Master and Slave sharing the charge

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Master and Slave sharing the charge

Postby Mauricio on Wed Apr 03, 2013 2:44 pm

I have posted something similar not long ago and I was really interested if anyone has found out this to be the case and if so if there is a possible solution.

My two VFX3648, are stacked OB Series. I have noticed that when in the charging mode, both the Master and the Slave evenly share the load throughout the Bulk cycle and into the Absorb cycle. During Absorb, as the charging current diminishes, eventually the Master drops out and all the charge is carried out by the Slave. Not a big deal, but I would like to know if there is a way to make both 3648 continuing to charge simultaneously and share the charge load.
If there is a charger 'sensitivity' setting somewhere, I have not found it.

Thank you
Mauricio
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Re: Master and Slave sharing the charge

Postby gww on Wed Apr 03, 2013 4:53 pm

I don't know the answer but what is your consern with it working this way? Are you worried about uneven inverter ware? I am not making lite of it, I just don't know what kind of problim it might cause. Does it equal out if the inverters also are running a load?
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Re: Master and Slave sharing the charge

Postby Mauricio on Thu Apr 04, 2013 8:37 am

Thanks for getting back GWW.
No, I was not worried for the inverters, only on the phase balancing of the generator. I guess if I only use the generator during Bulk and let the Sun do the rest, I'll avoid the problem, but at time weather does not cooperate and I am left with completing the charge with the generator.

Mauricio
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http://www.arribadelaroca.com
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Re: Master and Slave sharing the charge

Postby gww on Thu Apr 04, 2013 1:13 pm

Ok, now we have to wait for someone smarter then me.
Good luck
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My RE system: 1. installed
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6. 1 mx 60 charge controller, 1 fm 80 and 1 morningstar ts60 pmw
7. outback mount, panel and sub panel
8. mate
9. 4 place hub
10. couple more cc, doc wattson
11. 800 ah, 48 volt fork truck battery

Re: Master and Slave sharing the charge

Postby tallgirl on Fri Apr 19, 2013 8:23 pm

I typically see the slaves drop out as the system gets further into absorb. This seems to be a part of the firmware.

If you're worried about generator phase balancing, add a PSX240 on the input side.
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Re: Master and Slave sharing the charge

Postby SteveHiggins on Mon Apr 22, 2013 6:07 am

No way to fix it... It's just the way that most inverters charge the batts...

At that point its all about Ohm Law and who has the better connection.

Usually it is the slave that drops out, but I wouldn't worry about it..
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Re: Master and Slave sharing the charge

Postby tallgirl on Mon Apr 22, 2013 8:38 pm

Steve,

I thought the Master was responsible for commanding the Slave to charge or not, so the Slave would always stop first.
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Re: Master and Slave sharing the charge

Postby SteveHiggins on Tue Apr 23, 2013 6:27 am

When It comes to charging... the Outbacks and for that matter most every thing else its a "free for all" At this point because Inverter manufactures don't put shunts to measure how much exactly they are charging on the DC side. Someday inverter manufactures will do this as it will help with AC coupling. If you have this measurement capability then when you are ac coupling you can regulate that amount of current going to the battery rather than depending on a blackout relay or diversion load to prevent runaway charging.

Keep in mind that when inverters are charging, it's voltage driven... This means that the inverter raise the voltage potential to the batteries until they hit the target voltage, then current drops based upon how fully the batts are. You can force voltage into a battery but the battery is going to regulate the amount of current it can take once you get past the bulk stage. It's in the bulk stage you have be careful about how much current you provide the battery bank as you don't want the cells to overheat, typically this is 35% of the C20 rate. If you get over this rate then that extra current can cause overheating and premature failure of the positive plate.
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Re: Master and Slave sharing the charge

Postby tallgirl on Thu Apr 25, 2013 5:55 am

Steve,

The need to avoid "blackout" relays is why I've been looking at microinverter-based AC coupling as an alternative to the (much more expensive) solutions from "Brand S". However, this thread wasn't about AC coupling ;)

It would be nice if someone from OutBack would answer the question about how the Master and Slave decide to "share the load" during normal inverter charging. What I've seen is that the Master turns on first. Once the rate of charge reaches a certain amount (I'd have to try it and see what happens) the Slave's charger is turned on and the two of them proceed to bulk charge the batteries. Once Absorb is reached and the charging current declines, the Slave turns off. The only systems that don't behave that way are "Magic Stacked" 3-phase systems. On those systems it's a complete free-for all and if the Absorb voltage is set greater than SellRE, one inverter can charge while the other sells ...
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Re: Master and Slave sharing the charge

Postby timmartin on Thu Apr 25, 2013 3:38 pm

Yet another situation where it's impossible to balance the load on your generator? I never got a great answer from outback regarding autotransformers on the output side. viewtopic.php?f=17&t=6324&start=0

Seems like everyone just shrugs it off. I feel like my kohler gen won't last nearly as long as I hoped.
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Re: Master and Slave sharing the charge

Postby tallgirl on Thu Apr 25, 2013 5:07 pm

Did you calibrate the AC outputs with a high quality DVM? That's one thing that can cause that -- one inverter is calibrate (or just plain set) different from the other and the imbalance is being made up by the autotransformer.
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Re: Master and Slave sharing the charge

Postby Mauricio on Sat May 04, 2013 1:38 pm

Thanks you all for taking an interest. I guess there is no answer to my dilemma. The master shutting down before the slave was/is kind of puzzling, like TG said I would have expected to be the other way around, but since everything works this peculiarity was placed on a back burner; something to tackle on a later date. What bothers me is the actual shutting down of one inverter. I understand that at this stage the batteries will absorb the current they require, I'm not trying to push more current in, but I would have expected both chargers to continue the charge at a reduced rate. Installing a PSX 240 on the input side could do the trick, thank you TG.
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http://www.arribadelaroca.com
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Re: Master and Slave sharing the charge

Postby tallgirl on Fri May 10, 2013 6:57 am

Mauricio,

It really is better for one inverter to shut down at low levels of charge. The equipment is more efficient at higher power levels, so turning one off at lower levels increases the overall efficiency.
Julie in Texas

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