LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[newhaul]

Originally Posted by Ny-vern

Hi All ,

thank you for your feedback. I understand the impracticality of such a large bank and draw - precisely why I wanted to confirm the numbers before relaying the information and suggesting alternatives.

Cheers!!

it all comes down to a simple fact for the best cost ( not just dollars ) if you will have this large of a load for more than a few minutes a day at the full stated 6000w the correct and best option would be a 7k generator.

[OceanSeaSpray]

Originally Posted by Ny-vern

Hi All ,

thank you for your feedback. I understand the impracticality of such a large bank and draw - precisely why I wanted to confirm the numbers before relaying the information and suggesting alternatives.

Cheers!!

Actually, I know of a case where someone built a 2400Ah/24V bank to feed air-conditioning systems overnight on a large charter motor yacht.

Since they were out steaming by day, they had time to recharge it and it then saved them running a stupid genset all night. Peace and quiet at last!

[rom]

Originally Posted by newhaul

it all comes down to a simple fact for the best cost ( not just dollars ) if you will have this large of a load for more than a few minutes a day at the full stated 6000w the correct and best option would be a 7k generator.

Would your advice be to use a 3.5kW generator for a 3kW load then ? Shouldn't we consider the size of the boat, other energy production means, etc ...?

[newhaul]

Originally Posted by rom

Would your advice be to use a 3.5kW generator for a 3kW load then ? Shouldn't we consider the size of the boat, other energy production means, etc ...?

the issue I posted to just said a 6k total assumed constant load
Size of the vessel doesn't really play into the matrix other energy production means?
The question was bank size to run a constant 6k inverter load.
AC generators operate at their best with an approx 70% to 80% loading. So for a 3k load I would recommend ( just my personal experience and opinions) that for a 3k load you would be better served with a 4k generation capability. . 3.5k is a bit on the light side but if its what you got it will work . ( potentially having issues with startup surges)

[OceanSeaSpray]

Originally Posted by john61ct

2 different stop-charge setpoints, say at 77°C and a .2C current rate:

A: Absorb / CV set at 3.65Vpc, taper to zero current, no amps flowing at all, call that 100% SoC.

then B: Absorb is set to 3.45Vpc, and charge stops when trailing amps have tapered to .06C

I imagine you may think that SoC is significantly lower?

It is in reality 98.69%

It is only if you perform an accurate **load test** comparing A and B that you see just how much (in fact just how little) you are sacrificing by avoiding the top shoulder.

It is true, using **even an accurate** Ah counting current totalizer, the difference between A and B seems much larger than 1.3%.

However a large portion of those "incremental Ah" are just dissipated, probably mostly as heat, not actually **stored** as usable energy in the bank.

LFP is over 99.9% current efficient. The Coulomb counter measures a bigger difference because IT IS THERE and the "98.69%" claim is garbage. The weak-voltage stop-early scenario B fails to charge properly and the more incomplete recharge cycles the battery does, the worst this gets.
Even charging to 3.65V and C/30, the cells stay with 1-2degC of ambient temperature. There is no heating.

Originally Posted by john61ct

The bottom cutoff for load testing should be 2.99Vpc, which I also use as my definition of 0% SoC and the lowest last-ditch LVD cutoff.

The "actual SoC difference" between that and some other lower "industry standard" definition is again, very low single digits.

Obviously, when a LFP bank is not being cycled, it should sit at a low SoC, as long as there is no risk of it getting discharged **below** that 3Vpc.

Using an unduly high LVC just means that the system will trip when it still has usable capacity left if a heavy load is turned on. More and more so as the cells get older and their internal resistance increases. I can see it on my bank, which is in its 5th year of continuous service now and runs 0.6C loads for short periods.
There is hardly any capacity difference between 3.0V, 2.8V or 2.5V, so claiming that cutting off at 3.0V is so much better and leaves "reserve" is nonsensical.
The chemistry is stable all the way down to about 2.0V. The manufacturer's 2.5V end voltage limit is conservative. 2.8V provides additional margin for self-discharge on top of that should it remain tripped for a long time without charging and, at some point, higher and higher a LVC just for the sake of looking "conservative" has to become idiotic.

