14.2, 14.4, 14.6 - seems like there is no agreement on charging voltage

[archjeb]

Hi Folks,

I know there were some threads in the past on this I've read through. The ones I read didn't seem definitive on this and they were a while back; so I thought I'd post since I'm at a cross roads.

I have 3 - SOK Marine grade 100Ah LiFePo4 batteries that are wired in series for 36v. This is for an electric trolling motor application. I have two chargers I'm bench testing.

1. Minn Kota MK345PCL --- 3 Bank x 15amp Charger - manual profile selection
2. Noco GENPRO10X3 --- 3 Bank x 10Amp charger - manual profile selection

On testing these in the Lithium profile, I see that the Minn Kota has a charge voltage for the Lithium profile of 14.2volts
With the Noco, the Lithium charge profile is 14.6 volts.
The Minn Kota shuts down once the BMS disconnects at full charge or C MOS gets enabled.
The Noco goes to sleep once the BMS disconnects at full charge or C MOS gets enabled...but then it appears to wake up again every few minutes to try to charge, and then goes back into sleep. It appears to continue in this cycle.

At any rate, when I call SOK, they said they really like to see 14.4-14.6v so the cells can be properly balanced.
When I call Minn Kota and ask why did you settle on 14.2v, they blamed the LiFePo4 companies saying, "they many didn't provide the charge voltages in their documentation and therefore, we settled on a 'safe' voltage of 14.2v. If you need 14.4-14.5, you can use the AGM setting."


So I'm at a cross roads. I need an onboard waterproof marine rated charger. I would like the 15Amp with the Minn Kota, because it will reduce my charge time (5amps more). But I am worried about the lower voltage in the Lithium profile. In the AGM profile, it will drop to a float charge of 13.8 volts after the end of bulk of 14.5v...so not sure what kind of impact this has long term running in that profile?

Typical fisherman behavior, you come home, you plug it in to charge and then go out the next time to go fishing. Or you go to camp, plug in the charger to the generator and charge the batteries for the next morning of fishing.

Especially in the first case, I'm not fishing everyday....so I'm not baby sitting the batteries so to speak.

Any recommendations here? I'm getting confused when I read some battery sites that say you have to charge about 14.4. Then others say you can charge just fine all the way down to 14.0v? I even saw one battery manufacturer that said max is 14.8v. I don't know what is real or folklore at this point.

 

[sunshine_eggo]

14.0-14.6V can all get a LFP battery fully charged. Lower just takes longer.

13.8 is enough to get them to 98%+ SoC...

14.4V is the sweet spot because 14.6V requires perfect cell balance to achieve safely.

14.8V is likely to trigger BMS over-voltage protection - not something you want to count on for routine charging.

SOK's comments are the most relevant. Batteries must be fully charged regularly to allow the BMS to balance the cells and balancing is only meaningful at higher voltages.

LFP is not a good mesh with the described "fisherman behavior." Storing LFP at 100% charge can cause degradation particularly in warm conditions. If you're talking about a few weeks at a time in a climate controlled human-comfortable environment, it's not a big deal.

Optimal scenario:
14.4V absorption
13.5V float
Turn off after 24 hours.

 

[archjeb]

Thank you for the quick and VERY informative post.

I'm good with disconnecting after 24hours; heck I can even plug this into a timed / switched outlet to address that.

As far as storage, other than camping, the boat is stored in an insulated garage. So temp might get up to 90degrees in really hot weather. Not sure if that is the temp range you'd be concerned about.

I could store at 95% SOC so its ready to go - sometimes, fishing trips are last minute

So are you suggesting the AGM charge profile then? Or should I use the Lithium profile on the Minn Kota at 14.2 and then once a month or so, run it in AGM @ 14.5v to help balance the cells?

 

[sunshine_eggo]

So are you suggesting the AGM charge profile then? Or should I use the Lithium profile on the Minn Kota at 14.2 and then once a month or so, run it in AGM @ 14.5v to help balance the cells?

This. See how it goes.

It helps that your charger is a 3 bank charger. Where balance would be a real problem is if you were charging the 3 in series with a 36V charger.

 

[archjeb]

This. See how it goes.

It helps that your charger is a 3 bank charger. Where balance would be a real problem is if you were charging the 3 in series with a 36V charger.

Thank you!

I was looking at another post you mentioned:
"""
Do not do this. You can't target SoC by voltage, and you don't want to use your BMS for routine cut-off. The BMS is a safety device, and it should only be used to prevent damage. Repeated triggering can wear out the FETs.
"""

Charging Solution for Lifepo4: AGM Charger? 13.5V PS? 15V PS?

