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Old 24-11-2019, 16:26   #1
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What size LiFEPO4 bank

I currently have 420ah bank, with 580 W of solar and 2 80A Hitachi alt. On sunny days, with a hour of motoring I can put 100-200As back into the battery bank, though I am conservative and try to keep to using 120A a day. Refrig, navs. All LED lighting.

I would like to up these numbers, knowing I can use 150 to 200A a day, and be able to put that back in. Would be ok if maybe have to run the engine an extra hour.

Was looking at getting 600 Ah of Trojans, but now thinking of Li.

What size bank Li should I be looking at if I want to be able to use 200A a day.

Assuming the 2 80A alternators, how much can I put back in to a Li bank in what time? Can I get 150 A in in an hour or so?
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Old 24-11-2019, 16:34   #2
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Re: What size LiFEPO4 bank

Will be watching with interest as I am in the early stages of getting as much info/knowledge to make a similar upgrade. Most interested to hear about how much charging is preferable as a percentage of total Ah capacity of the lithium bank. Just because these batteries can accept 1C (or more in some claims), does that mean your total charging capacity should be 1C?
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Old 24-11-2019, 16:43   #3
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Re: What size LiFEPO4 bank

If we get good basic info, maybe we should make a sticky thread with this type of basic info?

Does mainsail have a blog post on Li fundamentals?
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Old 24-11-2019, 17:21   #4
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Re: What size LiFEPO4 bank

To sort of answer your ?, from what I've read (no real experience), you can take a LiFePO4 bank down to 20% SOC consistently with little effect on life cycle expectancy. Simple math says that 200Ah is 80% of 250Ah.
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Old 24-11-2019, 17:31   #5
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Re: What size LiFEPO4 bank

And I've seen the specs on many lithiums that claim very good lifespan taken down all the way to 100%
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Old 24-11-2019, 17:33   #6
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Re: What size LiFEPO4 bank

Quote:
Originally Posted by jbinbi View Post
I currently have 420ah bank, with 580 W of solar and 2 80A Hitachi alt. On sunny days, with a hour of motoring I can put 100-200As back into the battery bank, though I am conservative and try to keep to using 120A a day. Refrig, navs. All LED lighting.

I would like to up these numbers, knowing I can use 150 to 200A a day, and be able to put that back in. Would be ok if maybe have to run the engine an extra hour.

Was looking at getting 600 Ah of Trojans, but now thinking of Li.

What size bank Li should I be looking at if I want to be able to use 200A a day.

Assuming the 2 80A alternators, how much can I put back in to a Li bank in what time? Can I get 150 A in in an hour or so?
LFP will absorb whatever amps you feed to the bank. If I have used 300 Ah, and I am recharging at 200 amps, it is pretty much 90 minutes before the current acceptance rate starts to decline, and it falls off a cliff pretty quickly. So to your last, yes, if you ran 2 80 amp alternators for an hour, your LFP bank would absorb 160 amps per hour. HOWEVER, most alternators can't handle that kind of output for extended periods of time, so having a smart regulator that allows you to throttle the alternator output to around 75-80% is important.

To the question of sizing, it is pretty much a function of how long you want to go without recharging and your usage. I replaced 1280 Ah of AGM with 600 Ah LFP, and I probably could have gone to 400 Ah since we use 300 per day and I don't mind firing up the genset for 90 minutes to replace that 300 Ah. I have no solar.

For you, because you have solar, and assuming a daily recharge is ok, I'd deduct your reliable solar charging capacity from 200 Ah and multiple that value by 1.25 to 1.5.
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Old 24-11-2019, 17:40   #7
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Re: What size LiFEPO4 bank

Quote:
Originally Posted by jbinbi View Post
I currently have 420ah bank, with 580 W of solar and 2 80A Hitachi alt. On sunny days, with a hour of motoring I can put 100-200As back into the battery bank, though I am conservative and try to keep to using 120A a day. Refrig, navs. All LED lighting.

I would like to up these numbers, knowing I can use 150 to 200A a day, and be able to put that back in. Would be ok if maybe have to run the engine an extra hour.

Was looking at getting 600 Ah of Trojans, but now thinking of Li.

What size bank Li should I be looking at if I want to be able to use 200A a day.

Assuming the 2 80A alternators, how much can I put back in to a Li bank in what time? Can I get 150 A in in an hour or so?
ok a few questions . You have 2 80 amp alternators Internal or external regulators
580 watts solar that's good now PWM or mppt controllers?

You say you use 120ah a day but with Lfp you will use more once you realize how good they are and how they recharge vs Fla batteries.

