AC watts, DC watts, VoltAmps, Kwh, battery runtime, huh?! I've confused myself, newbie needs a math lesson please!!

[cyberfed]

Hello bright minds,

First off let me say that have a degree in computer engineering, which is essentially the same as an electrical engineering degree but with more programming courses (at least when I was in college in anyways 2000-2005). That said, I should probably turn in my engineering card because I have failed miserably!!

I thought I had a handle on this but clearly I don't.

So I have mostly built 48V 16S packs using LEV60 cells that have a capacity of 74Ah. Which works out to about 3.7KWh of juice. I know that number may come out differently when calculated but I'm going with what is stamped on the server rack style metal case designed for it, it says 3.7KWh so that's why I'm stating that number.

I bought a Kill-a-watt meter to measure how much power my office uses when I work from home and I left it plugged in few days to see what it would come out to. Between an active day of work (9hrs) plus the rest of the day/night with the monitors off. I noticed during "work" hours I was pulling 170-190 watts or perhaps I should say Voltamps thats what the Victron says in one of the screens and when I'm not working it would drop down to about 50 watts or so. (Monitors man!!) So this was obviously AC wattage. The Kill-a-watt hit 3.7KWh of use in about 2-3 days or so.

• So I assumed my DC battery at 3.7KHw hrs could handle the job for 2 days before needing a recharge. HA!!! Boy was I so wrong....

• I was led to believe that the unit of measure 'watts' was not impacted by AC or DC, meaning if something reads 100 watts AC it was the same as 100 watts DC. Sounds like not the case.

• My battery fully charged (3.5-6V) lasted slightly over 4 hours before it was out of juice (3.0V). Looking at the inverter it said it was pulling 170-190 VA. I've always been confused by this Volt Amps unit of measure.

• So I need a math lesson.

How do i properly calculate the wattage of my battery pack in DC watts? Then how do I calculate the amount of usage of energy of a an AC device over a period of an hour. I thought that if a device uses says 1000 watts of AC energy when in use then say 1 hour would be AC 1KWh of power consumed. Yes? No? Help?

Can someone explain how I can translate DC watts of my battery pack total to the amount of AC watts correctly?

Clearly watts are not just watts (DC vs AC) or there's some other factors in play I'm missing.

I figured since the Kill-a-watt meter didn't break 1KWh for along time that it would be no problem using my battery to run my office, instead a ended up with a dismal 4 hours approx runtime before I ran out of juice ( assuming its not plugged into the incoming solar energy ).

How do I properly do these conversions, looking for the equations and explanation not a link to a website that does it all for you.

Thanks!

For example I wanted to power my fridge during the day when I have plenty of excess power from the panels, the fridge uses 900 watts. I have no idea how to properly calculate say 8 hours of usage and translate that into how many DC watts my pack needs to be.

Not sure how much any of this extra info helps but here's the setup I'm using:
400watt panels 6x of them
Victron 250/60 SCC
Victron 48/800 Inverter
16S 74AH Batteries, brand new. Charged to 3.55 (full) and shutoff at 3V. Current config settings on the BMS. (JBD BMS)

No sure if I have my solar power input connected, then mostly my batter is just sitting there at float and the panels are powering everything.

I was thinking of building a 16S pack using these EVE batteries that seem to be what most people use, I was looking at the 314AH ones. But this gets expensive and I don't know how long I could power say my office or my fridge, if its just a few hrs then jeez how big are the ESS banks people are building to power way more things for way longer times??! I've never seen one of these EVE or CALB batteries (whatever bateryhookup or jag35 carries they are my main sites for batteries, got a better place please tell me!!) in the flesh so I don't know how big they are. Yes I've read the specs of them but that doesn't compare to holding one.I have a feeling they are huge.

Long post I know, but please help

 

[sunshine_eggo]

Hello bright minds,

First off let me say that have a degree in computer engineering, which is essentially the same as an electrical engineering degree but with more programming courses (at least when I was in college in anyways 2000-2005). That said, I should probably turn in my engineering card because I have failed miserably!!

I think this may be a bit overstated as I can't fathom a EE having any confusion concerning VA math.

I thought I had a handle on this but clearly I don't.

