Equalization with dual XWMPPT60's (individually they can't)?

OK, I'm sure part of my problem is that the search terms are common, as are dual CC questions, so I can't find a good answer.

I have dual Xantrex MPPT60's (everything connected by XANBUS). They work great together for BULK, ABSORB, and FLOAT, but it looks like only one can do the equalize while the other kicks into float. I can see why this makes sense, but I can't get the 41 AMPS (5% of 820Ah) I'd like for equalizing, from just one CC.

I have switched to the "boost" mode that Xantrex mentions. Basically if you set the BULK voltage higher than than the ABSORB voltage, the CC then uses that higher voltage for the first hour of the absorb cycle. A mini equalize, if you will. Works great for my particular case. NY weather, shallow DOD, short days (plus trees). These Surrettes DO NO LIKE TO BE BABIED. They've been much healthier (SG) since I started using boost.

Rarely do I need to equalize, but when I do, I'd like to use the sun instead of the Generator. Any way to get the other CC to help with equalizing?


Thanks

Comments

  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    When equalizing, CC keeps voltage, not current. When you try to equalize, do you get the voltage that you specified for equalization?

    In XW SCCs equalization must be enabled. Is it enabled on both controllers?
  • ww_kayak
    ww_kayak Registered Users Posts: 8
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    I know it's a voltage game, and it does hold the ~64 volts, no problem. Surrette, and Xantrex also say that the best current during equalization is around 5% of Ah capacity. When I use the generator and the inverter/XW6048 it does hold 40~45 amps the whole time, so I know the batteries will take it. Each CC carries half the panels. @ 64 volts that ends up being right at 40 amps per CC... with clear blue sky. But, with contrails, cirrus clouds, and battery/charger losses, it's closer to 25 amps most of the time. Enabling EQUALIZE on both CC's doesn't seem to matter. One always takes over equalizing, and the other drops to float. I guess that's my question: does the Xanbus communication mandate a slave / master relationship? If so how can I override that behavior. OR is it not working properly?

    Do yours both go into equalize mode when you tell them to?
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?
    ww_kayak wrote: »
    I know it's a voltage game, and it does hold the ~64 volts, no problem. Surrette, and Xantrex also say that the best current during equalization is around 5% of Ah capacity. When I use the generator and the inverter/XW6048 it does hold 40~45 amps the whole time, so I know the batteries will take it.

    You cannot increase current without increasing voltage. When you specify an equalization voltage it tries to keep that voltage, regardless of what current it produces.
    ww_kayak wrote: »
    Each CC carries half the panels. @ 64 volts that ends up being right at 40 amps per CC... with clear blue sky. But, with contrails, cirrus clouds, and battery/charger losses, it's closer to 25 amps most of the time.

    If they're networked correctly, they display the total current for the system, so I would assume that is what 40A is, correct?

    If you see 40A current and 64V, then when the cloud comes and current drops to 25A, voltage also should drop below 64V. Does it?
    ww_kayak wrote: »
    Enabling EQUALIZE on both CC's doesn't seem to matter. One always takes over equalizing, and the other drops to float. I guess that's my question: does the Xanbus communication mandate a slave / master relationship? If so how can I override that behavior. OR is it not working properly?

    They work together. They decide by themselves how to share the load. If one is enough to keep 64V, the other does not need to contribute.
    ww_kayak wrote: »
    Do yours both go into equalize mode when you tell them to?

    I do not use built-in equalizing because it only works for an hour. My charging current is already 64V. When I need them to go at 64V for longer, I set float voltge to 64V. They cooperate well.
  • ww_kayak
    ww_kayak Registered Users Posts: 8
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    Nope, I think the confusion is coming from the fact that they are not working together, or sharing the load properly(during EQ only) . I'm not trying to "increase" current. I understand that it maintains constant voltage regardless of current. BUT, because they are not working together, it is reaching it's physical limit for ONE CC (~2600W/64V = ~40A) because the other CC drops to float and doesn't contribute. (even though it's capable of 2600W too). The "system" puts out ~80 amps(total system current) during bulk charge no problem.

    So lets say it takes around 3000 Watts total to maintain that 64 volts at 40 amps (at the battery), because of system/battery inefficiencies. BTW that 40A during equalize is not an arbitrary number I chose. 5% of capacity seems to be built into the Xantrex equalize algorithm. When I use the genny, or have enough panels on 1 CC, it seems to always maintain 64V@~40A during equalize. Maybe it's just a characteristic of my battery bank, which is, coincidentally, 5% of my capacity?

    Now lets say a hazy sky is reducing reducing irradiance by 25% ( not an actual cloud blocking the sun). If both CC's were contributing they should still be capable of maintaining the 64V @ 40A easily... but only one is, so 64 volts is maintained, but current drops to ~25A to compensate (the benefit of MPPT). It's still working, but the higher current seems stir/ desulphate the batteries better.

