Jump to content
Thank you for the great forum, Safe Driving over the weekend. Sincerely Jason

weber

Members+
  • Content Count

    142
  • Joined

  • Last visited

  • Days Won

    8

weber last won the day on November 9 2019

weber had the most liked content!

2 Followers

Profile Information

  • Location
    Brisbane, Australia

Recent Profile Visitors

1,952 profile views
  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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
  6. 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
  7. 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.
  8. 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.
  9. 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
  10. 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.
  11. @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.
  12. @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.
  13. @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.
  14. 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.

Topics

Contact Us - Power Forum South Africa

×
×
  • Create New...