So I have mostly built 48V 16S packs using LEV60 cells that have a capacity of 74Ah. Which works out to about 3.7KWh of juice. I know that number may come out differently when calculated but I'm going with what is stamped on the server rack style metal case designed for it, it says 3.7KWh so that's why I'm stating that number.

16 * 3.2 * 74Ah = 3789Wh assuming those cells actually deliver 74Ah.

I bought a Kill-a-watt meter to measure how much power my office uses when I work from home and I left it plugged in few days to see what it would come out to. Between an active day of work (9hrs) plus the rest of the day/night with the monitors off. I noticed during "work" hours I was pulling 170-190 watts or perhaps I should say Voltamps thats what the Victron says in one of the screens and when I'm not working it would drop down to about 50 watts or so. (Monitors man!!) So this was obviously AC wattage. The Kill-a-watt hit 3.7KWh of use in about 2-3 days or so.

Did the kill-a-watt also measure the inverter/chargers load?

• So I assumed my DC battery at 3.7KHw hrs could handle the job for 2 days before needing a recharge. HA!!! Boy was I so wrong....
• I was led to believe that the unit of measure 'watts' was not impacted by AC or DC, meaning if something reads 100 watts AC it was the same as 100 watts DC. Sounds like not the case.

DC Watts = AC Watts, because

P = I * V

Note how neither specify AC or DC voltage/current.

You're assuming:

1. Kill-a-watt is truly accurate.
2. Conversion is 100% efficient (never)
3. There are no other loads.
4. Your cells are actually 74Ah

• My battery fully charged (3.5-6V) lasted slightly over 4 hours before it was out of juice (3.0V). Looking at the inverter it said it was pulling 170-190 VA. I've always been confused by this Volt Amps unit of measure.

Voltages reported by th BMS and confirmed with a voltmeter?

What did the JBD say with regards for Ah used/SoC?

• So I need a math lesson.

How do i properly calculate the wattage of my battery pack in DC watts? Then how do I calculate the amount of usage of energy of a an AC device over a period of an hour. I thought that if a device uses says 1000 watts of AC energy when in use then say 1 hour would be AC 1KWh of power consumed. Yes? No? Help?

The math involved isn't the issue. Your assumptions are the issue.

You confirm the assumptions you have made are correct or incorrect and then change your assumptions.

 

[zanydroid]

(There was way too much narrative bloat and really invited a roasting ; OP is lucky they're getting any traction; I refuse to go back and read it carefully point by point LOL)

EDIT: apologies to the OP if this was justifiable in the sense of coming from being overly excited or clinically manic

FWIW I have a software engineering degree within a program that does both EE and CS, and I would never consider that equivalent to an EE degree when I only took 3 classes in basic circuits and signals/systems. And symmetrically I don’t allow the EE side of the degree to claim the also CS in the absence of work experience or having taken the courses. I learned way more from electrical and solar DIY since 2021 than I did in my degree.

I concur that you:

• need to check the assumption that the cells meet the promised
• get a corroborating reading for the total current drawn, IE via the shunt in the BMS

Watts are watts whether they are AC or DC. The main difference is that they are somewhat more difficult to compute for AC (this is related to the concept of VA). Since the voltage varies, the power delivered for a given current will depend on the voltage at the time the energy is drawn.

My battery fully charged (3.5-6V) lasted slightly over 4 hours before it was out of juice (3.0V).

This is a weird voltage. What is the cell type?

 

[OzSolar]

I've met a lot of electrical engineers and not one of them understood power vs. energy. That took some getting used to. But what really continues to blow me away is how few EE's are able to use Ohm's and Watts laws to get from watts and volts to amps or vice versa. Truly breathtaking, like I'm on Candid Camera sort of experiences.

Anyway....

Until you get power vs. energy you're really going to struggle trying to do this stuff.

 

[Mattb4]

W=VA
However DC battery based Watts that you then run through wires, breakers, fuses, and inverters to become AC Watts do not equal. Nor for the reverse does AC Watts that you run through wires, breakers, fuses, battery charger, overcoming battery chemical conversion losses, mean your battery has equal amount energy stored.

Most folks will allow about 75% difference. Might be higher might be less.

Edit: BTW 4 hour run time for a 74ah rated battery would imply 74ah/4h=18.5A If your battery is 48vDC nom. that works out to a System watt load of ~880W if you are using ~200W after the inverter it would imply your battery is not up to snuff capacity wise or you have some real hot component somewhere.

