3v batteries?

Rv_Adventure_Couple

Marc Kurth said:

Based on your diagram, it looks like you have (8) 150 AH, 3v batteries?

correct.

Rv_Adventure_Couple

BB. said:

I highly suggest that you measure your loads (will a Kill-a-Watt type power meter) to figure out what you need for energy. And that you focus on conservation (almost always cheaper to conserve vs generate power).

Are you looking at a LiFePO4 battery bank? If so, the bank needs to be kept above freezing when charging/discharging--Or you will kill your cells (or worse).

For the battery bank, if not using LiFePO4, I would suggest the first bank use 6 volt @ ~200 AH "golf cart" batteries. 2x 6 volts in series for 12 volts, and 1-3 parallel strings (depending on your loads). Cheap, rugged, relatively forgiving. And most people probably end up "murdering" their first bank or two.

A 3,000 Watt AC inverter is (in my humble opinion) serverily oversized for your battery bank. For flooded cell lead acid, a 300 AH battery bank @ 12 volts would suggest a 750 Watt maximum AC inverter. And for an RV, I would suggest that a 300 Watt PSW inverter is a better fit.

For a 12 volt battery bank, suggest around 1,200 to 1,800 Watt AC inverter is the "practical limit" for 12 volts (cable size, voltage drop). A 3kW inverter at full load takes:

• 3,000 * 1/0.85 AC inverter eff * 1/10.5 battery cutoff = 366 Amps max draw

That would drain your 300 AH battery bank in less than an hour (even Li Ion batteries).

You don't list any backup genset--A 400 Watt solar array, mounted flat to an RV roof does not collect that much energy... For example, a flat mounted array in Austin Texas:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Austin
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.83	3.41	4.40	5.25	5.62	6.36
Jul	Aug	Sep	Oct	Nov	Dec
6.56	5.94	4.97	4.05	3.07	2.63

Say, 3 hours minimum sun (spring/fall) per day:

• 400 Watts * 0.52 off grid system eff * 3.0 hours of sun per day = 624 Watt*Hours per day

A large AC inverter can take 20-40 Watts "just turned on" (vs a 300 Watt inverter with 6 Watts of Tare losses)... Your solar array in fall would just run your AC inverter:

• 624 Watt*Hours per day / 30 Watts Tare loss = 21 hours (no other AC loads)

Your thoughts?

-Bill

Thank you for the in depth input.

The panels are on the sides of our RV so tilted slightly, I plan on adding 2 more 200 - watt panels eventually. I've got the inverter at 3000 watts so that I could power our ac for short periods from time to time. The battery bank is LifePO4 and depending on how well it impress's the misses, I will add another 300ah to it eventually. The DC side of the camper will stay as it is and wont connect to this battery bank, I have a 100ah deep cycle 12v lead acid for the DC side of my breaker box that has been doing the job for a year now.

I suppose I could return the 12v 3000 watt inverter and grab a 24v 2 or 3000 watt inverter. I just want to be darn certain I can run the ac on a 2000 watt inverter as that is what I promised the system could do. The AC unit itself uses 1,000 watts when running. I also installed a hard start capacitor in our AC unit so that the inverter can kick it on.

Our charge controller has low voltage disconnect, plus we hate freezing weather so we travel around it.

Rv_Adventure_Couple

Thanks

BB.

RV solar power systems and A/C. Not a good match in general. If you can find an "inverter-compressor" type AC (or heat pump) system, that can help a lot... Inverter type systems tend to have no surge current, and when you dial back the cooling, the smaller units can get down to 400 watts or less. Much easier on the power system. LG is making a window based inverter A/C. And many of the mini-splits (such as Panasonic) also have inverter based models.

Hopefully, some folks here can add their comments if they have an A/C equipped RV and what they are doing.

