Understanding MPP Solar charge settings for LifePO4 batteries

I have a LV5048 and am running 16S 150Ah LifePO4 aluminum cased prismatic cells. I have been running the system for a few weeks now and the cells are all well balanced. (I'm using a "320amp" ANT BMS)

I'm not understanding the behavior of the bulk and float voltage settings on the LV5048. I have program 26 set to 56.4v (bulk) and program 27 set to 54v (float).
I never see the battery voltage go above the float setting (54v). This seems of makes sense because that is an individual cell voltage of 3.375v - well within the safe max voltage and close to 100% SOC. However the bulk voltage setting translates to 3.525v per cell which is borderline high. I never see a voltage that high during charging. It never see the battery go above the float voltage setting...

The issue I am having is that the float charge voltage setting causes the charge current to drop off long before full charge is reached. The battery can be at 80% SOC and it is drawing well below the current that is available from my panels and hence charging very slowly. I know that the power is available because if I increase the load on the inverter the PV power rises to meet the load requirements while the charge current (reported by the BMS) remains unchanged.

When and how is the bulk mode activated? I have program 32 (Bulk charging time) set to Auto - but have no idea how that is supposed to work. There is an option for manually setting a bulk charge time, but that makes no sense to me because the SOC could be at any level when charging starts, so a specific bulk charge time seems useless. (I suspect this might be more useful for Lead Acid batteries)

With the days getting shorter now, my batteries are never reaching close to a full SOC. The low float charge voltage causes the charge current to taper off so early that I run out of sunlight long before the battery is charged. The problem is - there is still plenty of PV power available to fully charge the battery if only the charge current didn't taper off so early.

This is tickling a vague memory about this being an occasional issue with MPP units.

In most cases, a bulk time is an overriding maximum as a safeguard. I would try increasing the bulk time to something big like 8 hours to see if you can drive it to the absorption voltage.

Also play with float voltage - push it as high as it will go (up to absorp) to confirm it will go that high.

snoobler said:

This is tickling a vague memory about this being an occasional issue with MPP units.

In most cases, a bulk time is an overriding maximum as a safeguard. I would try increasing the bulk time to something big like 8 hours to see if you can drive it to the absorption voltage.

Also play with float voltage - push it as high as it will go (up to absorp) to confirm it will go that high.

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I have pushed the float voltage a little higher, but if I do several of the cells will hit the BMS limit (3.65v) due to cell voltages diverging at full SOC. That results in the BMS cutting off to protect those cells. So, I can't go much higher on the float voltage.

I think what is needed is a bulk charge mode that will increase the voltage to the cells to make them draw more current until the battery closely approaches full SOC. Then it backs down the voltage to the float level.
I thought that was the idea of bulk charge mode. But, I've never seen a charge voltage across the cells greater than the float setting (54v).

Maybe I can force bulk charge by setting the manual bulk charge timer. But I don't think I'd want to leave it that way because it could easily overcharge and damage the cells if it it waits too long to kick into float.

This is a completely new twist. You now have two problems:

1) malfunctioning MPP
2) imbalanced battery

What makes you think that going higher in bulk won't have the exact same problem?

Why do you think the MPP is malfunctioning? I'm not sure I understand.

The battery cells seems well balanced. Up to about 98% SOC the voltages track very closely. At that point - "the upper knee" - the voltages will start to diverge. I spent quite a bit of time trying to top balance the cells perfectly with a resistor across the cells that are higher in voltage at full SOC. They always settle back to where they are now. They stay in very good balance except at the extreme low and high end. I thought this is typical behavior.

I have three of the lv5048's..The time at absorbtion can be set in the menu, the rub is that the absorbtion clock starts counting as soon as float voltage is reached. You would think you would want the clock to start once the bulk voltage is reached, but sadly no.. The automatic setting basically goes straight to float once float voltage is reached.

This does not work well for lifepo4 batteries as they need different absorbtion times for different depth of discharge..

My work around is to set a high float voltage and change the absorbtion time depending on the time of year.

End amps would be the way to go but unfortunately the lv5048 are not capable of this..

Th

Mtour said:

I have three of the lv5048's..The time at absorbtion can be set in the menu, the rub is that the absorbtion clock starts counting as soon as float voltage is reached. You would think you would want the clock to start once the bulk voltage is reached, but sadly no.. The automatic setting basically goes straight to float once float voltage is reached.

This does not work well for lifepo4 batteries as they need different absorbtion times for different depth of discharge..

