I purchased two 500W grid-tie inverters to go into the system, with a plan to expand upto 4, 6 and 8 depending on how quickly the battery discharged in the evening. (not draining the battery completely (to my pre-defined level) means I'm not utilising all the power the system captures during the day)
These came from China via EBay, so I gave them the usual strip down, inspect, rebuild treatment.
Quite a few solder joints were lacking in solder, albeit not actually broken or loose, but they were re-soldered anyway.
They had attempted to apply some vibration resistant measures (basically squirting special sealant around and under anything that has enough weight to move due to vibration), but in quite a few places this was woefully inadequate so their 'stuff' was removed and enough new sealant applied to do the job.
One worrying thing was in one location the sealant they had used was covered with small particles of metal swarf, they must have applied it and then somehow the board got metal swarf blown over it, and it stuck to the still sticky sealant - rather worrying!
One thing to pay special attention to in these things is any extruded heatsinks. They come in very long lengths and the manufacturer cuts them up into the required length. During cutting up this can leave sizeable bits of swarf that are only just attached to the heatsink that are hiding between the fins. There were a few inside these items. They won't cause problems if they stay where they are, but the slightest vibration can dislodge them and you then have a 0.5 ~ 1cm long sliver of metal floating around inside, just what you need to short out something and let the magic smoke escape.
The only other thing I found was a few screws clamping the diodes and transistors to the heatsinks not done up particularily tightly, an easy thing to sort.
So after checking them over and reassembling both inverters, I connected one up to the battery, it lit up, performed MPPT and promptly said the input voltage was too low.
Damn!
MPPT can be implemented in numerous ways, constant voltage, open circuit voltage, short circuit current, perturb and observe, incremental conductance and others.
See this PDF from TI for the technical details www.ti.com/lit/an/slva446/slva446.pdf
In essence a PV panel is a constant current device and has to be operated at the highest voltage possible, just where the current starts to dip, to get the maximum power out of it.
To do this the MPPT circuit varies the resistance it presents to the PV panel, the higher the resistance, the higher the voltage, lower resistance, lower voltage.
In one way of operation it will start at it's lowest resistance, lowest voltage, lowest power transfer, and increase the resistance, up and up, until it sees the power just start to dip. When it sees the dip it knows it is working as efficiently as that design can.
If you connect a battery to such a MPPT device, it will move through the range of resistance values it has, trying to find where the maximum voltage is, but of course we're using a battery which is not a constant current device, so the voltage will not change.
If we have a 24V battery, do we choose an inverter that works from a voltage range of 22V to 36V so our battery voltage is at the bottom of the range? Or 14V to 28V so our battery voltage is at the top of the range?
I thought about what would happen at the "other" end of the range and made my decision on that.
For the 22V - 36V MPPT circuit, if it raised its resistance up to the maximum (so the PV panel voltage would be ~36V), with a 24V battery the current would be at maximum at the 22V end, and a lot lower at the 36V end.
For the 14V - 28V MPPT circuit, it would be OK at the high end, presenting a high resistance so the PV panel produces 28V, our 24V battery would be feeding in just below the maximum rated current.
BUT at the bottom end, the 14V PV voltage end, the MPPT circuit would have a low resistance, and a 24V battery would be feeding in far more current than the inverter was designed to handle, and would either blow the fuses or die.
That was my theory anyway, so I chose inverters that have this spec:-
* Vmp 35 - 39V
* Voc 42 - 46V
* MPPT range 18 - 48V
* Input DC 15 - 60V
Trying to get the supplier / seller to provide me with meaningful specs was fruitless, so it was something of a guessing game.
Now I've found out that these do NOT work with a 26V battery I am more confident in saying that the above specs mean the following:-
* Vmp : Voltage where the unit is designed to transfer maximum power
* Voc : The maximum open circuit PV panel voltage.
* MPPT range : The voltage range the MPPT circuit can "track".
* Input DC : Less than 15V DC and the unit won't even turn on. And 60V is the absolute maximum voltage the inverter can handle (but do not run it at this voltage for any length of time)
I suspect that the MPPT voltage goes as low as 18V is because it takes more voltage for the unit to start up and "lock on", but once working it can continue working until the voltage drops below 18V. Once stopped I suspect it needs something close to 35V to start working again.
So I have two very nice 600W (or 500W depending which label you read) grid-tie inverters I can't use.
Back to EBay to buy one with Vmp that covers the operating voltage range of the battery.
Oh, every single manufacturer and seller I spoke to on AliBaba and EBay said grid-tie inverters just cannot be used with a battery. I can see why, you need to think carefully how the MPPT circuit works, take a gamble that it uses the method you think it does, and pray it does NOT use the short circuit method!
Richard
Friday 30 May 2014
Monday 26 May 2014
Solar storage project 26-5-14
Inspected the variable 100A charger today.
