Upcoming Solar/LiFePO4 project

**Mike90250** · 04-11-2016, 07:42 PM

Whoa. Better look at what your winter loads will be. Pellet stoves have an electric glow coil that keeps the pellet cup hot & burning, There are only a few that require only a little bit of power for the fans and auger motors.

As far north as you will be - I would suggest look into a Masonry Heater. It will need a re-design of a house to allow the radiant heat out to the rooms, and the foundation needs to be extra beefy under the heater. But the quality and efficiency will not matched. It has to be experienced to be believed
http://www.mha-net.org/ We put a water heating SS loop in ours, and get preheated water for our tankless heater too.

**extrafu** · 04-11-2016, 08:00 PM

Originally posted by Mike90250

Whoa. Better look at what your winter loads will be. Pellet stoves have an electric glow coil that keeps the pellet cup hot & burning, There are only a few that require only a little bit of power for the fans and auger motors.

As far north as you will be - I would suggest look into a Masonry Heater. It will need a re-design of a house to allow the radiant heat out to the rooms, and the foundation needs to be extra beefy under the heater. But the quality and efficiency will not matched. It has to be experienced to be believed
http://www.mha-net.org/ We put a water heating SS loop in ours, and get preheated water for our tankless heater too.

Thanks for the quick reply and valuable info. From what I've read, the Froeling pellet boilers (~40kW) take about 88 watts of electricity to suck pellets in. There will be also a pump for the radiant floors. I don't know yet how much energy these will take but that is why I estimated 6 to 7 kWh. So far my list comes up to 5.75 kWh but I want to give myself some room.

We also indeed think about having a wood stove insert in the house. The heating requirements should still be quite small, being a solar passive house, aiming at R-60 wall insulation.

What do think about the components of my "solar design" ?

Thanks!

**karrak** · 04-11-2016, 09:56 PM

Firstly to answer your questions with the caveat that I don't have any of the Schneider/Conext equipment you mention and my answers are based only on a quick look at the manuals.

Yes, you can connect the charge controllers in parallel at the battery terminals.
Yes, the Conext XW+ will charge the battery from an external AC source which could be a generator.
Good quality Inverters will have good quality capacitors which should last more than ten years

I am intrigued as how you have come up with this list of equipment. To me, the Schneider/Conext XW+ 6848 inverter looks like it has much more functionality than you will ever need being off-grid. Have you looked at other off-grid equipment?

It is very important to do your energy budgets as accurately as possible before you start. I would find out how much power the underfloor heating pump uses. Your figure of 5.75 kWh per day is very low by most peoples standard. Have you calculated what your peak instantaneous load is likely to be.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

**Mike90250** · 04-12-2016, 02:31 AM

With the XW gear (easier to type than Schneider/Conext) you are really going to want a ComBox. It costs a bundle for what it is, but it does a nice job of logging and presenting data, And, you can use it to copy settings to a flash drive, and restore them
I don't think the inverter is overkill. (maybe because I loved an old video game called Overkill) The inverter is rock solid, nothing I've used makes it blink. 3Hp floor sander, 24V welder, pumps, workshop tools, it runs everything, And the Generator interface, while clunky to figure out, works great, which is going to be used a lot for 6 months of the year. In pure off-grid mode, it's pretty efficient, only in the hybrid grid selling modes, does it suck that 120watts .
In your combiner box, use DC breakers, not fuses, if you have the choice. I might go with 3 charge controllers and 3 PV strings. Not knowing what your gear replacement time is, loosing half your solar to a bad controller for 1,2 or 4 weeks waiting for replacement, could hurt more than buying a 3rd controller.
I don't see ANY surge protection gear (I suggest Midnight SPD) for either DC or AC lines.

Mounts
Roof or pole mount? Can you change angle for snow shedding in winter?

LFP & BMS:
Most BMS systems interface with the charge controller. you have at least 2 controllers. When the battery gets full the BMS normally signals the controller to shut off, Not sure how that works with this gear, does the controller interface with the MODBUS ?
Have you considered Bottom Balance on the battery bank, and no BMS. Use the controllers to shut down charge (they are programmable with the SCP/Combox) and the inverter has it's own programmable LVD. Battery protected, no extra wires, just a initial set up, 20 day checkup, and annual check of the balance of the cell pack.

