"Floating" LFP batteries

[john61ct]

Originally Posted by newhaul

just might be me

They keep iterating designs, new units coming in, old ones dropped, so I think engineering more active R&D than most mainstream more established makers.

So perhaps feeding them ideas for what our community needs may help steer them in the right directions, open up new markets for them.

Hope we see more innovative stuff from the Baltics, great people.

[200MPH]

The adjustable voltage switch is easy- got me one for $18 off eBay. Very configurable to do whatever you want for high/low/time delay voltage switching. Then just hook up to a large switching relay as John suggested.
I use one to protect our datalogger LFP batteries from over charge/discharge when left in remote locations. This is for my day job, not fun stuff like sailing though.

I have the BP-100 on the discharge side of our system, not on the battery terminal. You are right in that they don't like reverse current (charging current flowing into battery) passing through them.

What sometimes happens is that the shore power at marina fails or son idiot switches it off. With solar and wind chargers switched off, its only a matter of time before the fridge/freezer will over discharge the house battery. This is most likely what hurt the previous Pb battery bank. So I would have installed a BP-100 or similar no matter what new batteries were installed.

[nebster]

A few additional thoughts:

1) Traveller's warning about BP-100 is very important. Do not use asymmetric power FETs for an emergency disconnect between a battery and a dual charge/discharge source. (You can buy two BP-100s and point them at each other, if you want.)

1b) FETs fail closed-circuit and typically in a fast but silent cascade. This means they can "fail unsafe" while they are in production, and you wouldn't know it. It is trivial to accidentally overload one of these FET gates (I did it on the test bench by not applying a precharge to the buffer caps in my inverter) and then have a defective safety device sitting there in your circuit not doing its job.

2) There is some discussion here about using another small, cheap battery as a "buffer" so as to avoid float. I contend that there is almost zero evidence that suitably low float voltages have any deleterious effect on LFP. It would be a shame to go to great lengths to avoid something that isn't worth bothering to avoid, so research carefully. (Almost everything you'll find is likely attributable to confusion/inertial thinking about putting lead acid chargers into float stage at higher voltages.)

2b) For some of us with large residential loads and/or undersized shore power feeds, hybrid inverters are a real blessing. A power-assisting inverter can't assist via the battery if the battery isn't hooked up! So, even in shore-power scenarios, there may be good reasons to have the big LFP battery online and "floating."

3) If you really do want to take your expensive lithium pack fully offline during a stable float scenario, anyway, you may not need a lead acid buffer battery at all. Many modern inverters don't care if a battery is present or not. They will pass through shore power either way. So, if you're also willing to move your DC loads to the inverted side (via a 12VDC or 24VDC power supply), you could skip the extra battery in exchange for a ~20% conversion efficiency hit on the DC loads. I don't know if that ever makes sense -- you would need to be passionate about offlining the lithium in float AND have a relatively small total DC load compared to total AC load for it to pencil out, but maybe?

3b) One other advantage of this approach is that it makes it trivial to take the battery out of circuit for routine test and maintenance, leaving all the loads seamlessly online when shore or generator power is available. Yet another advantage is that it decouples your DC load voltage(s) from your lithium battery pack voltage. You can choose to run a 24V lithium pack on a 12V boat, or a 48V one on a mixed 12/24V boat, etc.


Cheers all!

[Delfin]

Originally Posted by nebster

A few additional thoughts:

1) Traveller's warning about BP-100 is very important. Do not use asymmetric power FETs for an emergency disconnect between a battery and a dual charge/discharge source. (You can buy two BP-100s and point them at each other, if you want.)

1b) FETs fail closed-circuit and typically in a fast but silent cascade. This means they can "fail unsafe" while they are in production, and you wouldn't know it. It is trivial to accidentally overload one of these FET gates (I did it on the test bench by not applying a precharge to the buffer caps in my inverter) and then have a defective safety device sitting there in your circuit not doing its job.

