Lithium Battery Charging Question(s)

[Roniszoro]

A buddy just replaced his AGMs with lithium batteries. I have a charging question:
With my AGMs when sailing off shore, I use them until they are at about the 55-60% level and then charge until they are at about the 85% level. I don't go beyond the 85% level, as it is my understanding that the last 15% takes much longer than the mid-15% level, as absorption is at a much slower rate as one get closer to "full". Is that true for lithium batteries as well? I had heard than one could (or perhaps should?) use the lithium batteries down to 30% or even less before charging, but have no idea if that is (1) true and (2) if absorption of the charge diminishes as one approaches 100% like it does with AGMs.


Thank you in advance for your input.

[MWGDVC]

Roniszoro,
I made the switch to lithium last year, since my lead acid batteries were in need of replacement. Regarding how far to take them down before recharging, there seems to be many opinions. I do know one of the features of LiFePo is that unlike lead acid batteries the internal resistance does not increase as the state of charge increases. This allows a more efficient charging of the batteries by my solar panels. Stated simply, I get to keep more of the harvested amps, as they aren't expended getting in the battery.
Another plus is the voltage remains nearly constant anywhere from 100% to 20% state of charge, so no dimming lights, or gremlins in microprocessor controlled devices due to low voltage.
The down side is many older charge systems need to be changed to supply the charge profile for LiFePo.

[SOLAR SUPPORT]

Lifepo4 chemistry has a very good efficiency, %100. Forget Phuckert effect. They don't like to be topped up when not in use unlike lead-acid batteries. 1/2 of the Ah capacity does the job when compared with LA. Always bulk charge no equalisation or float phases. You need to protect your charging devices against HVCutoff and LTCutoff events controled by the builtin BMS in the battery. Their charging Amps are too high. So you need to control alternator charge with an external regulator or by adding a dc-dc charger between the Lifepo4 batteries and the alternator. Other wise you cook your regular alternator in minutes..

[PaulCrawhorn]

First off yes lead takes a very long time to get to 100%, but for longevity you need to try to get there most cycles. Solar is good for that, maybe after using FF sources first in the morning to get to 85-90%.

Next , with LFP their acceptance rate is so high yes all the way to Full can be very quick, even less than one hour if high enough current made available. In fact current pulled by the bank may need to be limited.

But not only is there never any need to get to Full, it is better for longevity if you don't. Certainly don't let them sit there long.

[Pete7]

I was astonished how quickly our LFP can charge up able to take everything we can throw at them in terms of charging. Engine and solar together? not a problem. We do however, avoid charging the last little bit often called "the knee". This is nearly full and there is only a tiny bit of extra capacity to be gained using it. Instead 90 - 95% will do nicely we really don't worry about it so long as we are going to use it straight way.

Before leaving the boat we boil the kettle a couple of times to drop the state of charge (SOC) down to 60-70% or so before isolating the LFP so they are not on float.

[SOLAR SUPPORT]

There must be a relationship between the Lifepo4 service battery usage regime and battery life on boats. It is not possible to use these batteries on boats by first fully charging and then discharging them, as in EVs. Then, because we use these batteries as service batteries on boats, could their service life be negatively affected? Lifepo4 service battery has to be charged and discharged at the same time on the boat. This is usually the case during the daytime. Since solar energy production stops in the evening, it is only discharged until sunrise. Since the difference between daily electrical energy production and consumption is positive, my lifepo4 batteries are fully charged in 4-5 days. After this process is completed, I turn off the solar energy for 1-2 days. Lifepo4 service battery is only discharged until it reaches 20% SOC level in this process. I try to keep the charge of Lifepo4 batteries at 60% when the boat is not in use. I am not using alternator charging. I hope that Lifepo4 batteries will not disappoint boat owners in terms of life when used as a service battery.

[StuM]

Originally Posted by SOLAR SUPPORT

Lifepo4 service battery has to be charged and discharged at the same time on the boat. This is usually the case during the daytime.

That's either very poorly worded or wrong. A battery can't be "charged and discharged at the same time".

If the output from solar meets the needs of the house loads, it will supply the house loads and there will be no discharge from the batteries.
If the output from solar exceeds the needs of the house loads, it will supply the house loads (there will be no discharge from the batteries) and the excess energy will go to recharge the batteries if they are depleted.
If the output from solar doesn't meet the needs of the house loads, the additional energy required beyond what is supplied by the solar output will be supplied by the batteries.

After this process is completed, I turn off the solar energy for 1-2 days.

Why on earth would you do that. You are needlessly cycling your batteries and will be reducing their life.

