How much Battery Voltage Drop with Large Load?

MrM1

How much DC and AC Voltage drop should be occurring on a 24v battery bank at about 60% SOC? With a 700 Watt AC load I am seeing the battery bank drop to about 24vDC. When there is a 25 Watt AC load  the voltage at the bank is at 25.1vDC

The load is a 240v well pump. And when the pump starts and the SOC is around 60% I sometimes get a Low Battery Cut Off warning from my inverter. I have the Inverter set to 23.2vDC LBCO and I have a delay of 35 seconds for the LBCO delay. The delay keeps the inverter from shutting off for 35 seconds during any startup surge, so I do not lose power, but clearly at start up I am dropping below the LBCO setting. To avoid the warning, I set the LBCO to 22.5v, then it does not happen.

When pump startup occurs, the inrush amps of the well pump motor is probably about 25-30 amps AC (motor is 1/2 HP on 240v 2 wire with ground). When the pump kicks on, all my LED lighting flicker once very quickly at the startup surge. Would the startup be causing the inverter to drop AC voltage just long enough to cause the LED lighting flicker, especially when my SOC is low at 60%? (But the LED light flicker also occurs at 100% SOC). I know LED lights are very sensitive to AC voltage changes.

System Set up is in Signature

Photowhit

Depends on too many factors to give a quick answer. 

The type and size of battery bank is very relevant. 

Some types of batteries can provide high amounts of current with somewhat ease, Assuming Lead Acid, Some AGM can do high loads very easy, Some Batteries with fewer but thicker plates have a higher voltage drop such as Forklift batteries.

Size matters! A 700 watt AC load, which might well be a 800 watt DC load, could represent a very high percent of the amount of stored energy left in the battery bank. Looking at your profile a 435Ah battery bank with a 60% SOC would have about 260 ah left in the tank, You are trying to draw (800/25=) 32 amps or about (32/260=) 12% of that a very substantial load. 

In addition 700 watts sounds like a running load for a 1/2 hp motor, The inrush is likely much higher. Do you have a soft start ahead of the motor?

Also I see you writing this at dawn, Are you basing your 60% SOC on a shunt based battery monitor? 25 volts at rest is likely more than 60%, 25 volts with a slight charging is likely less than 50% SOC.

In addition the age and use of the battery bank might come into play, Batteries do lose capacity as they age. Heavy use may have created periods of time when held at a low SOC allowing sulfites to harden on the battery plates.

littleharbor2

25 - 30 amps @ 240 vac. is something like 250 - 300 amps @ 24vdc

MrM1

Photowhit said:

Size matters! A 700 watt AC load, which might well be a 800 watt DC load, could represent a very high percent of the amount of stored energy left in the battery bank. Looking at your profile a 435Ah battery bank with a 60% SOC would have about 260 ah left in the tank, You are trying to draw (800/25=) 32 amps or about (32/260=) 12% of that a very substantial load. 

In addition 700 watts sounds like a running load for a 1/2 hp motor, The inrush is likely much higher. Do you have a soft start ahead of the motor?

Yes that is the running load of the well pump.  And no, there is no soft start on the pump system. 

Photowhit said:
Also I see you writing this at dawn, Are you basing your 60% SOC on a shunt based battery monitor? 25 volts at rest is likely more than 60%, 25 volts with a slight charging is likely less than 50% SOC.

Did indeed write this just as charging was starting.  Probably had about 100 Watts coming into the battery from Solar.  So there was not much voltage change and not much input from solar at all.

And Yes the battery SOC is based on shunted MN Whizbang Jr and Classic 150 battery monitor settings.  But I tend to agree with you,  when my BMS says I am at 60% my SG reads have me more at about 65-70% SOC. 

Photowhit said:
In addition the age and use of the battery bank might come into play, Batteries do lose capacity as they age. Heavy use may have created periods of time when held at a low SOC allowing sulfites to harden on the battery plates.

Battery bank is 8 months old.  They have never been discharged more than 60% SOC with the normal being closer to 73% daily ... except for a half dozen times I did controlled discharged to 50% and then complete charge in the same day for deep cycling as  suggested by @Vic and others.  The batteries do get charged fully nearly every day.  Have been EQ'ed regularly (probably too often).  Cells balance with in 10 points.  Watered regularly. Plates never exposed.   And I set my target 100% fully charged SG to be the SG of my lowest cell after EQ.  So if after EQ the loswest cell is 1.288 (EQ'ed to the point where all cells stop rising),  I then try to target 1.288 on my hydro sampled from that lowest cell as my 100% daily fully charge SG reading.

MrM1

littleharbor2 said:

25 - 30 amps @ 240 vac. is something like 250 - 300 amps @ 24vdc

Agreed.  I am guessing at the pump surge amps.  I am using the table that  

mike95490 often posts.  I have tested the inrush of my pump with a borrowed amp meter that I was told measured inrush.  Not sure if it did not not,  but it measured my pump at 14.1 amps at startup.  But based on the table that mike posts, I am not sure I believe the low inrush number.  I do have a new inrush meter,  but have not had time to actually test yet. 

