DIY Lithium Guide

[john61ct]

Originally Posted by Maine Sail

Don't float

Since I've not yet seen any automatic charge source that accommodates this, I set Float V to 13.0

I realize just stop is best, but failing that, can you suggest a better setpoint?

And since actual amps flow appears to be zero, why would there be any impact?

Sorry for all the questions. . .

[john61ct]

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[OceanSeaSpray]

Originally Posted by Maine Sail

GEL would be closest voltage wise but bear in mind it is the "absorption duration" that is the killer. If you can't adjust the absorption time to 0 minutes to a max of 30 minutes you will see cell damage and shorten cell life.

By dropping charge voltage to 13.8V you can get by with a slight absorb duration at up to 30 minutes at CV. The higher the absorption voltage e.g.: GEL/14.1V the shorter the absorption duration needs to be.

For example: (12V nominal pack)

Charge Voltage 14.6V / CV Duration = 0 min
Charge Voltage 14.2V / CV Duration = 10 min +/-
Charge Voltage 13.8V / CV Duration = 30 min +/-

...

Duration at voltage is by far the bigger killer than voltage alone and lead acid chargers all do longer duration absorption then LFP can safely handle unless it can be duration programmed down to 0 - 30 minutes (voltage dependent).

The figures above might work for alternator charging on an average system. Absorption duration is primarily a function of the C-rate. Voltage alone doesn't have much to do with it.

If the C-rate is low, the maximum voltage is reached at low current (by definition) and there is no absorption time left. Any delay before terminating will harm the cells. At high C-rates, the internal resistance of the cells makes the voltage look higher before the cells are full... and we see an absorption period appearing, where the voltage is limited and the current tapers down. This absorption time becomes very important to charge properly.

If I charge a bank at 0.8C and up to 14.0V, absorption time is in excess of 35 minutes. At ~0.1C and same voltage, around 10 minutes. On a dull cloudy day, solar charging might amount to 0.03C only and then there would be no absorption left, because reaching final voltage at that current means immediate termination.

The bottom line is that charging systems need to be configured for absorption according to their current capacity: the higher the C-rate, the longer the absorption required.

Originally Posted by Maine Sail

Oh and LA chargers also float & float = BAD for LFP.

There is a great deal of confusion about "float". What is float? It means holding the battery at a voltage higher than it would show if it was left alone.
A full LFP battery settles a little over 13.35V. Anything less than that can't "float" a full battery. It might just ruin it slowly by keeping it full. 13.1V in discharge would be about 35% SOC. A charger "floating" at 13.1V will just prevent the battery from going any lower than that and will float by being turned off for as long as the battery is above 13.1V. No different than being disconnected. But sometimes it doesn't quite work this way and after a day or three in float, the unit switches back to bulk because the goal of LA chargers is always keeping the battery full - exactly what we don't want. Quite often, this behaviour cannot be prevented by programming, both with solar and shore power chargers.

Originally Posted by Maine Sail

Charge sources for LFP really need to be fully programmable including voltage and duration.

What they really need to do is terminate on a residual current condition at a given voltage. This is the only way to obtain correct charge termination regardless of C-rate. Unfortunately, I only know of two charge controllers that monitor an external battery shunt to do that.

[john61ct]

Originally Posted by OceanSeaSpray

The bottom line is that charging systems need to be configured for absorption according to their current capacity: the higher the C-rate, the longer the absorption required.

Since we don't care about getting "full" no absorb is required, ever, just terminate at target voltage (3.45Vpc, 13.8 for 4s)

Rarely will a large bank (600+ AH) get close to 1C rate, and if so I'd rather be conservative stop well before hitting the shoulder. A little less AH capacity, so what?

Originally Posted by OceanSeaSpray

What they really need to do is terminate on a residual current condition at a given voltage. This is the only way to obtain correct charge termination regardless of C-rate. Unfortunately, I only know of two charge controllers that monitor an external battery shunt to do that.

Links please, especially for shore power. Solar I know Whizbang Jr and Bogart, but really any SoC AH-counter with a relay 702-BMV can open the connection from any SC right?

[OceanSeaSpray]

Originally Posted by john61ct

Since we don't care about getting "full" no absorb is required, ever, just terminate at target voltage (3.45Vpc, 13.8 for 4s)

Here, we do care. Recharging properly is what makes for good long-term battery health without problems related to memory effects.

Originally Posted by john61ct

Rarely will a large bank (600+ AH) get close to 1C rate, and if so I'd rather be conservative stop well before hitting the shoulder. A little less AH capacity, so what?

Same as above. Additionally, I am interested in sensible banks, not capital-intensive dead lithium storage applications. The systems operate at C-rates where absorption matters.

[john61ct]

Originally Posted by OceanSeaSpray

Here, we do care. Recharging properly is what makes for good long-term battery health without problems related to memory effects.

So once in a while charge to 14, 14.1 if you must when doing your manual balancing, but you're dropping lots of cycles off EOL.

Nothing at all to be gained IMO holding Absorb with LFP, please link if you actually have a credible source.