My work around is to set a high float voltage and change the absorbtion time depending on the time of year.

End amps would be the way to go but unfortunately the lv5048 are not capable of this..

Click to expand...

That is good information. It sounds like automatic mode basically never goes into bulk absorption mode. I will try using the manual bulk timer and starting off with a short time period. I will then gradually increase the timer setting - watching to make sure I'm not overcharging. Does that sound correct?

I had hoped that the "auto" mode did something a little more sophisticated. If it briefly paused bulk charge every few minutes and checked the battery voltage at a zero charge current it should be able to figure out when to terminate the higher bulk charge voltage. Of course this would not work if there was a load on the battery when it paused to check the "rest" voltage.

Sounds like a good plan to play it safe and not try to get every last amp.. In the summer I had the float at 53.8 and bulk at 54v and 1 hour absorbtion... I would be a float about 11am every day..
Now with the limited sun I have float at 54v and bulk at 54.4 with 4 hours absorption.. I have not been to 100% charge in the last month...

KenDan said:

Why do you think the MPP is malfunctioning? I'm not sure I understand.

The battery cells seems well balanced. Up to about 98% SOC the voltages track very closely. At that point - "the upper knee" - the voltages will start to diverge. I spent quite a bit of time trying to top balance the cells perfectly with a resistor across the cells that are higher in voltage at full SOC. They always settle back to where they are now. They stay in very good balance except at the extreme low and high end. I thought this is typical behavior.

Click to expand...

It won't go into bulk mode.

If you're only going a little above float voltage, and "several" cells go to 3.65 before you get to bulk, they are not well balanced.

Typical behavior is all cell voltages are equal at 100% SoC, 3.65V.

Mtour said:

Sounds like a good plan to play it safe and not try to get every last amp.. In the summer I had the float at 53.8 and bulk at 54v and 1 hour absorbtion... I would be a float about 11am every day..
Now with the limited sun I have float at 54v and bulk at 54.4 with 4 hours absorption.. I have not been to 100% charge in the last month...

Click to expand...

This is making more and more sense. I believe the capacity of the battery would also affect the proper absorption timer setting. What is the capacity of your battery? My cells are 150Ah. I'll try your 54.4v bulk/absorption setting with a shorter timer setting, and then slowly work my way up to a longer absorption time.

snoobler said:

It won't go into bulk mode.

If you're only going a little above float voltage, and "several" cells go to 3.65 before you get to bulk, they are not well balanced.

Typical behavior is all cell voltages are equal at 100% SoC, 3.65V.

Click to expand...

The reason for not going into bulk mode appears to be the "auto" bulk charge mode. I didn't understand how it works (or actually doesn't). Thanks to Mtour I think I understand that now. (Too bad the MPP Solar manual doesn't explain that)

As far as proper cell balance is concerned, I've actually been chasing that for months. After reaching full SOC (~3.4v cell), I've tried using a low ohm resistor to try "help-out" the balancer and drag down those cells that creep higher than the rest. It seems to work briefly, but that effect is lost in relatively short order. When the cell voltages drop just slightly from full, the voltages all fall into near perfect equilibrium (.001 volt max difference). But when approaching the extreme low or high charge conditions, the voltages will diverge once again. Based upon numerous comments and charge/discharge graphs seen in these forums and elsewhere - this seems to be normal for LifePO4 cells. Across 90%+ of the operating range they are perfectly balanced, but I don't see a way to keep them perfectly voltage matched out at the upper and lower "knees" of the discharge curve.

Cells are not full at 3.4V. Cells are full at 3.65V.

The purpose of top balancing is to be able to fully charge all cells to their peak voltage. Top balancing explicitly means they are only balanced at ONE point in the curve - fully charged.

I have found it impossible to get a rest cell voltage that high (3.65v) without applying significantly higher (potentially battery degrading) voltage during charging. 3.65v is not a real sustainable cell voltage anyway. As soon as you apply any small load - even a cell balancer, the voltage drops to roughly 3.4v volts quite quickly. It also happens naturally if you leave the cells relax to a rest state after charging. There is virtually Zero mAh capacity between 3.65v and 3.4v.

The linked discharge graph displays this.

I have also observed that once you get into that region above 3.4v, the cell internal resistance appears to increase rapidly, and that "upper knee" of the charge curve varies significantly from cell to cell, while they remain solidly identical at 3.4v and below.
This has been my experience. Perhaps your LifePO4 cell model/formulation operates at a slightly higher voltage than the ones I have and the ones shown in the graph?

