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Old 31-01-2023, 16:50   #1
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Battery bank sizing with Lithium

So, on another thread I asked for ideas about running a dual 12/24 volt system for the boat. Long story short, it seems to be the way to go.

However, I am making some assumptions about the capabilities of lithium batteries that are based purely on what I've read, so I'd like to test those assumptions by running them past people who are using lithium batteries on their boats.

The assumption that is most critical is about voltage drop (or sag) under load.

My anticipated peak load would be 4kW, and that would be for maybe one or two minutes at a time. Sustained loads of up to 3kW for maybe twenty minutes at most.

This would be provided at 240 v AC by a decent quality pure sine wave inverter, probably rated at 5 kW continuous and running from 24 volts.

I would expect to use a battery bank of four 12 volt 100 AH(nominal) lithium batteries, each with a 100 amp BMS. So, two parallel pairs of two batteries in series. (At this stage I am assuming I will buy a decent set of commercially made 12 volt drop ins, but I have not ruled out making my own batteries)

So, that's asking up to 80 amps per series string for short bursts and 60 amps per string for longer periods of time.

In the experience of people using lithium batteries, how well are they likely to cope with this sort of use and is voltage sag likely to be an issue, given inverters tend to be a bit fussy about this sort of thing?
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Old 31-01-2023, 17:15   #2
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Re: Battery bank sizing with Lithium

(Not on the boat) We run 40Ah LFP in a 12P8S configuration at 0.5C for 2 hours continuous discharge. You're getting up to 0.6 to 0.8C, so not a direct comparison, but from 90% SoC to 10%SoC our battery voltage drops nearly linearly from 3.25V/cell to 3.15V/cell. That works out to 26V dropping to 25.2V for the string.

Here's the data from tonight's discharge, we started at 0.4C for the first hour (limitations of non-battery equipment) then went up to 0.5C for the second hour, then started tapering as SoC went below 10% (BMS function).

We're sitting at about 3.47V/cell at top of charge. As soon as we switch from charge to discharge that drops to ~3.27V where it stabilizes and then drops slowly with SoC down to about 20% SoC. Below 20% the rate of voltage drop increases, and eventually the BMS kicks in near 10% to limit our discharge rate (we will take it down to 2% for the night, then charge it tomorrow).

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Old 31-01-2023, 18:04   #3
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Re: Battery bank sizing with Lithium

The voltage of LFP will sag under load, but not nearly as much as with lead acid. I don't have any data to share. But I would expect the sag to be greater with drop-ins than with a battery you build yourself. The difference being smaller wires internally than you would likely choose if building yourself.

Also, you are starting at a higher voltage. So, a sag of 10% with LA (12.6-10%) drops the voltage to 11.34V, which would probably trip off an inverter. But a sag of 10% with LFP (13.8-10%) drops the voltage to 12.42V, which would not. And expect the sag to be much less than 10% with LFP.
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Old 31-01-2023, 18:17   #4
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Re: Battery bank sizing with Lithium

We see a voltage drop of maybe .2-.3 volts with those kinds of loads. I’m not 100% positive since it’s been a while.

We use a victron inverter and never had a problem.

The battery bank size seems a little low for those loads though, although that’s just gut feeling. I’m not sure what those loads are actually for.
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Old 31-01-2023, 19:16   #5
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Re: Battery bank sizing with Lithium

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Originally Posted by SV Confianza View Post
We see a voltage drop of maybe .2-.3 volts with those kinds of loads. I’m not 100% positive since it’s been a while.

We use a victron inverter and never had a problem.

The battery bank size seems a little low for those loads though, although that’s just gut feeling. I’m not sure what those loads are actually for.
I admit, the bank size feels small to me too. I arrived at the storage capacity based on five years of living aboard without an induction cooker, then added 1kWh per day for the induction cooker based on the experience of others and the volume of gas I have used over the time aboard. (Already had the inverter microwave and various other appliances, so those had solid historical data.)

The boat, up until now, has needed about 1.5kWh of power per day and had 4kWh of useable power storage. With 760W of solar panels that's been very easy to replace.

Now I make it 2.5kWh a day with the cooker, but increase the panels to 1200 - 1500W at the same time.

When I added up the new numbers I came up with 400AH@ (nominal)12v being just adequate. (This is assuming being able to go to 80% DOD on the lithium batteries from time to time.)

If I can afford it I will go bigger, maybe 4 x 120AH batteries, but they won't necessarily give me more current capacity, just more storage. And this is why I'm concerened about voltage sag from current.

