Lithium Batteries (for the rest of us)

[OceanSeaSpray]

Originally Posted by ebaugh

Hmmm...the results from that trip are not consistent with my charge data unless there is a long period of CV at 3.35. Thinking about it, I'm now curious.

I can't duplicate the temperature, or the current profile exactly like that trip. My alternators are not finely regulated, ie. loads like the microwave force a discharge, then they catch back up. But I am interested enough to explore 3.35V in the lab so to speak. I will let you know what it looks like. I will also recheck the cell voltage readings on my Elite Power BMS. As I recall it indicates a little low, so 3.35 might be closer to 3.37. Makes no difference at 3.6 or 2.5, but would mean quite a lot here.

I also don't remember the SOC at the beginning of the trip. If fully charged that would have an impact on the observed behavior too. I may have mentioned it in a post here at the time, I will go look. The Forzatecs came out of the box at 3.25-3.3 and were over 95%, don't ask me why they shipped them at 100%, I don't know.

Really appreciate the info and effort put into it. 95% for 3.25-3.3V/cell rested aligns well with published curves. I guess they want to ensure a longer shelf life without maintenance recharge.
Bench data is probably best than boat data here. Longer current taper at 3.35V/cell makes a lot of sense. There will be a "low" voltage still leading to a full charge over time, anything above only speeding up the path to 100% SOC, and there will be lower voltages giving 95%, 90% etc.

[goboatingnow]

Really appreciate the info and effort put into it. 95% for 3.25-3.3V/cell rested aligns well with published curves. I guess they want to ensure a longer shelf life without maintenance recharge.
Bench data is probably best than boat data here. Longer current taper at 3.35V/cell makes a lot of sense. There will be a "low" voltage still leading to a full charge over time, anything above only speeding up the path to 100% SOC, and there will be lower voltages giving 95%, 90% etc.

I dont understand this, why would a difference from 3.3 to 3.35 cause a long current taper. In my tests its a function of fast charging rather then termination voltage

dave

[ebaugh]

Originally Posted by goboatingnow

I dont understand this, why would a difference from 3.3 to 3.35 cause a long current taper. In my tests its a function of fast charging rather then termination voltage

dave

I think he meant terminating at 3.35 versus 3.55 or 3.6 causing a taper at C/3 that's not observed at the higher settings.

[ebaugh]

Originally Posted by OceanSeaSpray

Really appreciate the info and effort put into it. 95% for 3.25-3.3V/cell rested aligns well with published curves. I guess they want to ensure a longer shelf life without maintenance recharge.
Bench data is probably best than boat data here. Longer current taper at 3.35V/cell makes a lot of sense. There will be a "low" voltage still leading to a full charge over time, anything above only speeding up the path to 100% SOC, and there will be lower voltages giving 95%, 90% etc.

I would hypothesize that you can't make a voltage setting precise enough to "float" at a fixed SOC. Even the Chinese presentation you posted might allude to this based on the .02V difference between a resting voltage for charge versus discharge. None of the chargers I've seen are quite that well regulated, especially in a system with some variable loads. It's impossible anyway unless the charger has a separate voltage sense wire to the battery, and few do. Holding a fixed 80% SOC might be like riding a 20 ft high unicycle...

Now say holding 80-90% is probably doable, but that brings back the issue of mini-cycles Dave has been preaching to us about for some time now....I'm just finally coming around to see it.

[goboatingnow]

Originally Posted by ebaugh

I think he meant terminating at 3.35 versus 3.55 or 3.6 causing a taper at C/3 that's not observed at the higher settings.

But at c/3 has anyone seen any significant taper time. I certainly haven't, which is why , way back , I said that voltage cut-off alone was an acceptable method of terminating Li , low C charging , as experience has shown that there is little or no taper, i.e. no significant Ah needs to be added after voltage cutoff.

Dave

[T1 Terry]

In fact the pulse effect from a PWM controller and a reasonable size solar system speeds the final charging up considerably. Most good quality PWM controllers float the on/off at 0.5v either side of the target voltage, add a load that switches in and out like multiple fridges cycling and constant loads like the hot water cyl heating element, lights, little transformer type charger for a remote battery, my computer and modem, my pack would mini cycle all the time the sun is out, no sign of a problem yet.

