Charge Controller Current Limit Does Not Account For Power Going to Inverter (Opportunity Load)

mcnutt13579

Folks,

Just recently completed installing a "new" system for some people new to off-grid.

They already had some [mismatched] parts but we built it for future expansion.

I will give the specs but don't complain to me, I didn't pick [most] of these items, they were already there.

SW4024 inverter
Midnite classic 150 CC
2 strings 235 watt Solar World panels (1880 watts)
220 AH FLA string [yes I know]
1.5 HP well pump [yes I know]
House that uses extremely little.

Here's the rub.  We have the CC current limited at 30A so as not to burn up the [tiny] battery bank under normal conditions.  This is ~800 watts.

If we run the well pump it takes a consistent 2600 watts.  It would be nice for future reference if nothing else, is there any setting, or any charge controller or other set of hardware on the market, where we could somehow release this current limitation restraint when pumping water or some other big load comes on (air conditioning is one that would make sense)?

Obviously we have net negative 1800 watts with the existing setup but it seems silly that the sun is shining and we could theoretically only have net negative 760.

And yes, bigger batteries and more solar are in their future when finances allow, but I wanted to throw this out there to see how others might have dealt with these kind of opportunity loads.

Photowhit

Do some research into the Midnite Whiz Bang Jr., I think they have it implemented now to regulate by the current going into the battery bank.

The Whiz Bang Jr is a shunt based monitor, for the Midnite Classic, to measure the current going into the battery bank. It might require you buying a shunt if there isn't one already installed.

http://www.midnitesolar.com/productPhoto.php?product_ID=519&productCatName=Charge Controllers - Classics&productCat_ID=21&sortOrder=9&act=p

NAWS sells them;

https://www.solar-electric.com/midnite-solar-whiz-bang-jr-current-sense-module.html

mcgivor

mcnutt13579 said:

Folks,

Just recently completed installing a "new" system for some people new to off-grid.

They already had some [mismatched] parts but we built it for future expansion.

I will give the specs but don't complain to me, I didn't pick [most] of these items, they were already there.

SW4024 inverter
Midnite classic 150 CC
2 strings 235 watt Solar World panels (1880 watts)
220 AH FLA string [yes I know]
1.5 HP well pump [yes I know]
House that uses extremely little.

Here's the rub.  We have the CC current limited at 30A so as not to burn up the [tiny] battery bank under normal conditions.  This is ~800 watts.

If we run the well pump it takes a consistent 2600 watts.  It would be nice for future reference if nothing else, is there any setting, or any charge controller or other set of hardware on the market, where we could somehow release this current limitation restraint when pumping water or some other big load comes on (air conditioning is one that would make sense)?

Obviously we have net negative 1800 watts with the existing setup but it seems silly that the sun is shining and we could theoretically only have net negative 760.

And yes, bigger batteries and more solar are in their future when finances allow, but I wanted to throw this out there to see how others might have dealt with these kind of opportunity loads.

The pump if 1.5 HP should be in the region of 1200 W, how did you get 2600 W?

If the battery is not deeply discharged overnight the current required to charge the battery  may be less than 30A, also bear in mind the sun doesn't suddenly come with full intensity, it will gradually increase the voltage with a limited current potential, so by  10 am for example  the battery  voltage may be high enough to limit the current demands below the limitation programed, 30A in your case.

Having a system similar in size, 1500W PV, 260Ah FLA battery, 60A CC, I conducted a test over a one month period  to see what current the battery would demand with no limit to current (60A max) and found it never rose above 27A during bulk or absorbtion.  The excess production could be put to use handling small to medium loads whilst still sufficiency charging the battery. After float there is reached there is around 1200W available of otherwise wasted energy, this is when water pumping, clothes washing, and electric cooking an so forth  are conducted. Obviously cloudy days restrict use of opportunity loads.

Try increasing the current limit and closely monitor what the actual current is going into the battery with a clamp on ammeter, a healthy battery which is not deeply discharged  may not need to have a limitation because it will self regulate, only taking what it needs, I must emphasize this needs close monitoring on a daily basis and probably not something to hand over to the uninitiated or inexperienced. Of course the best solution would be to have a larger battery bank to provide a cushion of safety, but having said that, those who live off grid really need to understand intermitaly the system they are dependent upon.

