Classic Over Voltage Set Point

[Wxboy]

I have my Classic 150 on a 48 volt system set to absorb at 58.9 volts and the voltage limit set at 60.4 volts.  The ambient temperature where the batteries are located is 45 degrees F and the temp compensation is set at -2.5 mv.  Typically the batteries absorb around 59.6 give or take a few tenths.  This has worked well for several months but yesterday when monitoring remotely I saw 60.0 volts then 60.4 volts.  This was not normal behavior but I didn't pay that much attention.  Today it was working normally for most of the day absorbing between 59.6 and 59.9 and then I saw 60.4, 60.8, 61.2 and higher.  When I got home I looked at the graph on the Classic and the high reading on the graph was 61.8.  It was above the 60.4 volt limit for approx. 30 minutes.  Given that I have agm batteries I can not afford to charge the batteries outside of their recommended charging parameters so this has me very concerned.  Especially when you see it happening remotely and there is nothing that can be done about it.  I don't have the newest firmware that was recently released but I have the previous version and I don't think this behavior has anything to do with firmware because I've been running that firmware for many many months and I've never seen this before. 

Other than setting the voltage limit which doesn't seem to be working what can I do to prevent this from happening again?

[Westbranch]

WXboy, what is the service history of those batteries? Age? Max DoD achieved? How often do you achieve Float?

[Wxboy]

Westbranch, the batteries are healthy.

[Wxboy]

It just dawned on me what caused this and I went down to check the Classic graph and my suspicion was correct.  A year or two ago I saw the Classic do the same thing on my 12 volt system and I think I have an old post about it. 

Every once in a while my Classic's power output gets out of calibration by 80 to 100 watts(I also have a post about this).  What happened today, and it's the same thing that happened a while back, is the calibration went off and the Classic was in absorb but it was registering that it was outputting zero watts of power.  What that meant is that the batteries were taking less than 100 watts of power but the Classic was registering zero.  For whatever reason when the Classic is in absorb and it thinks it's putting out zero watts it causes the voltage to go over it's voltage limit.  I do not have another charging source connected to these batteries. 

The reason I know the Classic gets out of whack is because my Trimetric and Classic are usually relatively close but when the calibration goes off the Trimetric will show between 80 and 100 watts more going to the batteries than the Classic displays.  Normally not a big deal until it causes a an over voltage situation.  The calibration gets corrected when the Classic goes into resting and wakes back up but when the voltage is high it doesn't rest and it doesn't get corrected.  Luckily we ran out of daylight today otherwise the condition could have gone on for more than 30 minutes. 

I think it's time for a call to Midnite. 

[Vic]

Hi Wx ...,

Do not run AGMs here,  but it has been reported in a number of places,  that when AGMs are nearly fully charged,   that there appears to be a Surface Charge effect,  which causes the battery terminal voltage to rise.  This is especially noticeable with MPPT CCs when they perform a Sweep,  and the loads from an inverter or other loads are low.

At times,  when AGMs are essentially fully charged, it has been noted that  AGM batteries display an apparent over-voltage condition during these Sweeps.  A battery that is fully charged or nearly so,  represents a high impedance.  When the MPPT CC sweeps,  there needs a place for the current that the CC delivers to the batteries/loads to go during Sweeps.  These Sweeps can cause Vbat to rise above Set-points.

Folks smarter than I have noted that this situation should not be detrimental to the AGM batteries.  I do not know enough to know all of the ins and outs of this situation.

Of course,   this may not be the exact situation with your system.

FWIW,   Vic

[boB]

It may overshoot for a moment but should not see that for 30 minutes.

If you saw it in Absorb for very long at 000 watts or 0.0 amps, that USUALLY means that there is another
charging source......   But you don't have another charging source...

I'm not sure about what Vic was saying about the voltage going up by itself but maybe ?
Maybe some kind of temperature variation thing ?