    Anyway, this conversation would be easier in the real world :). So, after a long winded explanation I guess I have my answer. They are not working together the way they should. And, I can get around it by changing the float voltage. I'll try that next time and see what kind of current they accept.

    Thanks,
    BTW, just for kicks, after you reach absorb, try activating equalize on both and tell me what they do :)
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    What I'm trying to say is opposite. May be I don't explain it very clear. By what you describe they're working together. They're maintaining 64V. That's what they're supposed to do.

    At any given state of battery, there's a relationship between current and voltage. If you apply small voltage, say 55V, you'll get very little current. If you apply higher voltage, you'll get more current. If you apply 64V, you'll get certain current. Current is simply a result of a voltage applied.

    The charger does not have a choice to apply 40A@64V or 25A@64V. It simply applies 64V. What current you get from it only depends on the state of the battery, and does not depend on the charger. The only way for the charger to increase current is by increasing voltage.

    If there are clouds or some other situation which causes lack of power, the charger simply fails to rise the voltage to the desired level. And because there's not enough voltage current decreases too.

    The relationship between voltage and current changes gradually as battery charges. If you're applying the same voltage, you're getting progressively less and less current. And reversely, if you try to maintain the same current you need to apply more and more voltage.

    That's probably the best explanation I can master.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    to add to what NG just said so clearly, it is like the Absorb phase, Voltage is maintained and Amps drop . When the amps reach ~ 1 to 2% of C Absorb is finished
     
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    There may be some voltmeter calibration issues, both are "programmed" for 64V, but one is actually 64.05 V and the other is 63.95V, and will output much less . Is there a way to fine tune their internal volt meter, or slave one to the other ?
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • ww_kayak
    ww_kayak Registered Users Posts: 8
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    I guess I'm the one who just can't explain it in an email :)

    Yes, I REALLY do understand the relationship. The internal battery resistance will determine the current. I'm not saying equalize magically forces more current, I think Xantrex is actually LIMITING it to around 40 amps (maybe not, doesn't matter). This is an empirical number that I have seen over, and over, and over, except when I try to combine charge controllers to achieve the required power.

    My XWMPPT60's do seem to finish the ABSORB cycle at 2% (XW6048 seems closer to 1.2%). I think 2% is too high, and that is the reason the batteries accept a fairly high current when it switches to equalize.

    The following is what I have actually, physically, done:

    Scenerio #1 - (Max number of panels I can get on one CC (3500W), Full Sun):

    ABSORB finishes @ 60V, 16A (2% capacity triggers equalize)
    EQUALIZE ups the voltage to 64V, current starts to rise and stops right around 40A(+-10%) (whether this limited by CC, or not, doesn't matter, this is what happens)

    Scenerio #2 - (Generator, with XW6048 charging the batteries):

    ABSORB finishes @ 60V, ~10A (~1.2% capacity triggers equalize)
    EQUALIZE ups the voltage to 64 volts, current starts to rise and stops right around 35A(+-10%)

    Scenerio #3 - (Hazy day, two CC's: each with ~2500W worth of panels attached, but each only producing ~1700W each (3400W total), Both CC'a have EQ enabled):

    ABSORB finishes @ 60V, 16A (2% capacity triggers equalize)
    CC #1 switches to EQ
    CC #2 drops to FLOAT (like Mike said, they are not identical, so one of them is going to win the race)
    EQUALIZE ups the voltage to 64 volts, current starts to rise and stops right around ~20A*(+-20%)

    * My thought is a single CC: 1700W / 64V = 26A, 26A * 80% efficiency loss == ~20A

    ** If I turn on the generator at this point and equalize the batteries, with the inverter/CC, it will quickly go through the ABSORB cycle and equalize @64V, ~40A

    Hopefully that explains what I'm trying to say better ;)

    The whole conversation could be moot, because you are correct about an hour not being enough. I should just change the float voltage and EQ until the SG's level off. This is almost always more than an hour. And it will work for scenerio #2, because the CC's work together(share the load) in BULK, ABSORB, and FLOAT, just not EQUALIZE?

    Thanks
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?
    ww_kayak wrote: »
    I think Xantrex is actually LIMITING it to around 40 amps (maybe not, doesn't matter)

    The docs say it would.
    ww_kayak wrote: »
    EQUALIZE ups the voltage to 64 volts, current starts to rise and stops right around ~20A*(+-20%)

    If I turn on the generator at this point and equalize the batteries, with the inverter/CC, it will quickly go through the ABSORB cycle and equalize @64V, ~40A

    That shouldn't be happening. I can offer few possible explanations

    1. You haven't waited long enough. At the beginning of equalization, current may be dropping rapidly, so in 5-10 minutes it would be 20A

    2. Your SCC thinks that it is doing 64V, but in fact it does less than this. This can be a voltage drop in cables or a bad SCC. You need to figure out what the real voltage is. Look what voltage the other SCC displays. And, look at the voltage that XW6048 displays (you would need to find XW6048 screen on SCP, don't confuse with main screen or MPPT's screens). And, measure the voltage at battery terminal with voltmeter. They all must be 64V (give or take)