2) There is some discussion here about using another small, cheap battery as a "buffer" so as to avoid float. I contend that there is almost zero evidence that suitably low float voltages have any deleterious effect on LFP. It would be a shame to go to great lengths to avoid something that isn't worth bothering to avoid, so research carefully. (Almost everything you'll find is likely attributable to confusion/inertial thinking about putting lead acid chargers into float stage at higher voltages.)

2b) For some of us with large residential loads and/or undersized shore power feeds, hybrid inverters are a real blessing. A power-assisting inverter can't assist via the battery if the battery isn't hooked up! So, even in shore-power scenarios, there may be good reasons to have the big LFP battery online and "floating."

3) If you really do want to take your expensive lithium pack fully offline during a stable float scenario, anyway, you may not need a lead acid buffer battery at all. Many modern inverters don't care if a battery is present or not. They will pass through shore power either way. So, if you're also willing to move your DC loads to the inverted side (via a 12VDC or 24VDC power supply), you could skip the extra battery in exchange for a ~20% conversion efficiency hit on the DC loads. I don't know if that ever makes sense -- you would need to be passionate about offlining the lithium in float AND have a relatively small total DC load compared to total AC load for it to pencil out, but maybe?

3b) One other advantage of this approach is that it makes it trivial to take the battery out of circuit for routine test and maintenance, leaving all the loads seamlessly online when shore or generator power is available. Yet another advantage is that it decouples your DC load voltage(s) from your lithium battery pack voltage. You can choose to run a 24V lithium pack on a 12V boat, or a 48V one on a mixed 12/24V boat, etc.


Cheers all!

Very interesting, thank you. One comment...many, if not most all cruising boats, have two bank systems with a small starter bank and large house bank. For those vessels, converting to lithium means replacing the house bank, so you can leave the LA in place as the charge current end point with little to no effort in system design or implementation. While floating Li at low voltages may be not be harmful, letting them sit disconnected from a charge is definitively not harmful, so if it's easy to do, why not? We only use our Li bank when at anchor, or when recharging. The rest of the time they just sit. Pretty simple.

[john61ct]

Below is IMO and only pertains to my use case

Originally Posted by nebster

I contend that there is almost zero evidence that suitably low float voltages have any deleterious effect on LFP. It would be a shame to go to great lengths to avoid something that isn't worth bothering to avoid, so research carefully.

Members whose expertise I greatly respect have stated it a bit differently.

There is no evidence that a low-voltage float is harmless wrt maximizing longevity.

If that last is a lower priority than "simplicity", then of course, your boat your choice

> hybrid inverters are a real blessing

I personally avoid AC as much as possible, 90+% of my consumption is DC, and if I ever hook up to shore power the only connection is to the mains charger.

But I see separating the Loads buss from the Charging buss to be a requirement in properly managing LFP,

therefore combi charger/inverter units are eliminated, unless there is some design option I've missed.

Finally, sorry but running DC loads off AC current coming from a DC battery powered inverter seems just whackadoodle to me.

A setup like that, if large would waste more power than many boats consume in total.

[tanglewood]

Originally Posted by Delfin

Very interesting, thank you. One comment...many, if not most all cruising boats, have two bank systems with a small starter bank and large house bank. For those vessels, converting to lithium means replacing the house bank, so you can leave the LA in place as the charge current end point with little to no effort in system design or implementation. While floating Li at low voltages may be not be harmful, letting them sit disconnected from a charge is definitively not harmful, so if it's easy to do, why not? We only use our Li bank when at anchor, or when recharging. The rest of the time they just sit. Pretty simple.

Nebster hit on one key reason that applies to us, and that's the inverter boost function. The inverter(s) can supply loads great than the shore power connection or generator, by drawing on the batteries. Then when the load subsides, the batteries get charged back up again. It's a great way to load level, and use shore power or a generator that is sized to your average load, not your peak load.


Not everyone will care about such a function, but for those who do, you need batteries connected to use it. It's definitely something I care about.