So you have your LIMITS MAX Voltage set to  60.4 volts ?  61.8 should not occur unless it
is ~maybe~ for a quick moment IF there may have been a load (and Absorb was being held),
and THEN that load turned OFF.  But it would not stay at this high voltage.

The fact that you showed 000 watts means, even if the current sense if off slightly, that the
current to keep it at Absorb is VERY low....   I do not see how a Classic that has been
working fine for quite a while can mess up like this...  I have not seen a problem with
the MAX voltage limit not working correctly  unless the Classic is not quite calibrated
correctly and the reading is coming from another meter.  If you ARE reading it on
another meter and not the Classic's meter, it may be that you need to calibrate the
voltage reading.   If so, what I would do would be to set the TWEAKS voltage offset to
say, +0.4 volts and then adjust the trim-pot battery voltage adjustment to read
the same as your Fluke meter or Trimetric....  Assuming the Trimetric is correct to
the tenth of a volt, and it most likely is.

But, if you really needed your Classic to be fixed/swapped/looked at, then maybe it's time ?

boB

[Vic]

Hi boB,

I claim to nothing specifically about these reports,   but,  for a number of years this has been a not infrequent Topic on other Forums.   AGM  batteries seem to be the most-reported battery type,  but there have been some reports of this when Gel batteries are used:

Please plug this Search Term into Bing:
agm battery charge voltage overshoot  site=outback power forum

There are many Threads on the Outback Forum on this behavior.

It seems difficult to post a direct Link to (at least for older Threads,  using a Search Engine these days).

The Wind-Sun Forum also has had a number of discussions on this this as well.
   FWIW,   Vic

[Westbranch]

the batteries are healthy.

that is an assumption, based on ?

I did some guessing and I think you have 3 - 255W pv's... for 765W 
and 765W / 48v = ~15.94A max so that is about a  15.94A/ 200A  =~ 7.9% charge rate or
at 60.4V, (765W/60.4V ) +~ 12.64A=~6.3% charge rate.

Therefore depending on the DoD you appear to have a low charge rate,
so what makes you believe your batteries are healthy?
Do you have a WBjr, or similar meter,  to verify the actual amount of charge your bank gets?

[mike90045]

Does the BTS (Battery Temperature Sensor) compensation cause this ?  I ask, because in really cold weather, the chargers really boost up the voltage a bit (on a 48V bank) and that surprised me.  I had to tweak things back a tad to keep the inverter out of OverVolt shutdown.
The meters/settings are not always plain if it's actual voltage or TempComp voltage.

But reading, maybe it's an AGM thing, or something totally different.

[Vic]

WX ..,

You did start a Thread on this behavior about 23 months ago - go to the top of this:
http://midnitesolar.com/Forum/index.php?topic=1120.msg8482#msg8482

In that you Thread,  you seemed quite adamant that this behavior was NOT Surface Charge effect.   In my opinion,  it probably is surface charge.  When all loads are removed from the battery by turning the inverter off,  and the voltage rises a bit,  this is exactly what surface charge looks like.  Your configuration was different at the time of this other Thread,  I realize.

In VRLA batteries,  electrolyte migrates away from the plates slowly.   This often means that the effects of surface charge are visible for quite some time,  IMO.

Not to try to put too-fine a point on this.   But,  there are many,  many Threads of other Forums about very similar behavior with AGMs,  and Gels.

FWIW,   Vic

[Wxboy]

the batteries are healthy.

that is an assumption, based on ?

I did some guessing and I think you have 3 - 255W pv's... for 765W 
and 765W / 48v = ~15.94A max so that is about a  15.94A/ 200A  =~ 7.9% charge rate or
at 60.4V, (765W/60.4V ) +~ 12.64A=~6.3% charge rate.

Therefore depending on the DoD you appear to have a low charge rate,
so what makes you believe your batteries are healthy?
Do you have a WBjr, or similar meter,  to verify the actual amount of charge your bank gets?