-Bill

Rv_Adventure_Couple

Yea I understand, I recently had 2 6v 225ah deep cycle batteries on our RV and while it would power the ac it would only power it for about 15 mins due to lead acids huge voltage drop. I have done some research on camper AC units and I have seen people with 13500 BTU units and only 200ah of LifePO4 run for an hour and a half safely. I do not intend to run ours that long, just long enough to cool the camper down in times of extreme heat before bed, or if it is raining and we cannot open the windows. So I figure that just to start our 300ah with our 12500 BTU unit will be good. LifePO4 cells on aliexpress are pretty darn cheap so I will prob be adding another 300AH for less than $900 sometime in the future, as well as 2 more 200 watt panels.

I found dometic makes a invert-compressor model but it also uses 500 more watts at its lowest compared to ours according to the website.

littleharbor2

Looks like the neg. output of the CC connects directly to the battery. It really needs to connect to the shunt to monitor the battery charging.

Rv_Adventure_Couple

littleharbor2 said:

Looks like the neg. output of the CC connects directly to the battery. It really needs to connect to the shunt to monitor the battery charging.

I had been told on another solar forum to run the BMS on the charge side only, to not connect it to the inverter because a 3000 watt inverter would need two 250a BMS's in parallel turning that pack into 2 12 batteries.

Rv_Adventure_Couple

littleharbor2 said:

Looks like the neg. output of the CC connects directly to the battery. It really needs to connect to the shunt to monitor the battery charging.

Like the first or second new diagram, or neither if you could explain.

Rv_Adventure_Couple

Rv_Adventure_Couple said:

littleharbor2 said:

Looks like the neg. output of the CC connects directly to the battery. It really needs to connect to the shunt to monitor the battery charging.

I had been told on another solar forum to run the BMS on the charge side only, to not connect it to the inverter because a 3000 watt inverter would need two 250a BMS's in parallel turning that pack into 2 12 batteries.

Or do you mean like this? I was told like this the inverter would burn up the bms. is this true, if so can you tell me why?

mcgivor

Using 12V nominal is a mistake with large loads, my opinion naturally, 24V would be better, allowing the same batteries to be utilized in a 8S arrangement. The BMS should be sized appropriately to handle the loads as it is responsible for over discharge protection, the low voltage disconnect of most inverters, designed for lead acid batteries, is far to low for LFePo4 and will cause permanent cell damage.

LiFePo4 are very sensitive to low and high cell voltages, the extreme speed at which the voltage collapses near the discharge knee is incredible making overdischarge protection essential. This phenomenon will be amplified at the high discharge currents associated with lower voltages.

Based on experience using an inverter type 9000 btu 26 SEER  AC unit which uses 500W maximum,  powered by a 24V nominal 400Ah  LiFePo4 bank in a 200 square foot room, my opinion is your useage would be very limited. Actually I'm surprised a 12500 btu unit  would be 120V not 240V, to tame inrush when the compressor starts up.

You really need to think this over if the system needs to be reliable.

.

mike95490

If an inverter is capable of burning up a BMS, then the battery bank/BMS (they work as a pair) is too small or the inverter too big.

 a 3Kw inverter on a 12V system is STUPID DANGEROUS.  3Kw at 12V 90% efficient is 275 amps. I defy you to find a 12V 3kw inverter under $500, that has internal wiring that is safe at nearly 300A.

littleharbor2

Rv_Adventure_Couple said:

littleharbor2 said:

Looks like the neg. output of the CC connects directly to the battery. It really needs to connect to the shunt to monitor the battery charging.

Like the first or second new diagram, or neither if you could explain.

The bottom pic looks right to me. 
I agree with others here regarding the 12 volt inverter. Can you sell it and get a 24 volt version?

Rv_Adventure_Couple

littleharbor2 said:

Rv_Adventure_Couple said:

littleharbor2 said:

Looks like the neg. output of the CC connects directly to the battery. It really needs to connect to the shunt to monitor the battery charging.

Like the first or second new diagram, or neither if you could explain.

The bottom pic looks right to me. 
I agree with others here regarding the 12 volt inverter. Can you sell it and get a 24 volt version?

