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Forum Index : Solar : My first LFP battery install
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| Georgen Guru  Joined: 13/09/2011 Location: AustraliaPosts: 462 |
This cost goes on top of kWh charges and makes SOLAR system look cheaper, if you got mixed up with intentions of my post. What surprises me that every time I kind of want to support the farmer on those "Growers' markets" I am very disappointed with their prices. Suspect some sellers get their produce from supermarkets and take customers for mugs. George |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
It looks like we have a state of charge discrepancy between the two shunts in the system. I think the midnite controller is not resetting to 100% when it goes to float during the day. Even taking account of this the logs show a 100 amp difference over 7 days between what goes into the battery and what is coming out. I will have to check a few saved settings and the voltage calibration offset on the shunt or perhaps the earth strap to work out why it is doing this. it is very peculiar. there is basically 350 watts/day of battery input missing from the logs or 350 watts of output overcounted. I'm confused, no wait... maybe I'm not... |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi yahoo2 This is a problem when using separate shunts for bi directional readings, I am assuming an Allegro type shunt, when using a resistive shunt such as a piece of stainless rod it is possible to synchronize both very close to the same , but with electronic shunts they have a % accuracy problem built in that cannot be modified easily, I did see a balancing circuit but it would not fit inside the loggers you are using as from memory it used several IC circuits to read and modify, I will see if I can find it as it was in an old EA magazine . All the best Bob Foolin Around |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi, Bob, they are both 500A resistive shunts, the electronics on the side in the photo is to convert the voltage reading to a data signal, I am sure it is something simple that I have missed. It will be some good mental exercise to chase it down, these things have a habit of happening repeatedly if I dont find the cause the first time it occurs. Long term it should ideally have only one shunt with two outputs but at this stage the wiring is temporary, the SSR's and some other stuff is still in transit somewhere so I have to sit on my hands until I have all the gear. I'm confused, no wait... maybe I'm not... |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I think I will have to call it. We are just to far south to make a purely EAST/WEST panel configuration work for off-grid. The sun is too low in the sky in June/July to meet the energy targets I set for this system. It was looking so promising, then the power levels on the E/W test panels started to roll off in the last week. The goal was to limit the battery charging amps to 150A and provide 7kWh of battery storage for night use and 7kWh minimum direct from the panels during the day. Next month we will fall short of that daily target by 3.3kWh, with no north facing panels at all we would be 5.7 kWh short of where we need to be. It is disappointing because it has done a great job of taking the massive power spike at midday away with north facing panels only. Works perfectly for the rest of the year. I have an old copy of Doug's solar array configurator spreadsheet that I have modified quite heavily, I have tried a few dozen panel combos in that to see what could work. The best I have come up with so far is NORTHEAST/NORTHWEST with a 45 degree pitch angle on the panels. This will give me a minimum of 20 kWh per day, every day of the year. Looking at the numbers there is quite a few weeks in August and September we will be well over the amp limit if it is cloudy till about ten o'clock then the sky clears up. Not a big problem, I can work around that with the relay driver and SSR's. The problem is how to build it! I have limited unshaded area to work with and panels at a skewed angle might look very odd. I'm confused, no wait... maybe I'm not... |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Yahoo2 A configuration I used in NE Victoria was to set three panels middle one direct to the midday sun with 30 Deg lay back and the east and west ones at 30 degrees to the east and 30 degrees west of median line with 30 Deg lay back , This I found gave the greatest yield from the panel banks over a 10 hour cycle and it did reduce the large inrush of amps at lunch time and spread it out more easily. If you where going to run Air conditioning it would be better to increase the panels on the northern and western wings. Space is always a problem but you can put various panel banks where the free space suits the angle. All the best Bob Foolin Around |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Thanks Bob, it is good to know someone else thinks the way I do! 
