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Old 29-09-2018, 20:20   #1
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Good thesis on preserving the life of LiFePo4 lithium batteries

This is a good examination of the detriments of high temperature and storing lithiums at a high SOC.

https://uwspace.uwaterloo.ca/bitstre...pdf?sequence=3


The study focused on LiFePo4 batteries in electric cars, but the results appear applicable to house batteries on boats, so far as aging under varying storage conditions is concerned.

My own lithium battery is stored at the coolest location in my cabin (it's never colder than +10 degrees C here) and I store my battery at 20% SOC. That is the SOC where my lithium battery is automatically taken off the bus and my AGMs take over. (I only use the range of 20% to 80% SOC.) I've just installed a breaker to keep the lithium off the bus when I return to the dock so only the AGMs are immediately recharged.

I charge my lithium at the dock before going out, not after coming back. I have a small one amp 12 volt charging source (a "wall wart" supply) that won't allow the lithium battery terminal voltage to drop below 11.7 volts (12 volts minus the forward drop through a schottky diode) to prevent self-discharge from ever discharging the battery too low at the dock.

I know of people who store their lithium fully charged by just firing up the charger on shore power when they return, and I don't believe that is a good practice. They don't have to wait for their battery to charge - but they are sacrificing battery longevity with that storage practice.
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Old 29-09-2018, 22:44   #2
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

Yes absolutely right about storage, at least in general terms.

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I only use the range of 20% to 80% SOC
How do you define those points?

For that matter how do you define your 0% and 100% points?

> that won't allow the lithium battery terminal voltage to drop below 11.7 volts (12 volts minus the forward drop through a schottky diode)

That 12V is my usage 0% too, but I would not rely on any device with a significant voltage drop, since that will change depending on current levels.

My usage 100% is when charging reaches 13.8V, but that point is only a few AH below where the vendor defines it at much higher voltage, and still yields a higher than rated AH capacity.

And the 12V point is not many AH above dangerously low at 10-11V, far lower than 10% in practice.

My point is, I doubt that 20-80% is actually true in AH terms, there is no need to sacrifice anywhere close to 15% to avoid the shoulders, much less your 40%.
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Old 29-09-2018, 23:43   #3
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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Yes absolutely right about storage, at least in general terms.

How do you define those points?

For that matter how do you define your 0% and 100% points?

> that won't allow the lithium battery terminal voltage to drop below 11.7 volts (12 volts minus the forward drop through a schottky diode)

That 12V is my usage 0% too, but I would not rely on any device with a significant voltage drop, since that will change depending on current levels.

My usage 100% is when charging reaches 13.8V, but that point is only a few AH below where the vendor defines it at much higher voltage, and still yields a higher than rated AH capacity.

And the 12V point is not many AH above dangerously low at 10-11V, far lower than 10% in practice.

My point is, I doubt that 20-80% is actually true in AH terms, there is no need to sacrifice anywhere close to 15% to avoid the shoulders, much less your 40%.
I have a 100 A/H battery. I defined 100% SOC initially by charging with a constant voltage of 14.6 volts until the charge current reached 0.5 amps (0.05C) and the open-circuit cell voltages were 3.65 volts each. I then calibrated my battery monitor at "100%". Every few months, I recalibrate (and verify cell balance). Between recalibrations, I use the monitor to measure amps in and out. I define 80% SOC as 20 amps below a full charge, and similarly 20% SOC as 80 amps below a full charge. The accuracy of this method using the monitor's amperage totalizer is +/- 5% of the results measured at recalibrations.

Voltage means almost nothing as it pertains to a full charge. Given enough time, you can overcharge a 12 volt lithium with a constant-voltage recharge of 13.65 volts. It's amps in and out that matter. You need an amperage totalizer to do this (any good battery monitor using a shunt with the peurkert ratio set to 1.00). I do not cut off either the load or charge using voltage - I use the amperage totalizer instead to set the parameters, with backup high/low voltage parameters of 13.8 (about 80% SOc) and 12.7 (about 30% SOC) volts in case something goes wrong with the totalizer measurements (that hasn't happened yet over a few hundred cycles).

Using voltage alone to measure 100% SOC is a major engineering error in my opinion. The battery can be at 100% SOC with a voltage of only 13.65 volts under conditions of a sustained low current charge -- such as when charging with solar panels. Continuing to charge in those conditions up to 13.8 volts will severely degrade the battery over time.

My goal is to get more than 2,000 cycles out of the battery. That's why I don't take it anywhere near the "knees." I have twice the battery capacity I need, so I can afford being very conservative by using only 60% of its capacity. Being this conservative also allows me to avoid using cell balancers because I'm no where near the knees. My system is far less complex without cell balancing, I don't have a cobweb of wires on top of my battery, and complexity is the enemy of reliability. Trying to squeeze out those few final amp/hours adds too much complexity for my taste.

So far as the diode is concerned: I may not have explained its purpose clearly. The diode is merely a steering diode in an auxiliary charging path to preserve the battery during long term storage. Normally, that diode is reverse-biased by the higher battery voltage and passes no current at all.