    3. What's the batterty temperature? I noticed with my SCCs that they don't go above 64V even if batteries are cold and they would need higher voltage. The XW6048 does go above 64V. So, if your batteries are cold (20C or less), XW6048 will maintain higher equalization voltage than 64V while SCCs are capped at 64. This may explain it.
  • ww_kayak
    ww_kayak Registered Users Posts: 8
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    To me it seems like the 20 amp shouldn't be happening because every other method levels off near 40A. If I equalize, it's because they need it, so they'll only drop from say 44 to 35 for the entire hour(which proves they need more). I also have a Trimetric meter which lets me see what actually going in and out of the batteries. All devices show the same relative values. Trimetric jives with the SCP system view.

    That's a good point(3),the batteries are cold. I keep them in a heated shed, but most of the winter they are 50-60 degrees. and I have seen the 6048 bumb the voltage above 64 volts... substantially. I assumed the SCC's would too (the 6048 and SCC's have BTS's hooked up). However, it was 50 degrees yesterday, batteries were 65 deg. And it still happened :(
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?
    ww_kayak wrote: »
    To me it seems like the 20 amp shouldn't be happening because every other method levels off near 40A. If I equalize, it's because they need it, so they'll only drop from say 44 to 35 for the entire hour(which proves they need more). I also have a Trimetric meter which lets me see what actually going in and out of the batteries. All devices show the same relative values. Trimetric jives with the SCP system view.

    That's a good point(3),the batteries are cold. I keep them in a heated shed, but most of the winter they are 50-60 degrees. and I have seen the 6048 bumb the voltage above 64 volts... substantially. I assumed the SCC's would too (the 6048 and SCC's have BTS's hooked up). However, it was 50 degrees yesterday, batteries were 65 deg. And it still happened :(

    There could be a combination of the factors.

    First, temperature. At 60F, the temperature compensation should be about 1.2V. If your have your BTS properly installed, and equalization voltage is set to 64V, your XW6048 should keep 65.2V. When XW does that, you're getting 40A.

    When you use SCCs, the voltage that we see on SCP (which comes from XW6048) is 63.5V. This is 1.7V less than when you do it with XW. If, at 65.2V with XW6048 you get 40A, it is quite reasonable to see only 25A at 63.5V.

    Now. Your SCP shows 63.5V while SCCs are set to 64V. This is 0.5V drop. A little bit too big to be an XW error. I would suspect a voltage drop in the wiring between SCC and the rest of the system.

    Worse yet. Trimetric, which I guess is connected at battery terminals, shows 62.6V, this is 0.9V difference. Could be a voltage drop in the cable that leads to batteries. So, your batteries actually see much less than 64V.

    You may have too big voltage drops in your cables.

    I would suggest checking all these voltages with a precise voltmeter (capable of measuring voltages to 0.1V). You can measure at battery terminal, at the point where SCCs connect, at the SCCs terminal, and at the XW6048 terminals. You can do these measurements when equalizing with SCC and also when equalizing with XW6048 and then compare.

    At any single point of time, you shouldn't see any differences between these voltages more than 0.2V. If you do, your cables are likely too thin or too long.
  • ww_kayak
    ww_kayak Registered Users Posts: 8
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    My mistake: that was a snapshot I just took, it's still in BULK. I was just showing the relationship between Trimetric and the SCP. The Trimetric cable runs almost 200', the battery cables are only 3' @ 4/0. My Fluke shows that the SCC's/Inverter are correct. The loss (or calibration difference) is with the Trimetric, which is fine, I just use it as quick reference for my relative loads. I don't use it for anything critical. But the compensation being capped at 64V with the SCC's is something I 'll check next time.
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    If first XWCC gets to EQ and the second XWCC goes to Float before it has EQ'd for its programmed amount of EQ time, that is a problem...
    That is, unless it has been programmed to do that for whatever reason.

    You might try removing the xanbus network cable and try it again. Complete independent CC's.

    Maybe the xanbus is telling that second XWCC to go to Float for whatever reason it thinks it should do that.


    boB
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    I have two XW SCC's and I can only EQ with one of them at a time. One goes into EQ and the other goes into float. Even if clouds drop the output from the one doing the EQ, the other array will still be in float. According to the "guru's" at tech support, this is normal. According to me it's a bug.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    One thing to watch--Many controller will display the temperature compensated value of battery bank voltage, not the "as measured" value. And some parameters are temp compensated and others are not (EQ voltages tend not to be temp compensated?).

    I would use a good quality/accurate volt meter to verify the "as measured" values at the battery bank/bus to ensure you understand what is really happening.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?
    Joe_B wrote: »
    According to me it's a bug.

    I would tend to agree with that, Joe.
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    Re: Equalization with dual XWMPPT60's (individually they can't)?

    Yup, and now you know why there are several of us on this forum working on hacking the xanbus. If they wont fix it we will.