Alternator Size with LiFePo4

[Q Xopa]

Originally Posted by john61ct

Well to me, just buying packaged systems and following vendor charge specs would be "most conservative".

I'm all for Science Experiments, especially if expanding knowledge for the community, but in the back shed if not as safe and reliable as they need to be

For production use in a boat or other small off grid living space I'm pretty risk-averse. But not perhaps as much as the professionals selling packaged systems advise.

I guess Battery cell vendors charge specs arent included in that statement about being the most conservitive. I though I heard you say they are all too high, or something to that effect.

[Maine Sail]

Originally Posted by Q Xopa

I think John has asked this, but how do you do the 'stop at .03 - .025'?

When you know your net charge current, accounting for house loads, you can now count how long it takes from the time you hit the target voltage until the time current drops to your desired tail cut off. You now set the absorb duration to do that. I have been doing this with the Balmar MC-614 for many years and well over 1000 cycles...

[john61ct]

Originally Posted by Q Xopa

I guess Battery cell vendors charge specs arent included in that statement about being the most conservitive. I though I heard you say they are all too high, or something to that effect.

Yes they are way too high, and many members here think that is a radical position.

I was demonstrating that the word "conservative" has many connotations too open to interpretations.

Once it is established that all sides agree on

"maximizing longevity is a high priority compared to getting to a high SoC" and "avoiding the voltage shoulders promotes longevity",

then I suppose "conservative" as a positive stance, can be used in contrast to (too) aggressive wrt charge profiles.

But otherwise, people might associate "conservative" with "unquestioningly following established mainstream authority".

I understood you to mean that my support of Open Source non-commercial devices shows I'm not conservative, in the latter sense.

[Q Xopa]

Originally Posted by Maine Sail

When you know your net charge current, accounting for house loads, you can now count how long it takes from the time you hit the target voltage until the time current drops to your desired tail cut off. You now set the absorb duration to do that. I have been doing this with the Balmar MC-614 for many years and well over 1000 cycles...

Ok, sounds like a good approximation. Obviously house loads are usually regular patterns but can still vary somewhat. Of course setting it conseritively for worse case scenerio.

[john61ct]

Originally Posted by CatNewBee

No, John, you just get the job done quicker. If you stop charge at 3.55V and STAY there until there is no current flowing any more as indication you reached the point, the battery will have the same SOC. No matter if you initially charged with 10A or 100A.

With 100A you will reach the 3.55V earlier, than you can watch how the Amps start to drop keeping the voltage.

With 10A it will take 10 times longer until you reach the 3.55V at 10A and then probably the same time until the current drops to 0A.

The SOC at the end will be the same.

Thanks. If true, then when spec'ing CV / endAmp based profiles, there is no need to specify starting current level.

> 3.55V, taper to .02A absorption

is sufficient

> The goal is to have the cell charged to 3.55V.

Not mine.

The above is just a possible "vendor / theoretical 100%" (benchmark B), to understand how much my actual profile choices are sacrificing potential maximum capacity, in these discussions with members who consider that important, since "A. nameplate AH rating" is so variable by vendor.

And that B point is **much** higher than I would ever use as 100% in calibrating a BM

My "benchmark C, actual usage 100%" setpoint is currently

3.45V taper to .03C

While in normal cycling I do not CV taper at all, just **Charge To** a setpoint V and Stop.

That setpoint may vary by current level, and (**if** I were concerned about getting to a consistent SoC or anywhere close to Full, which I'm not, would) just shoot for a ballpark close to benchmark C.

Which with a trustworthy BM just means checking its SoC readout, or as Cpt Pat recommends even allowing the BM to control the stop-charge point.

> there is a minimal absorption time (5..15 minutes) you cannot cut to 0 on many chargers

Hence Cpt Pat's strategy, at low enough current, that failing can lead to overcharging harmful to longevity.

Or the voltage can be lowered to compensate.

Thanks very much again for engaging with civility, I recognize you do not necessarily agree with our premises.

[john61ct]

Originally Posted by Q Xopa

Of course setting it conservatively

Yes, calibrating AHT with target SoC can be more of a challenge with lead, especially with VRLA like AGM or GEL.

With FLA we can be very aggressive, to ensure getting to 100% Full nearly every cycle, as long as you keep the water topped up.

