Global Current Limiting

[Vic]

In March of this year,   HC Ryan posted a Global Current Limiting White Paper on this site.

Is it safe to ASSUME  that this function is still in the latest Firmware,  and that each Classic that is on the system needing Limiting still will need its own WBjr?

Believe that it is OK to Attach this document,  as should be shown below.

A friend is Commissioning a LARGE Backup system to Grid power with 13.7 kW STC PV and 7 Classics,   and really needs this Limiting.

Thanks,   Vic

[Vic]

OK,   the friend that needs Global Limiting is probably working with MN Tech Support,   he is fairly technical,   but   does not have a lot experience with battery-based PV systems.    I have tried to advise him on this recent rehab of the system,   but  I know nothing of the Classic Global Limiting,   so guess that I will be of no help to him.

While I do have systems with more than one Classic,   would prefer to not hack into other systems,   adding Shunts,   and ditzing with the Cl programming,   as it is difficult to spend very much time at remote sites in watching system behavior after any of these changes,  and so on ... usually like to spend several days with systems that have had significant changes made to them.

Assume that no one here is using this potentially great function in the Classics,   and therefore are unable to comment on this important function.

IMO  this function is/would be another feather in the Classic's cap,   but am reluctant to note this as a cool function elsewhere,   without a confirmation (other than that White Paper),   of its existence,   and what are the observations of its performance ... (although,   I did mention it recently on the Wind-Sun Forum,   FWIW.

Just prattling.      Thanks for any feedback,    Vic

[SolarMusher]

Hey Vic,
I should have an off grid system upgrade (I hope) with two classics 7kw/PV  next fall where I would need to use it, but right now I'm with you and know really nothing about it except this white paper from Ryan, sorry. I will be very interested by your next comments about it.
Good luck!
Erik

[Sunshine]

Throwing a complicated one at us Vic! We could use a little more info so that we can get you the correct answer.
What inverter system is your friend using?
I would guess that it is a 48V system, is that correct?
What is the battery capacity?

[Vic]

Hi Sue,

Thanks for the reply;

This chap is already working with Jim in MN Tech,   so was not trying to have the very busy MN folks try to support this project,  was hoping to make sure that this Limiting still exists,   any added caveats about its function,   AND  if Members here were using it.   Some here seemed to have been begging for it in the past.

Unfortunately,   the target system is only 24 V,   has  Surrrette  1124 Ah L-16s IIRC,  and is running two SMA Inverters that total 10 - 11 kW.

But  really Doug is already pinging MN Tech,  so let's not consume any more of Tech bandwidth  --  THANK YOU for offering the added support,   Sue.

Wish I had a capable Bench system that could be used for checking out some of the whizziest capabilities of the Classic and other MN products.   But  not in the cards just now.

Thanks,  too,  Erik.   Hope all is going well with you,   and look forward to info on that next system next system when it is operational.

This Global limiting  could be helpful for a number of folks with a lot of PV ...   think that Doug got some great deal on a couple pallets of PV,   so all went from there ...   he was running AIMS CCs,   which blew up when he upgraded all of the PV cabling to them ...   and so on,  'as the world turns'.

Thanks again for the comments!   Vic

[Halfcrazy]

Vic,
It still exists. You basically need one WBjr per Classic that you want to back off. So lets say you have 4 Classics and each can do 50 amps (200 total) but you want to limit to 50 amps.

Put a WBjr on Classic 1, 2 and 3 and set all 3 to 50 amps as shown in the attached document. Essentially it is not truly global as it only manipulates the one classic that is why the need for multiple WBjr's

[Vic]

Hi Ryan,   GTEAT to see you back here,   Thanks for the info;

You are saying that all but one Classic would need a Shunt/WBjr,  primarily because in your example of 50 A per Classic,  the maximum current from any one PV string is about 50 A,  so the fourth Classic would realistically be somewhat limited by the available current almost all of the time ?

Will think about this a bit more ...

Thank you very much,   Ryan,   and it was very nice to meet you at the Intersolar show in SF,  last month!    73,  TU,    Vic  --   kilo six italy charlie

[mike90045]

It just struck me, that the title of this thread  "Global Current Limiting"

sounds like a movie from Al Gore.

[Westbranch]

GOOOOD one Mike!

[Vic]

Strategy for Limiting Total Maximum Battery Charge Current with seven Classic 250s using 7 Strings of 7ea 180 W 48 Cell PVs.

Have been  trying to help a friend with a pretty good sized Grid Backup power system.   The Classic 250s  have recently replaced some AIMS CCs which got toasted.   The PV array is sufficiently large,  that some battery charge current limiting seems in order.

System parameters  --  Seven Classic 250s,   each with 7ea 180 W 48-cell PVs,  charging a 24 V Flooded 1070 Ah battery.  Total STC PV power is 8820 W.  The MidNite White Paper on Battery Charge Current Limiting with Multiple Classics,  and the MN String Sizer output for a single Classic 250 using 7ea of the 180 W PVs is are attached.

Using customary Rules-Of-Thumb for estimating the amount of PV power routinely available from PVs,  each string of  180 W PVs can produce about 32 Amps of current into the battery bank  (180 X 7 X  0.75  = about 945 W (typical).   945/29.5 = 32 A.   This takes into account reasonable heating in the PVs,   some losses in wiring and in the Charge Controller,  etc.

At the NV location the system could see times where this output is considerably higher  --  cold Winter days with wind,  and especially with reflections from snow,  cloud Edge Effects,  etc.  But 75% of rated STC power  is a good general  rule.