Originally Posted by john61ct

But in daily use cycling, there are tremendous longevity cycle gains from not dropping to too low an **average** SoC point.

Bring up data to support this "tremendous" claim for prismatic cells and average discharge at low C-rates.

All the failed banks I have seen succumbed to either:
1/ Stupidity - no cell-level protection
2/ Incompetence - voltage limited, but no charge termination
3/ Beliefs and unsupported claims - weak charging and almost no capacity left in the end
4/ Bad cell - factory defect: replace and carry on.

Originally Posted by john61ct

If you are going to "oversize" a bit for that reason alone, or perhaps even more, just to have a decent reserve for suboptimal conditions, that is entirely rational. And up to each owner to make the judgment call, even if it is an "expensive" 40% capacity sacrifice.
As long as the vast majority of that sacrifice is off the bottom end, not the top.

Oversizing makes no sense either. It is called dead lithium or excess cash syndrome. In reality, installations don't cycles evenly every day. The depth of cycling varies all the time and going all the way down at times is part of normal operation.

[newhaul]

Here is a serious question concerning battery vs loads.
I currently have a 100ah 4s4p bank with approx 15 ah daily use now when I install the fridge that will triple. I intend on eventually having 200ah. The fridge is just over a year away. Should I just add the other 100ah now or wait in till I have the need power wise for it? Cell age matching ?
Or it really doesn't matter

[john61ct]

Originally Posted by OceanSeaSpray

the "98.69%" claim is garbage.

From Maine Sail's hard data, will search for the link.

> The weak-voltage stop-early scenario B fails to charge properly and the more incomplete recharge cycles the battery does, the worst this gets.

Pretty sure from a longevity POV an LFP bank is perfectly fine even if it never ever get close to full.

Especially if your balancing strategy is not at the top.

> Using an unduly high LVC just means that the system will trip when it still has usable capacity left if a heavy load is turned on

2.99V is not unduly high. I would put in a bigger bank to support such loads. As you say


> There is hardly any capacity difference between 3.0V, 2.8V or 2.5V,

>so claiming that cutting off at 3.0V is so much better and leaves "reserve" is nonsensical.

You are conflating two separate ideas there. I was very specifically **not** advocating any oversizing for longevity

I **was** saying some owners may choose to have a reserve at the bottom, for reasons like solar variability, up to them.

> The depth of cycling varies all the time and going all the way down at times is part of normal operation.

I choose to terminate at 2.99V.

[JmanC]

I have some questions if you could answer them all thanks.

Is there any big difference in charging speed with 13.8V vs. 14V vs. 14.6V?
If the charge rate is fixed say at 0.4C.

Do people find it's impractical to not float? If your bank is full my noon with solar & you terminate charge you are now draining the bank instead of having the sun supply much of the loads. I have read a few comments on other forums like Solartalk that you can float at 13.2-13.6V depending on battery without much degradation.

The sterling pro ultra charge goes into standby/conservation mode straight after absorption as far as I can tell, when battery voltage drops below 12.8V it re-enters float. This sounds like a Lead Acid setup & not lifepo4. So where is 12.8V under load in the SoC of a lifepo4 battery? Looks to me like it will drain a fair bit before charger re-enters float. ie by the time the sun is gone.


How should I use the Sterling Pro for Lifepo4? Lifepo4 preset #1 is only 13.8V with too high a float of 13.8, #2 is 14.6V-too high. But the custom preset by the looks holds absorption time way too long like 3+hours and no way to shorten. Some say use PS mode, but that is just one single voltage to work with.

If sterling adds a custom absorb time, will this be a firmware type update or you gotta buy a whole new unit deal?

[EmPrep]

Originally Posted by JmanC

I have some questions if you could answer them all thanks.