But with chemistries sensitive to overcharging, you need to back off at least a bit.

Since LFP has **no** need whatsoever to get anywhere near Full, it is easy to just default to a lower SoC endpoint when concurrent loads are highly variable from day to day.

Of course ideally one day we won't need to rely on eggtimers, even with alternator VRs; but for now that's the best most of us can do.

[tanglewood]

Originally Posted by nebster

Yeah, something like that. I think it was about 70% at more like 0.425C, but I don't actually have access to the notes right now where I wrote down the exact numbers.



Yeah, their "SOC" is arbitrarily low, which is just the Winston way I think. You get a cell that has ~15-18% more capacity than the nameplate -- which is great, except when trying to make sense of their charts.

The real 100% SOC is even farther out to the right, probably at 118 or something in that chart, because those rates are quite high.



Right, it will definitely be at least somewhat higher than that, because...



That's exactly why, yes. Here is a some charge/discharge data that Simon shared on another forum:



This graph is one of my favorites, because it shows the same exact cell charged and discharged at a number of relevant (low) rates.

If we pick the 3.400V line there, we can see that we reach 95% SOC at 0.05C, 80% SOC at 0.1C, and about 70% SOC at 0.2C. If you want to tighten down that spacing, you can of course go to a higher V.

What I found in my tests was that my bottom-balanced cells couldn't tolerate much higher voltages. My 3.388V -- which is a value so precise as to be almost meaningless for another system because of variation in wiring resistance and sensor accuracy -- turns out to be a comfortable stopping voltage that doesn't allow any cells to start to think about spiking up.

It's worth running these tests on your own to see how it behavea, since my data is limited by my own design decisions. I think a top-balanced pack could reduce this gap a good bit, but I suspect it will still be meaningful at rates > 0.2C.

OK, so I think all this comes down to your chosen end charge voltage. That chart shows it all.


If you stop charging at 3.4V, there is a pretty wide spreed in SOC between charging at 0.2C (70% SOC) vs .05C (95% SOC). This is your key point.



But if you stop at 3.45V, the spread narrows to 87% to 98%


And if you stop at 3.5V, the spread further narrows to 92% to 99%


If you don't charge into the knee, you will need voltage and acceptance current to get a reasonably full charge (say over 80%).


But if you do charge into the knee, voltage alone can be an acceptable indicator to stop charging.


And if you want to reach a specific SOC, you will need voltage and current to determine that.


I think you have said all along that you want to get back to a pretty specific SOC, so I understand your approach and think it makes good sense. It also seems to be partly motivated by wanting to maintain a lowish end voltage to guard against inter-cell voltage spread, which also makes sense.


But it seems equally fine to just charge to a target voltage provided;


1) that voltage is deep enough into the knee to have reached acceptable SOC across your anticipated charge current range. And you get to decide what's "acceptable". If 70-90% is OK, then a lower charge voltage is fine. If you want to always get over 80% or 85%, then a higher voltage will be called for for higher charge rates.



2) You have a means of monitoring (and possibly correcting) cells that are enough above average voltage to risk over-charging. But all battery management systems, from manual to fully automated, need to do this, so nothing new.


I also think it's a perfectly good strategy to set different target voltages for different charge sources. For example, a .5C engine powered charge source might be set to charge to 3.5Vpc, where a solar source that can vary from 0 to maybe .1C could be set to 3.40Vpc or 3.45Vpc.


This whole discussion serves to remind me that there are lots of perfectly good strategies for charging and maintaining LFP, and that before dumping on any particular approach, it's important to understand who it fits in with their overall strategy. Significant different approaches, just off the top of my head, are:


1) Bottom balance or top balance


2) Charge to a voltage and stop, or charge to a voltage & current then stop


3) What does "stop" mean? Disconnect the LFP, or lower the charge voltage to stop charge current flow?


4) Active charge control from BMS, i.e. under normal operating conditions the BMS is turning charge sources on and off? Or program the chargers to behave within acceptable limits, and use the BMS strictly as an abnormal condition alarming and protection device.


5) What portion of the SOC range to use, and why? And is the "why" based on science or belief?


Fun stuff, I have to admit. It's just like boating - lots of right answers. Maybe that's why we boaters love it.

[john61ct]

Originally Posted by tanglewood

But if you stop at 3.45V, the spread narrows to 87% to 98%

And remember, that 98% is well over all the other definitions of "100%", in that case especially the rated AH paid for.