Where is "here" in the above context?

Originally Posted by OceanSeaSpray

I am interested in sensible banks, not capital-intensive dead lithium storage applications. The systems operate at C-rates where absorption matters.

No idea what any of that means, please explain like I'm five.

Edit: we are talking House bank usage not EV right?

[200MPH]

Originally Posted by OceanSeaSpray

The figures above might work for alternator charging on an average system. Absorption duration is primarily a function of the C-rate. Voltage alone doesn't have much to do with it.

If the C-rate is low, the maximum voltage is reached at low current (by definition) and there is no absorption time left. Any delay before terminating will harm the cells. At high C-rates, the internal resistance of the cells makes the voltage look higher before the cells are full... and we see an absorption period appearing, where the voltage is limited and the current tapers down. This absorption time becomes very important to charge properly.

If I charge a bank at 0.8C and up to 14.0V, absorption time is in excess of 35 minutes. At ~0.1C and same voltage, around 10 minutes. On a dull cloudy day, solar charging might amount to 0.03C only and then there would be no absorption left, because reaching final voltage at that current means immediate termination.

The bottom line is that charging systems need to be configured for absorption according to their current capacity: the higher the C-rate, the longer the absorption required.



There is a great deal of confusion about "float". What is float? It means holding the battery at a voltage higher than it would show if it was left alone.
A full LFP battery settles a little over 13.35V. Anything less than that can't "float" a full battery. It might just ruin it slowly by keeping it full. 13.1V in discharge would be about 35% SOC. A charger "floating" at 13.1V will just prevent the battery from going any lower than that and will float by being turned off for as long as the battery is above 13.1V. No different than being disconnected. But sometimes it doesn't quite work this way and after a day or three in float, the unit switches back to bulk because the goal of LA chargers is always keeping the battery full - exactly what we don't want. Quite often, this behaviour cannot be prevented by programming, both with solar and shore power chargers.



What they really need to do is terminate on a residual current condition at a given voltage. This is the only way to obtain correct charge termination regardless of C-rate. Unfortunately, I only know of two charge controllers that monitor an external battery shunt to do that.

I tend to think that OceanSeaSpray is pretty much correct here. Based on my test charging and discharging of my new 300Ah Li house bank with my Victron 12/3000/120 charger/inverter, and careful monitoring of volts and amps, it correlates with his statement above.

Because I haven't upgraded the firmware in the Victron as yet, I can't select a charge profile where it cuts absorbtion phase based on tailing current, though according to their latest documentation I should be able to.

So, I can only fool around with absorbtion time parameter, which is fixed, but only in increments of 1 hr! Not fine control at all...

But in first test with battery voltage sense wires connected, and bulk charging at 90A, by the end of the 1 hour absorbtion period (I can't set it less than 1 hour), the tailing current was about 16A, though some of those amps were going into running the fridge, so probably more like around 12A or so going into battery. So pretty much full charge, though perhaps a percent or three under 100%. This is based on charging up from around 67% SOC
(207 Ah remaining capacity going by the battery meter, yes, I know it's only an estimate).

Took 90 mins to charge; 30 in bulk and the 60 in the fixed time absorbtion phase.

To my way of thinking, I could fine tune it by altering the absorbtion voltage settings up or down a little, say 13.85V if I wanted to get the battery that tiny bit closer to 100% full. Or, better still, adjust the bulk amp settings up or down a bit. But I'll run some more tests first, and am not bothered by missing out on 'full' by a couple of percent or so.

Of course, the correct approach is that I upgrade the firmware and see if it will taper charge cut off on absorbtion.

Reading your above post did give me an idea however. I can put an adjustable volt switch I have (thanks ebay!) across the current shut and use that to trigger the virtual switch input to the Victron charger, which can be programmed to switch to float charge upon activation. In that way I could have a tailing current switch to float charge without firmware upgrade. I should just lose my fear of upgrading the firmware I think!

Oh, and I've gone with a 13.0V float voltage for now also as described above by Ocean Spray, so that it allows me to leave fridge/freezer running while in dock and on shore power. Can't see that as hurting the Li battery, it's just holding it at a midpoint SOC. I've programmed the routine absorbtion of the Victron to the maximum number of days, about 45 I think it is. So it shouldn't try charge up the battery at all.

[OceanSeaSpray]

When the cells are new, any charging voltage from 3.4V/cell up is enough to reach a full charge, but playing voltage against absorption time is not a great idea.
If the cells have seen a lot of incomplete charge cycles, it gradually gets harder to recharge them to full (http://www.nature.com/nmat/journal/v.../nmat3623.html). Basically, the upper knee starts shifting to the left.



The more cycles the battery does without getting recharged properly, the stronger this effect becomes and the more difficult it gets to overcome it afterwards, especially when all the cycles finish at the same point because the charging strategy is conservative incorrect. The bump on the red curve keeps building up.

It is important to have enough voltage to be able to charge properly and overcome any early rise in cell voltage, but this is not apparent when everything is new and fresh of course. My experience with a number of installations has been that absorbing at 14.0V before termination seems to be enough to avoid problems on the long run, even with a few months of cycling in winter each year without a full charge.

