Jump to content


  • Posts

  • Joined

  • Last visited

  • Days Won


Everything posted by weber

  1. Anyone thinking they might take a risk on these so-called "super-capacitors" on the basis that LTO is a long-lived battery chemistry, should forget it. That's only the case if the LTOs are properly manufactured and are protected and balanced by a proper BMS. The BMS in this device is rubbish. The photos of the fire should have been enough to convince us of that. But some more information has recently come to light, thanks to a brave volunteer guinea-pig. I received the following email 6 weeks ago. I asked the author to please post the information himself directly, but because he hasn't yet done so, and I'm concerned that people might still be thinking it's worth the risk, I post the email below, unedited. "Arvio" is the Australian agent for these devices. The executive summary: It lasted about 2 years. The warranty was not honoured. Disassembly found bulging cells and leaking electrolyte.
  2. We have already released patched firmware for the King which fixes the premature float bug (and nothing else). That's presumably what Lionel is referring to below. It can be found here: https://forums.aeva.asn.au/viewtopic.php?p=76178#p76178 I assume you meant to write "PIP-5048MK". Yes, Coulomb recently obtained one, and we plan to start porting more of our PIP-5048MS patches to it in a few weeks time.
  3. See Coulomb's excellent posts via the first 3 links here: https://forums.aeva.asn.au/viewtopic.php?t=4332#commands
  4. No. You should not let it go to 3.7 V. Use a headlight bulb to bring it down to where the others are.
  5. Yes. If you need to set the absorb voltage lower than your usual float voltage then you should set absorb and float the same. You typically won't be allowed to set absorb voltage lower than float voltage, so you'll probably have to set the float voltage down first.
  6. Only one cell needs to be above 3.4 V for it to be worthwhile doing balancing. It doesn't matter if the others are greater or less than 3.4 V. And you should stop (or the BMS stops you anyway) when any cell goes above 3.6 V. So you need to extend the time between the first cell going over 3.4 V and when it goes over 3.6 V. To do that, you need to reduce the charging current. Ideally, you would reduce it to the same as (or a little less than) the current that you are able to bypass around a cell by connecting a headlight bulb across it, which is typically 2 A for one filament. With the 5 kVA Axpert inverters, if you are able to charge from the utility, you can use the lowest utility charge current setting [11] of 2 A. This is ideal. If you are not able to charge from utility, you will need to set the absorb voltage [26] down to whatever will reduce the current to about 2 A by the time the first cell exceeds 3.4 V. This could be as low as n × 3.36 V where n is the number of cells. As you progress with balancing, you will need to keep raising setting [26] to keep the charge current close to 2 A. And if a cell does go over 3.6 V and so you have to stop charging, (or it stops automatically), then you leave the headlight bulb on that cell until its voltage goes below that of the lowest cell, then you start the charge cycle over again, and keep repeating this until you get all cells between 3.45 V and 3.6 V at the same time, and within 0.05 V of each other. Hopefully, after you've done that once, you'll be able to set your charge voltage and current settings back to normal, and the BMS will be able to keep them balanced from then on. I note for other readers, that these voltages only apply to LFP cells. Also, Ojeysky, I wonder if you have the correct Daly BMS — i.e. whether you have a model designed specifically for LFP cells and not other lithium-ion cells. Other lithium ion cells have a more linear voltage vs SoC curve, where LFP has some very flat plateaux. With a linear curve, the BMS can balance at any SoC, and can therefore get by with a few tens of milliamps of bypass current. This will not work with LFPs. With LFPs, such a scheme will actually unbalance the cells due to inaccuracies in voltage measurement. With the LFP cells you can only balance when they are near 100% charge (or less conveniently, when they are below 30% charge) and because the time available is so much shorter, the bypass current must be so much greater, typically half an amp or more.
