Shore Power Charger Settings for Lithium Iron Batteries, or ...

[DiploStrat]

Polishing a brick.

Looking at settings to use with my Magnum inverter/charger in CC/CV mode. Really only use it for maintenance/top balance, etc. So the goal is to do the lithium iron equivalent of an equalization charge.

Battery bank is my home brew, 840Ah, made up of three 280Ah batteries, each of four 280Ah prismatic cells. All appear to be well matched and balanced.

Home now and I plugged in. Got what appears to be a perfect charge, all three BMS's reporting within 1.0% of SOC and voltage. Individual cells are typically within about 0.003v. When I went to bed the BMS's reported the SOC to be be 99%. (The Magnum reports the SOC as 100% and the actual hour count as +250, but that is probably because it has not had enough real cycles to calibrate itself. That is part of the frustration - dueling monitors.)

Current setting is throttled back to 60-70A (of a possible 100A), 14.2v absorb/target voltage, and trailing amps set at 6. These are scaled back from the AM Solar recommendations. As this is a home build, I am trying to be careful.

As the charger is in CC/CV mode, it shut off when the trailing amps hit 6A. There is no float; recharge is set at 13.2v. Of course.

Several gurus that I have spoken with have recommended a lower charge rate, especially in "maintenance" mode when I have all night - that is, I'm not in a rush. (Just because it will take a higher charge, doesn't mean you should do it.) The 14.2v was guesstimated from Victron's recommended settings and the 6A trailing voltage was backed off a bit from the recommended 4A, or 0.5A per 100Ah of battery.

Again, if the goal is full restore/top balance between trips - NOT a fast daily recharge, would love to hear your thoughts.

-- Raise the target voltage to 14.4v? This corresponds to Battle Born's target voltage.

-- Change the max charge rate? Obviously, during the day, the solar contributes as well. It has been suggested that with a capacity of over 800Ah, even 100A is low.

-- Lower the trailing amps to 4A. This would effectively increase the absorb time.

I think I am pretty close, but all thoughts and comments welcome.

N.B. Those of you with commercial batteries should always follow the recommendations of the manufacturer unless you are out of warranty and have some reason to believe that they are seriously mistaken.

N.B. 2 For this discussion I am not worrying about solar controller settings - only the shore power charger settings.

 

[john61ct]

the goal is to do the lithium iron equivalent of an equalization charge.

There is no equalization charge with LI chemistries. Lead needs higher voltages to stay healthy, prevent / counteract sulfation. LI is only stressed by going to high SoC, no advantage to be gained only downsides, sacrificing cycle longevity.

And sitting at high SoC even more so.

Note that "full restore" as a concept is holdover lead thinking, in lithium land not a thing, actually harmful.


Below applies to any charge source not just the Magnum, but obviously may need compromises depending on a source's control possibilities.

And my priority is cell longevity, assuming quality cells 10000 cycles is do-able, only follow vendor data sheets if you are happy with only a few thousand.

Obviously needs of the use case or your convenience other preferences etc may override, long as informed decisions, not just murdering the bank out of ignorance or slavish credulity wrt datasheets.

> Battery bank is my home brew, 840Ah, made up of three 280Ah batteries, each of four 280Ah prismatic cells. All appear to be well matched and balanced.

Would be **much** better to first wire the 3P at the 1S level, and then do the 4S. You will get imbalanced wear paralleling 3 strings like that, shorter overall cycling lifespan.

> all three BMS's reporting within 1.0% of SOC and voltage.

> That is part of the frustration - dueling monitors.

Forget SoC% guesstimations from the BMS not trustworthy at all.

Magnum - with the shunt based coulomb counting BM add-on - is likely close - meaning within 4-6% - but stays that way only when the 100% point is reset frequently - ideally manually, by you,

often verifying with a known good calibrated ammeter, that the trailing amps setpoint is high enough, in reality not allowed to go too low - never blindly trust charge source readouts, for voltage either.

Measure at the posts.

Following my layout advice will mean only one BMS needed, have spares. BMS are very unreliable long term, very commonly the cause of pack failure.

> Individual cells are typically within about 0.003v

Way overkill, totally unnecessary, staying at high SoC so long harmful as stated.

within ~10mV (.01V) of one another is fine, but if balancing sessions go too long, I'm OK with 20mV.

Target for most good balancers is 5-10mV, ideally this setpoint is adjustable, loosen it up to shorten the balancing time required.


> Current setting is throttled back to 60-70A (of a possible 100A)

Why? No longevity gain going below 0.4C

> 14.2v absorb/target voltage

Nothing at all to be gained going higher than 13.80V, just stressing the cells without adding any significant capacity utilisation,

anything over 13.3-13.4V resting isolated is just surface charge, not additional usable stored energy.

The key for optimising longevity is using the minimum voltage/AHT combinations to get to that point

or even allowing a bit lower if your use case does not require every mAh of cap utilisation.

