MPPT or PWM charger? For LiFePO4 battery?

[steveg]

SunriseSunsetChCh.jpg

Hi,

Here's Sunrise/Sunset for the rest of summer above

Yeah my record so far is close to 27kWh - I have the excess once my batteries are mostly full going into running my hot water cylinder element (3kW) which is 300litres, previous to this our average was much less even on a sunny day because we just didnt use what was available.

20kWh is only 6.5hrs sunshine at 3kw average - we get many more hours than that in summer.

We have sunrise 5am and sunset 9pm .... we get the sunshine hours even if the edges are at some strange angle and get only 400-500W incoming for the first few and last few hours

Of course we are in the summer months when I get these sorts of numbers - on a fine or only partly cloudy day. My regulator is feeding me these numbers, not my inverter, but there will be 1-2kwatt hours difference between these two which would be loss over the day.

On a fine day (mid summer) by the time I wake up and have breakfast at 7:15am we have already collected 500wh - Winter I have had breakfast and driven to work by the time the sun comes up, so big difference between the two.

So on the 15th Nov the total kWh counter was 5439kWh and on the 13th Dec it was 6010kWh so for those 28 days it averaged 20kwh for example.

Real numbers from my Morningstar TS-MPPT-60 (yes that's a max of 3200W full sun) - I over rated my array to collect more on cloudy days.

Rember we are quite south here - and in summer the sun sets say 8:30pm we have far more than 180 degrees of sun movement mid year (looks close to 240 degrees) but it bites in winter (probably only about 100 degrees) - the max we got mid winter was approx 13kWh on a fine sunny day.

This is all aside - we use LiFePO4 which is what the thread was about.

[steveg]

Steveg it hard to believe you can use 20kwh a day with a 4kw solar array??
If the 4kw array is the rated figure its never going to produce 4kw in just about any real life scenario.and even if it did there are going to be losses ,and you are never going to have a totally cloudless day.Will not anywhere I know on this planet of that is a livable area.

My calculated figure would be about 13kwh per day in summer. but then you have charger losses and battery losses and inverter losses..

Losses - well worst case numbers : the regulator specs 4% (20kWh = 800Wh), inverter specs 8% (20kWh=1.6kwh) and battery charging/discharge loss is 2% (worst case 12kWh = 120Wh) => *max* total loss of 2.5kWh

With PbA will be more due to more charge and discharge loss - correct me if I am wrong.

[steveg]

PV Watts shows data for three locations in NZ - they vary between 4500 and 5000 kWh/year projected average production - a far cry from 20 kWh/day.

With annual average insolation figures of 4.2 to 4.6 high production would be difficult.

The peak is 5.6 and we run from 5.08 to 4.9 Nov through Feb, which is the period I am talking about ... i.e. now. We did 4MWh last year before we attached the hot water diversion earlier this year, so I expect 5-6MWh in the next 12 months. Our cloud and clear sky numbers are not too bad too ...

We are noth facing at 25 degrees roof angle, so read the 28 degree row on the attached JPG

http://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi?&num=353047&lat=-43.4&submit=Submit&hgt=100&veg=17&sitelev=&step=2&p=grid_id&lon=172.11

Youll need to log in to get all the numbers (great NASA site btw)

ChChNZ_Insolation.jpg

[T1 Terry]

Actual daily kWh for house?

Hi steveg,
Thanks for the post and link to your blog. A few questions if you don't mind.
What is you daily kWh use for the house only as in, if you had absolutely no sun, what would you draw from the batteries?

Do you change the angle of the panels to better gain max benefit from the sun depending on the seasons?

Do you have any large body of water nearby?

The reason for the questions is I want to take my house off grid dependancy as the meters are going to be changed to "smart meters" soon so the distribution authority can load shead as required and the peak price is going to be 44.9c per kWh between 2.30pm and 8.30pm (think they were the hrs).

T1 Terry

[steveg]

Hi steveg,
Thanks for the post and link to your blog. A few questions if you don't mind.
What is you daily kWh use for the house only as in, if you had absolutely no sun, what would you draw from the batteries?

Do you change the angle of the panels to better gain max benefit from the sun depending on the seasons?