> I also think it's a perfectly good strategy to set different target voltages for different charge sources. For example, a .5C engine powered charge source might be set to charge to 3.5Vpc, where a solar source that can vary from 0 to maybe .1C could be set to 3.40Vpc or 3.45Vpc.

**Nope** it's the opposite relationship, non-intuitive, I know!

With CC-only no-Absorb profiles, the **higher** the current, the safer a given voltage is, much lower SoC than a low current.

Those very low "trickle" sources are the dangerous ones, better to set at a much **lower** voltage to prevent going to too high a SoC.

If you (somehow?) have gear that reliably automates stop-charge based on endAmps even in highly variable usage / weather conditions, then you can use CV/Absorb based profiles safely.


> What does "stop" mean? Disconnect the LFP, or lower the charge voltage to stop charge current flow?

No, even a low-voltage Float is not "stop". Just because no coulombs are flowing, does not mean no impact on longevity.

But the "What does stop mean?" question remains as a design question, disconnect bank from charge buss, or turn off charge source?

If there is a non-proprietary central charge controller one day, you still need BMS functionality to protect the bank from that controller failing.

[newhaul]

Ok here is something to consider .
I have read all of this but have not seen anybody actually post any specs from the cell manufacturer. Or the specs for the bms they are using .
Here are the spec sheets for both of mine
Battery camel 25ah cells ( 4s4p) standard charging is to be at .5C with max being 5C
My bms specs are also attached as an fyi.

[tanglewood]

Originally Posted by newhaul

Ok here is something to consider .
I have read all of this but have not seen anybody actually post any specs from the cell manufacturer. Or the specs for the bms they are using .
Here are the spec sheets for both of mine
Battery camel 25ah cells ( 4s4p) standard charging is to be at .5C with max being 5C
My bms specs are also attached as an fyi.

OK, you've shown yours, so I'll show mine....


CALB CA 180 cells, wired 2P16S (360Ah @48V nominal)

Max charge current 1C
"Standard" charge current .25C

Charge Cut off voltage 3.65V
Discharge cut off voltage 2.5V


Max charge current is the inverter/charger powered by generator yielding 140A charge rate, or .38C.


Solar charge rate is typically 45-60A, so .15C, give or take.



My BMS voltages are all user programmable, but there are high and low disconnect points, and high and low warning/alarm points. Monitoring is per-cell voltage, and currently only one temp probe point, but I have the channels available to monitor every other cell. I'll be going on-line with the bank and BMS in the next few days, and plan to start operation with a very conservative SOC range (via voltage settings), then widen up as I get some cycles on the system and see how everything works.

[tanglewood]

Originally Posted by john61ct

And remember, that 98% is well over all the other definitions of "100%", in that case especially the rated AH paid for.

> I also think it's a perfectly good strategy to set different target voltages for different charge sources. For example, a .5C engine powered charge source might be set to charge to 3.5Vpc, where a solar source that can vary from 0 to maybe .1C could be set to 3.40Vpc or 3.45Vpc.

**Nope** it's the opposite relationship, non-intuitive, I know!

With CC-only no-Absorb profiles, the **higher** the current, the safer a given voltage is, much lower SoC than a low current.

Those very low "trickle" sources are the dangerous ones, better to set at a much **lower** voltage to prevent going to too high a SoC.

I think you misread what I wrote. We are saying the same thing

Originally Posted by john61ct

If you (somehow?) have gear that reliably automates stop-charge based on endAmps even in highly variable usage / weather conditions, then you can use CV/Absorb based profiles safely.


> What does "stop" mean? Disconnect the LFP, or lower the charge voltage to stop charge current flow?

No, even a low-voltage Float is not "stop". Just because no coulombs are flowing, does not mean no impact on longevity.

At the risk of repeating the whole "Floating LFP batteries" discussion, that's exactly what it means. No current flow is the same as disconnected. So unless a disconnected but powered up charger will impact longevity..... Or perhaps you know something different about electricity?

Originally Posted by john61ct

But the "What does stop mean?" question remains as a design question, disconnect bank from charge buss, or turn off charge source?

If there is a non-proprietary central charge controller one day, you still need BMS functionality to protect the bank from that controller failing.