 

[MrSparkle]

@zanydroid
Quick search on LEV60, 74Ah points most probably to Japan made LiFePO4.

As stated by others, OP, please specify battery type.
(You write "battery" but provide "cell voltage", that can add to some confusion.)
How and where from did you get the numbers 3.5 down to 3.0?

My battery fully charged (3.5-6V) lasted slightly over 4 hours before it was out of juice (3.0V). Looking at the inverter it said it was pulling 170-190 VA

The strange part is ~180W at 48V setup is about 3,5A?
If that is constant all the way from 100-0 SoC, then it would indicate cells are only ~20% of their original capacity?

Totally degraded cells?
Unaccounted loads?
Not balanced pack?

 

[cyberfed]

****SKIP TO BOTTOM FOR WHAT I NEED HELP ON IF YOU DON'T WANT TO READ THE REST****

I don't care about getting roasted by strangers on the internet honestly we all have 3 options right? 1. Ignore no response, 2 make fun of me because of whatever is going on in your life you feel that makes you feel better, or 3 you can provide me with help. I'm looking for help.

I wont bother explaining college. Think what you want.

Batteries confirmed to dump out 74AH. Tested each one by charging them full and then discharging them each individually, the delta of each battery during use is usually more that .3mV per BMS.

Did the kill-a-watt also measure the inverter/chargers load?

No it did not. The kill-a-watt was plugged into the house AC power and equipment plugged into it to get the consumption value after a few days I checked the overall usage total.

Battery (each cell) voltage confirmed on quality bench top MM. The MM has been tested against several power supply units and is accurate to 3 decimal places.

BMS SOC at shutdown shows just shy of 74AH used. I've also used a high 48V charger to charge the pack with house power and it also reported 74AH when it cutoff when they were full.

Apologies on the wording on the battery voltage I see why you got confused there. They are LEV60 LFP cells 3.2V 74AH brand new. I charge them to 3.5 - 3.6V via solar that's the cutoff. (ALL batteries were top balanced prior to use and voltage verified on each cell to be at 3.65). BMS is configured to shutoff at 3.0V.

BMS SOC at shutdown shows just shy of 74AH used. I've also used a high quality 48V (even has a profile for 16S LFP) charger to charge the pack with house power and it also reported 74AH when it cutoff when they were full. I think I've confirmed the batteries perform per spec.

--------------------------------------------------------------------------------------------------------------------------------------------------------------------
So let's assume for 8 hrs wattage used is 220 watts ( 190 + 30 for the inverter ) and then 16 hours at 80 watts (50 for the equipment that's left on, and 30 for the inverter).

1. What would be the total watts consumed for a 24hr period? I need help please.

Lets go with the battery having 3.7KWh.

Going with an estimated 80% loss due to the inverter on the battery we are down to 2.96KWh

1. What would be the expected runtime? How do I calculate all that correctly?

For reference I get just over 4 hours of runtime before I'm out of juice.

Forgive me for providing a lot of information, just trying to give you all the variables I have.

Thank you in advance to anyone who helps me understand the way to calculate this correctly.

 

[cyberfed]

I agree @MrSparkle something's off. I'm going to tear the pack down yet again and review each cell individually and then compare them again with the BMS readings. I don't know what else to do especially since I don't even know what to expect for a runtime with this load because I'm dumb and no one feels like helping out.

 

[time2roll]

You need a DC battery monitor to measure power in/out of the battery.

 

[cyberfed]

I have that, and I will do so. Thanks @time2roll its tear down time...sigh. Question stills stands if things are good, what is the expected runtime and what is the total watts consumed.

 

[MrSparkle]

I'm dumb and no one feels like helping out.

Relax. Take a deep breath. Focus on the task.

Did you balanced the pack? Started balancing from 3.4V (per cell) and up? Did the lowest cell go above 3.45V when the highest hit OverVoltageProtection and ended the charging? If not, that could indicate the pack was not balanced.

Even worse if you started balancing WAY under 3.4V. That would dis-balance the pack.

 

[sunshine_eggo]

I agree @MrSparkle something's off. I'm going to tear the pack down yet again and review each cell individually and then compare them again with the BMS readings. I don't know what else to do especially since I don't even know what to expect for a runtime with this load because I'm dumb and no one feels like helping out.