Do you have any large body of water nearby?

The reason for the questions is I want to take my house off grid dependancy as the meters are going to be changed to "smart meters" soon so the distribution authority can load shead as required and the peak price is going to be 44.9c per kWh between 2.30pm and 8.30pm (think they were the hrs).

T1 Terry

Hi Terry,

Our baseline load is of the order of 4-5kWh per 24 hours (that includes showers) but doesnt include cooking, TV, and incidental usage. Without showers say 3-4kWh (chest freezer, fridge, septic, lights)

No point in angling panels, for the extra $ for chassis etc might as well purchase another panel - right now Suntech panels are US$1.7 or less / watt - we are in a high snow and wind area so extra for chassis is significant.

Define large body of water nearby - we have the pacific ocean approx 50km away

The biggest challenge of off-grid is heating water to be honest - so you can either have a wood fire with wetback and solar HW (like us) or some other solution (gas boost etc).

We have approx 3 days of storage in our cells and used only a small number of hours during the year for generator running as per my blog - cost me NZ$60.00 from memory.\

My recommendation is PV is cheap - battery is expensive - so size battery to usable say 300-400Ah and panels like 4-5kWh for a larger household like ours - this means that on dimmer days you have power to heat water cook etc and on a sunny day you wont need more than say 3kW (max regulator load) to charge cells and heat water - this was my calculation and it seems to hold for our sunshine hours etc.

We have no need to use power for heating as we are then running the fireplace to keep warm and heat water - and we can cook on the fireplace also as it has a cast iron top.

Regards,

Steve.

[T1 Terry]

Thanks Steve
I have 720Ah @ 12v nom of LYP cells in use at the moment and 2kw of solar so you have twice as much of both, I have more battery but I'm light on for room to fit more solar that won't be shade effected by all the big trees here.
The reason for the large body of water question is because our place is bordered (with 1km each way) by a large lake on one side and the Pacific ocean on the other. With the right cloud cover my panels will sometime put out near their rated output even though I'm only using a PWM controller to regulate a solid state relay. The panels are on adjustable frames and I move them up and down till they give the highest output into deeply discharged batteries. Takes me a while to disconnect each set and do the adjustments but it worth it 3 times a yr. The angle never matches that given by the various authorities and I think this is to do with the reflected sun form the water and back off the clouds.

5kW per 24hrs, wish I could get ours down that low, if I hit 10kW per 24hr I'll be happy and seriously consider dropping off the grid. I'm weighing up whether the $220 yr is worth keeping the grid connected just so the off peak 19c per kWh is available over night. We have a gas stove and gas heating but the water is solar/electric. Maybe a mod to use the solar panel excess to heat the water could be a worth while addition. I was looking at a method of using that as a load for controlling cell over voltage. The Jusi Cell logger 8 has an external alarm output and I could use that to drive a relay to turn the water heater on for 5 mins at a time but I don't have an inverter that would handle the 3.6kW element at the moment.

T1 Terry

[steveg]

Thanks Steve
I have 720Ah @ 12v nom of LYP cells in use at the moment and 2kw of solar so you have twice as much of both, I have more battery but I'm light on for room to fit more solar that won't be shade effected by all the big trees here.
The reason for the large body of water question is because our place is bordered (with 1km each way) by a large lake on one side and the Pacific ocean on the other. With the right cloud cover my panels will sometime put out near their rated output even though I'm only using a PWM controller to regulate a solid state relay. The panels are on adjustable frames and I move them up and down till they give the highest output into deeply discharged batteries. Takes me a while to disconnect each set and do the adjustments but it worth it 3 times a yr. The angle never matches that given by the various authorities and I think this is to do with the reflected sun form the water and back off the clouds.

5kW per 24hrs, wish I could get ours down that low, if I hit 10kW per 24hr I'll be happy and seriously consider dropping off the grid. I'm weighing up whether the $220 yr is worth keeping the grid connected just so the off peak 19c per kWh is available over night. We have a gas stove and gas heating but the water is solar/electric. Maybe a mod to use the solar panel excess to heat the water could be a worth while addition. I was looking at a method of using that as a load for controlling cell over voltage. The Jusi Cell logger 8 has an external alarm output and I could use that to drive a relay to turn the water heater on for 5 mins at a time but I don't have an inverter that would handle the 3.6kW element at the moment.

