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

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?

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

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

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

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.