The idea of balance boards with lots of wires, being able to shunt 50 amps around a bunch of cells as they fill up, scares me, Failure rate for 1 board might be low indeed, but when you have bunches of them, your chance one will die and take a battery pack out with it, puckers my sphincter.

**karrak** · 04-12-2016, 10:59 AM

Originally posted by Mike90250

LFP & BMS:
Most BMS systems interface with the charge controller. you have at least 2 controllers. When the battery gets full the BMS normally signals the controller to shut off, Not sure how that works with this gear, does the controller interface with the MODBUS ?

Extrafu, Have you seen this document http://cdn.solar.schneider-electric....troduction.pdf?

Connection of a BMS to the charge controller and inverter in off-grid power systems is the weak link. If Schneider/Conext has sorted all this out that would be great, but I would think will come at a price.

Have you considered Bottom Balance on the battery bank, and no BMS. Use the controllers to shut down charge (they are programmable with the SCP/Combox) and the inverter has it's own programmable LVD. Battery protected, no extra wires, just a initial set up, 20 day checkup, and annual check of the balance of the cell pack.

The main problems I see with bottom balancing and no BMS are:-

the battery will not be balanced at the top end, so can't be charged to 100% without a BMS shutting off the charge controller.
to re-balance the battery you will have to completely discharge it, a bit of a pain if the battery is part of your power supply.
the re-balance will have to be done manually.
You won't know how far out of balance the battery is unless you do fully discharge it. With top balancing you will know if it is balanced every time it is fully charged.

The idea of balance boards with lots of wires, being able to shunt 50 amps around a bunch of cells as they fill up, scares me, Failure rate for 1 board might be low indeed, but when you have bunches of them, your chance one will die and take a battery pack out with it, puckers my sphincter.

I am not sure where the 50 amp cables comes from. If the battery is balanced to start with, balancing currents of only a few amps are needed. I am yet to hear of any stationary LFP batteries being killed by a fault in a BMS. I would think it far more likely that an LFP battery without a BMS will be damaged because of a fault or stuff-up. The BMS on my battery has saved my battery from damage due to a stuff-up. I suppose this makes me slightly biased.

The Orion Junior looks like a very good BMS. Another one you might want to look at is from Batrium.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

**extrafu** · 04-12-2016, 11:34 AM

Hi guys,

karrak, to answer your question, I indeed saw this document from Schneider. Unfortunately, it doesn't tell much except "please call us to discuss". It is fine by me and I will eventually call them

Mike90250 I was indeed considering a ComBox and some surge protection gear - I left these out of the picture in my initial post. Regarding the mounts, we haven't decided yet where we would put the solar panels. They will most likely NOT be on the rooftops - so we can have easy access to remove any snow that could build up on them.

Regarding the connection between the BMS and the charge controller, it is indeed a mystery for me. What I originally thought was that the charge controller will still try to charge the batteries, but the BMS would "ignore" incoming current in order to avoid overcharging the batteries. But if the BMS can tell the Schneider MPPT CC to stop charging, that is even better! The Orion BMS has a CANBUS interface, but the Schneider doesn't - so back to square one there if I want to make them talk to each-other.

I haven't yet considered bottom balancing, since I don't know anything about it but eager to learn (will do research on it later today).

Thanks!

**Mike90250** · 04-12-2016, 01:46 PM

Each MPPT charge controller puts out - say 20A when charging, near full. When the battery is 90% full (before it pops open) the controller stops charging.
you have to program the controller to the correct voltage. No BMS needed. The inverter has programmable LVD. You program it, and live with your choice. It either works or fails.
2 controllers and 1 inverter = 3 failure points.
Now if you use BMS with 40 balance boards (82 failure points), as each cell fills up, that balance board has to bypass the charging current, or the cell overfills and pops. If the charge current is 10 amps, the board HAS to bypass that, or else, Generally, BMS is designed to work with a specific size pack, specific load, and specific charger, so it can throttle charging back, stop overdischarge (LVD) and balance all cells. Normally balance boards manage 1A. I've not seen a off the shelf system to handle true off-grid usage, they are designed for cars that need full range/battery capacity.

**extrafu** · 04-12-2016, 03:06 PM

Thanks Mike90250 for your valuable input.

