Alternator Size with LiFePo4

[newhaul]

Originally Posted by john61ct

Do you know your per-day 12V AH consumption?

That will dictate your charge time more so than the bank size.

actually it will depend on my bank size . I will end up with 300ah Lfp . I do not intend on charging with the engine in till the bank is about 75% depleted or down about 250ah .
Which with solar I hope doesn't happen except in extreme circumstances. ( daily usage will be approx 35 to 45 ah )
looking to keep engine run times under 3 hours for full charging.
Which means at a minimum a 120amp alternator.
Only even mounted when needed.

[rgleason]

Originally Posted by john61ct

Do you know your per-day 12V AH consumption?

That will dictate your charge time more so than the bank size.

Yikes, that is an excellent point!!!! That is going to help a lot.

So say it is 100 amps (80amps is my cruising budget, 140a for sea), so with a 150-160a alt charging at hopefully 100a, the worst case might be 1 hour? Also LiFePo requires that it not be charged to 100% at all, period! Hooray.

I can live with that, but not 2hours, or 3hrs, or 4 hrs, or 5hrs (all req'd by FLA).

[john61ct]

And there are times the engine's already being run for other reasons.

Take advantage of any available sources, better than cycling deeper DoD than necessary.

[newhaul]

Originally Posted by john61ct

And there are times the engine's already being run for other reasons.

Take advantage of any available sources, better than cycling deeper DoD than necessary.

its called a sailboat for a good reason.
Besides DoD is not an issue with Lfp . It seems it's actually the opposite. Lfp don't seem to like being 100% fully charged but do like being between 20% and 80% charged.
I did my math earlier before I was awake . My max discharge to 25% would actually give me approx 225ah not 250ah. So 4 to 6 days with no charging available. ( That's a lot of really bad weather to have with no wind either.)

[tanglewood]

Originally Posted by rgleason

Tanglewood
Thanks for clarifying the alternator heat issue.

• Diode Rectifier heat linear (1.2w/amp for .6v drop) = 6 diodes x 1.2w/amp x 140amp= 1008w= 3500 btuh (Is this right? It feels too big. Are all the diodes working 100%?)
  
• Rotor and Stator heat is exponential = How to get a handle on that?
  

Certainly the remote rectifier could be improved with a "Smart High Efficiency Rectifier" using a Texas Instruments LM74670-q1 and Mosfet Rectifier which is completely detailed here

• www.ti.com/lit/ds/symlink/lm74670-q1.pdf

What exactly would change with a generator?

• Another complicated system.
• Another engine.
• More heat from generation.
• Where on a 32' boat?

I started a thread called Tiny Powered Alternators & DC Generators - Cruisers & Sailing Forums
which was very informative, but I never really found an answer for me for the long tail on charging FLA until I found EFOY, but it was very expensive, another fuel and took some living space in a side locker.

So now I am trying to get a good alternator setup that really does work for a LiFePo4 Bank of between 200-300 ah that will charge quickly, not 3 hours, preferably not 2 hours. I will have about 50-100 watts of solar (max) to ensure the LiFePo are maintained at about SoC60% and to offset some daily use.

I'd like to have an alternator that fits in 3ym30 that will put out 130-140a continuous, if changing to 24v will help then maybe, if not, it is not worth it because the low rpm charge on the hook, scenario might suffer. I doubt that one of the Delco 24si (160a) or 28si (160a - better at low rpm than 200a) will fit, but I am going to certainly try. I kind of doubt those will achieve my goal anyway.
I've made these kinds of google searches to find a 12v Alternator Texas Instruments LM74670-q1 Mosfet Rectifier

• Smart Mosfet Rectifier
• High Efficiency Mosfet Rectifier
  
• Alternator mosfet Rectifier

And I found these projects

• Bridge rectifier made with MOSFETs
• Motorcycle Mosfet Rectifier
• Switched Mode Rectifiers for EV

You would think that these components would be off the shelf for 12v Alternators by now with Toyota, Nissan, Tesla, Gm and all the rest into EV. Maybe I am not looking in the right place or maybe the mosfets are not big enough or robust enough for 200a and just 16v


https://www.analog.com/en/technical-...techclips.html
https://www.powerelectronics.com/pmi...dge-controller

I don’t think your math is correct. Power loss is current times the diode voltage drop which is nominally 0.7v. So for a 140A alternator that would be 98W. See, not a huge part of the problem.