I'm in the final stages of building a 280ah LIfepo4 system. BMS is a JK 200A with all the goodies, high and low temp cutoff, full programable, 2a balancing, etc. I'm still a bit foggy on the technicalities of charging, and it's the last piece of the puzzle. Can someone answer the...

diysolarforum.com

Is this the major reason why you are suggesting the Minn Kota charger I already have, since it goes into error mode when the BMS reaches cutoff...and does NOT cycle again into charge mode?

The Noco is the one that seems to go to sleep and awakes in a continuous cycle.

 

[sunshine_eggo]

These are separate issues. Battery specs/datasheets often list 80-90% as the maximum recommended charge state for increased cycle life. This makes people think they can simply charge to a lower voltage and attain this.

The other issue is you don't want to use the BMS as the means of stopping charge every time you charge. The equipment should operate inside the BMS limits.

I'm a big proponent of "use what you have if it works." Your proposed equipment/method works, and you don't have to spend anymore $.

That's why I recommend it ?

 

[niktak11]

For a 4S Lifepo4 battery, both of those settings are considered high. I'd go with 13.8V bulk charging and occasionally do maybe 14.2V to balance.

 

[Pierre]

For a 4S Lifepo4 battery, both of those settings are considered high. I'd go with 13.8V bulk charging and occasionally do maybe 14.2V to balance.

and 13,5v float and store at 60% SOC.

 

[archjeb]

So this is what I mean by confusion. We're back to multiple voltages now.
SOK's datasheet says Recommended Charge Voltage is: 14.4V-14.6V

SK12V100P SPEC SHEET.pdf

drive.google.com

When I ask SOK support can I store them fully charged, they say no problem. I'm super confused now...

 

[sunshine_eggo]

So this is what I mean by confusion. We're back to multiple voltages now.
SOK's datasheet says Recommended Charge Voltage is: 14.4V-14.6V

SK12V100P SPEC SHEET.pdf

drive.google.com

When I ask SOK support can I store them fully charged, they say no problem. I'm super confused now...

Why?

I think I know. Maybe you believe in magic - Somehow SOK has produced a LFP battery unlike every other one on the market exempt from all limitations of the chemistry, which charges differently and is immune to the high SoC storage characteristics of ALL Lithium battery chemistries.

Again, they haven't.

You are also likely dealing with English-as-a-second-language folks that care way more about perception than technical accuracy.

Please consider that SOK doesn't manufacture these products. They assemble cells made by other manufacturers into batteries. These same cells are used in other brands of batteries. SOK is just a packaging outfit.

 

[John Frum]

Another perspective on charge voltage.
3.65 volts is dead full
Its typically counter productive to charge a battery to 3.65 volts per cell.
The reason being, statistically zero batteries are perfectly top balanced.
If you charge to 3.65 volts per cell, at least one cell would be charged over 3.65 volts which is not good for them.
Also the BMS should trip to protect the high cell(s).
Many BMSs only balance in the high knee and only during charging.
So if the BMS trips, your battery is severely hampered in maintaining the top balance.

Given the above...
3.65 volts per cell = 0 mv headroom
3.55 volts per cell = 100 mv headroom
3.45 volts per cell = 200 mv headroom

I don't recommend charging below 3.45 volts per cell.

 

[Pierre]

So this is what I mean by confusion. We're back to multiple voltages now.
SOK's datasheet says Recommended Charge Voltage is: 14.4V-14.6V

SK12V100P SPEC SHEET.pdf

drive.google.com

When I ask SOK support can I store them fully charged, they say no problem. I'm super confused now...

Why don't you start off conservatively and do capacity discharge tests. If you are not satisfied , move up the the next higher settings. Your happy spot will be between 13,8 and 14,2v . So what if your battery is only 98,5% full.

 

[Sverige]

Please consider that SOK doesn't manufacture these products. They assemble cells made by other manufacturers into batteries. These same cells are used in other brands of batteries. SOK is just a packaging outfit.

It reminds me of Audi, who assert that the transmission in my car is a sealed unit requiring no fluid changes “for the lifetime of the vehicle”. ZF, who supply the transmissions to Audi state unequivocally their product accrues damage if the fluid is not changed every 80-90k miles. One statement encourages vehicle sales, especially to the maintenance cost conscious fleet market, the other prioritises length of useful service, so I know who I believe!

Battery manufacturers probably are not at all incentivised to discourage users reducing the lifespan of their product.

 

[archjeb]

Honestly, I'm upgrading from 3 x 100Ah AGM batteries. So even 95% full with these LiFePo4 is going to be WAY more usable capacity than what I had before.

I just don't want to damage them or impact the longevity. So interpolating everything on this thread, it appears that the Minn Kota charge profile of 14.2volts is actually better for longevity of the batteries then...compared to the Noco that is full on 14.6volts?

 

[sunshine_eggo]

Honestly, I'm upgrading from 3 x 100Ah AGM batteries. So even 95% full with these LiFePo4 is going to be WAY more usable capacity than what I had before.