The relay driver is here. it is going to be wired as a stand alone device switching the 4 outputs based on the voltage of the batteries. relay 1 switching a bank of solar panels wired as 24 volt relay 2 DC loads relay 3 ac loads relay 4 power dumping ac loads Aux 1 is still free on the midnite controller, not sure what I will do with that yet. One feature I like is that every voltage setpoint can be programed with a time function to stop the system hunting on a day with intermittent cloud. Looks like I have a few soft starters to build for a couple of motors and switchmode power supplies that have some heavy startup amps, truth is, I should have already installed them  I have just been a bit slack!I'm confused, no wait... maybe I'm not... |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Yahoo 2 Looks like a good approach to the system, Look forward to the final set up. I had some trouble with High amps on startup with a friends system and to keep it simple we used 80 uf caps designed for boom boom amps in a car to deliver the amps direct to the inverter terminals, as we found that the wiring to the inverter was the problem not the inverter. The high load was only for several seconds, but without the caps it would drag on and cause problems, with the caps the load fired up in a second and no problems. Another little mod was a couple of elimination relays to eliminate the power to non essential loads while the main load was working, this allowed us to keep the max loading well within the rating of the inverter. The inverter was a 24 volt Australian made unit rated at 5 KVA not sure of the max load rating. All the best Bob Foolin Around |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Bottom Balancing. I have been slowly trawling through all the discussion on the interweb regarding bottom balanced LFP's for the past 9 months, it is a pretty slow and frustrating process! Finally I gave up and went straight to the source and downloaded one of Jack Rickard's weekly videos from EVTV.me (or at least a youtube copy) the video is more than 2 hours so there is a fair bit to sit through And what do you know! Now that I have listened to Jack Rickard explain the reasoning and demonstrate the technique it makes perfect sense. https://www.youtube.com/watch?v=HAsmQ2O4qHY 17 april 2015 friday show EVTV WARNING, NOT LIGHT ENTERTAINMENT dont expect a slick show. the gist of it is thinking about what happens when a battery pack approaches empty under load. with a top balanced battery one or two cells will go over the "Bottom Knee" before the others and its output will drop to almost nothing at the same time the rest of the cells are still powering on and can push the empty cell into reverse and destroy it. The only coherent argument from some detractors of this method is that the BMS cell voltage monitoring will catch this situation and disconnect the load. Jack maintains that the BMS is simply not fast enough, a cell can go over the knee and be pushed into reverse in a fraction of a second. Having seen what these cells can do I believe him! With a bottom balanced pack all the cells go over the knee roughly together and there is not enough charge in any of the cells to drive a load OR damage their neighbours, they are all equally weak. I have to say that once I got my head around not top balance charging more than once every 3-6 months and keeping the charge voltage under the 100% level for the daily charge cycles it is not a big leap to think about installing a bottom balanced pack. I'm confused, no wait... maybe I'm not... |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Have been doing quite a bit of reading on LiFePO4 as well and for a house battery situation it appears that you just stay away from the areas that get even close to "dangerous" voltages ... top or bottom. Referring to a chart you posted last year ... 2.6V for BMS cell cutout seems the to quite a bit higher than the "dangerous" bottom cell voltage. Do you see the cell voltages at the low end start to diverge significantly? Dave |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
truth is, of all the banks I have done I have only seen one hit the Low Voltage Disconnect once and it was almost brand new so we were still running it in. All of the battery banks I have installed so far are all CALB brand and all sequential serial and batch numbers. They are extremely even in charge and capacity. if the cells are different capacity they will diverge at one end or the other top balance diverge at the bottom bottom balance diverge at the top we are not talking about a slow curve in voltage here, from memory it takes 2 watts per cell to go from 3.55 volts to 4.0 volts, that is less than 0.002% of the capacity of the cell and only takes a few seconds even at low charge amps. the curve is not as steep at the bottom but if we are sucking 300-500 amps in short bursts things are going to happen super quick. I think it