So just to make sure I fully got what you said. Say I go with the CALB CA400 batteries (3.2V), 16 of them. The specs for each battery is as follow:

NOMINAL/MINIMUM CAPACITY	400Ah @ 0.3C
STANDARD DISCHARGE CURRENT/AMPERAGE	0.3C-120
MAX DISCHARGE CURRENT/AMPERAGE	2C-800
STANDARD CHARGE CURRENT/AMPERAGE	0.3C-120
MAX CHARGE CURRENT/AMPERAGE	1C-400
MAX CHARGE VOLTAGE	3.65
DISCHARGE CUT-OFF VOLTAGE	2.5
CYCLE LIFE (80%)	2000
SELF DISCHARGE RATE (30 DAYS)	3%
INTERNAL RESISTANCE (milliohm)	0.4
CHARGE WORKING TEMP (F)	32 to 113
DISCHARGE WORKING TEMP (F)	-4 to 131
Metric
CHARGE WORKING TEMP (C)	0 to 45
DISCHARGE WORKING TEMP (C)	-20 to 55

So, if I wire them in series, I'll end up with a 51.2V / 400Ah for a total of 20,480 Wh.

Say I have 24 Canadian Solar CS6P-260P panels (30.4V Vmp and 8.56A Imp). Wire them in 3 arrays of 8 panels, 4 in series + 2 series in parallel - that will give me 121.6V/17.12A per array, or 2802W. The Conext MPPT 60 150 CC can handle 140V and 60A - so one should have no problem handling an array. Make it 3 CC like you suggested.

I would then connect the 3 CC in parallel to the battery bank - the 3 will be connected to the same two battery terminals.

Based on the battery specs, I would set the MPPT to stop charging the batteries when they reach 3.65V - or 90% of that value? (3.285V)

Since it's the inverter that is using power from the battery bank, I would set the inverter to not draw anymore if the voltage gets below 16x2.5V or 40V?

Before connecting all that and start using them, I would carefully bottom balance each battery to 2.5V and check for balancing as you described?

Thanks and sorry if I sound stupid - been reading on this for many weeks but I know I still have lots to learn.

ps. One other 'interesting' question I have is regarding the connection of the inverter to the battery bank. Should I use the same battery terminals as the MPPT charge controllers or the other two (ie., - / + on opposite batteries)?

**karrak** · 04-13-2016, 12:04 AM

Originally posted by extrafu

Thanks [USER="1076"]
So just to make sure I fully got what you said. Say I go with the CALB CA400 batteries (3.2V), 16 of them. The specs for each battery is as follow:

So, if I wire them in series, I'll end up with a 51.2V / 400Ah for a total of 20,480 Wh.

That sound reasonable to me, gives you around 3 days storage capacity which is similar to my setup. If you are prepared to use a generator on a regular basis you could get by with a smaller battery of around 260Ah, see posts by steveg for more information. By the way, I don't have a generator and with some load management haven't needed one for the three years I have had the battery installed.

Say I have 24 Canadian Solar CS6P-260P panels (30.4V Vmp and 8.56A Imp). Wire them in 3 arrays of 8 panels, 4 in series + 2 series in parallel - that will give me 121.6V/17.12A per array, or 2802W. The Conext MPPT 60 150 CC can handle 140V and 60A - so one should have no problem handling an array. Make it 3 CC like you suggested.

If you use the Midnight Solar Classic 150 you can get away with two charge controllers instead of three. The Classic 150 can also be shut down via an external BMS.

Based on the battery specs, I would set the MPPT to stop charging the batteries when they reach 3.65V - or 90% of that value? (3.285V)

I think Mike means 90%SOC, not 90% of the voltage. With reference to the charge graph below, charging to 3.285 volts will only charge the battery to less than 30%.
WinstonChargeCurve.jpg

I have done tests with my battery and charging up to 3.3 V/cell only charges to around 30%SOC. I have found that it is impossible to reliably charge an LFP battery to around 90% using solar power. The reason for this is that the current from solar power is not constant. I have found that charging at a charge voltage of 3.375 V/cell and terminating the charge at a current of C/20 results in a battery SOC of anywhere between around 75%-90% dependent on the weather conditions.