I just don't want to damage them or impact the longevity.

Difficult to do.

So interpolating everything on this thread, it appears that the Minn Kota charge profile of 14.2volts is actually better for longevity of the batteries then...compared to the Noco that is full on 14.6volts?

Yes.

 

[Aly_Oops]

I would follow SOK settings. Absorption 14.6 and Float 3.8. SOK batteries will protect the cell at 3.65 or so and overall total of 14.6 vdc. If you plan to use this for over 10 years, (not likely for most people) then go 14.4 absorption.

Not sure about this, but it seems SOK chemistry is safe up to 3.7 cell, but set their cutoff at 3.65. Overall total 14.8 vdc. For low end, 2.6 is cutoff but can go 2.5vdc. They are giving 7 year warranty with those settings. Check your balancing voltage, if only 3.35 is the best you see or can do, I would suspect an issue.

Once bulk, absorption, then float, it will settle at 13.8 vdc quickly. I would do at least one full, proper charge and check the voltages, balancing, etc, then decide how conservative you want to be.

Those are the specs that they published for their parameter. There are differences with each cell and how they match them, makes a difference.

 

[John Frum]

Not sure about this, but it seems SOK chemistry is safe up to 3.7 cell, but set their cutoff at 3.65.

Do you think that SOK batteries have a different chemistry from the big blue aluminum case prismatic cells that we use for DIY?

 

[sunshine_eggo]

I would follow SOK settings. Absorption 14.6 and Float 3.8. SOK batteries will protect the cell at 3.65 or so and overall total of 14.6 vdc. If you plan to use this for over 10 years, (not likely for most people) then go 14.4 absorption.

Not sure about this, but it seems SOK chemistry is safe up to 3.7 cell, but set their cutoff at 3.65. Overall total 14.8 vdc. For low end, 2.6 is cutoff but can go 2.5vdc. They are giving 7 year warranty with those settings. Check your balancing voltage, if only 3.35 is the best you see or can do, I would suspect an issue.

BS. You're a slave to specs because you lack a fundamental understanding of how LFP works. To quote your own words back to you: Do the research.

SOK doesn't make their cells. There's nothing magic about them. Charging over 3.65V isn't good for them just like every other LFP cell on the planet.

 

[Browneye]

The challenge is to keep all lfp cells balanced at high knee - near full charge. If one reaches full before the others its voltage will shoot up and the bms will cut off charging. So a lower charge voltage means current tapers off at that lower voltage and most bms have a balance feature that will cut current to the high cell, allowing the others to 'catch up', or reach a higher full charge. Trying to do this at 14.6V on a 12volt battery is more than likely to result in charging termination by the bms for one or more cell over-volt. Those cells simply reached full charge and their voltage shoots up.

LFP has such a flat charge-discharge voltage that it isn't a good gauge of state of charge (SOC). But full IS full - 3.65Vpc is max for LifePo4.

Through much experimentation it has been found that a charge voltage of 13.8V to 14.2V with bring lfp cells to full charge, or at least 99% of capacity. That last one or two percent can take a long time. The higher voltage rate will just get it there a little faster since the charge source will taper current as the voltage (resistance) comes up. LFP has such low internal resistance that this peak can happen pretty quickly as they all reach full.

Most LFP batteries will over-volt if you try to charge them to 3.65Vpc in a bank. Works fine for individual cells, but once you assemble them together any variances will readily show up in the charge profile.

SOK tries to compete directly with Battleborn, the latter uses a different (cylindrical) cell that is better suited for higher charge voltages, hence their claim of being a ready drop-in replacement for lead acid. SOC uses the same prismatic cells as just about everybody else. The girl at the front desk really doesn't know much about batteries - she has literature provided by management, whom has a priority focus on sales volume. SOK does seem to make a decent product, lots of happy customers. But then, the same can be said for BB - but at twice the price.

 

[archjeb]

So, I've been bench discharging/charging these battery packs...and 2 of the batteries, the cells are fairly balanced. I'm around 30mV between the 4 cells.
But the 3rd battery, I have one cell that I just can't get closer to the other cells. I've tried multiple discharge/charge cycles. The last charge cycle, SOK recommended charging at a low current. I used a 4Amp charger (I have a 2 amp charger, but the 50+hr charge time was driving me nuts) and I still have this single cell (cell3) that is tripping the BMS on charge - even with the 4amp charger.

Rookie question here, but should I have stuck with the 2Amp charger? Or is 4Amps low enough to see if the BMS could actually balance that cell?

Or should I take the battery cover off and try to drain that single cell to try to force it some into balance?
On full discharge, its actually lower then the first 2 cells...so it makes me think that maybe there is something wrong with that cell. But since this is my first LiFePo4 rodeo, I have nothing to compare this to other than the 2 other SOK batteries that are all part of the same shipment.