T1 Terry

Hi T1 Terry,

I lived in Sydney for 10yrs - where are you in NSW?

What you should look at is putting a timer on and using it to switch a charger when you need it.

Possible to move to a 48 or 24V system? - higher dc system leads to less loss and lighter loads - 10A at 12V is 120W 10A at 48V is 480W it's the amps that lead to loss in your wiring for example, you can also get larger inverters at a higher voltage. and ontop of all that higher current leads to faster battery aging as you are pulling more current out of it - 100A 12V = 1200W, 100A 48V = 4800W, which number would you hit more often...

Manually (or timer etc) switch your hw only to turn on during off-peak maybe? (if you dont do this already) - also if you can live with it make it so it only switches on once solar HW has done all it can to heat your water - use electrical from grid only to top up HW.

If you can get more panels and a higher voltage inverter ie.. more Watts then you can use extra power to boost HW - works well here, and we have less sun on average than you. We burn plenty'o wood otherwise to heat water - can you move to a wetback also?

Efficient fridge is a good plan too - and lower power PC's i.e. laptops

Regards,

Steve.

[T1 Terry]

Hi Steve, I live not far from the banks of Lake Illawarra just past Wollongong, fairly heavily housed so wood heating isn't really an alternative these days, wood is too expensive.
I have a 48v 5kw UPS inverter charger still in it's box but I still need to better organise how to charge the battery pack at 48v nom. The higher amp draw has little effect on Li batteries, their 3,000 cycles rating is at 0.5C discharge/charge routine and
I certainly couldn't pull that sort of current in a house system.
If I link 4 panels in series to create the 48v nom. the Mpv would be 60v to 68v while the battery voltage would be 51v to 55.2v , not very efficient, then there is the loss of output from 4 panels if one is shaded. I've already been through the MPPT scam and that is the biggest rout since the R12/R134 scandal.
I could run 18 cells for nom. 54v, this would give me a battery range of 57.6v to 62.1v, fairly close to Mpv but running very close to the wire for the inverter over voltage of 64vdc. Something to look at in the new year.

The hot water is solar with manual electric back up, it's a matter of making a switchable system that is triggered by individual cell voltage so I know the batteries are near full before solar generation is wasted heating water. 18 cells creates a logistics problem monitoring and balancing them without going over board with electronic circuitry. After quite a few years in the industrial fitting world and a life time in the automotive world I know just how unpredictable micro processor control is so I'd much prefer not to go down that path.

T1 Terry

[steveg]

Hi Steve, I live not far from the banks of Lake Illawarra just past Wollongong, fairly heavily housed so wood heating isn't really an alternative these days, wood is too expensive.
I have a 48v 5kw UPS inverter charger still in it's box but I still need to better organise how to charge the battery pack at 48v nom. The higher amp draw has little effect on Li batteries, their 3,000 cycles rating is at 0.5C discharge/charge routine and
I certainly couldn't pull that sort of current in a house system.
If I link 4 panels in series to create the 48v nom. the Mpv would be 60v to 68v while the battery voltage would be 51v to 55.2v , not very efficient, then there is the loss of output from 4 panels if one is shaded. I've already been through the MPPT scam and that is the biggest rout since the R12/R134 scandal.
I could run 18 cells for nom. 54v, this would give me a battery range of 57.6v to 62.1v, fairly close to Mpv but running very close to the wire for the inverter over voltage of 64vdc. Something to look at in the new year.

The hot water is solar with manual electric back up, it's a matter of making a switchable system that is triggered by individual cell voltage so I know the batteries are near full before solar generation is wasted heating water. 18 cells creates a logistics problem monitoring and balancing them without going over board with electronic circuitry. After quite a few years in the industrial fitting world and a life time in the automotive world I know just how unpredictable micro processor control is so I'd much prefer not to go down that path.

T1 Terry

Hi T1 Terry,

You live in a lovely part of the word there!

you are best to stick to 16 cells - I use a MPPT controller only because it has current limiting and feedback features not because I expect more power from my panels MPPT benefits are very questionable - I erred to the side of MPPT because it was the newer unit, but I guess PWM would work just as well?