  7. This is not mere theory. Coulomb and I do this with every new battery (set of cells). The typical 55 W headlight bulb draws about 4 A at 13.8 V, so you might expect it would only draw a quarter of the current at a quarter of the voltage, so about 1 A at 3.4 V. But in fact it will draw about 2 A at 3.4 A. This is because the filament has a much lower resistance when it is only glowing a dull red compared to when it is white hot. A fan will behave in the opposite way. i.e. it will draw much less than a quarter of the current at a quarter of the voltage. I doubt that the fan will draw enough current from a single cell to be useful in balancing the cells in a reasonable time. But it can't hurt to try it. If you only do this manual balancing after you stop charging, then the cell you connect the load to, and in fact all the cells, will drop below 3.4 V almost immediately, so you will only have a very short time of balancing. You would have to repeat this many many times.
  8. One way is to use one or more car headlight bulbs with alligator-clip leads, and a multimeter. With the battery on charge, and close to full charge, clip the bulb(s) on to the cell(s) with the highest voltage(s), provided that voltage is greater than 3.4 V (for LFP cells), to burn off some charge and let the other cells catch up. You can leave the bulb on a cell until its voltage drops below that of the lowest voltage cell. You can also work in the other direction. If you have an adjustable-voltage current-limited power supply (a lab power supply) you can set it to 3.6 V, and when the whole battery is being charged and some cells are over 3.4 V, connect it to the lowest voltage cell until it goes above the voltage of the highest voltage cell. If there happens to be two to four high cells next to each other, you can clip one bulb across the lot to burn off charge faster. You should put each bulb in a porcelain cup or mug to stop it blinding you and melting things. This is a @Coulomb innovation. Likewise if you have a number of low cells next to each other, you can adjust the lab power supply to (n × 3.4 V) + 0.2 V. But this is more dangerous. You must monitor the individual cells often to ensure none goes over 3.6 V. 3.4 V is an absolute minimum for balancing. The higher you go, the more accurate the balancing will be, but you shouldn't let any cell go over 3.6 V. I note that connecting LFP cells in parallel will not balance them unless you charge them to more than 3.4 V while they are connected in parallel.
  9. If this inverter accepts the same serial commands as its big brother, then I think it will reset its charging algorithm if you send it a command to change the float voltage or absorb voltage. You only need to change it by 0.1 V and then you could change it back. You could automate this, using Node-RED on a Raspberry Pi. The commands are PBFT and PCVV. http://forums.aeva.asn.au/uploads/293/HS_MS_MSX_RS232_Protocol_20140822_after_current_upgrade.pdf
  10. I thought I was a bit more nuanced than that. I did give at least one suggestion for making it work. https://powerforum.co.za/topic/4614-axpert-settings-for-lifepo4/page/2/?tab=comments#comment-70618
  11. I love how you wrote that with a straight face. The idea that an Axpert would have a 25% safety margin for anything, strikes me as hilarious.
  12. Yes. It's important, when designing a battery system, to try to predict the maximum storage requirement over the life of the batteries, and include enough capacity right from the start, preferably without paralleling. When this hasn't been done, some workarounds are: (a) Connect the new batteries to a separate inverter/charger and separate solar panels, and parallel the AC outputs of the two inverters. Unfortunately you can't do this with two Axpert inverters. To be able to parallel their AC outputs, they must be connected to the same battery. (b) Connect the new batteries to a separate inverter/charger and separate solar panels, and use them to power separate loads, e.g. separate light and power circuits. (c) Parallel the batteries but measure their current separately and deliberately add resistance, in the form of extra cable length, to approximately balance the currents in proportion to the capacities.
  13. Successfully paralleling strings of identical new cells is difficult enough. It requires extreme attention to detail. Successfully paralleling different makes, models or ages of cell is almost impossible. One string will end up carrying all the current during the middle of charge or discharge, and the others will carry all the current at the beginning and end. So they will all be abused, So it's pointless, unless the second lot of cells are free, or nearly so. More information here: https://powerforum.co.za/topic/2736-the-multiple-string-battery-riddle/?tab=comments#comment-43501
  14. Alternating the connections would be better than nothing. But you will need to swap it once a week (or once a day) for the entire life of the batteries. Otherwise one will age and die long before the other.
