AC Coupling: How to stop Battery Charging using GT interters

mnittler

Using AC Coupling with Grid Tied Solar Inverters in a Grid Down condition:
I am making an assumption that if you know how to AC Couple then you know that there are also pit falls in using AC Coupling.
How do you stop the wild uncontrolled battery charging after the batteries are fully charged?
How do you determine the batteries are fully charged?  My Xantrex XW6048 has a contact closure that can be programmed to close at a particular battery voltage.
How do you knock the Grid Tied inverters offline when AC Coupled after the batteries are fully charged and there is more energy being produced that is being consumed.
Based on 20 AHr rating of a battery bank what is the maximum allowable charge rate. (I think Trojan recommends not over 10% so is that 20AHr rating X 0.1 X battery voltage to get maximum watts allowed?
As far as I know when using AC Coupling the battery charge controller is out of the circuit.

bill von novak

mnittler said:

Using AC Coupling with Grid Tied Solar Inverters in a Grid Down condition:
I am making an assumption that if you know how to AC Couple then you know that there are also pit falls in using AC Coupling.
How do you stop the wild uncontrolled battery charging after the batteries are fully charged?

Several ways.
1) Change the operating frequency of the system.  Some grid tie inverters (like the SMA system) will throttle back as the frequency rises.
2) Shut down segments of the array as voltage approaches maximum.
3) Shut down the entire array once voltage hits maximum.

How do you determine the batteries are fully charged?

In general you don't.  You just regulate voltage.  You will, in general, not get good charging with simple AC coupled power systems - which is one reason they are rarely used, and even when used, used only for emergency power.

Based on 20 AHr rating of a battery bank what is the maximum allowable charge rate. (I think Trojan recommends not over 10% so is that 20AHr rating X 0.1 X battery voltage to get maximum watts allowed?

C/8 is a good rule of thumb for flooded batteries.  Some AGM types can handle C/4.

As far as I know when using AC Coupling the battery charge controller is out of the circuit.

AC coupled systems do not have a charge controller between solar and battery.

Dave Angelini

Agree with above post and add that the correct firmware and your settings are fairly critical.

Marc Kurth

Bill, I will respectfully add that some AGM's can accept a whole lot more than C/4. It is design and temperature dependent.
I do not want to derail this thread, but I can detail industrial app's charging at C2 and C3. Yes, 400 ah L16's at 1220 amp rate.

Not all AGM's are alike so they should be selected for the application.

Marc

Dave Angelini

Marc, while one can do this, why would one want to when long life and plenty of time is what many apps desire and have.
Hi C's do sell batteries As long as they get charged why hit them hard?  One of the problems with AC coupling is it just has too much room for overcharging as the OP noticed. It can be set-up and work reliably, but it certainly is not part of keeping things simple.

Marc Kurth

Howdy Dave,

My point wasn't this app or the typically gentle stuff that off grid PV folks deal with. I'm sure that high charge rates seem silly when your focus is off grid PV. 

I was talking about understanding battery capability. I'm a battery guy who deals with an extreme range of applications. Alternate energy systems occupy one little corner of my world.

The "why" is that there are many applications that need it. I can give you a long list of real world systems needing super fast charging.

Note that Concorde specifies a bare minimum charge rate of .2C for repetitive deep cycle applications. They suggest going higher for maximum battery longevity.

Marc

mnittler

Marc,
I have a stupid question.  What is C? I am figuring that if I have this question then probably there are others out there that might benefit from a definition.  Can you give an example of a 20Ahr battery rating and use a C rating to calculate charge amps allowed if you would.

Mark

bill von novak

Marc Kurth said:

Bill, I will respectfully add that some AGM's can accept a whole lot more than C/4. It is design and temperature dependent.
I do not want to derail this thread, but I can detail industrial app's charging at C2 and C3.

Agreed.  As always, the manufacturer's data should be the ultimate authority.  However, it can be hard to find such high rate batteries, and they almost always make compromises on energy density to achieve such high rates.

bill von novak

I have a stupid question.  What is C? I am figuring that if I have this question then probably there are others out there that might benefit from a definition.  Can you give an example of a 20Ahr battery rating and use a C rating to calculate charge amps allowed . . .

C is charge rate relative to the capacity of the battery.  C/4 for a 20ah battery would be 5 amps.

Estragon

There are no stupid questions. Like anything else, there are abbrieviations and shorthand used in battery/solar that may not make sense initially.

The "C" is literally short for capacity. Like Bill said C/4 on a 20ah battery would be 5a.

It gets a bit wierd though, because C is itself a function of charge/discharge rate. For instance, the battery might be rated at 20ah at a 20hr rate (C/20) rate of discharge. In other words, it could sustain a 1a load for 20hrs. The thing is, if you discharge the battery a 5a/hr, it likely won't last 4hrs, because the capacity will be lower at C/4 than at C/20. C is also affected by temperature. My 350ah bank spends a lot of the year at more like 300ah because it's in a normally cool location.

In most cases, just using C as given by the manufacturer is fine for our purposes, but it doesn't hurt to understand it's a bit of a squishy term.

Marc Kurth

bill von novak said:

Marc Kurth said:

Bill, I will respectfully add that some AGM's can accept a whole lot more than C/4. It is design and temperature dependent.
I do not want to derail this thread, but I can detail industrial app's charging at C2 and C3.

Agreed.  As always, the manufacturer's data should be the ultimate authority.  However, it can be hard to find such high rate batteries, and they almost always make compromises on energy density to achieve such high rates.