[Maine Sail]

Originally Posted by Maine Sail

I just programmed in a charge to 3.4VPC test. I will then discharge to 0% SOC and let you know the SOC attained at charge up to 3.4VPC. We already know what charge to full then drop to 3.3VPC is, it's in the graphs...

To summarize the *data I spoke of:

CALB SE 100Ah Cells @ 87.7% SOH:

Capacity Test #1 - Parallel top Balance at 3.65V and allow current to taper to 0A = 87.723 Ah (100% SOC *Baseline)

Capacity Test #2 - Charge to 13.80V (3.45VPC) and allow current to taper to 0A = 87.673 Ah (99.94% SOC)

Capacity Test #3 - Charge to 13.80V (3.45VPC) and allow current to taper to 5A (.06C) = 86.570 Ah (98.69% SOC)

Float Capacity Test #1 - Charge to 13.6V (3.4VPC) and allow current to taper to 0A & float for 24 hours = 74.836 Ah (85.3% SOC)

Float Capacity Test #2 - Charge to 13.8V (3.45VPC) and allow current to taper to 0A then drop to 13.6V (3.4VPC) and float for 24 hours at 13.6V with a 10A load and 30A charger capability = 86.428Ah (98.5% SOC)

All of these capacity tests, except for Float Test #1, essentially result in the battery attaining, for all intents and purposes, 100% SOC. As can be seen "CHARGE TO 13.6V" is a very different outcome at 85.3% attained SOC, than "CHARGE TO 13.8V THEN DROP TO 13.6V" which essentially maintains the battery at darn near 100% SOC.

Interesting Observations:

Using a lead acid type charger that charges to 100% then drops to a "maintenance voltage" of 3.4VPC will hold the battery at 100% SOC.

Charging to 3.4VPC or 13.6V (from below 85% SOC) results in a battery being floated/maintained/stored at less than 100% SOC or approximately 85% SOC but, charging to 100% SOC then dropping back, to the same voltage, results in holding the battery at near full.

In a top balance where all cells went to 3.65VPC and 0A then followed by a capacity test, the difference between 3.45VPC tapering to 0A and 3.65VPC tapering to 0A resulted in a 0.06% difference in attained capacity or well within measurable resolution error range.

When I get more time I will post the discharge graph data..

Hope this helps...

*This data was not created specifically for this thread it is data collected from Alpha testing the Balmar SG-200 battery monitor. If you want more info on the SG-200, or to be part of the Beta test part of the product launch, see Balmar at the Annapolis show.

[tanglewood]

Yes, very interesting. The Float Test 1 is not what I would have expected. It's interesting that holding a high float voltage reduced capacity. I might have expected that long term, but not in a single cycle.


Float Test 2 is as expected.

[Maine Sail]

Originally Posted by tanglewood

Yes, very interesting. The Float Test 1 is not what I would have expected. It's interesting that holding a high float voltage reduced capacity. I might have expected that long term, but not in a single cycle.


Float Test 2 is as expected.

It did not "reduce capacity". At max charge voltage of 3.4VPC it could only get to 85% SOC when charged to 3.4VPC from a lower SOC than 85%.

There is a difference in attained SOC between charge to 3.4VPC, and by this I mean from a lower SOC, and never go above that level, and charge to 3.45VPC and then drop back to 3.4VPC.

Setting a peak charge voltage of 3.4VPC (13.6V) gets you to about 85% SOC

Setting a peak charge voltage of 3.45VPC (13.8V) gets you to 100% SOC

In Float test #1 the battery was only ever charged to 13.6V (3.4VPC) and it could only attain about 85% SOC in doing so.

In float test #2 the battery was charged to 100% SOC at 13.8V (3.45VPC) then the voltage was dropped to 13.6V, just as a lead acid charger would do. This resulted in the battery attaining and then being held at near 100% SOC.

In test #2 once the battery dropped from 13.8V to 13.6V a 10A load was applied to the "bus" to simulate house loads at a dock. The charger carried that load, it was capable of 30A, and the battery charger maintained 13.6V or 3.4VPC.