The HW switch over - mine is a 20A insulated triac which is being driven by a opto isolated triac driver - so a small 3v signal can now turn on/off my 3kw element - happy to share the details if you are keen on making a small micro controller board to measure battery voltage and turn element on/off - mine turn on at 55.3V and of at around 53.2V (55.3 is pretty close to full charge)

The 53.2V means that if something else turns on the voltage drop will cause the micro to turn off the element and wait till the voltage gets back to 55.3V.

Regards,

Steve.

[T1 Terry]

Cell balancing

Hi Steve,
If you use accumulative cell voltage as the trigger how do you use any other method to keep the cells in balance?
I'm leaning toward using the alarm port on the Cell Log8 monitors and using 3.8v as a switch point and using a Jay Car timer circuit running the load for 10 mins but I may bring that back to 3.6v and drop the load back to 5 mins each time high voltage is reached, I will need to trial it first. On another forum a member is trialling a kit to control the 240vac side to 1,000w, if this is a winner I can simply switch the water heater on and off and hard wire it into the 5kW output from the inverter. The element in the HWS is a 3.6kW unit so I'm concerned about this load switching on while other loads are already running like the fridges and/or microwave. Possibly making an issue of a non problem as the inverter is 20% overload rated for 30 mins operation making it 6000w max output, allowing for 20% inefficiency at the overload stage makes that a 7200w battery load absolute max. Worst case scenario, at the moment all the loads turned on a cloud blocked the sun stopping any solar input the max battery load would be 140 amps for 10 mins, 23Ah...... If the inverter can handle it I'm sure the batteries will. Inverter temp derating could be the issue...... Might need to keep the 100w controller for the HWS as a back up plan B.

Sorry for the rambling, funny how typing it through here sorts the issues as I go Think I'll copy this post into my personal folder as a reminder of what plan A & B were so if you see bits I

[steveg]

Hi Steve,
If you use accumulative cell voltage as the trigger how do you use any other method to keep the cells in balance?
I'm leaning toward using the alarm port on the Cell Log8 monitors and using 3.8v as a switch point and using a Jay Car timer circuit running the load for 10 mins but I may bring that back to 3.6v and drop the load back to 5 mins each time high voltage is reached, I will need to trial it first. On another forum a member is trialling a kit to control the 240vac side to 1,000w, if this is a winner I can simply switch the water heater on and off and hard wire it into the 5kW output from the inverter. The element in the HWS is a 3.6kW unit so I'm concerned about this load switching on while other loads are already running like the fridges and/or microwave. Possibly making an issue of a non problem as the inverter is 20% overload rated for 30 mins operation making it 6000w max output, allowing for 20% inefficiency at the overload stage makes that a 7200w battery load absolute max. Worst case scenario, at the moment all the loads turned on a cloud blocked the sun stopping any solar input the max battery load would be 140 amps for 10 mins, 23Ah...... If the inverter can handle it I'm sure the batteries will. Inverter temp derating could be the issue...... Might need to keep the 100w controller for the HWS as a back up plan B.

Sorry for the rambling, funny how typing it through here sorts the issues as I go Think I'll copy this post into my personal folder as a reminder of what plan A & B were so if you see bits I’ve got wrong please let me know, personal blinkers to issues can be a real problem sometimes There is no one around here I can bounce ideas off, mention contemplating a 7200w load on a battery bank for 20 mins and their eyes roll.

T1 Terry

Hi,

My pack came wtih a BMS built in it - blog has all the details - http://offgridnz.livejournal.com all I do is hook the solar reg directly to the charge in terminal of the pack and the inverter to the discharge terminal - easy.

The pack has it's own cell balancing circuit - justa few 100 mA current balance, but seems to do the trick lovely - if you want make an flyback converter that'd be the way to go - I was thinking of making such a beast as there seems to be a lack of high performance balancers out there.

You will find the voltages can be tuned fo ryour system for on/off - be sure not to 100% charge your cell too - from what I have read 90% full and 30% empty are good way points for longer life - my full charge (set by regulator) is 56.4V

btw - I have found someone else here in NZ just yesterday with a 48V LiFePO4 system - I will see how his system is configured.