Everyone's helping out. We're just not necessarily doing it the way you want because you're insisting that the problem is "A", when it may be "B" or "C". You're assuming it's a math issue, and it's not. It's likely a measurement, assumption or expectation issue.

When the inverter was reporting 180W, did the BMS report the same (probably need to multiply DC current * voltage)?

Did you leave the kill-a-watt plugged in when drawing from the inverter? If so, did the power numbers line up with the inverter reported output?

Worth noting that BMS can be pretty darn inaccurate at low current.

Need to confirm DC current with a clamp DC ammeter.

Batteries confirmed to dump out 74AH. Tested each one by charging them full and then discharging them each individually, the delta of each battery during use is usually more that .3mV per BMS.

In the operating range of 3.1-3.4V, this is expected even if the cells are imbalanced. Voltage only correlates to balance outside the operating rang.

No it did not. The kill-a-watt was plugged into the house AC power and equipment plugged into it to get the consumption value after a few days I checked the overall usage total.

Okay.

BMS SOC at shutdown shows just shy of 74AH used. I've also used a high 48V charger to charge the pack with house power and it also reported 74AH when it cutoff when they were full.

Okay.

So let's assume for 8 hrs wattage used is 220 watts ( 190 + 30 for the inverter ) and then 16 hours at 80 watts (50 for the equipment that's left on, and 30 for the inverter).

1. What would be the total watts consumed for a 24hr period? I need help please.

30W for the inverter seems high

24h * 220W = 5280Wh

Lets go with the battery having 3.7KWh.


Going with an estimated 80% loss due to the inverter on the battery we are down to 2.96KWh

1. What would be the expected runtime? How do I calculate all that correctly?

For reference I get just over 4 hours of runtime before I'm out of juice.

Forgive me for providing a lot of information, just trying to give you all the variables I have.

Thank you in advance to anyone who helps me understand the way to calculate this correctly.

3789Wh / 5280Wh * 24h = 17.2h

We've already established that the math isn't working. It's remains time to move on from the math to check all the inputs into the equations and confirm them. Until you've confirmed each value with an additional instrument, all results are inconclusive.

Also, we are assuming that all of your wires are properly sized and torqued and are of high quality. Any crimps you have made are solid. May need to test that assumption as well.

 

[time2roll]

I have that, and I will do so. Thanks @time2roll its tear down time...sigh. Question stills stands if things are good, what is the expected runtime and what is the total watts consumed.

One cycle with the DC battery monitor will give the story. Actual load, actual capacity. Capacity / load = time.

 

[cyberfed]

Thanks @MrSparkle I actually needed to hear that. I appreciate your kindness.

So I cracked open the pack (server rack style box) just now, before doing so I checked each battery's voltage from the BMS they were all within .002V of each other hovering at 3.340V. All 16 of them. I then checked them manually with a MM and it returned the same value. So the only thing I can think of is it got unbalanced somehow. I did top balance it to start, I put them all in parallel and charged them to 3.65V @30 Amps, took a long time. That was awhile ago as this is the first pack I built.

So I'm thinking of discharging each cell individually to about 2.7V and then putting them in parallel again and charging the pack to 3.65V with CV with no BMS in the picture. That should ensure things are even again yes? In terms of SOC?

There has to be an imbalance in the only thing that makes sense, even testing the IR of each cell yielded almost identical values super low and within spec. Using a quality IR tester.

Unless you have a better suggestion to ensure we get each sell to a 0 SOC and then to a 100SOC so everything is even and balanced. I'm open to suggestions on the best way to do that. I've just seen the "top balancing" way several time by respected you tube battery/solar guru's. So that's what I did.

When I got the batteries they were all at 3V. Charged CV 3.65V @ 30A , again they were 16 cells in parallel for this. I measured each one 2 days later and they were all almost identical in terms of voltage.

If there's a better way or I'm plain just doing wrong please share! I thought 4 hrs seemed super wrong. I pulled the Kill-a-watt meter back into my office (no inverter) at 5:30PM today and I am going to check it every 24 hours. Just so I know the power consumption, of course I don't work tomorrow, but still.

Thank you kindly!

 

[robbob2112]

Lemme take a stab at it -

16s - LEV60 cells (3.7v Lithium Ion) = 59.2v nominal with a full charged voltage of 67.2 (4.2v per cell) and a low cutoff of 48v.