[john61ct]

Originally Posted by newhaul

Besides DoD is not an issue with Lfp

Actually the avg DoD vs lifetime cycles is very much the same shape as lead.

But I agree that isn't (shouldn't be) a big factor in system design, just something to keep in mind.

[rgleason]

Originally Posted by tanglewood

I don’t think your math is correct. Power loss is current times the diode voltage drop which is nominally 0.7v. So for a 140A alternator that would be 98W. See, not a huge part of the problem.

Oh, it is not the full amps of the Alternator (140a)? - Why not?
6 diodes x 1.2 watts/amp x 140 amps = 1008watts = 3500 btu/hr
It seems a bit high, but I think the basic calculation is fine.

You say Power Loss=I(amps) x diode voltage drop (0.7v)
98w is 334 btu/hr which does not account for all the diodes and is too little IMHO.


Thanks

[rgleason]

Sinopoly SP-LFP200AHA - Lithium Cell LiFePO4 (3.2V/200Ah)
Max discharge current (A) 600
Optimal discharge current (A) 66
Max charging current (A) 400
Optimal charging current (A) 66

Winston WB-LYP200AHA LiFeYPO4 (3.2V/200Ah WIDE)
Max discharge current (A) 2000
Optimal discharge current (A) 100
Max charging current (A) 600
Optimal charging current (A) 100

Note different "Optimal charging currents"

In reading the Orion Jr BMS documentation there are Relay settings for max current discharge and charge. What values should be used? Optimal or somewhat above?

I had understood that LiFePo4 could be charged at between .4C and 1C


Additionally the Orion Jr BMS has a Canbus connection to enable EV Fast Charging which conforms to the industry standard for CHAdeMO integration. This mentions a max charge of 125amps.

Do I need to enable and wire for CHAdeMO to be able to utilize a high amp alternator of between 110-125a?


Also, it appears that if I were to conform to the CALB optimal charging current of 66a I would not have to change my alternator, and one of the reasons for choosing to install LiFePo4 just disappeared!


What is the long term penalty for charging at a rate above "optimal"?


This is a high charging, low or slow discharge application. Unlike EV Cars which is the reverse.

[john61ct]

Reduced bank longevity.

Going above say .6C, definitely need careful attention to over temperature protections and robust wiring infrastructure.

[rgleason]

It appears the BMS actually calculates the maximum charge and discharge limits, based on a number of parameters, including temperature, state of the battery, physical characteristics of the battery and other set parameters.

Orion jr Manual

"To be able to control more than one parameter at once, the BMS incorporates different parameters into a maximum allowable charge and discharge current limit. Charge and discharge limits can be thought of as the realistic maximum amperage limits that a battery can discharge or charge at any given moment (expressed in 1 amp increments)."

To what extent are these limits and the BMS calculation going to affect the Alternator charging rate?

Has anyone bumped up against this in a BMS?

[rgleason]

Originally Posted by john61ct

Reduced bank longevity.
Going above say .6C, definitely need careful attention to over temperature protections and robust wiring infrastructure.

John these terse statements do not really help. The reader fails to understand the full intent and underlying assumptions & conditions...

Is it "reduced bank longevity" when charging above optimal charging current?
or is it "reduced bank longevity" when charging above max charging current?
PS: I guess you are saying reduced bank longevity above .6C.

To what extent does fast charging above "optimal" affect longevity?

This application is purposefully a Fast Charge/ Slow Discharge application, unlike Electric Vehicles which are the reverse.

Perhaps the number of cycles is much the same as EV Slow Charge/Fast Discharge applications. It is just the reverse.

Interesting that different manufacturers have such different optimal charge / discharge values.