When the battery was discharged back to 0% SOC the data showed the battery had been held at darn near 100% SOC by first charging to 100% then dropping back to 13.6V. This is because the charger carried the house loads not the battery. The data indicates that 3.4VPC is enough to maintain 100% SOC, if the battery had already been charged to that point, but 3.4VPC was not enough to get to 100% SOC, from a lower SOC, on its own.

The point is that charge to 3.4VPC and charge to 3.45VPC then drop back down to 3.4VPC results in a difference in the SOC the battery will be stored or floated at.

Perhaps this is why CALB says charge to a max of 3.4VPC for standby use? Is it because they know the battery won't be stored at 100% SOC if only ever charged to 3.4VPC?

In test #1 the battery was stored/floated at 85% SOC and in test #2 it was being stored at essentially 100% SOC, despite the same float or storage voltage.

Confusing stuff for sure..

[nebster]

Maine Sail, how is your device measuring the voltage? And, have you checked its calibration with another known-good meter at the same measurement point?

Just a few millivolts can make a big difference in charging up into the knee, and my experience is that (a) the meters on the charger usually don't match my reference equipment's measurements too well, so an offset to compensate is usually needed, and (b) a lot of times people are not taking the voltage measurements at the cells directly, so we end up trying to compare apples and oranges.

Also, charge acceptance is definitely related to charge rate. Can you share with us your continuous-current charge rate for these tests? (And, was it pretty much the same for all of the tests?)

[nebster]

Originally Posted by Delfin

Very interesting, thank you. One comment...many, if not most all cruising boats, have two bank systems with a small starter bank and large house bank. For those vessels, converting to lithium means replacing the house bank, so you can leave the LA in place as the charge current end point with little to no effort in system design or implementation. While floating Li at low voltages may be not be harmful, letting them sit disconnected from a charge is definitively not harmful, so if it's easy to do, why not? We only use our Li bank when at anchor, or when recharging. The rest of the time they just sit. Pretty simple.

That's a great point. It's also common in large RVs (where I am mostly coming from with my experience), and it is usually easy to cross-bus the starter batteries with the house bus for house DC loads. So, same thing could be done there.

I forgot about that, since we have our starter batteries completely isolated to just starting engines.

[tanglewood]

Originally Posted by Maine Sail

It did not "reduce capacity". At max charge voltage of 3.4VPC it could only get to 85% SOC when charged to 3.4VPC from a lower SOC than 85%.

There is a difference in attained SOC between charge to 3.4VPC, and by this I mean from a lower SOC, and never go above that level, and charge to 3.45VPC and then drop back to 3.4VPC.

Setting a peak charge voltage of 3.4VPC (13.6V) gets you to about 85% SOC

Setting a peak charge voltage of 3.45VPC (13.8V) gets you to 100% SOC

In Float test #1 the battery was only ever charged to 13.6V (3.4VPC) and it could only attain about 85% SOC in doing so.

In float test #2 the battery was charged to 100% SOC at 13.8V (3.45VPC) then the voltage was dropped to 13.6V, just as a lead acid charger would do. This resulted in the battery attaining and then being held at near 100% SOC.

In test #2 once the battery dropped from 13.8V to 13.6V a 10A load was applied to the "bus" to simulate house loads at a dock. The charger carried that load, it was capable of 30A, and the battery charger maintained 13.6V or 3.4VPC.

When the battery was discharged back to 0% SOC the data showed the battery had been held at darn near 100% SOC by first charging to 100% then dropping back to 13.6V. This is because the charger carried the house loads not the battery. The data indicates that 3.4VPC is enough to maintain 100% SOC, if the battery had already been charged to that point, but 3.4VPC was not enough to get to 100% SOC, from a lower SOC, on its own.

The point is that charge to 3.4VPC and charge to 3.45VPC then drop back down to 3.4VPC results in a difference in the SOC the battery will be stored or floated at.

