Alternator Size with LiFePo4

[evm1024]

Let me toss out some absurd numbers.

Say you have a daily usage of 200 AH and you wanted to replenish that each day.

If you has a 10,000 AH LiFePO4 bank you could cycle between 51% and 49% SOC. You have a 7+ KW charging system that charges at 500 amps.

In those cases you end up charging at 0.05 C for about 24 minutes to recover your full usage.

Of course we are ignoring the cost, weight etc of such a system.

Now lets move from the unreasonable to something more likely.

Same 200 AH/day use and same 0.05 C charge rate.

We end up cycling between 75% and 25% (of course this assumes that you design to have an average SOC of 50%).

The 0.05 C charge rate turns out to be 20 amps and takes 10 hours. This is a rate that can be done with "smallish" solar.

Up your charge rate to 0.2 C (80 amps) and you get your usage back in 2.5 hours. 80 amps is doable with "largish" solar. Around 1.1 KW.

Of course 10,000 AH of capacity is crazy (at least today) but it has some advantages. You could run your boat for 50 days without any charging at all for example. You could forget to shut off your charger for a few days without overcharging your bank and such.

A 400 AH bank gets you perhaps 2 days without charging and you do need to watch that it does not get overcharged and such. But of course it is much more cost effective.


Why toss out these numbers?

The idea is to get us thinking about the trade offs we make when we go to "right-size" our bank. There are advantages to having a bank of 400 AH up to the point that the cost is out of line.

There are disadvantages with having a bank of less than 400 AH with a 200 AH usage. Drop the usage and a 200 AH bank becomes more desirable.

I have a 700 AH bank. The price was right (on sale at the time) and 400 AH just appeared to be too small for my projected (future) usage.

All about trade-offs.

[john61ct]

Originally Posted by 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?

Dunno why you've chosen those two numbers, nothing's black and white.

Yes very conservative, but if solar is then getting you to your daily cycling Full definition after that, sure, nothing lost.

[evm1024]

Originally Posted by john61ct

SNIP!

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. . .

I've got one but have not wired it in to try it out yet. I am a bit of DIY hacker but do value tried and true....

I will get to using it when I have time to "play".

[john61ct]

Originally Posted by evm1024

I've got one but have not wired it in to try it out yet. I am a bit of DIY hacker but do value tried and true....



I will get to using it when I have time to "play".

I posted text from my email to Al here

http://www.cruisersforum.com/forums/....php?p=2794301

[nebster]

Originally Posted by john61ct

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

The main thing, in my opinion, is to understand what the real capacity is, not what the manufacturer writes on the box. Some manufacturers are conservative with their rating, perhaps in an attempt to allow for degradation while staying above the nameplate capacity.

My particular cells were sold as 100Ah, but they're all between 104 and 105Ah using reasonable endpoints (3.5V to 2.9V). We've seen examples of Winston cells being >10% overprovisioned.

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 ?

Yeah, I don't think there's any need to push so high. The capacity limits are reached very quickly (the function is strongly asymptotic), so additional testing at ever more exotic voltages yields no meaningful additional return. As long as we pick a voltage fairly high (higher than any of us should be using day to day), we will measure a stored energy that is very close to the maximum capacity.

[john61ct]

Originally Posted by evm1024

Let me toss out some absurd numbers.

Great approach to clarify the variables.



> 200 AH and you wanted to replenish that each day.

Actually real world.


> If you has a 10,000 AH LiFePO4 bank you could cycle between 51% and 49% SOC.
Silly but cool

> You have a 7+ KW charging system that charges at 500 amps.
Not that far from what I'd like in some use cases.



> charging at 0.05 C
silly low due to crazy high bank size



OK for solar-only, but with ICE available (this thread topic) in the "realistic" scenario, why not go to .3C ?



> We end up cycling between 75% and 25% (of course this assumes that you design to have an average SOC of 50%).

Expensive, when extreme longevity can be attained cycling between 5% and 90% anyway. And % of which defined benchmark becomes relevant at the top.

Of course without convenient on-demand ICE, solar requires a bigger reserve buffer.