[rgleason]

From the Orion Jr BMS:

"The maximum current a cell can discharge (or charge) is defined by the cell manufacturer, and the value in the BMS should never exceed the maximum limit given by the cell manufacturer, though in some cases, it may be desirable to use a lower value than specified by the cell manufacturer for increasing the lifespan of the cells."


Then it adjusts for temperature and other things (temp, SOC, cell resistance, pack resistance, pack voltage, cell voltage, critical faults). I would set it above optimum charging values, but not at the manuf. max., probably at .6c x 200a = 120a, provided the other adjustments don't interfere with alternator charging too much.


PS: I might set it at .7C

[john61ct]

Originally Posted by rgleason

Is it "reduced bank longevity" when charging above optimal charging current?

That was the specific question you asked, just above my answer.

Exceeding the spec'd max rate would not be a good idea at all unless you really had hard data.

There is no quantified data as to the longevity hit from fast charging, but Maine Sail confirmed the general statement pretty strongly in a recent thread.

I have no idea if high (C+) **discharge** rates are harmful to longevity.

And note, my usage of "harmful to longevity" is not code for "will murder the bank", just that it is not optimal coddling.

You may well still get vendor spec'd cycles charging above, say .4C, e.g. 2000+ cycles.

You just may not get the 2x 3x 4x lifespan we're shooting for, by keeping to "lower / gentler than vendor specs" usage.

But that is not even a goal for many owners.

[tanglewood]

Originally Posted by rgleason

Oh, it is not the full amps of the Alternator (140a)? - Why not?
6 diodes x 1.2 watts/amp x 140 amps = 1008watts = 3500 btu/hr
It seems a bit high, but I think the basic calculation is fine.

You say Power Loss=I(amps) x diode voltage drop (0.7v)
98w is 334 btu/hr which does not account for all the diodes and is too little IMHO.


Thanks

OK, for starters I'm not sure where your 1.2W/amp comes from. 1 amp across a 0.6V drop is 0.6W. So that's issue #1.


Next, you don't have 140A flowing though all the diodes all the time. For any of the three phases, only one of the diodes will be conducting at any given time, one on the positive sweep of the phase, and the other on the negative sweep.


And the current, and hence power dissipation, is a sine wave so of varying amplitude. RMS voltage or current is used to get the DC equivalent of an AC wave form to simplify the math. The RMS current for each of the three phases would be 1/3 of 140A, with all 3 totaling 140A. In other words, you don't have 140A running through 6 diodes at once, which is what you calculated.


On the other hand, let's look more closely at the stator heating losses. An example LN 185A alternator has a stator winding resistance of 0.020 ohms. Here's the heating at different loads:


50A: 50*50*.020 = 50W
100A: 100*100*.020 = 200W
150A: 150*150*.020 = 450W

185A: 185*185*.020 = 684W


And for comparison, here's the diode heating:


50A: 50 * 0.6 = 30W
100A: 100 * 0.6 = 60W
150A: 150 * 0.6 = 90W

185A: 185 * 0.6 = 111W


And here is the total heating power as compared to the output power @12V, and associated efficiency.


50A: 80W heat, 600W power, 88% efficient

100A: 260W heat, 1200W power, 82% efficient

150A: 540W heat, 1800W power, 77% efficient

185A: 795W heat, 2220W power, 74% efficient


You can see the rapid decline in efficiency as the current goes up


Note that these numbers just calculate the electric heating in the alternator. There are also losses in the rotor, and in mechanical drag, especially from the fans. Power to drive a fan increases as the cube of the speed. Typical alternator total efficiency is in the 50%-60% range.

[evm1024]

Just a note, To complete the circuit there are 2 diodes and thus 2 diode voltage drops. Even though there are 6 diodes in a 3 phase alternator we can view it as a single phase for calculations sake.

Two diodes with a nominal 0.6 volt drop each gives 1.2 volts drop across the pair. 140 amps through a 1.2 volt drop gives 168 watts of heat to dissipate.

Try this paper if you want to see how simple it is to calculate alternator heat generation:

https://dspace.mit.edu/openaccess-di...e/1721.1/86970

And this one for general application:

http://www.delcoremy.com/documents/h...ite-paper.aspx