Is there any big difference in charging speed with 13.8V vs. 14V vs. 14.6V?
If the charge rate is fixed say at 0.4C.

Do people find it's impractical to not float? If your bank is full my noon with solar & you terminate charge you are now draining the bank instead of having the sun supply much of the loads. I have read a few comments on other forums like Solartalk that you can float at 13.2-13.6V depending on battery without much degradation.

The sterling pro ultra charge goes into standby/conservation mode straight after absorption as far as I can tell, when battery voltage drops below 12.8V it re-enters float. This sounds like a Lead Acid setup & not lifepo4. So where is 12.8V under load in the SoC of a lifepo4 battery? Looks to me like it will drain a fair bit before charger re-enters float. ie by the time the sun is gone.


How should I use the Sterling Pro for Lifepo4? Lifepo4 preset #1 is only 13.8V with too high a float of 13.8, #2 is 14.6V-too high. But the custom preset by the looks holds absorption time way too long like 3+hours and no way to shorten. Some say use PS mode, but that is just one single voltage to work with.

If sterling adds a custom absorb time, will this be a firmware type update or you gotta buy a whole new unit deal?

I'm interested in these answers as well given that my friend has an older Sterling Pro (Ultra) model which was sold to him on the premise that these same questions were but a thing of the past.

[transmitterdan]

Higher voltage delivers more current which charges faster. But saying 0.4C means current is fixed so not sure what you are asking.

Floating reduces battery life. So consensus is to not do that unless you have to. For a small bank you may have to float. With a large enough bank it won’t float until end of solar day which is about optimal. If it floats at noon and you need the solar power all day then increasing bank size would be the optimal solution.

[Delfin]

Originally Posted by JmanC

I have some questions if you could answer them all thanks.

Is there any big difference in charging speed with 13.8V vs. 14V vs. 14.6V?
If the charge rate is fixed say at 0.4C.

Quite a few variables, like the length of absorption, which will be longer at lower max voltages to achieve the same level of charge. For that reason, some, like me, choose to charge to 14.2 but terminate the charge when current acceptance drops to 2 - 5% of capacity. That is the fastest charge cycle I have found.

Originally Posted by JmanC

Do people find it's impractical to not float? If your bank is full my noon with solar & you terminate charge you are now draining the bank instead of having the sun supply much of the loads. I have read a few comments on other forums like Solartalk that you can float at 13.2-13.6V depending on battery without much degradation.

As Maine Sail and others have pointed out, what matters for Li batts is not the charge voltage, but the duration at max charge voltage. You can kill a Li bank with a lower voltage if the battery just sits there and cooks away, or charge to a higher voltage without harm if you stop the charge cycle when the target voltage and CAR is reached. The argument against floating is based on the fact that you can kill an Li battery at low charge voltages, but others argue that as long as the float voltage is below what the resting voltage of a fully charged battery is you should be ok. For mine, that would be 13.6 volts. However, there doesn't seem to be consensus on this point.

Originally Posted by JmanC

The sterling pro ultra charge goes into standby/conservation mode straight after absorption as far as I can tell, when battery voltage drops below 12.8V it re-enters float. This sounds like a Lead Acid setup & not lifepo4. So where is 12.8V under load in the SoC of a lifepo4 battery? Looks to me like it will drain a fair bit before charger re-enters float. ie by the time the sun is gone.

Assuming a minimal "load", 12.8 under load would be less than 20% SoC for my bank, but it is best to measure voltage at no load. The simple solution that some use to address all of the above is to use a small starter bank as the destination source for on-going charge capacity, like solar. Use the Li bank to the DoD you want, re-charge, then take it off line and let charge sources provide on-going power needs via a lead acid starter bank. Sun goes down, bring the Li bank online. Rinse and repeat.

[rgleason]

OceanSeaSpray wrote:

The weak-voltage stop-early scenario B fails to charge properly and the more incomplete recharge cycles the battery does, the worst this gets.

Isn't this FLA battery thinking? Title says "LiFePo4" I think you have your battery types mixed up!