[john61ct]

Originally Posted by nebster

The main thing, in my opinion, is to understand what the real capacity is, not what the manufacturer writes on the box.
…


Yeah, I don't think there's any need to push so high. The capacity limits are reached very quickly (the function is strongly asymptotic), so additional testing at ever more exotic voltages yields no meaningful additional return. As long as we pick a voltage fairly high (higher than any of us should be using day to day), we will measure a stored energy that is very close to the maximum capacity.

THANK YOU!!

So many posters have consistently attacked me for that view.

Exotic to us, is exactly what the cell makers write in their specs, so noobies to the "avoid the shoulders" approach see those (IMO crazy) high voltages as "mainstream"!

[nebster]

Originally Posted by john61ct

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

Well, in an example charging strategy being discussed earlier, the endAmps was set to bulkAmps. As in, no CV time. My point is that it can then have a substantial impact, if the rate is high.

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.

Right, that is the theory. A lower voltage seems to work fine.

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

I'm not sure. Probably? I don't think higher voltage thresholds are a good idea no matter what, though. The threshold should be set as low as possible.

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.

I don't think this is a good idea, but it's so easy to get great results at lower voltages that I've never spent any time trying to understand whether there's some other corner of the configuration envelope that also works.

My current boilerplate:

Your boilerplate doesn't include any tail current values, so I'd suggest it's underspecified. Unless you just mean*to have no CV interval at all?

[nebster]

Originally Posted by john61ct

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

It might not be your goal, but I'm sure there are many folks who would like to charge their battery to the same fullness each time.

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.

How is that simpler if the charger's supply is variable?

If I plug into a shore supply that can only deliver 1500W, my achievable rate is going to be very different than if I plug into a shore supply that can deliver 9600W tomorrow. Am I really going to be reprogramming my charger every time I move to a new supply?

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.

A shunt or battery monitor has nothing to do with a charge strategy that involves an absorption phase. One of the best parts of the CC-then-CV approach is that it avoids depending on an (unreliable) coulomb counter. It makes the charging very straightforward.

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

Most owners will be buying an engineered pack with a BMS that manages all of this. I'm not sure why that means we should feel okay with discussing only a portion of the relevant information needed to achieve an excellent result when DIY.

[Maine Sail]

I see a lot of focus on "replacing Ah" with ideals and thoughts of getting back to full .... (not what nebster is discussing, he gets it)

This is a lead acid mentality. Instead simply focus on replacing only what you need for the next 12, 24, 36, 48 hours etc. and shut it down.

If you have an opportunity to get back to near full, by all means take it, but don't fuss about it daily. Spend the money now to size the bank for a few days worth of silence and you'll really begin to appreciate LFP even more.

For example we leave our bank at less than 50% almost every-time we leave the boat. When we get back to the boat I fire up the engine & alt first. By the time we stock the fridge, bar, ready the vessel, stow our gear, leave the hook and get out of the harbor, about 40 minutes +/-, we have replenished more than an entire days worth of energy + what ever SOC we left it at.. The motor, if fully warm, then gets shut down and the PV flipped on.

With the addition of the new Balmar SG200 we will no longer need to guess at the accuracy of our SOC via a Coulomb counter. I doubt our bank will see 100% SOC all that often. It's just no longer necessary... LFP is really, really quite simple if you can leave the lead acid mindset in the 60's.

[john61ct]

Originally Posted by nebster

The main thing, in my opinion, is to understand what the real capacity is

I agree, as long as it's understood that "real capacity" number, to the extent it is **precise** is not "objective".

And any given precise AH number, can be reached from an infinite combinations of Charging Amps / Absorb Voltage / endAmps.

Setting what combination of those values a given owner wants to use as their "theoretical / vendor" benchmark is an arbitrary decision.

As is in fact the AH capacity number reached by those settings.

But if .1C / 14.6V / taper to Zero A

is within 4% AH capacity of

.1C / 14.2V / .02C endAmps

then the latter works just fine as a "theoretical / vendor" 100% SoC benchmark

since my "normal daily cycling 100%" is another 2-3% lower in AH capacity reached.