[goboatingnow]

Lithium battery technology has several factors affecting cycle life , ie the number of charge discharge cycle you will get before battery capacity drops below 80 %

So assuming your abiding by the charging stipulations

1. particular Li chemistry , LFP has one of the best cycle counts of all Li tech , > 3000 cycles and many capable of >10000 cycles

2. Charge and discharge rates , keeping both under 1C has significant benefits and <= 0.5C is best

3. Depth of discharge , ( DID) the lower the battery is allowed to be discharged to has a significant effect on cycle life

4. Short cycles affect cycle life but the effect in LFP is minimal

5. Internal parasitic actions cause Li to age just sitting there


LFP in a boat is likely to well outlast EV battery usage , EVS are subjected to very high DOD , very high charge and discharge rates and extremes of temperature

Whereas on a boat , typically charge rates will be well below 0.5 C on a decent size bank , and average discharge current will be fractional C in the main. With sufficient recharge, DOD is often quite small , say 30% over night

Hence the portents for LFP boat batteries is excellant and assuming you don’t expose them to (a) DOD > 80 % , overcharging , or long periods at close to 100%SOC , then one could reasonably expect 10 years life span to 80% SOH ( state of health ) even more perhaps. This is well beyond even high quality Lead Acid

[Stu Jackson]

Read this, please:


https://marinehowto.com/drop-in-life...ated-consumer/

[goboatingnow]

Originally Posted by SOLAR SUPPORT

There must be a relationship between the Lifepo4 service battery usage regime and battery life on boats. It is not possible to use these batteries on boats by first fully charging and then discharging them, as in EVs. Then, because we use these batteries as service batteries on boats, could their service life be negatively affected? Lifepo4 service battery has to be charged and discharged at the same time on the boat. This is usually the case during the daytime. Since solar energy production stops in the evening, it is only discharged until sunrise. Since the difference between daily electrical energy production and consumption is positive, my lifepo4 batteries are fully charged in 4-5 days. After this process is completed, I turn off the solar energy for 1-2 days. Lifepo4 service battery is only discharged until it reaches 20% SOC level in this process. I try to keep the charge of Lifepo4 batteries at 60% when the boat is not in use. I am not using alternator charging. I hope that Lifepo4 batteries will not disappoint boat owners in terms of life when used as a service battery.

Your approach is not optimum

There is a trade off between short cycling and depth of discharge ( DOD) , for LFP battery life cycles are affected much more by high DOD cycling , then short cycles ,

Given an upper charge limit of 80% for long life , it’s much better to always prevent discharges from occurring where a recharge source is available rather then allowing high DOD to occur before recharging. Hence LFP is better subject to short recharges ( and consequently short discharges ) then allowing high DOD to occur before recharge.

You are correct that absorption or float charging Lithium is not a good idea , there is no practical absorption phase in lithium , so in effect Constant Current charging is the order of the day with a removal of charge sources at the designated charge stop point.

And as you say , for long term storage such cells should be kept at around 60 % SOC to reduce internal stresses and parasitic actions.

[UFO]

Originally Posted by Pete7

We do however, avoid charging the last little bit often called "the knee". This is nearly full and there is only a tiny bit of extra capacity to be gained using it. Instead 90 - 95% will do nicely

What Voltage would you call Fully Charged? I have ours set to 13.6V as 100% charged (at which point the BMS turns all charging sources off), but we happily sit for weeks at a time at around 90-95% or less - Especially in Queensland at the moment which all it seems to do is rain or be over cast

[SOLAR SUPPORT]

Originally Posted by StuM

Originally Posted by SOLAR SUPPORT View Post
Lifepo4 service battery has to be charged and discharged at the same time on the boat. This is usually the case during the daytime.
That's either very poorly worded or wrong. A battery can't be "charged and discharged at the same time".

My dear sailor friend, the use of Lifepo4 batteries in EVs is quite different from the use of Lifepo4 service batteries in boats. Discharged EV batteries are continuously charged with constant current. This is the case at least until you reach 90% SOC. On our boats, on the other hand, it is not possible to keep the charging current constant at a certain level and to charge them with constant current, especially since the changing weather conditions in the solar energy system and the angle at which the sun rays reach the panels are constantly changing. In addition, it is not correct to think that the energy going from the solar panels to the batteries during the day will always be more than the energy consumed instantly. Because both energy production and energy consumption change during the day. Therefore, while the lifepo4 battery is being charged during the day, under variable energy production and consumption conditions, it may switch to discharge while being charged frequently. Therefore, the charge-discharge status of the lifepo4 battery may change very frequently depending on the balance of daily energy production and consumption. For example : Consider the operation of an electric bread maker running for two hours in the middle of a sunny day during peak solar hours. While the 12 Volt system is charging with 30 A, some of the 120 amperes needed by this device, which is fed from the inverter, is met from the solar energy and the rest from the batteries. Since the device will keep the temperature constant while baking the bread, it turns the baking resistance on and off continuously. In order to say whether the lifepo4 batteries used in boats will perform as long as expected in EVs under the variable consumption and production conditions I have mentioned, I think it is necessary to continue to observe for another four to five years.