Regards,

Steve.

[T1 Terry]

Not real keen on full time active BMS, too many reports of lost cells from those type BMS units going astray. I'm more into passive monitoring individual cell voltages with an alarm for over and under voltage of the individual cells, it gives a clear warning if anything needs attention. We found a loose cell linking plate on a 24v nom. bank this way because one set of paralleled cells hit over voltage first every time yet had the lowest voltage every time under high load, basically it was one cell down on capacity.
I set max boost voltage to an average of 3.5v per cell, absorption to 3.75v average and float to 3.45 average. The alarm function catches any runaways but with the way the system set up as a 12v nom. the charging amps when/if the sun appears is between 75 amps and 125 amps so runaways happen very quickly if the cells get too much out of balance. This is the reason for looking at a dump load control into the HWS, better to use the excess solar than waste it. I guess that means I will need to lift the max. boost voltage to 3.6v per cell to ensure the solar excess is used by the HWS, more trialling to do.

T1 Terry

[steveg]

Not real keen on full time active BMS, too many reports of lost cells from those type BMS units going astray. I'm more into passive monitoring individual cell voltages with an alarm for over and under voltage of the individual cells, it gives a clear warning if anything needs attention. We found a loose cell linking plate on a 24v nom. bank this way because one set of paralleled cells hit over voltage first every time yet had the lowest voltage every time under high load, basically it was one cell down on capacity.
I set max boost voltage to an average of 3.5v per cell, absorption to 3.75v average and float to 3.45 average. The alarm function catches any runaways but with the way the system set up as a 12v nom. the charging amps when/if the sun appears is between 75 amps and 125 amps so runaways happen very quickly if the cells get too much out of balance. This is the reason for looking at a dump load control into the HWS, better to use the excess solar than waste it. I guess that means I will need to lift the max. boost voltage to 3.6v per cell to ensure the solar excess is used by the HWS, more trialling to do.

T1 Terry

Hi,

I have just measured my cells - with 60A coming in they are all within 70mV of each other - I would rather have a BMS that should wsork for the life o fthe cells than worry about checking them once a month, but hey that's just me. My pack came with everything all ready to go, so this was the easy path US$7k for 48V 300Ah including metal housing and BMS - not too bad I thought.

LiFePO4 should never have absorption - and never above 3.65V per cell, so maybe set absorb to off or to 3.65V/cell and float to same - as there is no 'float' wtih this technology cell - just a thought.


My regulator is set to 56.4V constant voltage with a 60A current limit - simple setup and the BMS handles the over/under charge if that happens.

Regards,

Steve.

[T1 Terry]

Just realised I missed a digit in the absorption voltage, it should have been 3.475v. So, 3.5v boost, 3.475v absorption and 3.45v float. surprisingly the batteries will take another 20Ah to 30Ah during that absorption phase but the cells are protected from over voltage, float just keeps them full till the sun goes down.
You can only measure the cell balance of fully charged cells, voltages above 3.45v while being charged at less than 10 amps, if 60 amps were going in it was no where near fully charged at the time you measured the cell balance. If they were 70mV ot of balance at that point you might get a big scare when you see what the imbalance is at fully charged and just how high the high cells go. Anything under full voltage my cells remain within 10 mV

T1 Terry

[steveg]

Just realised I missed a digit in the absorption voltage, it should have been 3.475v. So, 3.5v boost, 3.475v absorption and 3.45v float. surprisingly the batteries will take another 20Ah to 30Ah during that absorption phase but the cells are protected from over voltage, float just keeps them full till the sun goes down.
You can only measure the cell balance of fully charged cells, voltages above 3.45v while being charged at less than 10 amps, if 60 amps were going in it was no where near fully charged at the time you measured the cell balance. If they were 70mV ot of balance at that point you might get a big scare when you see what the imbalance is at fully charged and just how high the high cells go. Anything under full voltage my cells remain within 10 mV

T1 Terry

Hi,

No big scare? - they are always <70mV at any point, the worst case will be under full charge load as this accentuates the cell differences.

My full charge voltage is 3.525V per cell

Regards,

Steve.