FLA
- Sulfation if not charged to 100% frequently, say once a week, or equalized, say once every month or two.
- Healthy FLA require extra energy to keep at 100% and to combat sulfation.
- It is a pretty inefficient process. The closer to 100% you get while charging (above 80%SOC), the slower it goes and there tends to be more heat and loss of energy at these upper charge levels, depending on how fast your are trying to charge the batteries (actually the regulator has to back off, because resistance goes up and the voltages get too high and the amps being accepted drop lower and lower).
- There are real limits on charging rates, 25%C is max in bulk mode. In acceptance mode it goes down from there. The battery is the governor here, not the charge device. LiFePo4 has generally faster charge and discharge characteristics and is more efficient (CEF).
- FLA combined with solar is a good relationship! +++


LiFePo4
- Don't overcharge. Damage or loss of battery life.
- Don't float charge. Likely damage or loss of battery life.
- Don't charge to full, there is no benefit to this, due to no sulfation problem with these batteries. Not required!
- At the upper end small changes in Voltage make a big difference in charge, so best to leave a little room there for inaccuracies of sensing and charging devices.
- Lower end is similar. Don't let it go dead!!
- Also leaving 20-30%SOC in the battery may increase the number of life cycles (my understanding)
- These batteries are happiest left in the middle SOC! Say 30-95%SOC or 20-95%SOC if you don't care about having 5000 cycles and can accept 4000 or so.


This is my understanding.


Delfin wrote:

The simple solution that some use to address all of the above is to use a small starter bank as the destination source for on-going charge capacity, like solar. Use the Li bank to the DoD you want, re-charge, then take it off line and let charge sources provide on-going power needs via a lead acid starter bank. Sun goes down, bring the Li bank online. Rinse and repeat.

I like this setup. It's John's too (I think)

[Delfin]

Originally Posted by rgleason

OceanSeaSpray wrote:

Isn't this FLA battery thinking? Title says "LiFePo4" I think you have your battery types mixed up!

I think this question hinges on whether LiFePO4 batteries exhibit memory effects, resulting in lower capacity over time from under charging. Some research says this effect is real:

https://phys.org/news/2013-04-memory...batteries.html

Not knowing the answer, I follow the regime I described above, which takes the battery to 95-98% charged state (assuming one can rely on CAR as the best indicator of that state, and given lithium's charge efficiency, I think you can).

FWIW, when I polled the techs at Lithionics, they recommended re-charging "monthly" to 14.6 volts to trigger balancing via their built in shunting. However, they didn't seem to have a strong reason for why this was better than terminating charging at a lower voltage, so I'm only doing that annually under the assumption that a balanced pack really doesn't go out of balance all that easily.

[rgleason]

Delfin, I think your batteries are getting great care. Thanks for the explanation.

[CatNewBee]

Hi there, there is a lot of misinformation and fear regarding LFP charge regimes.

First of all, do you use your battery or are you talking about storage?

When using it, there is no problem with float at all. you stop charging between 14.4...14.6V, the battery is then almost (97%)/ full (100%). Charging current depends only on the inner resistance / capabilities of the charger, because impedance of LFP is very low, and it takes all current you throw at it. I float mine at 13.5V, what means the battery discharges to 94...95% to the loads on board before it then stays around 95% without delivering more current, all loads run then on solar alone as long as the sun yelds more power than needed, the battery does not take any current on float.
There is no overcharging at all, cell voltage is then around 3.375V.

The battery is cycled during the evening and night for the fridges, freezer, lights, entertainment, a/c, watermaker, cooking, hot water for the shower, whatever.

Most of the logevity posts are based on fear and misinformation / lack of understanding of basic principles of electric circuits, voltage and current.

It is true, that LFP cells like high currents and being cycled, they are more robust than FLA, and it is true, that for storage when not in use for a long time, they are happyer when staying around 50% SOC (pack voltage 13.2V, cell voltage 3.3V). But this is not relevant when you use them daily.