MPPT or PWM charger? For LiFePO4 battery?

[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]

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]

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]

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]

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

[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]

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.

[russ]

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.

[john p]

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..

[steveg]

The easiest way for you to do that, is to obtain the 12V automotive "drop in" replacement batterys that have the BMS built into their case. They are quite pricey !

Hi,

Have been using LiFePO4 for 1.5yrs now - using TS-MPPT-60 on custom mode 60A current limit. I have a 300Ah pack at nominal 48V. Empty voltage 44V full charge 56.4 (I dont charge to 100%)

My Blog is here : offgrignz.livejournal.co.nz

More folks should consider it - I use no gas and cook with electricity - currently in summer we use 20kWh per day on average - we have a 4kW array

I got my pack wtih BMS from Seiden in China for US$7k - have under and over charge protection and balancing.

Regards,

Steve.

[john p]

Terry at work we have tested almost every MPPT and PWM and I can tell you . in real life the differences are not great.. in many cases no improvement at all. just a lot of extra expense. one of the good PWM chargers is the Steca copy sold by Jaycar for about $130 its available in 12 or 24v and 20 and 30a .they are what I use for lithium phosphate cells.

[T1 Terry]

The answer is NO but they "know" they are better

Even the electrical engineers have problems accepting that what happen in a lab under unrealistic conditions won't duplicate in the real world. The theory is correct, if you leave out the fact that the MPP drops as the panel voltage increases, if you use 12v or less as the acceptance load, iff you ignore the fact that there is no free lunch, all these electronics are going to convert some of the energy to heat because nothing is 100% efficient..... if you ignore all those things the it will work as described. When you try to tell them that panel in full sun on a 30degC plus day will be over 50 degC and the MPP is now around 14v they refuse to believe you, they refuse to test it just incase you might be correct and are much happier to live in their world of denial, they are engineers, how could they be wrong.
Eventually you just shake your head and walk away.

T1 Terry

[john p]

Terry if you had read all my post you would have seen I put on each cell a 3.9v zenner and a 10 ohm resistor to make sure no cell goes above that voltage and use a low voltage cut off switch set to 10v..This is more important as the cell voltage drops very quick below 2.8v and if a cell drops to below 2.2v it will die a sad death.Im only using 10ahr cells so 1w zenners are more than adequateas voltage levellers.. Make sure your charger cant do an :equalizing" charge.

I have found when you ask the people that are always saying how wonderful MPPT chargers are ,if they have done a direct comparison between a MPPT and a PWM on their panels and batteries and what were the differences,The answer is NO but they "know" they are better.

[T1 Terry]

Hi John,
I found the Plasmatronic PL series really good for any sort of battery charging. I'm into converting a bus into a mobile home and I've been experimenting with running household 240vac appliances via an inverter for around 5 yrs. I discovered the Li batteries by accident while browsing electric vehicle stuff, found the AEVA forum and later the DIY EV forum and that lead to an interest in Li batteries.
I have been researching these batteries and cells for some time before lashing out and buying them, $10,000 takes a while to save
From that I have set up a system for a mate who lives full time on the road, it's a 180ah 24v system and it will be my mobile test bed. We have 720w of the evil bay 120w no name panels fitted to the roof.

From my testing so far I've found that 13.8v to 14v is a good flat constant voltage for charging a 12v nom. system as the cells are fully charged by 3.4v or 13.6v in a 12v nom, battery pack.
Not trying to pick a fight on my second post but, your previous post recommending 14.8v made me wince a bit, even though 3.7v per cell is within their tolerance range it doesn't leave much room for safe if a cell goes into runaway before the other cells are fully charged.
Do you have a (shudder) BMS?

T1 Terry

[john p]

TI TERRY What you have said is mostly what ive been trying to tell people or ages .MPPT in many cases put less than 5%extra into batteries compared to good PWM ones and in all cases ive found adjustable voltage PWM ones work just fine with lithium phosphate batteries. The same cant be said for MPPT chargers and they cost at least double of PWM ones (in Australia)and come with shorter warranties..