Westbranch, the batteries are about 2 years old, they get cycled an average of 15-25%, and they've never been lower than 55-60% DOD.  They never sit in a discharged state for more than 24 hours and they get a full charge at least twice a week.  The batteries are healthy.  My older smaller bank is 7 years old, they are the same type of batteries, and even those are doing well for their age. 

As for the other ideas I value all insight but it isn't surface charge in my case.  Other people may experience this but watching my batteries every day for years I can say they don't behave this way.  As soon as the charging current stops the voltage drops quickly.  If the charge controller was resting it would draw about 5 watts and my inverter draws about 7 watts while in standby.  With a combined 12 watt draw my batteries would never be above 60.4 volts for 30 minutes.  I have a Fluke meter, a Trimetric, and a Mate 3 all to verify voltage.  The Mate 3 is what first tipped me off about the high voltage when I was monitoring the system remotely.  The charger on my Radian was off as it is every day.  I only use the charger to top off the batteries if there is a storm coming that could knock out power.  Also the Radian was in Grid Zero mode which is a mode that requires the charger to be turned off to operate. 

Maybe I'll get lucky and it will happen on a weekend when I can take a video and show with a clamp meter what's happening.  I will be the first to say that this is a rare event(3 times that I know of in 3 years, although 2 days in a row) so chances are I won't be lucky enough to get it on video.  I suspect the Classic was putting out between 1.1 and 1.5 amps when this occurred because I have the whiz bang jr. set to trigger float at 1.0 amp. 

I don't claim to be a solar expert but this was not normal for my system. 

Bob, I am about ready for a swap out if you guys are willing.  I'm working with Roy on this situation and I sent him a few pictures.

[boB]

Bob, I am about ready for a swap out if you guys are willing.  I'm working with Roy on this situation and I sent him a few pictures.

That is good.  Roy is  a pretty sharp guy and we're very glad to have
him and the other great support people here at MidNite !

boB

[Wxboy]

Another event today.  Voltage limit set at 60.0 volts.  Classic registered 60.8 volts at one point on the MNGP display and 60.5 at the next interval.  I knew to check it today because I saw the high voltage while monitoring Optics RE which shows the Mate 3 voltage.  This was a short lived event because shading hit the panels about 10 minutes after the event started.

Same exact conditions as last time.  No loads on the system, batteries almost at 100%, Classic showing Zero watts out(real watts out would have been in the 60-80 range), voltage overshoots the set limit by an unreasonable amount.  No other charging source. 

[boB]

Wx,  Just looking at the code...  It looks like it should be limiting OK.  Have you seen the battery voltage
stay more than a couple of tenths of a volt above the maximum voltage set point for any length of time ?

When the battery voltage was above the maximum voltage set point, have  you gone into the T-Comp--VIEW
menu and looked at the temperature compensated target voltage at that time    If you catch the
voltage above the set MAX T-comp'd set point, I am very curious as to what that value is at that time.

That value may  give a clue as to what is happening.

[Wxboy]

boB, I've seen it go more than a few tenths above the set point for about 30 minutes on one occasion.  I believe it would have gone on longer but I usually run out of sun this time of year when the batteries are getting topped off. 

I haven't looked at the temp. compensated target voltage while this is happening because I'm typically at work but I've checked it during normal conditions and it will be in the 59.8 range.  I believe this problem is linked to the fact that the Classic thinks it is outputting zero watts and maybe because of that reason it doesn't try to scale back it's power output.  If it was showing that it was putting out even 20 watts I'm confident that it would not overshoot the voltage.  Every time I've seen this happen the Classic was registering zero watts out.  It sees that it's still at or above the absorb voltage so it doesn't go into resting.  If it went into resting and woke back up it would reset it's calibration and when it woke up it would see it was outputting power and I believe it would correct the problem.  Of course at the heart of all this is the problem where my Classic's power is sometimes spot on and sometimes it reads low by approx. 75 watts.  I would prefer to have that corrected and that would also solve the problem.

Thanks boB.