  15. @Adri, Sorry to take so long to respond. Using heavy cables isn't sufficient, because much of the resistance is in the crimps, the plug contacts and the bolted connections. If you have the same numbers of those on the path through each battery, then you may be OK. http://www.smartgauge.co.uk/batt_con.html As Chris Gibson shows in the above, there are only two ways to do it properly, his method 3 and method 4. He shows 4 batteries, not 2 as you have, but his method 4 is just "diagonal takeoff" applied hierarchically. Diagonal takeoff only works for 2, 4, 8, ... batteries (powers of 2) and, strangely, 5 batteries. But method 3 can work for any number. Your present arrangement may be the equivalent of method 3, or might easily be made so. I still don't find your description entirely unambiguous.
  16. @Adri What you have may be fine. Diagonal takeoff isn't the only way to balance currents between two batteries in parallel. It will be fine if you are using one of these: https://www.redarc.com.au/anderson-parallel-cable-2 provided the cables going from it, to each battery, are the same length.
  17. @Adri, I already answered most of your questions, starting here: https://powerforum.co.za/topic/4614-axpert-settings-for-lifepo4/?do=findComment&comment=68570 The capacity of LFP is usually measured between 2.5 V and 3.6 V per cell (28.8 V). By stopping at 3.4 V per cell (27.2 V) you only leave about 2% of the capacity unused, but you extend the life of your cells a little. For voltage versus SoC, see Figure 2 of: http://www-personal.umich.edu/~hpeng/DSCC2013_Weng.pdf For aging versus temperature and voltage/SoC see Figure 2 of: http://jes.ecsdl.org/content/163/9/A1872 Click the link to the Full Text (Free) on the right.
  18. The best hypothesis I can come up with is that the battery was sitting on the shelf for a while before it got to you. And in that time the cells got out of balance by maybe 2%, so that when the battery voltage gets to 27.6 V you have 7 cells averaging 3.41 V each and one cell at 3.73 V. The BMS would have switched on the bypass resistor for that cell when it passed about 3.5 V, but it can only bypass maybe 1 A, and you're charging at maybe 30 A, which is why it got up to 3.73 V. And then to save that one cell, the BMS open-circuits the whole battery using a solid-state relay so the charge current goes to zero. That's what I think is happening when the voltage suddenly jumps up to the absorb voltage setting of 28.4 V. After a few minutes of disconnection, the bypass resistor pulls the voltage of that cell below maybe 3.6 V and the BMS reconnects the battery, and the cycle repeats. As it's doing this, the high cell is coming more into balance. But if it was 2% ahead of the others, that's 4 Ah for a 200 Ah battery. And if the bypass is only 1 A, then it will take 4 hours to balance that out. So it may continue these crazy oscillations for several (sunny) days, but if I'm right, they will eventually reduce and finally cease. If you want to speed the process up a little, and be a little kinder to the battery, you could wait until the problem starts occurring each day, then set the max charge current [02] down to its minimum of 10 A, and then set it back to 30 A or 40 A each evening. But you don't need to do that. You can just let it go as it is now. Please let us know how it is going in say a week's time.
  19. When I click your link I get "Dataplicity is unable to connect to your device. It may be offline." Can we see battery current as well as battery voltage? It doesn't sound harmful, since it is merely cycling between absorb and float voltages, and not spending much time at the higher voltage. But I do not understand why it is happening. You could try dropping the absorb voltage setting [26] down to 28.2 V to see if it eliminates this behaviour. Normally, after achieving float, an unpatched 24 V Voltronic inverter would not return to bulk/absorb until the battery voltage had fallen 2 volts below the float setting [27], i.e. 24.9 V in your case. But that does remind me to tell you that, if you find that your battery voltage does not fall below 24.9 V over night, or when you boil the kettle first thing in the morning, and therefore it does not go back to absorb voltage on the following day, but stays at float voltage, then you may need to raise your float voltage setting a little, i.e. to 2 V more than whatever voltage it does get down to. But preferably no higher than 27.6 V. It is quite the balancing act, to get an inverter/charger designed for lead-acid to work sensibly with LFP, particularly when it has the Voltronic Premature Float Bug. Now I'm worried that this one has some other bug as well. The behaviour may be caused by the BMS, but I can't figure out how. I'll be able to comment further when I've seen the battery voltage and current curves.