But I guess you are saying,  that this kind of DC  amperage could indeed effect lighting and AC voltage slightly for a split second at startup?    I do have a 250 amp MN Solar breaker on the DC side between the battery bank and inverter. 

MrM1

In the past I had thought my issue with the lights might be the Conext SW 4024 inverter,  but I am thinking now it may be the battery bank instead.  The inverter is rated for 3400 Watts continuous, 4000 for 30 minutes and 7000 Watts for 5 seconds.  At least in theory, the inverter should be able to handle the well pump start.    I am starting to think it might be the size of the battery bank at 24v causing AC voltage to drop at large startups.

Dave Angelini

22 Volts is lights out after the hysteresis delay. That is really all you need to worry about for now
Independence day soon

MrM1

Dave Angelini said:

22 Volts is lights out after the hysteresis delay. That is really all you need to worry about for now
Independence day soon

Hi Dave,  thanks for your response.  So is my setting of 22.5 for an LBCO a correct number.  Not sure what you mean by "lights out" at 22v.     The system seems to work fine at the 22.5v LBCO.   But going back to my OP, just did not know if the voltage numbers I was seeing was correct when large-ish loads are applied.     And if that would be causing an AC momentary voltage drop, would that cause a single flicker of my LED lighting?

MrM1

Also got to wonder,  if I had a 48v system instead of a 24v system,  does a 48v battery bank and inverter handle AC voltage sags / drops better than a 24v system?  I am considering a change.

Raj174

I would say it does, Twice the voltage at half the current. But our lights dim when it starts the 2 ton heat pump in first stage, about 16000 BTUs, a 136 amp surge, with an OB 4000 watt Radian inverter with a 12 kWh LiFePO4 battery bank.

Rick

Dave Angelini

Really, you do not know what lights out are on an inverter based battery power system?  I think you do
The adjustments for this are a user setting. I use 22V for my clients and set at least a 60 second delay.

If your LED's are flashing without a dimmer being used, I would say buy better quality LED's.
As Rick said there will be more surge on a 24v nominal with a 400AH battery. Alot less with the 1,100 ah though.

Relax everything will be fine!  It is not as if you are Offgrid with the concerns that go along with that.

Horsefly

@MrM1 - I haven't been seeing your other threads, so you may have posted other things about (for example) your battery bank size in AH. That info might help this discussion.

My system has lots in common with yours:

1. My inverter is a Schneider Conext SW4024
2. The biggest load I have is a 1/2 HP well pump, although mine is a 120VAC, 2-wire pump
3. My battery bank is 310AH, 24V AGM (2x2, 12VDC, 155AH batteries) (not sure how this compares to you)

My in-rush current is about 34A at 120VAC, but obviously for only a split second. Although I was concerned about it, the inverter never really seems to have a problem with it. While the pump is running, right around 50ADC is being pulled from the battery bank (measured with a clamp meter). The pump runs for almost exactly 2 min and 40 sec each time, from when the pressure tank switch turns on until it is turned off.

I can watch the battery bank voltage drift down some while the pump is running. I've seen the bank starting at 27.1V float, and running the pump pulls it down to 25.6V. However, with only a little sun (340W from my 1560W array) the bank returned to 27.1V around 5 minutes after the pump turned off. At night, we typically can see the pump run 2 times (3 at most), and run the microwave (800W) maybe 8 minutes max. In the morning, I've never seen the battery bank at below 25V before the CC started charging.  I've never seen our lights flicker under any conditions.  I have our LBCO set to 24.6VDC, but that's pretty conservative. I know it has tripped twice in the year since we got everything hooked up, and that was when there were 5 hunters at the cabin who could not be persuaded to go light on electrical consumption. This forced them to run the generator for a few hours.

Based on my similar by limited experience, here's what I think of your situation:

1. Your battery bank is undersized for running your pump. 
2. The SW4024 works fine for a 1/2HP pump, and I don't think that is the problem.
3. You have your LBCO set way too low. When Dave says "lights out" at 22V, he means that 22V is 0% SOC. I don't think you want to let your system get anywhere close to that.

mike95490

Inverter sag is the reason to get a large inverter, high voltage battery bank (48V  vs 24V) heavy gauge interconnects and ample battery bank to handle the surge. 

To maintain voltage, the inverter has to sense a sag in voltage, and then it tries to catch up without overshooting and blowing all your light bulbs !   If it's choked off by cables, poor connections, and a battery that can't keep up, it's not good.

Many grid homes have a bit of light flicker from a fridge or air conditioner starting up, that's mostly AC wiring influence.

MrM1

Dave Angelini said:

Really, you do not know what lights out are on an inverter based battery power system?  I think you do
The adjustments for this are a user setting. I use 22V for my clients and set at least a 60 second delay.