In spite of my worries about it's construction, a close inspection revealed a couple of screws that needed an 1/8th of a turn to get them nice and tight.
Looking round for dry or solder joints didn't reveal anything.
A quick test with the meter for blown semiconductors showed everything was fine.
It also showed that the metal case of the unit was only connected to the -VE DC output via a small capacitor. Now that isn't to the UK spec (and I suspect most of the rest of the world) where anything with a metal case must have the case connected to earth just in case the internal circuitry shorts out and connects mains directly to the output terminals. This is easily sorted with a new 3 core mains cable with the earth connected to the right places.
Happy that all was good as far as I could tell, plugged in to the wall and turned on.
All good.
Max voltage is 29.2V, minimum charging current is as low as 6.3A which is better than I was lead to believe, having been told that the minimum was around 20A. That's good news as I can still charge the batteries (albeit very slowly) when the solar panels are producing as little as (6.3A @ 24V) 150W.
Connected to the battery and out with the digital thermometer to see how hot parts inside the charger get.
Set it to 20A and everything is room temp (20 deg C) apart from the main HF transformer (30 deg C) and a balancing resistor between the high V main push - pull transistors (50 deg C).
Set to 50A charge current and HF transformer is upto 33 deg C, balancing resistor is 65 deg C.
That resistor is a ceramic enclosed wire wound type with has a max operating temp of 250 degrees C, so it is well within it's rated power, although it is down between the main HF transformer and a huge choke on the other side, so the air flow provided by the twin fans won't get that much air past it. I'll test it a bit more later on and may move it up further into the air flow if it gets over 150 deg C.
The absolutely massive heatsink increased in temp by 5 deg C after 10 mins @ 50A.
Overall it runs a LOT cooler than I expected it too. I've only run it upto 75A for a short period, as I suspect a brand new LifePO4 battery will not appreciate being charged at its absolute maximum charge rate for the very first charge!
I'll do a few cycles of full charge & discharge with the inverters and then do a 100A charge and check the charger temperatures.
Overall I'm very happy with it.
Yesterday I realised I'd forgotten to buy any fuses for anything - they're on the parts list but got overlooked. So an order for the large DC fuses and smaller AC fuses plus holders and a nice little DC fuse panel which will be perfect was put in earlier today. Should be here towards the end of the week when I'll be able to lay out everything and sort out the cabling.
In spite of my worries about it's construction, a close inspection revealed a couple of screws that needed an 1/8th of a turn to get them nice and tight.
Looking round for dry or solder joints didn't reveal anything.
A quick test with the meter for blown semiconductors showed everything was fine.
It also showed that the metal case of the unit was only connected to the -VE DC output via a small capacitor. Now that isn't to the UK spec (and I suspect most of the rest of the world) where anything with a metal case must have the case connected to earth just in case the internal circuitry shorts out and connects mains directly to the output terminals. This is easily sorted with a new 3 core mains cable with the earth connected to the right places.
Happy that all was good as far as I could tell, plugged in to the wall and turned on.
All good.
Max voltage is 29.2V, minimum charging current is as low as 6.3A which is better than I was lead to believe, having been told that the minimum was around 20A. That's good news as I can still charge the batteries (albeit very slowly) when the solar panels are producing as little as (6.3A @ 24V) 150W.
Connected to the battery and out with the digital thermometer to see how hot parts inside the charger get.
Set it to 20A and everything is room temp (20 deg C) apart from the main HF transformer (30 deg C) and a balancing resistor between the high V main push - pull transistors (50 deg C).
Set to 50A charge current and HF transformer is upto 33 deg C, balancing resistor is 65 deg C.
That resistor is a ceramic enclosed wire wound type with has a max operating temp of 250 degrees C, so it is well within it's rated power, although it is down between the main HF transformer and a huge choke on the other side, so the air flow provided by the twin fans won't get that much air past it. I'll test it a bit more later on and may move it up further into the air flow if it gets over 150 deg C.
The absolutely massive heatsink increased in temp by 5 deg C after 10 mins @ 50A.
Overall it runs a LOT cooler than I expected it too. I've only run it upto 75A for a short period, as I suspect a brand new LifePO4 battery will not appreciate being charged at its absolute maximum charge rate for the very first charge!
I'll do a few cycles of full charge & discharge with the inverters and then do a 100A charge and check the charger temperatures.
Overall I'm very happy with it.
Yesterday I realised I'd forgotten to buy any fuses for anything - they're on the parts list but got overlooked. So an order for the large DC fuses and smaller AC fuses plus holders and a nice little DC fuse panel which will be perfect was put in earlier today. Should be here towards the end of the week when I'll be able to lay out everything and sort out the cabling.
Sunday 25 May 2014
The start of my Solar Storage project
This blog is where I'll document my DIY Solar Storage project.