As far a current tapering as the battery reaches 100% SOC, maybe the bit between 99% and 100%, but if it was plotted as a curve it would look much like the glide angle of a brick thrown horizontally from the roof of a tall building.

As far as I can see it, the mini cycling would only be a problem if the end voltage was set too high, there is no evidence of cycle life deteriorating in prismatic cells if the voltage remains below 3.4v per cell and above 3v per cell and the charge rate/discharge rate is below 0.5C no matter how often the cells are cycled. Heating the electrolyte is the killer, that only occurs with over voltage/under voltage, heavy charging and discharging will cause the voltage level to rise/fall, so limiting the voltage parameters is still the key to long cell life.

T1 Terry

[ebaugh]

Originally Posted by T1 Terry

Heating the electrolyte is the killer, that only occurs with over voltage/under voltage, heavy charging and discharging will cause the voltage level to rise/fall, so limiting the voltage parameters is still the key to long cell life.

T1 Terry

Or perhaps heat in a 119 degree engine room? I'm not so sure the cells are as tolerant to heat as the Thundersky/Winston specs indicate. But it could be the chemistry variation of my GBS cells.

Even so, they still look much longer lasting than Pb alternatives. Just not as long....

[OceanSeaSpray]

My feeling when it comes to longevity is keep them at reasonably low temperatures, and don't push current in that can't be absorbed. I have never seen any relevant data about harm from mini-cycles that didn't result in repeated voltage peaks and heating on recharge. I am going with Terry.
PWM voltage control really refers to fast switching between very tight limits to maintain a constant voltage output, if the limits are further apart, it becomes a floating point charger. Same principle, same operation, longer timeframe.

Finding the right upper voltage limit(s) is what we have been talking about. For the lower limit, I suspect the difference could be that if the boat is used, recharging should/could kick in earlier than if left unattended for a long time, so two lower voltage settings? Or a smart algorithm, no consumption for one day and drop into the other mode?

If the bank is deemed to be in "storage", then it could be allowed to deplete a long way down, greatly reducing overall cycles and time spent at a high SOC. 50% SOC can't be sensed reliably based on voltage (curve too flat), but 30-40% can, so recharge from there to a suitably low end of charge voltage? There could be months between recharges.
There will be a strategy to make them last on boats, but I would be concerned about accelerated aging if the bank is in an engine room that is hot a lot of the time. A pleasure boat might not steam often enough for this to be an issue however (if gensets don't cook up the place daily). On commercial vessels, watch out.
I am going to install my cells in the bilge, because it is at sea temperature there.

[goboatingnow]

All technical papers seem to agree that in Li capacity retention over time is a function of cycles.( outside of abuse) This is more so then other technologies. On top of that the battery simply ages even if doing nothing.

All agree that shallow ( 50-70%) discharges have less effect then deep ( >80%) discharges. But every paper Ive read essentially says that consuming cycles results in loss of capacity. ( its doesnt mean the battery has failed just its in effect smaller).

Hence attempting to minimise mini-cycles, i.e. unnecessary discharge recharge situations would seem to suggest that reductions in capacity would be reduced.

The fact that the battery does through 1000 cycles isn't the issue, what we need are loss of capacity over time against charge discharge cycles to come to a conclusion. The research suggests it is a factor .

dave

[goboatingnow]

interestingly heres a comment from i believe someone with li on board for a while ( 3 years ?)

I had one cell die on the way back from Hawaii.

The company I bought the battery from went out of business and I had an issue getting it replaced. Ended up with one 180 AH cell.

Even with a low voltage cut off, the battery will go dead if sitting at the dock for too many weeks.

It seems to be very important to keep them charged. I don't think I will get 10 years out of them.

It is also hard on alternators as it draws so much current. So you will need a regulation system.

dave

[noelex 77]

My LA batteries have not seen a battery charger input for two years.