These concepts are what I meant by "100% SoC" is arbitrary"

another position for which I was vehemently scorned.

But I am grateful for that opposition, as it has helped me to think things through, define the wording more thoroughly.

[nebster]

Originally Posted by john61ct

These concepts are what I meant by "100% SoC" is arbitrary"

I understand why you say it that way, but I think it is somewhat misleading.

The reality is, for any aggressive charge strategy -- none of which we should be using day to day, I hope we all agree -- the achieved stored energy will be very close to that of any other aggressive strategy. They won't differ by 4%, they should differ by under 1%. We don't care about being any more precise than that, so we can simply declare the true storage capacity as equal to the result from any aggressive cycle.

[john61ct]

Originally Posted by nebster

It might not be your goal, but I'm sure there are many folks who would like to charge their battery to the same fullness each time.

I have no idea why. The **only** time I can see any value in that, is resetting a BM that drifts, as all AH counters do, maybe SG200 will prove to be the exception if its SoH measure is accurate.

> How is that simpler if the charger's supply is variable?

Very few sources automate end-charge based on trailing amps. Victron actually removed that option from its firmware. Magnum does, but only offers Combi units which I avoid.

Of course stopping at Voltage reached is simpler.

> Am I really going to be reprogramming my charger every time I move to a new supply?

If that can be a couple taps on a screen, sure. But I was referring to one profile for solar, another for the genset or alt.

I very rarely have access to shore, but don't see that as radically different.


> A shunt or battery monitor has nothing to do with a charge strategy that involves an absorption phase. One of the best parts of the CC-then-CV approach is that it avoids depending on an (unreliable) coulomb counter. It makes the charging very straightforward.

Now I'm **really** confused. EndAmps is the **only** reliably safe stop-charge criteria if you are pushing into an Absorb stage.

Or do you just rely on the charge source regulator's Absorb Hold Time? Usually just an eggtimer! Not measuring acceptance, so varying concurrent Load levels make even a sophisticated algorithm or human manual control total guesswork.

> Most owners will be buying an engineered pack with a BMS that manages all of this.

You can be sure any such of quality are using amps measured by a shunt unless CC-only.

And, no I think the future lies not with proprietary solutions, but BMS functionality via OTS and Open Hardware standards, compatible with any maker's cells.

[Q Xopa]

Originally Posted by john61ct

Great approach to clarify the variables.



> 200 AH and you wanted to replenish that each day.

Actually real world.


> If you has a 10,000 AH LiFePO4 bank you could cycle between 51% and 49% SOC.
Silly but cool

> You have a 7+ KW charging system that charges at 500 amps.
Not that far from what I'd like in some use cases.



> charging at 0.05 C
silly low due to crazy high bank size



OK for solar-only, but with ICE available (this thread topic) in the "realistic" scenario, why not go to .3C ?



> We end up cycling between 75% and 25% (of course this assumes that you design to have an average SOC of 50%).

Expensive, when extreme longevity can be attained cycling between 5% and 90% anyway. And % of which defined benchmark becomes relevant at the top.

Of course without convenient on-demand ICE, solar requires a bigger reserve buffer.

Yes this last statement is what I was questioning you questionig last time. (If that makes sense).

Im not sure if you are suggesting the SOC cycling range makes no difference to cycle life, or negligable. And with practical economic considerations its not worth considering to any extent. I think the later?

Personally I dont know the answer myself, however I have heard from a number of sources recently that many consider it does. So there does seem to be a 'weight' of opinon adding to the possibilty.

[john61ct]

Originally Posted by nebster

they should differ by under 1%. We don't care about being any more precise than that, so we can simply declare the true storage capacity as equal to the result from any aggressive cycle.

Yes, but for noobs, they need a recipe, hard numbers, certainly nothing requiring load testing.

Therefore better to err on the "less aggressive" side, since so many will end up with secondhand or QA rejects, where real capacity is below the nameplate AH.

Until decent local supply channels are established.