With mppt you can see about 200ah out of the solar on an average good day . And ( it won't happen but ) on a perfect day you will get in excess of 250ah out of the panels ( these figures are with mppt controllers.
To be conservative I would say 400ah Lfp bank . That will run 2 or possibly 3 days with no charging.
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Old 24-11-2019, 17:43   #8
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Re: What size LiFEPO4 bank

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And I've seen the specs on many lithiums that claim very good lifespan taken down all the way to 100%
Compared to lead acid, yes, although discharging to 100% isn't something recommended. However, if you discharge to 50% rather than 80%, you'll get around 25% more cycles. In the real world of cruising, it is unlikely most people would ever be able to exhaust the cycle life of their LFP banks, which if the bank isn't abused would provide 10 years worth of daily charging at 75% depth of discharge and almost 20 years at 50% DoD.
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Old 24-11-2019, 18:19   #9
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Re: What size LiFEPO4 bank

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you can take a LiFePO4 bank down to 20% SOC consistently with little effect on life cycle expectancy.
The lifespan vs average DoD relationship has the same direction slope as with lead chemistries.

An average draw of 70% DoDwill yield **much** longer lifespan than 90%.

And 50% much longer than 70%.

However few owners bother with this,

2000-4000 cycles being long enough already, no interest in trying to get 7000 or even 10000.

While lead might get just 100-200 if avg DoD is too high, 50% is the conventional guideline.

LFP costing many times more, and already cutting weight/space per Ah by half are also strong factors.

So, I am not personally recommending oversizing as such just to get longevity, but want to clarify that much longer bank longevity would indeed be the result.

The other big factor is how close to "power on demand" do you have?

If trying to get by on mostly solar, I'd build in 3+ days of storage.

If I can replenish the bank from ICE power at will, and that's needed a few times a week anyway, then I'd size 3-400Ah to handle 200Ah per day average.
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Old 24-11-2019, 18:23   #10
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Re: What size LiFEPO4 bank

Quote:
Originally Posted by Delfin
if you discharge to 50% rather than 80%, you'll get around 25% more cycles.
Much more dramatic difference, the curve slope much steeper than that.

In a use case where 80% yields 4000 cycles, just moving to 70% DoD may yield 6000 cycles
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Old 24-11-2019, 18:28   #11
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Re: What size LiFEPO4 bank

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And I've seen the specs on many lithiums that claim very good lifespan taken down all the way to 100%
Completely false at least for LFP.

It does depend on which out of the dozens of LI chemistries you're talking about, true for LTO for example.

I would not pass the SoC meter showing 90% personally, except in a life threatening emergency. Or 3.2Vpc at a low C-rate, whichever comes first. At high rate discharge LVC can be set lower since SoC is higher.

Of course the system designer may set arbitrary "0%-100%" boundaries to only show usable capacity, but that's a separate issue.
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Old 24-11-2019, 18:32   #12
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Re: What size LiFEPO4 bank

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Just because these batteries can accept 1C (or more in some claims), does that mean your total charging capacity should be 1C?
Absolutely not.

The specs laid out by cell manufacturer are **absolute maximum** "stress ratings" only, the limits to which the cells can only be subjected for short times, without causing irreparable damage.

These ratings are not recommended for normal functional operating conditions. As with all electronics, regularly operating at these maximum rating conditions for extended periods will reduce longevity and reliability.

Charge rate safety is highly temperature dependent. As a general rule, 0.3-4C is OK.

But in warm to hot temps, pushing up to 0.6-8C even 1C will harm longevity much less.
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Old 24-11-2019, 18:34   #13
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Re: What size LiFEPO4 bank

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Does mainsail have a blog post on Li fundamentals?
https://marinehowto.com/lifepo4-batteries-on-boats

Best to use LFP or LiFePO4 rather than the umbrella term lithium or LI
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Old 24-11-2019, 20:36   #14
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Re: What size LiFEPO4 bank

Quote:
Originally Posted by john61ct View Post
https://marinehowto.com/lifepo4-batteries-on-boats

Best to use LFP or LiFePO4 rather than the umbrella term lithium or LI
one of the best posts on this page . References to a knowledgeable and well respected member and service providers page on the subject.
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Old 24-11-2019, 20:39   #15
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Re: What size LiFEPO4 bank

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Absolutely not.

The specs laid out by cell manufacturer are **absolute maximum** "stress ratings" only, the limits to which the cells can only be subjected for short times, without causing irreparable damage.

These ratings are not recommended for normal functional operating conditions. As with all electronics, regularly operating at these maximum rating conditions for extended periods will reduce longevity and reliability.

Charge rate safety is highly temperature dependent. As a general rule, 0.3-4C is OK.

But in warm to hot temps, pushing up to 0.6-8C even 1C will harm longevity much less.
that is not quite correct . The manufacturer of my cells states that an occasional 5C charge and up to 10C discharge is acceptable with no issues but also recommends .5C or lower charge rate for maximum longevity. It all depends on the manufacturers specs and warranty.
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