is quite simple to stay away from the top voltages, it is just a matter of setting the charger to the pack average at the point where the highest voltage cells are still below the top voltage knee. My gut feeling is that as the batteries age some cells will lose capacity more than others. I think this is one of the reasons that they regard the end of life when the cells hold 80% of their original rated capacity. I would be confident that with the right management a LFP bank should be perfectly serviceable down to 60% or 50% of its original capacity. God knows how many cycles that will be! At this stage I am happy to leave the top balance BMS systems in place on the existing systems and keep an eye on things for a while, if the owners start stretching the systems I will have a closer look at the bottom voltages. If I do a bank for a UPS, electric fence energizer or electric-hydraulic power pack or even an electric UTV or work ute where it is going to be run hard, I think I will build it as a bottom balance test bed. PS 2.75 volts is well into the curve of the bottom knee so I am thinking that 2.6 volts is scraping the bottom of the barrel I'm confused, no wait... maybe I'm not... |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Thanks for the comments. I now understand that one of your specific "drivers" here is that the system will survive long periods of inattention. I am trying to engineer my system to be able to survive for 1-2 months without me being on-site ... however a lot of the "fun" in building an off-grid system is that it can give you so much information that you can monitor and ponder over :) While I am waiting for Electrodacus to make his SolarBMS I want to have a good understanding of LVC and HVC for a new LiFePO4 battery pack. I have built some LVC units, rated up to about 10Amps for my existing system. Could you point me to any howto articles on, say a 100-200Amp LVC at 24V, that you have found useful? Cheers, Dave |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Yep, long term reliability and low maintenance is where the cost saving is for me. Remote monitoring through the cloud and such like is just fantasy land, we dont got much internet in the sticks  .
so it is nordlock washers, braided connectors, gigavac contactors and prebuilt off the shelf components. It is not cost effective for me to build anything electronic unless I have the parts on the shelf or it is a custom build, postage and freight is expensive and there is a lot of wrong parts sent and waiting involved, plus having accounts with lots of suppliers is a challenge. I think I have mentioned that it is on the "to do" list to build a emergency backup LVD/HVD to replace a failed BMS while it is repaired or replaced. I will use one of the morningstar relay drivers I have and a couple of contactors. I'm confused, no wait... maybe I'm not... |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
UPDATE LFP cells have been cycling for 9 months without a balance. I checked them with a multimeter on Saturday while they were in float mode. reminder of the settings charging to 3.4875 volts per cell then float @ 3.3375 volts per cell 6 cells were exactly the same voltage 2 cells were slightly higher, but still within 0.009 volts. 
so that all looks good to me, the cell balance modules are all drawing the same load (only a tiny green LED) and not bypassing from faulty components or anything strange. I pushed the manual equalise button when I left, so IIRC this will be the third balance since the bank was installed. No doubt those voltages stretched apart when the cells got to 3.6 volts but I wouldn't have thought there would be more than 5-15 watts between any of them. No logical reason to balance really, just ol lead acid force of habit kicking in. After having the fridge and freezer unplugged for most of the day while rerouting and upgrading some of the old AC wiring, they had to run at night from the batteries to get the temperature down again. Out of interest I checked the voltage before the sun hit the panels in the morning and the bank was sitting at 26.51 volts. that's down 0.19 volt from the float voltage of 26.7 volts. Pretty ridiculous, until you realise that the panels are still working till just before 8.00 pm, days are still LOOOOONG in March. The total overnight Load was fridge, freezer, LED TV, laptop, cordless phone charger and 2 x 9W LED globes. None of the evening meal cooking came from the batteries, the sun was still up! I still have not fitted the 240 V outlets for the "load when battery is full" circuit to plug the cordless powertool and lead acid battery chargers into. One of the channels on the relay driver is programmed ready but I am still dithering around looking for a colour coded dual outlet socket to make them stand out from the "permanently on" outlets. ATM I think there is roughly 20-25 Kwh going spare during the day, not exactly short of power this time of the year. I'm confused, no wait... maybe I'm not... |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