Perhaps this is why CALB says charge to a max of 3.4VPC for standby use? Is it because they know the battery won't be stored at 100% SOC if only ever charged to 3.4VPC?

In test #1 the battery was stored/floated at 85% SOC and in test #2 it was being stored at essentially 100% SOC, despite the same float or storage voltage.

Confusing stuff for sure..

OK, I completely misunderstood what you were reporting in the first place. The Ah figure is the charge delivered to the battery by the particular charge protocol. I thought it was the Ah drawn out after a subsequent load test.

[nebster]

Originally Posted by john61ct

There is no evidence that a low-voltage float is harmless wrt maximizing longevity.

Uh huh.

If that last is a lower priority than "simplicity", then of course, your boat your choice

But it's not simplicity... it's much simpler to leave the pack connected.

I personally avoid AC as much as possible, 90+% of my consumption is DC, and if I ever hook up to shore power the only connection is to the mains charger.

Yeah, for folks with small house systems and low demand, a lot of this discussion doesn't make sense. Those systems also have far less complexity. But lots of people living full time on boats want more.

But I see separating the Loads buss from the Charging buss to be a requirement in properly managing LFP,

The big reason to separate the two busses is to improve mission critical load support when there is a charge fault. This problem is unique to boating.

I hazard a guess that the vast majority of LFP packs, including in marine applications, do not yet benefit from a two-bus design.

It is certainly not necessary to have two busses to "manage" an LFP pack properly. The second bus simply hardens the upstream system.

Finally, sorry but running DC loads off AC current coming from a DC battery powered inverter seems just whackadoodle to me.

A setup like that, if large would waste more power than many boats consume in total.

Just because some of us have order-of-magnitude larger systems doesn't mean we're whackadoodles, yo!

Horses for courses...

[Maine Sail]

Originally Posted by nebster

Maine Sail, how is your device measuring the voltage? And, have you checked its calibration with another known-good meter at the same measurement point?

Just a few millivolts can make a big difference in charging up into the knee, and my experience is that (a) the meters on the charger usually don't match my reference equipment's measurements too well, so an offset to compensate is usually needed, and (b) a lot of times people are not taking the voltage measurements at the cells directly, so we end up trying to compare apples and oranges.

Also, charge acceptance is definitely related to charge rate. Can you share with us your continuous-current charge rate for these tests? (And, was it pretty much the same for all of the tests?)

It's lab equipment, not marine type chargers. It is all compared and checked against a NIST calibrated Fluke 289 which is checked against reference chips between sending it out to Fluke for calibration.

In other words if I set it at 3.400V it is a heck of a lot closer than any boat equipment would be. When I top balanced at 3.65V it measured 3.650V on the NIST calibrated Fluke as measured at the battery terminals.

Yes all measurements are at cell or battery + & - terminal measurements not at the power supply or load end. The charge and discharge equipment utilize dedicated voltage sensing circuits.

Charging for 12V nominal tests was at .3C based on cell Ah rating or 30A for the 100Ah rated cells. Discharging was done at 10A to more closely simulate a house bank. End voltage for all discharge tests was 3.0VPC as there is no real usable capacity, below that point in the lower knee, worth going after.

Each of the tests were repeated in the same way except for the differing target voltages and tail current parameters. All discharging was also identical & to the same end voltage.

It's easy to duplicate these tests if you feel you'd like to and have equipment capable of giving good repeatable results.

[nebster]

Thanks for that extra info. It's great that you are so exacting with your methodology and equipment here.

I'm surprised that holding 3.400V results in no further charge acceptance, since that voltage is higher than the generally-agreed-upon 100% SOC open circuit resting voltage for LFP. Do you have a graph of the SOC versus time for the 24-hour test at 3.400V by chance?

For what it's worth, I charge in CV at 3.400V for 1 hour at ~0.25C and achieve roughly 90% SOC on a typical charge. I guess your data suggest that I could just float at the same voltage and not really risk an overcharge. I certainly would like to float higher than I do today, because it would let supplementary charge sources (solar) contribute longer and more often when there is demand.