[Maine Sail]

Originally Posted by OceanSeaSpray

The figures above might work for alternator charging on an average system. Absorption duration is primarily a function of the C-rate. Voltage alone doesn't have much to do with it.

If the C-rate is low, the maximum voltage is reached at low current (by definition) and there is no absorption time left. Any delay before terminating will harm the cells. At high C-rates, the internal resistance of the cells makes the voltage look higher before the cells are full... and we see an absorption period appearing, where the voltage is limited and the current tapers down. This absorption time becomes very important to charge properly.

If I charge a bank at 0.8C and up to 14.0V, absorption time is in excess of 35 minutes. At ~0.1C and same voltage, around 10 minutes. On a dull cloudy day, solar charging might amount to 0.03C only and then there would be no absorption left, because reaching final voltage at that current means immediate termination.

The bottom line is that charging systems need to be configured for absorption according to their current capacity: the higher the C-rate, the longer the absorption required.

This is absolutely, positively & most definitely 100% true! I should have mentioned it myself.

However just getting folks to understand that it is not just voltage but also duration at voltage is difficult enough.

If I had a dime for every time I have read or had someone say to me; "Well, I am only charging to 13.8V or 14.0V so my bank can never get full." I'd be retired by now. Add charge rate the discussion, and I find it often goes right over most folks heads. Baby steps..

[200MPH]

OK, above info noted thanks.

[nebster]

Unfortunately, Victron appears to have removed the tail-current charge termination feature, even though it's still documented. I purchased based on this documentation, and all they say now is, "you should have your BMS manage charging."

Incredibly frustrating.

[Maine Sail]

Originally Posted by nebster

Unfortunately, Victron appears to have removed the tail-current charge termination feature, even though it's still documented. I purchased based on this documentation, and all they say now is, "you should have your BMS manage charging."

Incredibly frustrating.

If this is true it's likely due to a potential liability issue. Due to the inherent inaccuracies & drift with Ah counting, relying on for SOC termination could actually become dangerous with LFP...

It's one thing to be get out of sync with lead acid, so you over charge a bit or over discharge a bit, not the end of the world, but it can actually become dangerous with Lithium.

I have many, many hundreds of hours of testing a number of Ah counters against calibrated lab grade equipment, on LFP, and in as little as three-4 cycles they can get out of whack.

I've tested the *Link-Pro, Victron BMV's, Blue Sea and the Bogart. Sadly I have yet to find any SOC gauge, for LFP, that remains accurate for more than a few cycles, that I would relay on for anything to do with SOC termination.

*I currently use a Link-Pro on my own boat. I use it as a current & voltage meter, plus quasi -Ah guidance, up to 3-4 cycles away from full. I would certainly never use it to manage the banks charge characteristics nor for any sort of accuracy in SOC.

[200MPH]

Originally Posted by nebster

Unfortunately, Victron appears to have removed the tail-current charge termination feature, even though it's still documented. I purchased based on this documentation, and all they say now is, "you should have your BMS manage charging."

Incredibly frustrating.

Ah, that kinda makes sense. Their documentation runs through how to setup tail-current charge termination when using the Lynx Li configure assistant in VEconfigure program.

But when I downloaded the program just a few weeks ago, and then played around with how it worked using their option of 'full featured simulated connection', the option to set tailing amp charge cut off was not in the Lynx Li configure assistant. Strange me thought...

Well thanks for the information, I guess there is no rush to upgrade firmware in my 2009 model 12/3000/120 Multiplus. Seems like only option is to tweak bulk Amp settings up or down a little to get tailing amps about right at the end of the fixed duration 1 hour absorbtion phase. But it will only be approximate, which is a pity.

Back to plan B then. Hookup up a simple voltage sensing switch across current shunt and use output from the switch as the input to the Multiplus to tell it Absorption phase is over.

I'll have a look at doing that sometime - got a few days away a sea next week on board, so hopefully can see if the idea is feasible. I guess it will depend upon accuracy and resolution of voltage switch.

[john61ct]

But basing Absorb termination on falling amps remains accurate, no matter how much the AH-counting derived SoC reading has gotten out of whack.

In fact isn't that the only accurate way to get a precise Full with which to reset the SoC function?

This would lead me to question buying such a Victron product.

Is it possible to re-flash manually to a previous firmware version?

[nebster]

Originally Posted by john61ct

But basing Absorb termination on falling amps remains accurate, no matter how much the AH-counting derived SoC reading has gotten out of whack.

In fact isn't that the only accurate way to get a precise Full with which to reset the SoC function?

This would lead me to question buying such a Victron product.

Is it possible to re-flash manually to a previous firmware version?

Precisely. That's why it's particularly frustrating.

I still can't tell whether they just dropped the capability by accident, or if there's some (good) reason they regressed this functionality.

To be honest, for all the great features and programmability across their product suite... their actual documentation and especially their support for custom LFP banks is, at this stage, a disappointment.