I have had a test 12 volt 4 x Calb 400AH system running a 2.0 KW sinewave inverter powering my fridge and freezer for the past 12 months, whilst I play around with battery management systems, these 4 cells are spares for the main 50v system, not yet commissioned. As they are mostly fully charged by 2pm each day, it makes sense to top balance if needed, I have set solar charger charge cutoff to 3.48v per cell then switch to float 3.38v. There is a coulomb meter in circuit JLD404 to help keep an eye on things. When the charger switches to float, the battery discharges approx 2 AH then stays at that point until the sun goes down, the 3.38 setting for float just maintains capacity, if say both fridges are running, the charger increases current to compensate for the load, then when load has gone, charge currents essentially drop to inverter standby current. Once the cells reach 3.45v it only takes another 2 AH charge to reach 3.5, then approx another 1 AH to get to 3.6, these are 400AH cells so < 1% capacity after the 3.45 point, only reason I have used 3.48v setting is the accuracy of the solar charger. The programmable BMS has upper and lower cell limits and can turn off the inverter if any cell drops to 3.0v or turn off the solar charger if any cell exceeds 3.6v, I have set it to balance at 3.5v, but it rarely ever reaches that as the solar charger has switched to float by then. When initially setting up the cells, I charged each one to 3.50 using a CellLog8 then connected all in parallel and balanced to 3.6v, then placed in series as a 12 volt bank. Mike |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
So, I've got a question for you Mike. I have been reading other peoples charging experiences on the calb cells and some very knowledgeable people have commented that on this lower voltage charging regime that the charging slows to a trickle well before 90%soc and it takes half the day to completely fill. I am not finding that, I have done some tests on 5 systems I have installed and they all get really close to full very rapidly. it only takes a few minutes in absorption to cut to float then if I override it and take it to 3.65volt it takes 20-30 seconds to stabilise at the higher voltage. Much the same experience as you. it puzzles me, what is different? Are their batteries different? perhaps they use different electroyte in older cells? are they running to a lower soc? these are rarely going past 50% soc are they charging harder? I design to charge at 0.3-0.4C rate are their connections higher resistance? Im pretty fussy with that. are their batteries older? ie done more cycles have they been run hotter? I have fitted active cooling to four of these so technically they have never seen a summer. Am I going senile? * not compulsory to answer that!it all seems very strange to me. I'm confused, no wait... maybe I'm not... |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
No this doesnt happen, I'm charging up to 90 amps combined rate, the chargers (I have two in parallel with their own seperate PV arrays) can charge at this rate right up to the 3.45v point, where at that setting they taper off the charge down to about 6 amps above any load on the battery for the remaining 3-4 AH to finally reach the 3.48 ish setting, this takes anywhere from 20 min to an hour depending on sun\load etc. Dont really care at this point cells are > 95% fully charged. I think what others may be experiencing depends on their solar charger and how its set up, I use MPP Solar PCM60x, you have full control over various set points, been quite reliable, very well built internally - no fans. Cheers Mike |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
UPDATE - check voltage and balance time since last balance cycle 589 days  MY TAKE ON THIS measuring voltage at this resolution is a pain. the first row of float voltages is accurate because I had time, the bottom rows are just indications of relative voltages, it happens too fast to keep an eye on everything. Cell 3 was on the low side on float and was the first to shoot up under charge load. I think this is just some gas not being absorbed. they do that, take charge then plateau for a bit and take charge again. Balance charge finished in roughly 8 minutes. I could calculate how many watts transferred out of cell 3, I am guessing less than 0.6 watts. I would not let a battery bank with a lot of cells or one that has regular very heavy loads and fast charging or one that has trouble shedding heat in the middle go this long without an adjustment. TAKE HOME MESSAGE Dont panic to much about balance charging- provided the install is done to a good standard and all the safeties are regularly checked and functioning, it should be all good. I'm confused, no wait... maybe I'm not... |
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