Understood and agreed upon, Sir! 
That is exactly why I speak up at times. Not all AGM's are created equally anymore than all flooded cells are the same.

I would further suggest that all designs involve compromise and that applies to batteries and virtually all else

Respectfully,
Marc

mnittler

Bill,
I have 3 strings of Trojan T-105 batteries at a 20 Ahr rating of 225 Ahr each string @ 48Vdc.
So 3 strings would =  225 x 3 = 675 Ahr (at a 20 AHr hr rating).  Would I have to divide the 675 AHr number by 20 to get it to a 1 hr rating before applying the C rating discussed above or use the 20 Ahr rating number?
I am assumming that Trojan rule of thumb is 675 Ahr X 0.1 = 67.5 amps charge rate.
How would you calculate the maximum charge rate for the battery strings.  I know some go over the 10% maximum charge rate.

Plan A:
What I am thinking of doing is during a grid down situation is turning off all but one or 2 of my micro inverter strings and reroute the micro inverter circuits so they think the gird is on because of the hybrid inverter.  I plan on managing the micro inverters by battery voltage and house loads.  This will have to be closely watched.
The 3kw DC Solar charger controller will still be able to collect solar and charge the batteries as normal.

Plan B:
Same as Plan A above but use a Normally closed 30 amp contactor or relay in line with the micro inverters output.  Opening the inverter output will cause them to turn off for 5 minutes.
Have the Xantrex 6048 output a contact closure that will go the coil of the contactor to energize the relay that will break the micro inverter outputs when the Xantrex indicated that the batteries are full charged.  I just have to find a "normally closed" 30-40 amp contactor.

Another Question: Will causing the micro inverters to possibly shut down every 5 minutes  or so worst case cause any kind of damage to the micro inverters by turning off via UL1741 grid tied logic.  I would guess that is just the way they work during a power failure anyway but maybe not just not every 5 minutes.

bill von novak

I have 3 strings of Trojan T-105 batteries at a 20 Ahr rating of 225 Ahr each string @ 48Vdc.
So 3 strings would =  225 x 3 = 675 Ahr (at a 20 AHr hr rating).  Would I have to divide the 675 AHr number by 20 to get it to a 1 hr rating before applying the C rating discussed above or use the 20 Ahr rating number?

No!! The amp-hour rating of a battery is somewhat dependent on discharge rate but you can't just divide the number by 20.  For example, a Trojan T105 has a 20 hour rate of 225 amp hour.  The 10 hour rate is NOT 112.5 amp hours - it is 207 amp hours.  You have to find this number by looking on the datasheet or (worst case) by testing.  Also note that it would be a mistake to discharge a battery like a T105 at a steady 1C rate; if you have a design that calls for that your battery bank is likely too small.

When people talk about the C rate during charge they are usually talking about the 20 hour C rate, because that is the most commonly used spec for a battery.  But as Estragon mentions that's not universally true.

I am assumming that Trojan rule of thumb is 675 Ahr X 0.1 = 67.5 amps charge rate.

How would you calculate the maximum charge rate for the battery strings.  I know some go over the 10% maximum charge rate.

In your example, if you have a 675 ah pack, your maximum charge current (assuming C/8 max charge rate) would be 84 amps.  If you want to charge at C/10, or 10%, then you are correct - it would be 67.5

Now here's the caveat.  A 3 string battery will NOT share power equally.  The string with the lowest ESR (i.e. lowest resistance) will take more of the current, and differences in wire length can also cause unequal sharing.  This is one of the many reasons that lead acid strings in parallel is often not a good idea.

Plan A:
What I am thinking of doing is during a grid down situation is turning off all but one or 2 of my micro inverter strings and reroute the micro inverter circuits so they think the gird is on because of the hybrid inverter.  I plan on managing the micro inverters by battery voltage and house loads.  This will have to be closely watched.
The 3kw DC Solar charger controller will still be able to collect solar and charge the batteries as normal.


This can work.  Your maximum microinverter output must be less than the maximum rating of the inverter, and you have to watch battery voltages carefully.  Note that ideally you do not want to have to "reroute" anything - the microinverter array can live permanently on the downstream side of the inverter, and that is generally how AC coupled systems are wired.

Also note that Outback makes DC controlled breakers that can do this automatically (trip when the voltage limit is reached.)

Plan B:
Same as Plan A above but use a Normally closed 30 amp contactor or relay in line with the micro inverters output.  Opening the inverter output will cause them to turn off for 5 minutes.
Have the Xantrex 6048 output a contact closure that will go the coil of the contactor to energize the relay that will break the micro inverter outputs when the Xantrex indicated that the batteries are full charged.  I just have to find a "normally closed" 30-40 amp contactor.

Should also work.  It can be hard to find UL-rated contactors but they exist.

Another Question: Will causing the micro inverters to possibly shut down every 5 minutes  or so worst case cause any kind of damage to the micro inverters by turning off via UL1741 grid tied logic.  I would guess that is just the way they work during a power failure anyway but maybe not just not every 5 minutes.

That SHOULD not be a problem.  One issue is that the inverter inputs will see a much higher voltage than they are used to seeing, but provided your system was designed to the specs of the microinverter (most are) then it shouldn't be a problem.

jonr

An issue that I see is that one can't simply look at battery voltage and say that a battery is or isn't fully charged.  For example, a low battery might be charging at 14V while a fully charged battery might be resting at 12.7V.    So what value do you use to stop charging?

A better way might be to sense the rate of rise of battery temperature as compared to charging power - as a battery reaches full charge, it increasingly turns charging power into heat.

IMO, all parallel batteries should be checked for balance with a clamp DC ammeter.