Since it's the inverter that is using power from the battery bank, I would set the inverter to not draw anymore if the voltage gets below 16x2.5V or 40V?

If you don't have a BMS, I wouldn't go below 48 Volts (3.0 V/Cell). With a BMS 2.8 V/cell is OK. There is very little energy left in the battery below 2.8 V/cell

ps. One other 'interesting' question I have is regarding the connection of the inverter to the battery bank. Should I use the same battery terminals as the MPPT charge controllers or the other two (ie., - / + on opposite batteries)?

Not sure what you mean here. I thought there were only one positive and negative terminal on each cell?

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

**Mike90250** · 04-13-2016, 12:23 AM

So, you can't just use % of the full and empty values. But you are on the right track. As the batteries charge and discharge, they have a flat spot in the middle of the cycle, which has a cliff that it falls off of. Same with charging, goes along just fine, and then when it's full, you have only a couple minutes to stop charging before you over do it.
There is a long discussion at this thread https://www.solarpaneltalk.com/forum...iron-phosphate
of how to measure and set things up, Even between mfg's the voltages vary. Charging batteries to 100% will kill them prematurely, Discharging deeply can kill them in 1 day, and driving one cell in a pack into reverse charge can start a fire when recharging.
Read the thread, take your chances. LFP is still very young, and the specs are still being decided upon. If you can use a pack of golf cart batteries for a year, and let the LFP industry come further along, there may be a better recommendation at that time.
BMS was designed for cramming every last bit of power into a car battery for the most range.
Bottom balance was designed to be fool proof - as long as the initial set up and yearly test is done right. Much easier to do than watering flooded batteries every month.

Sample curve shown below
life-discharge-series-a.png

**Mike90250** · 04-13-2016, 12:25 AM

So, you can't just use % of the full and empty values. But you are on the right track. As the batteries charge and discharge, they have a flat spot in the middle of the cycle, which has a cliff that it falls off of. Same with charging, goes along just fine, and then when it's full, you have only a couple minutes to stop charging before you over do it.
There is a long discussion at this thread https://www.solarpaneltalk.com/forum...iron-phosphate
of how to measure and set things up, Even between mfg's the voltages vary. Charging batteries to 100% will kill them prematurely, Discharging deeply can kill them in 1 day, and driving one cell in a pack into reverse charge can start a fire when recharging.
Read the thread, take your chances. LFP is still very young, and the specs are still being decided upon. If you can use a pack of golf cart batteries for a year, and let the LFP industry come further along, there may be a better recommendation at that time.
BMS was designed for cramming every last bit of power into a car battery for the most range.
Bottom balance was designed to be fool proof - as long as the initial set up and yearly test is done right. Much easier to do than watering flooded batteries every month.

Sample curve shown below

**extrafu** · 04-13-2016, 11:14 AM

Hi,
Thanks to karrak and Mike90250 for your detailed and very valuable answers so far.
I carefully read the thread Mike suggested and also: http://www.powerstream.com/LLLF.htm
The Conext XW seems to have a DC under-voltage shutdown feature (LVD). The manual says “DC under-voltage shutdown (immediate) occurs if DC voltage is below 32 V. The fault clears and the inverter restarts when DC voltage reaches V+4 V.” - I do guess and hope that value is configurable - as I would need to set it to 48V (or 3V per cell) for my LFP battery pack.
I would configure my MPPT charge controllers to stop charging when the battery bank reaches 57.6 (16 X 3.6V) - or roughly 95% of the max charge voltage to improve/extend my batteries life.
From what I’ve read, I have two charging options. First option is constant current (CC) up to 60% SOC and then, constant voltage (CV) to 100% (3.65V per cell) - this should take 3 hours (1+2). The second option is CC up to 95% SOC - which should take two hours. If I set my MPPTs to 57.6, I guess forced charging (2nd option) is the way to go to maximize sunlight availability.
Now, if each of my MPPT CC receives about 121.6V/17.12A per array (or 2081W - at very optimal performance), I guess the CC will be able to convert this into a 57.6V current at 48A (yes, I need to factor some loss). Since the max charge current is 400A per battery and they are in series, does that mean I could safely wire the 3 MPPT CC in parallel, connect them to the same two battery terminals, and charge at 57.6V at 144A?
If so and SOC of the battery bank was at 30% (down to 48V) and I want to bring it to 95% (up to 57.6V), does that mean it would need ~3.5 hours to charge at maximum capacity from the solar panels? (unlikely to be possible!). My rationale for this is as follow: my battery bank will operate from 57.6V down to 48V. Say it provides me with 400Ah when it goes from top to bottom - or 20480W at 3.2V nominal (most likely less then that). If I’m pumping in 6243W per hour (60 minutes) using my 3 MPPT CC, it woud take 197 minutes, or 3h17 to charge the whole bank.
Am I still continuing making more sense?