> and trailing amps set at 6.

If you are holding CV at all - not needed as far as cell health is concerned - terminate charge when current falls to 20A (~0.02C) ,

8A would be the lowest I'd allow, and then only when accurate benchmarking is required, like cap testing for State of Health.

For normal daily cycling no CV at all is best, very easy to implement, "charge TO setpoint and STOP" or

the minimum AHT the charger allows.

No Float as you are doing, good. Only fill up just before loads need feeding, do not sit at high SoC any longer than necessary.

But that recharge setpoint is too high, 3.2V would be better.

 

[john61ct]

The above focuses on cell health only.

The requirements of the device used for balancing - most BMS are pretty bad at that function - really deserves its own thread

or such discussion held off until after the above points are clarified / debated.

 

[DiploStrat]

"Would be **much** better to first wire the 3P at the 1S level, and then do the 4S. You will get imbalanced wear paralleling 3 strings like that, shorter overall cycling lifespan."

BMS is 120A max. My target is 200A as my microwave/stove can draw up to 150A. I picked Overkill Solar as it is near idiot proof. Would love recommendations for a good 200A+ BMS that is as safe for amateurs.

Would have built two 560Ah batteries, but I don't have the room. Wanted to make all of the batteries the same size.

Don't understand the terms, "3P", "1S" and "4S". Sorry. Assume you are recommending one big battery with 200A+ BMS.

Everything else, we are in sync.

 

[john61ct]

your current layout is 4S3P

IOW three 4S strings to first get to 12.8Vnom, then

put into parallel to get to 840Ah.

I am suggesting do the 3.2V paralleling first, into 3P groups, and then series those into one string.

Thus 3P4S rather than 4S3P.

Either get a bigger BMS or one that does not pass current through the FETs, just monitors voltages and

controls external contactors which you can buy at whatever ampacity required, upgrade later leaving the same BMS in place.

Maybe your existing BMS can be repurposed that way, but details above my pay grade I'm afraid.

 

[DiploStrat]

So "S" = "Series" and "P" = "Parallel." Got it.

The cells are said to be rated at 6000 cycles. 1000 cycles is probably longer than my lifetime.

That said, will keep an eye out for a true 200A BMS. Overkill is said to be working on one. (The are simply importing Chinese BMS, but they test, guarantee, and replace locally within the US. A bit better than using Ali Baba.)

Thank you!

My next project is solar. As you have noted, we really need different settings (even "off") when the camper is in the drive, rather when actually in use.

Also a good point on long term SOC. Seems that the best is to let the bank drop to about 50% SOC, the use shore power to come up to full prior to a trip.

I didn't top up, so I was a week on the road with cloudy weather. Ooops! That said, we did not run out of power.

 

[bajajoaquin]

I asked my battery manufacturer what to do about this situation albeit with much less sophistication. In my case, I have an old legacy charger that is one stage of 13.6v. Their response was literally, “why bother?”

The batteries will do just fine at 80% pretty much forever. My solar controller (Rich Solar 40a) will top up as will my DC to DC charger (Victron 18a Isolating). There’s no advantage to topping up at home. Is this a situation of trying to do something well that doesn’t need to be done at all?

SOK 206ah battery, if that matters.

 

[DiploStrat]

Need to be done? As you noted, scholars debate.

Typically, these batteries will not balance unless lifted right to the high voltage knee. Also, some battery monitors want to "see" a full charge periodically to calibrate themselves. That is why I specified that I was looking at this as maintenance, not daily use.

On the road, with the air conditioner hammering, you want to be pretty full each afternoon. In storage between trips? 50% or so seems to be best. Counter intuitive for those of us will years of lead acid experience.

Also counter intuitive is the lower voltage, or simply the idea that higher voltage is not essential to shake the plates. (No plates to shake!)

I think that john61ct's comment about sitting at 50% and then using shore power before a trip is spot on. Like you, I have three charge sources, shore, solar, engine through a REDARC battery to battery charger.

A final point is background loads. On the road, the refrigerator, fans, heat, etc. can present up around 6A per hour. At home, with the refrigerator off, we are closer to 0A. My observation is that cell balance is a bit more even with some load. But that is something that I will keep watching - may not be correct.

 

[john61ct]

Typically, these batteries will not balance unless lifted right to the high voltage knee.

Rephrasing, some balancing devices in order to top balance, require a very high voltage to trigger that function.

IMO a poor design, if that start-balance setpoint cannot be brought down to line up with the user's preference for charge termination.

In fact you can choose to balance at any one point in the SoC/voltage curve. Including the midpoint which is how most cells are delivered.

Or at the bottom, say 3.05V.

Some balancers do not require any trigger, they do their job at whatever the voltage happens to be when you plug them in

periodic maintenance, not while charging.

High enough balance current rate, only takes a few minutes, maybe once or twice a year.