These were my observations and experience based on daily checks and close monitoring. YMMV 

Dave Angelini

Older Xantrex SW not a Conext SW correct?

Estragon

One caveat to lifting the limit on CC current - make sure the remote battery temperature sensor is installed and working properly. If so, even if the batteries take enough current to heat up a lot (I've never seen that with my banks), the classic should reduce target voltages based on BTS reading.

jonr

+1 on the Whiz Bang Jr.  Charging with an  "up to" .2C (~45A) limit should be fine - but, as recommended,  monitor temperature to verify. 

BB.

Yea--I would make sure the remote battery temperature sensor is installed and working. That will help prevent battery "melt down" with high available charging current.

Then go ahead and let the full charging current through from the charge controller.... What you will need to watch out for is--Monitor the battery bank voltage when the sun is up and the batteries are full. Worst case, folks have reported that some MPPT charge controllers (no Midnite reports that I recall) will dump full charging current into the battery bank during MPPT sweep and take the battery "out of regulation"--Potentially > 36 volts which can fault the AC inverter and/or start to cause damage to your DC battery bus connected loads.

Flooded cell batteries (I think) tend to "clamp" high current/voltages "better" than AGM (convert high voltage/current into heat and hydrogen+oxygen gasses).

If your MPPT controller does not "behave funny" during MPPT sweeps and your battery bank voltage does not exceed ~30-31 VDC, you should be OK.

The 1.5 HP pump is a wild card... Starting loads will be heavy no matter what. Running some extra wire between the inverter and the pump (50' or 100' extension cord to experiment) can reduce starting surges significantly.

And running loads depend on pump type and water flow. Centrifugal type pumps draw more current as water flow (gallons per minute) increases--You can put a water valve on the pump output and restrict the water flow to reduce running current (such as when pumping into a cistern).

For positive displacement pumps, the higher the pressure, the more current they draw. Placing a water flow valve on the output will just make current draw higher.

If they have a cistern--Use well pump to move water into the cistern (use gate valve to reduce flow if current draw is too high for centrifugal pump), and use a 12 or 24 volt "RV type" water pump + pressure tank for household/garden water pressure.

-Bill

PS: I should add that the WizBang Jr. is a good idea too. My suggestion is if the WBjr is out of their price range for now.

mcnutt13579

mcgivor said:

The pump if 1.5 HP should be in the region of 1200 W, how did you get 2600 W?

Measured with amp clamp going into step up transformer. 22 A @ 120 V.  S.F. max amps printed inside control box ~11.1 IIRC @ 240 V.  This is inline with what I have experienced, my 3/4 HP pump at home is around 8.5 A.  So I am not concerned about this particular installation.  Inrush measured at around 45 A @ 120V.  Seems to spin up rather slow which is good for the poor old SW (Trace).

Does anyone have access to a reputable chart of power requirement of well pumps?

BB.

In general, induction well motors with remote starting capacitor (3 wire motor, well top installed) have lower starting current than induction motors that do not have a remote starting capacitor (two wire motor).

Franklin well pump motors (50% higher starting current for 2 wire motors?):

http://www.franklinwater.com/media/110562/M1311_60_Hz_AIM_12-14-WEB.pdf

Then there are the "solar friendly" pumps like from Grundfos. They basically are 3 phase motor (usually permanent magnet?) with a variable frequency dive (VFD). They have very low (almost zero) "surge" current and can take AC or AC/DC input (depending on model--SQ vs SQFlex for Grundfos--DC input can be battery or even direct connection to solar panels). However, these pumps tend to be expensive (but well built). And there are other brands/solutions available (such as a 3 phase well pump motor with a VFD).

Lots of options out there.

-Bill

jonr

Note that an AC clamp ammeter will get you VA, which can be quite different than watts.

mcnutt13579

Correct that we don't know the power factor, however, I don't imagine it is much worse than 80%.  All the ratings seem to be within normal range.

In any case the pump is not a problem other than it is a lot bigger than it needs to be.  The old Trace starts it reliably every time.  Inrush as measured is well within inverter surge current capability.

I would not have put it together this way from the start, but we have to deal with what we've got.  Can't change out every "good enough" part for the perfect part, it would break the bank.  Little by little here.

I will look into the Whiz Bang unit and call the factory when I get an office day.  Too much "real work" going on in the daytime these days.