But, from what has been posted so far, it seems four batteries may cope.
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Old 31-01-2023, 20:29   #6
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Re: Battery bank sizing with Lithium

Quote:
Originally Posted by GILow View Post
So, on another thread I asked for ideas about running a dual 12/24 volt system for the boat. Long story short, it seems to be the way to go.

However, I am making some assumptions about the capabilities of lithium batteries that are based purely on what I've read, so I'd like to test those assumptions by running them past people who are using lithium batteries on their boats.

The assumption that is most critical is about voltage drop (or sag) under load.

My anticipated peak load would be 4kW, and that would be for maybe one or two minutes at a time. Sustained loads of up to 3kW for maybe twenty minutes at most.

This would be provided at 240 v AC by a decent quality pure sine wave inverter, probably rated at 5 kW continuous and running from 24 volts.

I would expect to use a battery bank of four 12 volt 100 AH(nominal) lithium batteries, each with a 100 amp BMS. So, two parallel pairs of two batteries in series. (At this stage I am assuming I will buy a decent set of commercially made 12 volt drop ins, but I have not ruled out making my own batteries)

So, that's asking up to 80 amps per series string for short bursts and 60 amps per string for longer periods of time.

In the experience of people using lithium batteries, how well are they likely to cope with this sort of use and is voltage sag likely to be an issue, given inverters tend to be a bit fussy about this sort of thing?
I guess I don't have the whole picture, but 12v100ah in 2S2P would only be 4800wh. At 3kw draw you've only got about an hour and a half of battery. What are you expecting to use this relatively small battery for? I had 12v300ah in 4P for 14400wh. And rarely had that high of a draw on it, and there were many days I wish I had more battery.
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Old 31-01-2023, 20:59   #7
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Re: Battery bank sizing with Lithium

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I guess I don't have the whole picture, but 12v100ah in 2S2P would only be 4800wh. At 3kw draw you've only got about an hour and a half of battery. What are you expecting to use this relatively small battery for? I had 12v300ah in 4P for 14400wh. And rarely had that high of a draw on it, and there were many days I wish I had more battery.
If you read the post I said the 3kW draw was for maybe 20 minutes.

At most.
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Old 01-02-2023, 09:48   #8
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Re: Battery bank sizing with Lithium

Also worth noting is that induction cooktops don't draw that power the whole time they are on. They kind of pulse if you will and the wattage varies quite a bit. So I do think that will work. But, I think you may want to try to overdesign just a bit if possible.

They're not cheap, and it doesn't need to be this battery. But maybe consider something like this and it's just a "one and done" kind of a thing as a drop in. Then if you needed to later you could parallel a second one and double your storage and current carrying ability.
https://www.amazon.com/CHINS-LiFePO4.../dp/B08Z3TV494

I personally prefer to roll my own but there are decent options like this out there and more every day pretty much.

But to answer your question about sag - it just isn't really a concern until you get to the bottom of the charge and even then it shouldn't be a problem.

My bank at 90% was 26.57v and at 31% 26.41v according to logs.
To be fair I've got 1120Ah in 4 banks of 280 so I'm not even hitting 1C if I pull 1000 amps which is to say I'm hard pressed to even reach .25 C so I'm not going to see much sag due to load anyway. But this brings up the point of having a single bank that can handle that load because when/if you do add a second bank you have a system that should be handle your loads even with a bank failure or cutoff. That was part of my design and truly I could lose 3 of my 4 banks and still run normal off just the one bank.

If you really want a hard number though, I'd be willing to shut off 3 banks and put a 200ish amp load on and see what the results are.
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Old 01-02-2023, 19:41   #9
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Battery bank sizing with Lithium

Quote:
Originally Posted by Jenta View Post
Also worth noting is that induction cooktops don't draw that power the whole time they are on. They kind of pulse if you will and the wattage varies quite a bit. So I do think that will work. But, I think you may want to try to overdesign just a bit if possible.



They're not cheap, and it doesn't need to be this battery. But maybe consider something like this and it's just a "one and done" kind of a thing as a drop in. Then if you needed to later you could parallel a second one and double your storage and current carrying ability.

https://www.amazon.com/CHINS-LiFePO4.../dp/B08Z3TV494



I personally prefer to roll my own but there are decent options like this out there and more every day pretty much.



But to answer your question about sag - it just isn't really a concern until you get to the bottom of the charge and even then it shouldn't be a problem.



My bank at 90% was 26.57v and at 31% 26.41v according to logs.

