Alternator Size with LiFePo4

[nebster]

Originally Posted by newhaul

with purely renewables I am actually charging with variable current and stopping at designated voltage Via the controller setpoints.

I'm with you on the variable current in real life, but I had to pick some static values to use as examples. "Stopping at designated voltage" isn't specific enough to tell what your actual charging strategy is, though.

If your charging strategy is the (simpler) first type, and your average current is north of 0.2C, you will end up reaching the stopping voltage point at meaningfully lower SOCs. Whether that lower energy storage is acceptable is situational, but these batteries are expensive, so I think it's worth understanding what you're getting.

[evm1024]

Clearly different aims at work here.

I charge at around 0.15C perhaps reaching 0.2C in special cases.

I am quite content to reach 90% SOC and shut off charging. Going to 95% SOC is meaningless to me. It represents 35 AH of additional charge. Nickles and dimes....

[newhaul]

Originally Posted by nebster

I'm with you on the variable current in real life, but I had to pick some static values to use as examples. "Stopping at designated voltage" isn't specific enough to tell what your actual charging strategy is, though.

If your charging strategy is the (simpler) first type, and your average current is north of 0.2C, you will end up reaching the stopping voltage point at meaningfully lower SOCs. Whether that lower energy storage is acceptable is situational, but these batteries are expensive, so I think it's worth understanding what you're getting.

the solar controller holds the set voltage of 13.8 ( in my system) as long as the sun is out.
The wind generator the same as long as wind is blowing ( if needed). Auto electromagnetic brake.

[nebster]

Originally Posted by evm1024

Clearly different aims at work here.

Definitely, we all will have different objectives.

I charge at around 0.15C perhaps reaching 0.2C in special cases.

I am quite content to reach 90% SOC and shut off charging. Going to 95% SOC is meaningless to me. It represents 35 AH of additional charge. Nickles and dimes....

Me, too... I like to hit around 92 or 93% of the true battery capacity.

But, if you were to have a charger capable of 0.4C, you might only achieve 80% SOC using the exact same charge strategy. Would that be acceptable to you? It certainly isn't for me.

For these scenarios, we do need a two-stage charge strategy in order to obtain consistent SOC across a wide range of rates. (If your particular system doesn't have that wide range, no worries then.)

[rgleason]

This is a very interesting discussion. I would like to use my ICE power alternator morning/evening for 30min to charge a bank of 200ah (approx) LiFePo4 effectively. My needs are approximately 100ah/day. I would like my batteries to last 10 years. What would the best charge regimen be for this use?
Would prefer not to use the engine more than one hour. Will have some solarPV, but not more than 100w at the start.

I had been sizing my alternator to charge at 0.3C to 0.35C and was assuming I would use John's values for stopping, but I believe that with 30min of ICE charging, the batts won't normally reach full, but we no longer care about that with LiFePo4.
I still regard batteries as long term consumable. The reason to change is to make life easier and save engine hours.
When I install these batteries I dont want to obsess over them, and I want to have a reliable system that works as expected for 10 years.

I no longer wish to deal with the FLA slow tail charge when cruising on the hook or mooring exclusively!

[john61ct]

Originally Posted by nebster

will not result in a full charge
...
can end up not storing as much energy as you planned
…
test and verify the charging results yourself before committing to a solution that only uses a CC charge strategy

Since I'm already getting very close to rated AH even avoiding the shoulders, losing a, little capacity is no sacrifice at all IMO.

But even if I were "giving up" a bit more, that would be well worth it for the gain in longevity.

The charge rate obviously depends on the source available at the time. Solar rarely gets too high, so my cutoff is now set to 3.41 - 3.43Vpc in good insolation conditions when it's likely to reach that.

Even with mains or ICE sources I usually stay between .2 and .3C, which is where my cutoff is 3.45. Also when I know solar isn't getting there anyway.

> A typical cell will reach only ~90% true SOC at 0.2C to 3.45V

How do **you** benchmark "true SoC"?

I might refer to that as "theoretical / vendor SoC", but if I see that, say

3.5Vpc @ .1C tapering to .02C

is already getting to rated AH capacity, then why push so high as, say

3.65V taper to 0A ?

[john61ct]

Originally Posted by nebster

Incorrect, for higher charging rates

The charging rate should only have an impact when endAmps is set pretty high, right?

If you allow the charge to taper down to say .01C or even lower, then the use of higher volts will be the stress factor.

If using endAmps of .03C, or no Absorb / CV at all, then higher volts will be less harmful, even at low current rates.

I can thus see scenarios where going higher than 3.5Vpc would be fine, but that really complicates setting guidelines for the vast majority of owners, who are not equipped or willing to bother doing capacity load testing.

My current boilerplate:

_______
Stop at 3.45Vpc / 13.8V for 4S, for amps rate of .2 - .3C.

At higher rates, to shorten ICE run-times, it is safe to go to 3.50Vpc / 14.0V.

At **very** low charge rates, as with many solar setups, back off to 3.40Vpc / 13.6V
________

is already complex enough, even too much so for noobs.