[StuM]

Originally Posted by SOLAR SUPPORT

...while the lifepo4 battery is being charged during the day, under variable energy production and consumption conditions, it may switch to discharge while being charged frequently.

Yes, that's what I said (more succinctly)

In order to say whether the lifepo4 batteries used in boats will perform as long as expected in EVs under the variable consumption and production conditions I have mentioned, I think it is necessary to continue to observe for another four to five years.

Everything I've seen says rhat they will perform longer if kept at a higher SOC. I'm fairly sure that repeatedly dischargng to 80% DOD and then going through that charge/discharge regime until fully charged will put more stress on the batteries than keeping them constantly at a higher SOC with a smaller cycling range.

Here's one good reference:
https://www.grepow.com/blog/what-is-...po4-batteries/
"The “cycle life” of a battery or the number of charge/discharge cycles in its life depends on how much battery capacity you typically use. If you regularly discharge the battery with a lower percentage of charge, it has more useful cycles than often draining the battery to its maximum DoD. For example, a battery may have 15,000 cycles at a DoD of 10%, but only have 3,000 cycles at a DoD of 80%."

And here's a graphic illustration of the same effect
https://www.powertechsystems.eu/home...phate-lifepo4/

Lithium Iron Phosphate Life Cycle

It certainly looks as though repeatedly turning off your solar and letting your batteries get down to 80% DOD wil shorten their life considerably (and if keeping them on charge as much s possible they should last a lot longer than they do in an EV with constant deep discharges.)

[SOLAR SUPPORT]

I must state that I am satisfied with the performance of Lifepo4 batteries. After using LA batteries for many years, the performance gap should be mentioned. I can say this with certainty because at least every month I test the active capacities of Lifepo4 batteries with my fixed Columb Meter. Two months ago, I performed this test after the first assembly. I have definitively confirmed the total battery capacity of 400Ah using powerful electrical devices that draw 80-100A. Since the coloumb shunt is annular, the Solar negative line and the common negative line pass through here. The Columb Meter works with astonishing precision and calculates the difference between consumption and production, showing the remaining battery capacity. I will complete the third test soon. I'll be posting the results here intermittently.

[fxykty]

Originally Posted by StuM

Yes, that's what I said (more succinctly)
Everything I've seen says rhat they will perform longer if kept at a higher SOC. I'm fairly sure that repeatedly dischargng to 80% DOD and then going through that charge/discharge regime until fully charged will put more stress on the batteries than keeping them constantly at a higher SOC with a smaller cycling range.

Here's one good reference:
https://www.grepow.com/blog/what-is-...po4-batteries/
"The “cycle life” of a battery or the number of charge/discharge cycles in its life depends on how much battery capacity you typically use. If you regularly discharge the battery with a lower percentage of charge, it has more useful cycles than often draining the battery to its maximum DoD. For example, a battery may have 15,000 cycles at a DoD of 10%, but only have 3,000 cycles at a DoD of 80%."

And here's a graphic illustration of the same effect
https://www.powertechsystems.eu/home...phate-lifepo4/

Lithium Iron Phosphate Life Cycle

It certainly looks as though repeatedly turning off your solar and letting your batteries get down to 80% DOD wil shorten their life considerably (and if keeping them on charge as much s possible they should last a lot longer than they do in an EV with constant deep discharges.)

Not according to https://nordkyndesign.com/charging-m...battery-banks/, so I guess we have a case of competing experts, sigh.

To address two points, Nordkyn says:
1) Storage is best at very low voltages, about 3.2V per cell. With our battery that is ~20% SOC. 60% SOC is really high for storage.
2) Charge to 80% SOC (and to 100% SOC once a month to recalibrate your BMS and to allow cell balancing to take place if needed), then turn off all charging and discharge “meaningfully” before re-enabling charging. Meaningfully is not defined but in context certainly means deeper than only discharging 10-20% SOC. There is no problem discharging to 30% or 20% or even lower on a regular basis, the same way it is not a problem to charge to 70% or 80% on a regular basis. The key to lithium batteries is to avoid the knees - the final 5% at either end.