59.2v * 74ah = 4376Wh --- this doesn't match the stated WH - so the cells must instead be LEV60F cells which are 3.2v cells

Now that we have the correct cells

51.2v * 74ah = 3788.8Whr - this matches - so they are indeed LEV60F cells


220w * 8hrs = 1760Wh
80w * 16hrs = 1280Wh

1760w + 1280Wh = 3040Wh

Watts are watts - ac or dc they are watts.

Power = voltage time current
Whr = power over 1 hour - so if you work the math the hours can come and go as stated above

Batteries with setups like yours have the BMS to protect them from over current, over voltage, and under voltage --- for each cell

So if your cells are not balanced properly one will hit low voltage cutoff and the battery is off --- when charging one will high high voltage cutoff and the battery stops charging

When you built the pack did you use a benchtop power supply to top balance them? If not there is your problem - poke around with a good multimeter capable of 3~4 digits accuracy and measure and write down the voltage of each cell then report back.

And by good multimeter I don't mean the $5 at harbor frieght that they give away free every other month - If you are building batteries you need a DC clamp meter that doubles as a multimeter so you can tell what is going on - they run $40 to $100 depending on if you want in the ball park quality or dead on quality - or there are the $400+ models that pro's use but you don't need that.



And of course your BMS could be malfunctioning - a loose or cut balance lead? a wire that is missing insulation or just a bad BMS.



And I know you don't believe it but it is true - CSE is a long long long way from EE - at least 4 math courses and then specialized courses - the CSE split off from the EE after the freshman year generally - the circuits 2 class where you start calculating 3 phase power is usually what does the splitting off.

If you school was turning out EE and CSE with similar skill sets that just means the EE were woefully lacking.

 

[sunshine_eggo]

Thanks @MrSparkle I actually needed to hear that. I appreciate your kindness.

So I cracked open the pack (server rack style box) just now, before doing so I checked each battery's voltage from the BMS they were all within .002V of each other hovering at 3.340V. All 16 of them. I then checked them manually with a MM and it returned the same value. So the only thing I can think of is it got unbalanced somehow. I did top balance it to start, I put them all in parallel and charged them to 3.65V @30 Amps, took a long time. That was awhile ago as this is the first pack I built.

So I'm thinking of discharging each cell individually to about 2.7V and then putting them in parallel again and charging the pack to 3.65V with CV with no BMS in the picture. That should ensure things are even again yes? In terms of SOC?

No point in discharge. If your goal is to top balance, head for the top. Simply recharge whole battery to BMS cut off, if all cells are above 3.45V, balance may not be needed. If needed, then charge each cell to 3.65V individually in place. Once you've been through all 16, then run back through them in the same order... the 2nd run through should only take 5-10 minutes for all 16.

There has to be an imbalance in the only thing that makes sense, even testing the IR of each cell yielded almost identical values super low and within spec. Using a quality IR tester.

If your BMS reported 74Ah used, then imbalance isn't likely.

You need to confirm:

volts and current out from battery BMS and a separate tool (voltmeter, DC ammeter).
Kill-a-watt VA and Victron VA are the same. Simply power the loads from the inverter through the kill-a-watt like you did with grid.
Confirm AC volts and amps out of inverter (voltmeter, AC ammeter).

Unless you have a better suggestion to ensure we get each sell to a 0 SOC and then to a 100SOC so everything is even and balanced. I'm open to suggestions on the best way to do that. I've just seen the "top balancing" way several time by respected you tube battery/solar guru's. So that's what I did.

Yes. Better suggestion above.

Excellent guide in the Resources section. My suggestion avoids the nuisance of breaking down the pack.

 

[cyberfed]

Thank you @robbob2112 , yeah they are LFP. Yes voltages were checked with a $600 bench-top MM capability of 4 digit accuracy, I even paid more the "calibrated certificate" version. See above for my plan and if you have suggestions.

I'm not starting anything tonight, time for dinner with the g/f and a movie, but I'll back at this tomorrow and however long it takes. Thank you kindly again. For your feedback. I cannot tell you how much I appreciate it.