Inspired by Christophe Huberts system which you can read all about here http://www.diyesskit.com/p/what-is-it.html
Brief specs of the first stage of the system:-
* 5KW LifePO4 battery pack
* 10A ~ 100A LifePO4 battery charger (adjustable constant current)
* 2 x 600W output Grid Tie inverters
* 100A solid state relays for the battery charger and battery* 40A solid state relays for the AC input to the charger, AC output of the inverters and DC input of the inverters
* 100A current shunt
* 2 x AC induction current sensors
* Raspberry PI micro computer controller
* 4 x 16 character display for the PI
* 32 digital I/O board for the PI
* 8 analogue input, 8 analogue output board for the PI
Some pictures:-
The 5KW LifePO4 battery pack
(it has 16 x 3.2V 200Ah flat pack cells in it in a 2P8S configuration for 5120W capacity, each cell is 2.5Kg, making the total weight 40Kg - not exactly nice and light)
The two 600W Grid Tie inverters
All the SS relays
The 24V, 100A battery charger
The charger is in bits as I want to check it over very thoroughly before using it. Two reasons for doing this, it was bought from China, and from past experience their QC isn't as comprehensive as we are used too, and secondly the journey from China to the UK may well have shaken things loose or caused some solder joints to crack - as you can see the components aren't exactly small and if they are jarred, the solder joints can easily crack.
Checking it over carefully now can make the difference between it going bang when I plug it in and it becoming a $600 paperweight, and it lasting for years.
After a quick check of the components in it (things like capacitors rated to 105 degrees C rather than 115, or 125, the additional thick wire soldered to the high current tracks on the PCB), leads me to believe that even though it may be able to charge at 100A, it will extend it's life a lot if I use it to, say, 75A maximum.
A variable current LiPo charger was chosen as I can drive the charge current control directly from the PI controller, which will allow me to exactly match the power taken by the charger to the excess power generated by the solar panels. The alternative was to have 'x' smaller (say 10A) chargers and turn more and more of them on or off as the excess power from the solar panels allowed. The latter would work fine but cannot perfectly match the solar excess power to the power going into the battery. Deciding to go with this type of charger is a bit of a gamble though, they aren't cheap and I hope the efficiency of the charger and the extra charge I can get into the battery on a dull day will be worth it. Time will tell.
After I've checked over the charger I have a couple more bits to get (better crimp connectors, 200A fuses, more cable), finish the initial version of the controller code, finish making the buss bars and decide how to lay everything out and find a heavy duty box to put everything in.
Richard
Inspired by Christophe Huberts system which you can read all about here http://www.diyesskit.com/p/what-is-it.html
Brief specs of the first stage of the system:-
* 5KW LifePO4 battery pack
* 10A ~ 100A LifePO4 battery charger (adjustable constant current)
* 2 x 600W output Grid Tie inverters
* 100A solid state relays for the battery charger and battery* 40A solid state relays for the AC input to the charger, AC output of the inverters and DC input of the inverters
* 100A current shunt
* 2 x AC induction current sensors
* Raspberry PI micro computer controller
* 4 x 16 character display for the PI
* 32 digital I/O board for the PI
* 8 analogue input, 8 analogue output board for the PI
Some pictures:-
The 5KW LifePO4 battery pack
(it has 16 x 3.2V 200Ah flat pack cells in it in a 2P8S configuration for 5120W capacity, each cell is 2.5Kg, making the total weight 40Kg - not exactly nice and light)
The two 600W Grid Tie inverters
All the SS relays
The 24V, 100A battery charger
The charger is in bits as I want to check it over very thoroughly before using it. Two reasons for doing this, it was bought from China, and from past experience their QC isn't as comprehensive as we are used too, and secondly the journey from China to the UK may well have shaken things loose or caused some solder joints to crack - as you can see the components aren't exactly small and if they are jarred, the solder joints can easily crack.
Checking it over carefully now can make the difference between it going bang when I plug it in and it becoming a $600 paperweight, and it lasting for years.
After a quick check of the components in it (things like capacitors rated to 105 degrees C rather than 115, or 125, the additional thick wire soldered to the high current tracks on the PCB), leads me to believe that even though it may be able to charge at 100A, it will extend it's life a lot if I use it to, say, 75A maximum.
A variable current LiPo charger was chosen as I can drive the charge current control directly from the PI controller, which will allow me to exactly match the power taken by the charger to the excess power generated by the solar panels. The alternative was to have 'x' smaller (say 10A) chargers and turn more and more of them on or off as the excess power from the solar panels allowed. The latter would work fine but cannot perfectly match the solar excess power to the power going into the battery. Deciding to go with this type of charger is a bit of a gamble though, they aren't cheap and I hope the efficiency of the charger and the extra charge I can get into the battery on a dull day will be worth it. Time will tell.
After I've checked over the charger I have a couple more bits to get (better crimp connectors, 200A fuses, more cable), finish the initial version of the controller code, finish making the buss bars and decide how to lay everything out and find a heavy duty box to put everything in.
Richard
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