Avoiding the voltage shoulders **prevents** imbalances from showing up, BMSs that require anywhere near 4.20Vpc

are actually creating the problem people think they are solving!


> Also, some battery monitors want to "see" a full charge periodically to calibrate themselves.

You should be able to specify the conditions, define 100% as you like, or best just press the reset button manually.

 

[john61ct]

Whoops LFP that "do not approach" stressful-maximum is 3.60Vpc

4.2Vpc is for li-ion, EV packs and LiPo, not to be used for mobile House banks

 

[DiploStrat]

"Some balancers do not require any trigger, they do their job at whatever the voltage happens to be when you plug them in"

In my case, I do not have a separate balancer. There is a balance function in the BMS with the following parameters:

Balance Configuration
● Balancer start voltage: 3400 mV (Note: in the Android app, this is called “Balanced turn-on voltage, and the units are volts, not mV”)
● Balancer delta-to-balance: 15 mV (Note: in the Android app, this is called “Balancing precision, and the units are volts not mV”)
● Balancer enabled: True
● Balance only when charging: True

The balancer is simply a small resistor. These settings can be changed, but the defaults are what seem to work in many installations.

So, to balance:

-- Battery voltage must be 3.4v (13.6v for the whole battery) or greater
-- Difference between cells must be 15 mV or greater.
-- Battery must be under charge.

This seems pretty tame. Thoughts?

 

[john61ct]

No indication of Balance Current?

If your observations confirm those specs are true IRL

then that 3.40Vpc is nice and low, so even if it requires say 12hrs to "finish" balancing (your call, say 30mV delta)

setting the charge CV / Absorb setpoint at 3.45V is a lot less stressful than 3.6Vpc even 3.55Vpc

And the main charge current rate could likewise be limited to 1A or less, which is far above what that resistance circuit is capable of bleeding, could be 30mA or less as the delta closes.

However, if you did need to balance "that much" frequently, more than say once a month

then getting a dedicated balancer in there at over an amp balance current, some claim 3+A

would be worth considering, cut that 12-hour job down to a few minutes.

The most excellent feature there, is that you can see what is going on, watch balancing start, the delta close over time, then balancing finished.

99% of people with factory-made banks/packs or drop-in unit are flying blind, have no idea what is going on at the per-cell/group level,

have to just leave the charger going a long time and cross their fingers.

Which IMO is a total ripoff when they've paid over say $500 per 100Ah

without even knowing if the maker used good quality cells on their battery's production run.

10 years is not a long lifespan when dealing with LFP, I personally want to leave my banks to my grandkids.

 

[john61ct]

I have an old legacy charger that is one stage of 13.6v.

At 3.40Vpc that is only **very** slightly below a safe/gentle definition of 100% Full

In fact, if you wanted to get to max capacity, 3.33Vpc isolated at rest, all you need do is hold that CV/Absorb until current drops to 0.01, the lower voltage just means that will take longer.

The difference between that "max Full" - which I am not recommending BTW, just an illustration

and a CC-only, "charge TO 3.4V and stop" profile using an HVC circuit

might be well under 5% of the nameplate capacity.

But that latter lower profile may also result in an extra 2000 cycles lifespan before hitting say 70% SoH, assuming the other care factors are at coddling level.

Do not believe that "80%" balderdash - totally exaggerated, and unnecessary to get good lifespan.

That said, stopping **discharge* at say 3.15Vpc rather than 3.05Vpc can double cycle count lifespan, so a 20% **total** sacrifice of capacity utilisation both top and bottom

is IMO a very good idea.

 

[DiploStrat]

To date, once I got a bad lead on one of the BMS's fixed, I have never seen a difference in voltage of greater than about 0.015v, so the cell to cell balance appears to be very good. Nor have I noticed the BMS go into balance for over a month.

So, it sounds like my base settings are not too far off. The goal for the shore charger, as you redefined it, is to get the batteries set for a trip, or to recover from a deep discharge while on a trip. Not hard to do as we usually find a campground with a laundry about every ten days and they usually have power.

The next step will be to continue to monitor the solar controller. I had some interesting problems with it reporting a higher voltage and thus dropping to absorb or float too soon.

Happily, I paid well below that price for my cells and my grandkids don't want 'em.

 

[john61ct]

>> Only fill up just before loads need feeding, do not sit at high SoC any longer than necessary.

Since you want to keep powering loads via Solar input, shore power, alternator whatever

even after the LFP bank is Full

just isolate the LFP bank from all charge sources when that point is reached.

A cheap "sacrificial" lead batt permanently wired into the charge buss

can still buffer the incoming energy and prevent any surge/spikes from sudden dis/connects

In a space/weight constrained scenario, that function "could" be combined with Starter cranking in the engine bay spot.

But with additional redundancy to avoid getting stranded, little li-ion jumpstarter powerpack kept charged in the glovebox

or even a 1/2/Both switch like Blue Sea's to enable use of your LFP House bank as a nice big Kahuna backup.