[newhaul]

Originally Posted by tanglewood

OK, you've shown yours, so I'll show mine....


CALB CA 180 cells, wired 2P16S (360Ah @48V nominal)

Max charge current 1C
"Standard" charge current .25C

Charge Cut off voltage 3.65V
Discharge cut off voltage 2.5V


Max charge current is the inverter/charger powered by generator yielding 140A charge rate, or .38C.


Solar charge rate is typically 45-60A, so .15C, give or take.



My BMS voltages are all user programmable, but there are high and low disconnect points, and high and low warning/alarm points. Monitoring is per-cell voltage, and currently only one temp probe point, but I have the channels available to monitor every other cell. I'll be going on-line with the bank and BMS in the next few days, and plan to start operation with a very conservative SOC range (via voltage settings), then widen up as I get some cycles on the system and see how everything works.

sounds good . Now lets see what everyone else is working with. So we can all discuss the subject from common ground so to speak

[tanglewood]

Originally Posted by tanglewood

OK, you've shown yours, so I'll show mine....


CALB CA 180 cells, wired 2P16S (360Ah @48V nominal)

Max charge current 1C
"Standard" charge current .25C

Charge Cut off voltage 3.65V
Discharge cut off voltage 2.5V


Max charge current is the inverter/charger powered by generator yielding 140A charge rate, or .38C.


Solar charge rate is typically 45-60A, so .15C, give or take.



My BMS voltages are all user programmable, but there are high and low disconnect points, and high and low warning/alarm points. Monitoring is per-cell voltage, and currently only one temp probe point, but I have the channels available to monitor every other cell. I'll be going on-line with the bank and BMS in the next few days, and plan to start operation with a very conservative SOC range (via voltage settings), then widen up as I get some cycles on the system and see how everything works.

For what it's worth, My starting management voltages (Vpc) will be roughly:


3.65 Emergency disconnect
3.55 Battery reconnect

3.50 High voltage alarm
3.45 charge voltage, two stage (bulk to 3.45, then to float)
3.35 Float voltage. Adjusted as needed for zero charge current.
3.15 Gen start after 15 min
3.00 Low voltage alarm
2.90 Battery reconnect

2.50 Emergency disconnect

[nebster]

Originally Posted by tanglewood

OK, so I think all this comes down to your chosen end charge voltage. That chart shows it all.

I agree with everything you have surmised, although I would add to this bit

But if you stop at 3.45V, the spread narrows to 87% to 98%
And if you stop at 3.5V, the spread further narrows to 92% to 99%

...that those are the deltas from 0.05 to 0.2C. My original comment/concern was with regard to charging above 0.2C... where the SOC deltas continue to increase to what become almost laughable values.

The other idea I would put forth is that there may be some cells that perform substantially better. I don't know what would make them different, because I'm not an electrochemist. But there are definitely high-rate lithium batteries (perhaps no iron phosphate ones?) that clearly can stay somewhat stable even under enormous rates. I'm not sure that matters for our practical purposes today, with what we can purchase in the marketplace, but I suppose in the totality of the problem it's worth keeping in mind.

Finally, with regard to very low-rate charging, I think this is a tricky problem. On my setup, I have bypassed having to really solve the problem, because my solar is so small relative to my pack that it has basically no chance of ever fully charging it in day-to-day use. Therefore, I leave the solar system at a safe "float" voltage full time, and it only serves to sustain the pack around 80% SOC when we are off the grid. This avoids the trickle-charge-of-death once you get up to any voltage high enough to fully intercalate the matrix.

(Okay, I admit, I just wanted to say "fully intercalate the matrix." But you know what I mean.)

Fun stuff, I have to admit. It's just like boating - lots of right answers. Maybe that's why we boaters love it.

Indeed.

[nebster]

Originally Posted by john61ct

No, even a low-voltage Float is not "stop". Just because no coulombs are flowing, does not mean no impact on longevity.

As we've discussed before, we don't really know that this is true. The preponderance of the evidence -- scant as it is -- and the superficial physics both seem to suggest that it does NOT have a measurable impact on longevity.

Moreover, the advantages of floating at a midpoint-maintaining voltage are so great in some systems that it doesn't matter. I would trade -10% lifecycles (I think it's unlikely it is that high, if it is nonzero) for the convenience of having the pack always ready for load support, and so do many others every day.