  20. It's only on cloudy days that it might matter. You can wait and see if you get premature float on cloudy days, and if so, drop setting 02 to 30 A.
  21. @ojeysky Sorry to have got your hopes up in regard to firmware. We only have patched versions for the 5 kVA 48 V models. So it's the same offer as for Adri. If you can find us a firmware update, we'll look at it, but no promises. I can't tell whether the IVCM3024 is a rebadged Voltronic or a copy made unethically by another company (a clone), but since we don't have a firmware update, it doesn't matter. I also said the wrong thing about the current setting (setting 02). I suggest putting it a little lower than the maximum that ends up going into the battery (after loads). Based on your battery charge current log, it looks like 30 A would be the best compromise between missing out on a little charge when the sky is clear and avoiding premature float when it is cloudy. Please let us know how this goes. It is quite possible that I'm completely wrong. The explanation for the strange behaviour may lie elsewhere.
  22. Coulomb asked me to look in here, as he is rather busy at the moment. I believe Coulomb has already correctly explained what's going on here, namely the infamous Voltronic Premature Float Bug, which is in every Voltronic inverter firmware we've ever looked at. Instead of going to float when the battery voltage is near the absorb voltage (setting 26) and the current has fallen below one fifth of the maximum charge current (setting 02), it goes to float when the battery voltage is near the float voltage (setting 27)!! and the current has fallen below one fifth of the maximum charge current (setting 02). @Adri If you can find us a firmware update file for your Axpert MKS 1K-24, we may be able to patch it to fix the bug, as we have done for several other Axpert models. But no promises. @ojeysky What model is your Axpert inverter? We may already have patched firmware for it. See the Firmware section here. Failing that, there are two ways of avoiding the bug: 1. (As you have discovered) Set the float voltage (setting 27) to what the absorb voltage setting should be, i.e. don't have a float mode. This is a terrible thing to do to lithium batteries. Unlike lead-acid, they age most rapidly when held at a high voltage. 2. Don't let the charge current fall below 1/5th of the max charge current setting (setting 02). That means you either have to keep the charge current high (until the absorb voltage is reached), or you have to set the max charge current setting (setting 02) low. For the Axpert MKS 1K-24 I suggest putting setting 02 to 20 A. For more powerful Axpert models I suggest setting it to a little less than what your PV array is capable of, but no more than 40 A for a 200 Ah battery (0.2C). What were your settings 02 and 11? And what charge currents do you normally see from your solar panels? Does the problem occur when you charge from utility? Coulomb also gave correct absorb and float voltages for 8S of LFP cells, namely 27.6 V to 28.4 V for absorb and 26.9 V for float. I suspect you will need to use 28.4 V for setting 26 for two reasons: (a) so that the series cells will get their SoCs balanced by the BMS, and (b) to keep the charge current high all the way to absorb, to avoid the premature float bug. But you will still be hit by the premature float bug on overcast days. There's another issue. Current sharing. When you have two batteries in parallel you have to ensure you have the same resistance in the link between their positive terminals as between their negative terminals, and the takeoff cables must come from the positive terminal of one battery and the negative terminal of the other (diagonal takeoff), with the same lug-stacking order (e.g. takeoff lugs on top). And the batteries should be stacked beside each other and have the same thermal environment, as internal resistance goes down as temperature goes up, and so a small current imbalance can become a larger one.
  23. Ah! I think I get it now. Despite the manual saying "Please insert USB disk into USB port", you can't actually do that because there is no type-A USB port on the display unit, hence the need for the OTG cable. What kind of B port is on the display unit? It looks like a micro-B. If so, then the OTG cable or adapter would be like this.
  24. The Axpert King manual page 23 (PDF page 25), suggests to me that you can upgrade the inverter firmware, simply by having the files on a USB stick (flash drive) plugged into the USB port of the display unit. No need for any kind of serial comms adapter or cable. I can't try this because I don't have an Axpert King. Has anyone tried this? http://www.mppsolar.com/manual/PIP-MK (PF1.0)/PIP-MK manual-20190717.pdf
  • Create New...