 

[cyberfed]

Thank you @sunshine_eggo, glad I don't have to discharge them all, but rather hit each one with a top balance of 3.65V. Don't quote me on this but I think the current out was around 3 amps maybe a little more with ammeter. I did check that. I can recheck that but simply checking the load again. The pack is "full" right now so to speak. But now I'm worried they aren't at the same SOC just the same voltage.

Thank you my friend.

 

[sunshine_eggo]

Thank you @sunshine_eggo, glad I don't have to discharge them all, but rather hit each one with a top balance of 3.65V. Don't quote me on this but I think the current out was around 3 amps maybe a little more with ammeter. I did check that. I can recheck that but simply checking the load again. The pack is "full" right now so to speak. But now I'm worried they aren't at the same SOC just the same voltage.

Thank you my friend.

I'm less of a dick than it might seem.

Once balanced, for the next discharge, get it set up and confirm all readings with a second instrument. Document them.

Deliberately add a notable load (200-400W) and confirm all those values as well.

Periodically check the BMS to confirm that the Ah/Wh used is consistent with what all instruments report. A faint recollection says that the BMS will revert to 0%/74Ah used arbitrarily at cut off regardless of actual usage. In other words, at 3.1V/cell, BMS should be reporting most of the 74Ah is used. If you see it goes from 30Ah used and then suddenly reports 74Ah used, the BMS is being arbitrary.

Others have had issues with those cells. Given the existing conundrum, any prior results should be confirmed as well. If you were relying on the BMS, and it arbitrarily reported 0%/74Ah used, you may have been falsely led to believe cells met spec.

Also, another fuzzy memory. They have a slightly atypical voltage range of 2.0-3.5V:

https://secondlifestorage.com/index.php?attachments/lithenergyjapan70-specification-of-lev60f-002-2-1-pdf.30416/

Recommend you lower BMS cell cut off to 2.5V/cell and set inverter cut off at around 10.5V. Might also try to turn the inverter off before the BMS triggers shutdown if you're getting close to 2.5V to avoid BMS reporting 0%/74Ah used falsely.

 

[robbob2112]

I'm less of a dick than it might seem.

Once balanced, for the next discharge, get it set up and confirm all readings with a second instrument. Document them.

Deliberately add a notable load (200-400W) and confirm all those values as well.

Periodically check the BMS to confirm that the Ah/Wh used is consistent with what all instruments report. A faint recollection says that the BMS will revert to 0%/74Ah used arbitrarily at cut off regardless of actual usage. In other words, at 3.1V/cell, BMS should be reporting most of the 74Ah is used. If you see it goes from 30Ah used and then suddenly reports 74Ah used, the BMS is being arbitrary.

Others have had issues with those cells. Given the existing conundrum, any prior results should be confirmed as well. If you were relying on the BMS, and it arbitrarily reported 0%/74Ah used, you may have been falsely led to believe cells met spec.

Also, another fuzzy memory. They have a slightly atypical voltage range of 2.0-3.5V:

https://secondlifestorage.com/index.php?attachments/lithenergyjapan70-specification-of-lev60f-002-2-1-pdf.30416/

Recommend you lower BMS cell cut off to 2.5V/cell and set inverter cut off at around 10.5V. Might also try to turn the inverter off before the BMS triggers shutdown if you're getting close to 2.5V to avoid BMS reporting 0%/74Ah used falsely.

Oh, he is a dick, believe it

 

[robbob2112]

It occurred to me what might be wrong. Is this the case from another thread that has pcb for balance leads verse wires directly from the BMS. If so all it would take is a crack in one of the runs to cause your problems.

If it isn't the pcb version I would ohm out all the balance leads to see if they all same value. If it is the pcb version I would check the runs on that.

Reasoning - since the balance leads are used to sense the cell voltage a lead being slightly off from the others could make it appear that a cell has hit low voltage cutoff when everything is fine... i.e. a crack or bad connection drops a few mv so the cell reads a few mv low to the bms.

 

[MrSparkle]

@robbob2112 Interesting lead.
Though it seems to me OP did test the leads when comparing BMS reads to MM reads. I might be wrong though:

I checked each battery's voltage from the BMS they were all within .002V of each other hovering at 3.340V. All 16 of them. I then checked them manually with a MM and it returned the same value.

When I got the batteries they were all at 3V. Charged CV 3.65V @ 30A , again they were 16 cells in parallel for this. I measured each one 2 days later and they were all almost identical in terms of voltage.