Living on solar mini cycles are very common. Every time the sun goes behind a cloud, a high load device is turned on, or a significant shadow like the mast falls on the panels we are likely to get a net discharge. Many cruising boats are in the same situation and it would be very difficult and inefficient to manage the system to significantly reduce these cycles.

If define a cycle by the strict definition of going from discharge to charge we have experienced over a hundred cycles today alone as my wife has been sewing with short high demand bursts that exceed our solar output. It is hard to imagine these very short cycles having much influence, but perhaps it is prudent to turn off the solar input with lithium batteries during this sort of activity. This still leaves many cycles per day that would be very difficult to eliminate.

We know how LA batteries cope with this sort of treatment. It is will be interesting to see how the lifespan of lithium is effected. I hope not significantly.

[T1 Terry]

All the data I have read so far relate to cycles being to a certain depth of charge, not just cycles. Unless heat is being produced at the end of each charge cycle causing the electrolyte to degrade, I can't see any reason why mini cycling would effect the over all cycle life, the lithium ions don't wear out, each cycle is not causing a chemical change, just lithium ion transfer in and out of the electrolyte. Any coating build up on either plate is a result of electrolyte break down, heat is what causes that break down. Maybe stored in a high heat area could be a problem, Winston suggest 60 degC is the upper limit, but heat soak through the battery box and then through the plastic casing would take a while. If the engine room gets very hot, a fresh air blower through the battery box while the engine or gen set is running would solve any issues. Maybe the battery box in a sea water cooled wet jacket for an excessively hot engine room.

The blog piece about the cell failing in a 3 yr old battery pack doesn't have any indication of what the charging parameters were for the cells or if cell voltage was monitored. This still leaves the question, did the cell die of natural causes or was it murdered?

T1 Terry

[goboatingnow]

Well T1 Terry , given your comments thats heat causes failure, would you comment on the fact that Li just dies slowly sitting on the shelf Thats a proven situation. Li death just isn't just heat buildup related.

We know how LA batteries cope with this sort of treatment. It is will be interesting to see how the lifespan of lithium is effected. I hope not significantly.

Yes we know that one of the advantages of LA is that life cycle isn't a function of small discharge cycles .

What we also know is that the capacity retention of Li is affected by cycles. WHat we don't have a handle on , is the degree of capacity loss. I mean if you suffer 10% of capacity say over 3 years on a 1000Ah bank , you're unlikely to be bothered. If you suffered 50% , things might be different.

dave

[T1 Terry]

I haven't heard of good Li prismatic cells dieing on the shelf. 50% of my test pack was 3 yrs in the box before I charged them and put them to work, there are over 800 cycles on them now, that's proper cycles, no doubt thousands of mini cycles or more, 3 fridges each cycling at their own rates and at any time all 3 could start up at the same time so a few big loads well over 0.5C and they are still going fine, so I can't say I've witnessed it but if you have witnessed it, tell us about it.

T1 Terry

[familycruisers]

So much conjecture on Lithium Ion. I have about six years experience with large voltage, high c discharge application. Typically using lipo, but also had the opportunity to test lifepo4 cells.
From what I've seen, don't charge above max cell voltage, don't discharge below min cell voltage and you'll bee just fine. They last a long time and even when they don't have the punch they had, they still are better than any alternative.
Now, to boats.
If you can't actively balance charge and have the charger monitor voltage, time, temp, etc, like the hobby industry already does, then you need to leave yourself headroom.
If the packs can be heated by exterior sources, ie engine, genny, sun, etc, you must leave headroom.
I don't cotton to the mini cycle theory being a detriment to longevity, just never exceed the limits in voltage, period. They will last a long time.

My setup
Midnite classic 150
510watts solar
4- 180ah calib cells

Replacing two group 31's that were barely maintaining the frig
all night.
So, lighter, less space, more usable capacity, and my controller is already setup to discontinue forthe lithium crowd) our can just be programmed to use a low voltage cut in, basically disabling charger after float is achieved (which is whatever the user has determined to be a safe number with lots of headroom).
Time will tell
thats my.02