Much like lead-acid, with it's nominal 12V requires over-voltage to charge, i.e., you won't get above 80% UNTIL you hit about 14.4V, and then you have to hold that voltage and taper current to "absorb" the full charge, Lithium is the same way.

It's not relevant that it doesn't "hold 3.65V." It requires 3.65V to achieve true 100% SoC.

snoobler said:

Much like lead-acid, with it's nominal 12V requires over-voltage to charge, i.e., you won't get above 80% UNTIL you hit about 14.4V, and then you have to hold that voltage and taper current to "absorb" the full charge, Lithium is the same way.

It's not relevant that it doesn't "hold 3.65V." It requires 3.65V to achieve true 100% SoC.

Click to expand...

Ok, I understand that lead-acid works that way. You have to apply a higher voltage to get it to absorb that last few percent of capacity. Forcing in that last little bit of remaining capacity is inefficient and ends up generating heat. A large Lead-acid battery does not seem to be harmed by this unless the overcharge current and heat buildup is excessive.
I figured that Lithium Iron was not like that because of my years of experience with Lithium Polymer batteries in RC applications. LiPo chargers do not employ any kind of absorption charge. The charger goes into constant voltage mode at the end of the charge and never applies more than 4.2v - the standard fully charged voltage of a LiPo. At the termination of a LiPo charge, the voltage holds very close to 4.2v after the charger is disconnected.
Perhaps LifePO4 is different. More like a Lead-Acid rather than a standard Lithiom Ion or Lithium Polymer.

Not sure if you maybe used chargers that didn't report the information, but LiPo does have a CV phase similar to absorption as well. As I'm sure you've charged dozens of those, the charger runs up to 4.2V/cell, holds it and tapers current until the shut off current is achieved, typically 10% of start current.

FLA/AGM, LFP and Li-Po all charge via a CC-CV profile.

The difference with LiPo is that its voltage settles very little after the charge current is removed.

This is what I find as well, I can get to 98% soc @ 3.37v per cell but it will take 4-6 hours of absorbtion, absorbtion will start at around 90% soc. If I were to charge to 3.65v I would be able to get from 90% soc to 100% in no time, but I would start to get imbalances(40-80mv) and could possibly get a runner cell. Just not worth the risk..

I do not run a bms, the Lv5048 sets the high and low voltages.

Lipos are a whole different animal 5c charge rates and 60c discharge...

There are some 1C charge curves on LFP here:


They hit 3.65 at about 25% SoC.

I think there might be voltage drop from your controller to your battery. The controller might think the battery is at 52 volts, when its actually much lower. Next time compare the voltage reading on the controller to what the voltage on the battery terminals are. If the voltage on the terminals is lower, you need to compensate by raising the bulk setting appropriately. Example if the controller reads 52 volts, but your battery terminals read 51, and your bulk setting is 52 volts, you need to increase bulk setting to 53 volts or higher until you get the terminals to 52 volts.
Your problem of not charging at high amps even though your panels are capable of doing it is something I encountered before. I have a 4s 220ah lifepo4, and I charge it to 14.6 volts every day (3.65v per cell) I demand max performance from my battery. But I have voltage drop from controller to battery (almost 1 volt) to compensate I had to increase the bulk setting on my controller(ecoworthy 20a mppt) to 15.5 volts, anything lower and the controller switches to float too soon (lower amps). I have float setting at 14.3 volts (max it will go) and battery charges at 2 or 3 amps. Too slow for such a massive battery.
You want max amps going into your battery you need to increase the bulk voltage, there has to be a large differential from the bulk setting to the what the battery terminals read. For me its the difference between charging at 12 amps or 3 amps. The higher bulk voltage is the only time my controller goes into absorb mode.
Some controllers (makeskyblue 60a mppt which I also have) have a setting where you can compensate for voltage drop, by adjusting the controllers voltage to match the battery terminals, if your controller doesnt have such a setting its when you need to increase the bulk voltage to compensate.
I have the chargery bms8 and also an overvoltage protection relay that monitor battery voltage and disconnect the solar panel to prevent overcharge. When you charge at max amps your for max performance your charging system has to be failsafe.
I also had to use active balancers due to balance issues at the topend, those are required if you like charging to the max 3.65 per cell at high amps.
Increasing the bulk time won't help since the bulk voltage is still going to be too low to fully charge your battery at high amps.