If your LED's are flashing without a dimmer being used, I would say buy better quality LED's.
As Rick said there will be more surge on a 24v nominal with a 400AH battery. Alot less with the 1,100 ah though.

Relax everything will be fine!  It is not as if you are Offgrid with the concerns that go along with that.

Yes (just making sure I'm seein what you're sayin),  And True / True / Ok / True / thanks     I do tend to overthink most of this stuff.  I am  still in my first year (8 months of system operation)..  Baby steps. 

I am really starting to settle into liking the SW more and more as I learn to work around it's quirks and limitations, and fool it into doing what I want it to do even if it does not want to.  (ie. charge block has become my latest friend when the charger will not stay in absorb long enough because it stops due to end amps - I just set Float and Absorb to the same voltage and set charge block as though it where the absorb timer). 

MrM1

Horsefly said:

Based on my similar by limited experience, here's what I think of your situation:

1. Your battery bank is undersized for running your pump. 
2. The SW4024 works fine for a 1/2HP pump, and I don't think that is the problem.
3. You have your LBCO set way too low. When Dave says "lights out" at 22V, he means that 22V is 0% SOC. I don't think you want to let your system get anywhere close to that.

Thanks,  The drift you describe when the pump turns on is pretty consistent with what I am seeing. 

But my battery bank is actually larger than your (if I understand your 310 Ah correctly).  My bank is 435Ah of L16s.  Not saying the bank is Not too small though.  It may very well be. 

MrM1

mike95490 said:

Inverter sag is the reason to get a large inverter, high voltage battery bank (48V  vs 24V) heavy gauge interconnects and ample battery bank to handle the surge. 

To maintain voltage, the inverter has to sense a sag in voltage, and then it tries to catch up without overshooting and blowing all your light bulbs !   If it's choked off by cables, poor connections, and a battery that can't keep up, it's not good.

Many grid homes have a bit of light flicker from a fridge or air conditioner starting up, that's mostly AC wiring influence.

Thanks Mike,  You've walked me thru cabling and lugs and connection and heat already.  I rebuilt all my lugs a few months ago based on our discussion about that.   Now also using 7ft of 4/0 battery cable rather than 10ft.    On grid I see some,  but a bit less to no light flicker from the same larger loads (pump, fridge, 1000w water heater) than I do when those loads are off grid. 

Dave Angelini

Horsefly is just a hair off on my 22V setting. I set this and have since my Trace inverter over 25 years ago because Trace was right about that setting. It has little to do with Soc but rather loads from big motors. Since they are temporary surge loads, the 22V is too keep the inverter on to start the load. This happens with smaller battery banks and also when close to the point where the sun will come out and start charging the bank.

It avoids running a generator if the sun is close to rising offgrid. As Horsefly said, if you have small loads and are getting close to 22V,
it is on the owner to make that decision as he will be the one writing the check for new batteries.

Good Luck!

MrM1

Dave Angelini said:

It has little to do with Soc but rather loads from big motors. Since they are temporary surge loads, the 22V is too keep the inverter on to start the load. This happens with smaller battery banks and also when close to the point where the sun will come out and start charging the bank.

it is on the owner to make that decision as he will be the one writing the check for new batteries.

Good Luck!

What  I thought,  and BTW you answered my original question, knowingly or unknowingly (but I assume the former).   As far as the 22v goes,  I'd never get there with constant casual loads,  only momentary surges.   I watch the bank and system way too closely and am to OCD.  

littleharbor2

Interesting Dave. Since your signature doesn't include components we don't know what your system consists of. Are you running a Trace SW currently? Pretty cool if you are, and a nod to Trace from a guy who installs state of the art systems for a living. If not... oh well, still a nod for Trace.

Dave Angelini

I do not list equipment because I have NDA's with  Schneider (now) and have done electronic field testing work for Outback, Array Technology, and a few others I can't mention. You can look at the picture link on my web page below and see what I had at one time recently. There are 3 identical trackers here with 2KW in each for testing mppt's and other BOS gear. Lot's of data logging also.

I have a Trace SW in my barn that might work still but it has been some time. I also store a spare XW (which came from Trace) for 24 and 48 clients who want to drive and pick it up. About 7 outback mx60's are in there for metal recycling.

How is the Sea this year? I miss the spear fishing the most, but it was a great decade we lived/floated there. 

littleharbor2

Dave Angelini said:

How is the Sea this year? I miss the spear fishing the most, but it was a great decade we lived/floated there. 

Here on the Island it's usually always clear for underwater activities. I've been in water so clear it gave the sensation of flying 100'+ visibility.
 Just got back from South of San Felipe. Water was 87 degrees and rising. With the huge tidal swings it is rarely too clear, but great for soaking in.    

Dave Angelini

I think we saw 92f one year in the Sea. Talk about a big hot tub. Enjoy!