Thanks!

**karrak** · 04-13-2016, 01:15 PM

Originally posted by extrafu

I carefully read the thread Mike suggested and also: http://www.powerstream.com/LLLF.htm
The Conext XW seems to have a DC under-voltage shutdown feature (LVD). The manual says “DC under-voltage shutdown (immediate) occurs if DC voltage is below 32 V. The fault clears and the inverter restarts when DC voltage reaches V+4 V.” - I do guess and hope that value is configurable - as I would need to set it to 48V (or 3V per cell) for my LFP battery pack.

You can set this value to 48 volts, see pages 3-7,3-8,B-3 for details

I would configure my MPPT charge controllers to stop charging when the battery bank reaches 57.6 (16 X 3.6V) - or roughly 95% of the max charge voltage to improve/extend my batteries life.

You don't need to go that high. Charging to 3.45 V/cell and terminating the charge when the current drops below C/50 will get your battery to within a couple of % of full and won't stress it. Only problem with this if if you bottom balance the battery and don't have a BMS to shut down the Charge Controller there is a chance you will damage the battery as you can't have the battery balanced at both the top and bottom at the same time unless all the cells are exactly the same capacity.

From what I’ve read, I have two charging options. First option is constant current (CC) up to 60% SOC and then, constant voltage (CV) to 100% (3.65V per cell) - this should take 3 hours (1+2). The second option is CC up to 95% SOC - which should take two hours. If I set my MPPTs to 57.6, I guess forced charging (2nd option) is the way to go to maximize sunlight availability.

I can't see that you have two options. As far as I can see the charge controller will only do the charging sequence outlined on page 4-6 of the manual. There is no such thing as CC charging with solar. The charge current is dependent on the amount of sunlight which is variable.

Now, if each of my MPPT CC receives about 121.6V/17.12A per array (or 2081W - at very optimal performance), I guess the CC will be able to convert this into a 57.6V current at 48A (yes, I need to factor some loss). Since the max charge current is 400A per battery and they are in series, does that mean I could safely wire the 3 MPPT CC in parallel, connect them to the same two battery terminals, and charge at 57.6V at 144A?

Yes, that is correct.A word of warning, you will need the correct circuit breakers/fuses, cabling, connectors etc. and be able to make all the connections properly to make this safe.

If so and SOC of the battery bank was at 30% (down to 48V) and I want to bring it to 95% (up to 57.6V), does that mean it would need ~3.5 hours to charge at maximum capacity from the solar panels? (unlikely to be possible!). My rationale for this is as follow: my battery bank will operate from 57.6V down to 48V. Say it provides me with 400Ah when it goes from top to bottom - or 20480W at 3.2V nominal (most likely less then that). If I’m pumping in 6243W per hour (60 minutes) using my 3 MPPT CC, it woud take 197 minutes, or 3h17 to charge the whole bank.
Am I still continuing making more sense?

Thanks!

Except under a high load 48 volts is an SOC less than 10%. You are unlikely to achieve the 6.2kW figure for your solar array output very often. You should de-rate this by 20-25%. To get an accurate picture of how many solar panels you will need you should try to obtain the Insolation data for the area you are going to placing the system.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

**extrafu** · 04-13-2016, 02:38 PM

Hi karrak

Thanks once more for your valuable comments. I also read in the thread that Mike90250 suggested that it is advisable to use passive BMS such as this one: http://ev-power.com.au/webstore/ev-power-bms/cm090.html

These seem to prevent battery damage in case my self-monitoring skills go bad. I would need 16 of them, one by battery.

Any opinions on them?

Thanks,

Upcoming Solar/LiFePO4 project

Upcoming Solar/LiFePO4 project

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