I like the proposition of @sunshine_eggo
With a controlled discharge test and periodic value reads during the test, the OP would test, rule out or identify BMS fault or cell capacity fault.

Two stones in one bird!

Also @robbob2112 you post #15, line 7. I think you mean 16hrs not 16w?
80w * 16w = 1280Wh

 

[Mattb4]

I agree @MrSparkle something's off. I'm going to tear the pack down yet again and review each cell individually and then compare them again with the BMS readings. I don't know what else to do especially since I don't even know what to expect for a runtime with this load because I'm dumb and no one feels like helping out.

Perhaps you have suss'd it out by now but runtime is going to be your total load (loads plus conversion losses) divided by your battery capacity with a fudge factor based on how the battery performs.

Thus 74ah 48vDC nom should provide at around ~3800Wh maximum. Run time then becomes your watt draw . Example: 3800Wh/200W =19h Standard algebra. That is the absolute number and in practice it is more likely to be 17-18 hours.

Your batteries being depleted in only 4 hours at your reported draw is why all the other posters are suggesting something is wrong with your batteries/Bms.

 

[cyberfed]

Guys I want to start off and say thank you. You ARE trying to help. I was frustrated because I was first looking for confirmation what the approx runtime should be and how to get those numbers. Thank you @sunshine_eggo @MrSparkle and EVERYONE really. I never assumed the batteries/BMS were not at fault, they absolutely could be. I was interested first in the formula to get the runtime to have a ball park idea what to expect, and THEN troubleshoot further I guess my mind was just off in phrasing and what I posted, I apologize I do suffer from mental health issues (I'm serious) and it can lead to me going off the ledge 'so to speak' easily. So I beg all of your pardons.

You have me very confused/interested on the battery specs, for example check out:

New LEV60F 3.2v 74ah Lifepo4 Power Cells

If you buy 6 they will ship in the original case. Otherwise we will repackage them safely into a custom box. They are Lithium Energy Japan cells model LEV60F. These cells are overstock and were originally intended for luxury vehicle 12v lifepo4 starter batteries. Cell Specifications Nominal...

batteryhookup.com

the image of the spec aligns with 2.0 - 3.5 but look at their own description:

Full Charge Voltage: 3.65V
Full Discharge Voltage: 2.5V

So this is something I definitely could something I need to change on the BMS.
For those curious here is the video I followed to build this pack.

I was brand new to this and I watched this video and figured I had enough skills to pull this off... I'm sure it will make some facepalm. Sorry but that's what I built.

This pack was #1 built, and you are 100% right I need check my crimps because for this pack I wasn't using a high quality crimper which was purchased later. Definitely need to check that out. The one I have now from TEMco that creates great cold welded crimps. So that may needs a "re-do" anyways regardless using the better crimper. Noted and good call!.

I liked the controlled test mention, this needs to happen and be documented with other instruments besides the BMS. I will also make the necessary BMS programming adjustments and i like the inverter being set to a number lower to see what the BMS does.

The balance wire leads I agree are worth an investigation.

I checked the torque before I took it apart and it was still tightened to spec. I have a high quality one to ensure things are tightened to spec. Well at least the spec its allegedly supposed to be...

I remember when I finish building it I was so excited and I remember testing with an ammeter to test the amperage leaving the pack (in DC) to the inverter and then amperage AC after the inverter, I had taken a heavy duty extension short (short 3 ft) and stripped away the outer sheath so I could get to the 3 individually sheathed AC wires so I could measure the AC amperage. Sure enough P=I*V was exactly right there were no numbers wrong in terms of leaving the battery and going into an A/C device. I can re-perform this test to ensure things are still behaving correctly.

Also agree about the balance leads as yes they are built into the PCB. I know I made sure the connections were inserted properly and tightened down but the PCB could be an issue. More measurement to rule that out as a suspected.

 

[cyberfed]

Ok I'm not sure if I hit send on my laptop, I was drafting a response that looks similar to this one but I'm not sure now. Out of respect to all of you I wanted to let you know that I am still working on this and I will post the update (if I have one drafted on my personal machine) later today after work. The very short version (which is well recieved here ) is I am taking all the considerations posted to investigate! I didn't want to leave things just hanging out there after so much help was provided. Again I dont recall if my post was the one above. Stupid medications I have to take