I am looking into returning it for a 24v, one of the two below

https://www.amazon.com/gp/product/B07JQKQQTP/ref=ox_sc_saved_title_4?smid=ACVAOOIC561IF&psc=1

https://www.amazon.com/gp/product/B081YVNZX1/ref=ox_sc_act_title_1?smid=A30YX8KII65IAA&psc=1

Then I will just run all of the cells in series for 24v 150ah bank.

So to be clear - instead of the bms connecting direct to the batteries main negative, I should run it through the shunt's loads side and connect the shunt direct.

The charge controller wont be an issue as it can switch between 12/24, so I guess this is what I am going to do.

softdown

Guess you already made the purchases. I'd be in favor of running a genset when boondocking. You face several challenges with solar and RVing and wishes to run A/C. 400 watts of panels is probably too little. Then again, I don't know the particulars of your RVing. The campsites I have used are generally heavily wooded - a disaster for solar. If you camp in the open the sun heats your RV faster than a 400 watt panel can cool it - I would think.

Why not use more solar panels? Another possibility is charging the batteries with genset or shore power.

mcgivor

The  BMS needs to be connected directly to the battery  negative in order to provide protection, the shunt should be downstream of the BMS, failure to do this will negate all protection if the connection of the shunt were to fail .

Should the BMS be incapable of supporting the required current then a suitable one should be sourced, don't mess around attempting to bypass the safety features, that would be a fools game. The BMS needs to have priority, that can only be achieved if it is first in line, better yet get a bi directional BMS with separate charging and dischargeing ports, they can be had for around  $100 

Rv_Adventure_Couple

mcgivor said:

The  BMS needs to be connected directly to the battery  negative in order to provide protection, the shunt should be downstream of the BMS, failure to do this will negate all protection if the connection of the shunt were to fail .

Should the BMS be incapable of supporting the required current then a suitable one should be sourced, don't mess around attempting to bypass the safety features, that would be a fools game. The BMS needs to have priority, that can only be achieved if it is first in line, better yet get a bi directional BMS with separate charging and dischargeing ports, they can be had for around  $100 

I have revised the diagram, am I correct in this draw up?

softdown

Much better. Use the smallest inverter that you can. Why not two 400 watt panels?

Rv_Adventure_Couple

softdown said:

Much better. Use the smallest inverter that you can. Why not two 400 watt panels?

I had already purchased the 2 -200 watt panels a year ago, I have been kinda building the setup in stages so it wasn't as much of a financial hit. I have room in my CC and roof to put 2 more - 200 watt panels if it comes to that.

softdown

I made a similar mistake at first. Too much battery and too little paneling. There are 100 pitfalls waiting for the newcomer. I really think you need more panels. How much are these 200 watt panels you bought and/or can buy? 

Rv_Adventure_Couple

softdown said:

I made a similar mistake at first. Too much battery and too little paneling. There are 100 pitfalls waiting for the newcomer. I really think you need more panels. How much are these 200 watt panels you bought and/or can buy? 

$185 US each, however the ebay supplier I got them from is out of stock currently

softdown

If you can get more panels they need to closely match what you have. 

Rv_Adventure_Couple

softdown said:

If you can get more panels they need to closely match what you have. 

Just checked, the same seller has some 210 watt panels same amperage, is that close enough?

BB.

You need to match Vmp for parallel connected panels within 10% or better.

For series connected panels, Imp needs to match within 10% or better.

Bill

Rv_Adventure_Couple

BB. said:

You need to match Vmp for parallel connected panels within 10% or better.

For series connected panels, Imp needs to match within 10% or better.

Bill

current panels are vmp 21.052 and imp 9.50, the ones i found are 22.4 and 9.30

So if I did my math correct I can series and parallel them?

Rv_Adventure_Couple

One more question. Being that my CC is 150v / 40a mppt does that mean that I could run 4 of those panels in parallel since they are only 9.5 and 9.3 amps