[john61ct]

Originally Posted by newhaul

Do I need to add another 100ah?

Just to reduce your C rate?

Buying a bigger bank IMO should only be driven by an actual need for the higher AH capacity, because you feel you're hitting too close to your 0% point too often, and don't have the ability or desire to replenish via ICE on demand.

Increasing your charging voltage a little will get you to higher SoC.

But the only way to calibrate the impact is through load testing.

[john61ct]

Originally Posted by evm1024

Clearly different aims at work here.



I charge at around 0.15C perhaps reaching 0.2C in special cases.



I am quite content to reach 90% SOC and shut off charging. Going to 95% SOC is meaningless to me. It represents 35 AH of additional charge. Nickles and dimes....

I'm with you there, but agree with nebster that it's great to actually know what you're sacrificing to the longevity goal.

I consider a CC-only charging strategy much simpler, even if that results in getting to a variable SoC point.

I think the fact my "boilerplate profile" spec'd above covers three current-rate scenarios strikes a reasonable balance.

Cpt Pat's advice to use AH counting to keep **very** low C rate charging from pushing SoC too high, I think is a decent solution, but only if you've benchmarked enough via load testing to really trust your BM in your usage conditions.

Perhaps the SG200 will become a silver bullet there?

[rgleason]

This is helpful information. So if the LFP soc is 25-30% I could charge at .4C x 200ah= 80amps for 30min to raise SoC by 40 amps or say about SoC 48%. I could even charge longer than that at .4C, say another 15min adding 20amps with no problems, staying well away from the ends of the curve, with no real issues about longevity or cycles.

It is just the maximum discharge points (ends) and max charge points (ends) that cause problems with cycles and longevity.

If this is true, I will pick conservative DoD and DoC and charge at a higher rate for the alternator.

Thus leaving the SolarPV some room to charge at a slower rate with its own appropriate settings, as suggested by the notes below.

So now I need to understand the differences between charging regimens. CC constant current does not reach the same SoC each time, but CV does?
What is appropriate for my use case?

[john61ct]

Originally Posted by rgleason

I would like to use my ICE power alternator morning/evening for 30min to charge a bank of 200ah (approx) LiFePo4 effectively. My needs are approximately 100ah/day. I would like my batteries to last 10 years. What would the best charge regimen be for this use?

No guarantees wrt lifetimes, nor even firm numbers are possible, too many variables even with lead, plus LFP' brief existence adds unknowns.

But keeping below a .3C charge rate will help, prioritize that over reducing ICE runtime, or at least realize you're balancing longevity of the bank with that of your engine.

A stop point of 3.45Vpc, or you could safely add some Absorb / CV time, but I would not taper below .02C myself, and you're likely not buying more than 3-5% actual SoC with the added complexity.

Add solar to reduce your ICE runtimes.

[rgleason]

Rick wrote:

It is just the maximum discharge points (ends) and max charge points (ends) that cause problems with cycles and longevity. (Do not know if this is true, totally unconfirmed)

If this is true, I will pick conservative DoD and DoC and charge at a higher rate for the alternator. (No Alternator charging below SoC 25% and no alternator charging at .4C above SoC 80% for example.)

Thus leaving the SolarPV some room to charge at a slower rate with its own appropriate settings, as suggested by the notes below.

[john61ct]

Originally Posted by nebster

we do need a two-stage charge strategy in order to obtain consistent SOC across a wide range of rates.

I don't see precise "end-charge SoC consistency" as a goal.

My above strategy of varying the stopping voltage for different current-rate charge sources I think strikes a reasonable balance, and is simpler than stop-charging using endAmps.

But for those willing to babysit their charging manually, or with all charge sources under the control of a central shunt-based BM, obviously adding some Absorb does no harm long as the end, charge is not allowed to go past a conservative taper A / voltage combination.

Most owners won't be doing repeated load testing required to calibrate these factors in either case.

[rgleason]

Does anyone have some thoughts about alternator "bulk" charging at some Constant Current, provided the voltage does not go above say 3.38vpc?
or 80% soc?

[john61ct]

Originally Posted by rgleason

No Alternator charging below SoC 25%

What is the rationale for that bit? Charging, only need to worry about the top shoulder.

Only need to avoid the bottom when Discharging.

Originally Posted by rgleason

no alternator charging at .4C above SoC 80%

Interesting idea!

For the longevity concern, yes, reducing current rate as SoC climbs would likely be a great strategy, 80% (of which definition of?) SoC is unnecessarily low.

Would be more reliable / simpler to just start scaling C rate back based on circuit voltage

If .4C, maybe start tapering down at 13.6V, to .1C as you approach 13.8V, then stop.

Great balance of reduced runtime and safely maximizing SoC.

Al Thomason's Viking Star open source Very Smart Regulator / "Smart Alternator Regulator" project should be able to handle that.

Most owners would not be willing to trust it, but a bleeding edge DIY hacker type who'd be willing to monitor closely in early days,

combined with trusted BMS layers of additional HVD protection. . .