I agree that 11.7 volts is low. But it's only a safety net for long term storage preservation. And it's only there in case I don't use the boat for a very long time (in which case my heirs have probably taken custody). The measured self-discharge rate so far has been 1% per month (it's that low because I don't have cell balancers eating power), so I've got 30 months to recharge the battery.
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Old 30-09-2018, 02:37   #4
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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I use the monitor to measure amps in and out. I define 80% SOC as 20 amps below a full charge, and similarly 20% SOC as 80 amps below a full charge. The accuracy of this method using the monitor's amperage totalizer is +/- 5% of the results measured at recalibrations.

Voltage means almost nothing as it pertains to a full charge. Given enough time, you can overcharge a 12 volt lithium with a constant-voltage recharge of 13.65 volts. It's amps in and out that matter. You need an amperage totalizer to do this (any good battery monitor using a shunt with the peurkert ratio set to 1.00). I do not cut off either the load or charge using voltage - I use the amperage totalizer
...
Trying to squeeze out those few final amp/hours adds too much complexity for my taste.



Please, before advising others - learn the difference between amps and amp hours and use te correct units.
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Old 30-09-2018, 02:44   #5
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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That 12V is my usage 0% too, but I would not rely on any device with a significant voltage drop, since that will change depending on current levels.


The beauty of a diode is that the voltage drop is a constant regardless of current, so a good pick in the application.
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Old 30-09-2018, 03:12   #6
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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Please, before advising others - learn the difference between amps and amp hours and use te correct units.
I used the term "amps" for the sake of brevity.

Is there something else you would like to contribute?
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Old 30-09-2018, 09:47   #7
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

I agree that the distinction is important - AH is in fact shorter than amps - but the (very common) misuse does not in itself signal incompetence.

That 14.x setpoint at the top is pushing well into the top knee, and will drastically reduce longevity if hit regularly.

Stopping at ~13.8V is much gentler, and will only sacrifice a few percent capacity at most. No absorb at all needed either, unless when precise calibration needed, and then .03-.05C will do fine.

Also helps prevent imbalance issues
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Old 30-09-2018, 09:49   #8
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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The beauty of a diode is that the voltage drop is a constant regardless of current, so a good pick in the application.
I have not found that to be the case, but apparently irrelevant here anyway.
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Old 30-09-2018, 09:56   #9
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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I used the term "amps" for the sake of brevity.

Is there something else you would like to contribute?

I think the term was misused as well.
I took it as you terminating charge when the voltage reached 3.65 and the charging current at that time was 20 amps.
Imho, that was a reasonable assumption.


I agree with John about diode voltage drop. Here's a diode voltage vs. current chart:



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Old 30-09-2018, 10:22   #10
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

That's the current change during a voltage switch, which is normal. For PN diode used in a forward biased voltage drop, it's ~0.7V.

I'm curious of the schottky selection vs. a typical diode. We normally use Schottky for their fast transitions in switcher supplies, and depending doping the voltage drop can be less than the ~0.7V. But their could be a design reason that has yet to be revealed, like "its what I had lying around".
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Old 30-09-2018, 11:58   #11
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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I think the term was misused as well.
I took it as you terminating charge when the voltage reached 3.65 and the charging current at that time was 20 amps.
Imho, that was a reasonable assumption.


I agree with John about diode voltage drop. Here's a diode voltage vs. current chart:




OK, sure, that's correct when reverse biased and before the diode goes into saturation. But in the context we are talking about, namely a diode-based battery combiner and the challenges of voltage sensing on the same, it's for all intents and purposed a constant voltage drop. In that sketch, it's operating in zone I on the right side of the curve, where the current line is straight up.


If you really want to split hairs, there is internal resistance in the diode, connecting wires, terminals, etc and those will create voltage drop proportional to current, just like all the interconnects if you don't have a diode in the ckt.
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Old 30-09-2018, 12:09   #12
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

Just use a VSR / ACR combiner for that purpose.

Or an Echo Charger to Starter if all significant sources are direct to House as they should be.
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Old 30-09-2018, 12:38   #13
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

Versus a single diode? No thanks, the OP has simple solution and simple works on a boat. We can split hairs on which type of diode, but for basic protection they are hard to beat wth that simplicity.

Plus it's hard to program a diode incorrectly.
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Old 30-09-2018, 12:42   #14
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

None of the devices I mentioned have any adjustability.
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Old 30-09-2018, 13:43   #15
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Re: Good thesis on preserving the life of LiFePo4 lithium batteries

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That's the current change during a voltage switch, which is normal. For PN diode used in a forward biased voltage drop, it's ~0.7V.

I'm curious of the schottky selection vs. a typical diode. We normally use Schottky for their fast transitions in switcher supplies, and depending doping the voltage drop can be less than the ~0.7V. But their could be a design reason that has yet to be revealed, like "its what I had lying around".
A schottky diode has half the forward voltage drop of a typical PN junction diode. On the order of 0.2 to 0.4 volts.

That is why you often see them used as steering diodes in PV systems: less voltage drop = less power lost (about half) for a given amount of current flow, when compared to ordinary PN junction diodes, as referenced above. They are also used in battery bus combiners for the same reason.

When dealing with low voltage systems (12 volts) any voltage drop becomes very relevant.

Schottky diodes have the disadvantage of higher reverse-biased leakage. But a few milliamps is often irrelevant in practice.

Here's a good brief article on the topic: http://www.electricalbasicprojects.c...unction-diode/. More details here: https://en.wikipedia.org/wiki/Schottky_diode
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