At 10% of the Surrette S-1475 20 Hour Capacity (C),  this maximum battery charge current is about 0.95 X 1124/10 = 107 A for 10% of Capacity (C),  and about 140 A  for the max rate recommended by most Flooded battery manufacturers of 13% of  C.  The multiplier  of 0.95 represents the Capacity reduction from the Data Sheet C of 5% due to electrolyte of 1.265 SG verses the Data Sheet SG of 1.280,  which is customary for Surrete Solar  batteries,   FYI

Believe that NOT every Classic needs to have its own WBjr and Shunt.   Seems to me,   that  three Classics need not have WBjrs used to limit battery charge current,   and the other four will need separate WBjrs and Shunts of their own  added to the single one that the system now has, (which  will measure total battery charge current).

Saying this another way,  you have seven Classic 250s,  three of the seven Classics should not need to have their output  current limited into the battery.   But you would probably want to add WBjrs and Shunts to three of the additional Classics   The Classic that has the Shunt directly connected to the battery negative counts as one of the Classics that will not be limiting the battery charge current.  The WBjr on this Classic is used to measure the Net charge/discharge current of the battery.

The "reasoning" for the above is that each Classic could contribute about 32 A of output current.   Based on  3 X 32 A = 96 A of maximum nominal Classic output current capability,   being less that the 107 A nominal 10%  of C limitation. The only real fly in this,  is,   that during some Winter conditions  that cold,   windy weather,  plus some possible snow reflections might allow the three Classics without WBs could produce more battery charge current than that calculated above.    However,   most Flooded batteries can accept up to 13% of C,  which allows for some headroom for situations with higher-than-normal PV production.

The above started as an e-mail to the owner of this system,   and now suffers from far too much editing ...   should have started over.

Below is the Classic Sizer output for one string of seven PVs on one of seven Classic 250s.

The brief MidNite White Paper on Battery Current Limiting is at the foot of the first Post in this Thread.

Thoughts?  Thanks,     Vic

[mike90045]

And, as the battery charge ramps up with the rising sun, the battery accepts less amps, and sort of self limits anyway.  But a cloudy morning, clearing at noon, can surprise you.

[harryn]

I am still wondering if your friend really has a problem or not given the tendency for these kinds of batteries to self limit.

Obviously a breaker could limit the charge current, but probably that isn't the desired path.

In theory, you could program a PLC, industrial single board controller or hobby controller (such as a beagle board)  to detect current levels and use relays to start shutting down controllers as needed. Current detection isn't that hard - even can do with with a hall effect sensor setup.

If extended temperature range is needed, that takes it more towards the single board controller approach.

[Vic]

I am still wondering if your friend really has a problem or not given the tendency for these kinds of batteries to self limit.

Mike,  harryn,

The maximum available battery current from the entire array may well not be too much for the battery on a typical day.

Without some form of limiting,   there could be problems  with intermittent clouds,  some shading etc.

If it were my battery bank,   would not want the battery charge current exceeding 10% of C,  as the battery Gassing voltage is approached,   and above it.

The self-limiting of battery charge current that I am familiar with,  is  the declining battery Acceptance,   once the Absorb stage has been reached.   IMO  at this stage of charge,   Flooded batteries are the most sensitive to currents exceeding about 10% of C.

Limiting could be performed by a fairly large Diversion load on the system,   perhaps by a relatively large electric heater,  perhaps mounted outside,  or something similar.

Initially,  if  had been stated that the PV array was huge relative to the Capacity of the battery,   after some checking,   the PVs were determined to be 48 Cell 170s,  as opposed to some unknown 280W PVs  ...

The reduction in the size of the PV array greatly reduced the magnitude of the issues,   in string Vmp,  Voc,   and  excessive CC current demands.   But,  it still seems that some Limiting is in order,   for the location of the system.

A number of variables in the system are still unknowns (like the elevation angle,  and Azimuth of the PVs, etc),   but  still would suggest some Limiting in battery charge current.

FWIW,   Thanks,   Vic

[krementz]

I don't understand why global limiting is an issue with Classics and SMA inverters.

On the SMA menu #222-01 is BatChrgCurMax, which can be set from 10 to 1200 amps. This is the maximum charging current to the battery.

Why wouldn't you just set it here, and not mess with the Classic settings at all?

I ask because I have 2 SMA (12kw total) and 2 Classic 150s, and am strongly considering adding more panels and another Classic. My total output on optimal sunny days could be over 10% C/20, but many days are cloudy in winter, which is when I need to catch every photon. Also I would have significant daytime only loads once absorption phase has started.

[Vic]

I don't understand why global limiting is an issue with Classics and SMA inverters.

On the SMA menu #222-01 is BatChrgCurMax, which can be set from 10 to 1200 amps. This is the maximum charging current to the battery.

Why wouldn't you just set it here, and not mess with the Classic settings at all?

I ask because I have 2 SMA (12kw total) and 2 Classic 150s, and am strongly considering adding more panels and another Classic. My total output on optimal sunny days could be over 10% C/20, but many days are cloudy in winter, which is when I need to catch every photon. Also I would have significant daytime only loads once absorption phase has started.

Hi Krementz,

Since I know nothing of SMA Inverters,  or Inverter-Chargers,  do not know.   But,  it would be surprising if the SMAs  know anything at all of Classics  or other Solar battery chargers.

Would assume that the setting in the SMA,  is only for its battery charger,  and not any other.

FWIW,  Vic