To be fair I've got 1120Ah in 4 banks of 280 so I'm not even hitting 1C if I pull 1000 amps which is to say I'm hard pressed to even reach .25 C so I'm not going to see much sag due to load anyway. But this brings up the point of having a single bank that can handle that load because when/if you do add a second bank you have a system that should be handle your loads even with a bank failure or cutoff. That was part of my design and truly I could lose 3 of my 4 banks and still run normal off just the one bank.



If you really want a hard number though, I'd be willing to shut off 3 banks and put a 200ish amp load on and see what the results are.


Hi Jenta, thank you for the detailed response.

All valid points, my numbers for power consumption were what I think (hope) is worst case. We went through all the sorts of things we cook and tried to imagine what power setting we’d need to cook them. Like you say, most of the time the system would be pulsing at a much lower duty cycle, as little as 30% for most stuff we think. (Hope.)

The real power hog was the paella, but what can I say, some comforts I’m not willing to surrender. So it forms our worst case power draw, full tilt for most of twenty minutes.

If you do get a moment to trial that battery isolation/sag test I’d be really keen to hear what you found.

On the issue of which battery, I’ve been reading lots, watching videos by a young lad on YouTube who seems to be quite credible, and I’m increasingly tending towards making my own. I found a good kit with Eve cells, claimed to be new grade A, from a reputable online retailer here in Oz.

I’m tending towards two 12v banks made from 280AH cells, each with a 200 amp BMS. (JBD). So that’s 7.2kWh, a lot more than I initially planned.

As you said, you prefer to roll your own, and a couple of credible posters here have expressed similar sentiments. Someone made a very good point about cable gauge in the prepackaged batteries and the young lad on YouTube showed just how much variation there is out there.

Fun journey, a bit of a learning curve.
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Old 01-02-2023, 21:45   #10
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Re: Battery bank sizing with Lithium

I am glad you found a satisficed answer this was quite informative for me too. I am a learner. I don't have boat but I wish I could start my own. I love cruising and interested to start my own one day.
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Old 01-02-2023, 22:32   #11
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Re: Battery bank sizing with Lithium

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The real power hog was the paella, but what can I say, some comforts I’m not willing to surrender. So it forms our worst case power draw, full tilt for most of twenty minutes.
Use a Remoska instead, 580w and 4L capacity so able to take a 2Kg duck or enough paella to feed a racing crew.

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Fun journey, a bit of a learning curve.
Indeed Btw we are drawing 0.5c from our LFP boiling the kettle 10 times a day. Will this shorten the battery life? don't know but if its 4000 cycles rather than 5000 if we nurse it, then I am not worried. Its the ability to draw a large load repeatedly that I want for electric cooking. I think the sag at 0.5c drops the voltage down to 13.1v, but the LFP recovers instantly when the kettle has finished boiling.

Bear in mind that any solar power being generated whilst you are cooking is used first and then the batteries top up the shortfall.

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Old 02-02-2023, 20:24   #12
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Re: Battery bank sizing with Lithium

Reads like another language to me. Can hardly understand a word of it! Too techo!
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Old 02-02-2023, 21:51   #13
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Re: Battery bank sizing with Lithium

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Reads like another language to me. Can hardly understand a word of it! Too techo!
Too many TLAs?
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Old 03-02-2023, 05:36   #14
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Re: Battery bank sizing with Lithium

My boat was designed for 2 4D house batteries. Shortly after I got the boat I changed to 4 GC2 golf cart batteries for 12V 440ah. I replaced those with same once there changed to 461ah of G31 FlyFly (what a lot of crap). I now have 4 100ah LFP drop ins because those fit the same space as the 4 G31.

I never have had one of those huge house banks I read about and even through I use 150-200ah/day have never been less than 50% SOC. So I could use a smaller bank, but see no advantage other than a few $$ that are meaningless in total costs to live on the boat.
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Old 03-02-2023, 06:52   #15
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Re: Battery bank sizing with Lithium

There is an important factor that has not been mentioned in this thread. If you are using solar as your primary means of electrical generation the amount of day to day variation in solar insolation is of great importance in determing the required battery bank size.

In places like the Caribbean and the Mediterranean (in summer) the solar insolation is very consistent so consequently you can manage comfortably with a very small battery bank. If you want to cruise some other parts of the world, a larger battery bank is needed to average out the days and sometimes even weeks of poor conditions, with the better times.

A “solar” boat cruising Scotland in summer will need a much larger bank than an otherwise identical boat, with the same electrical needs cruising the Caribbean in winter. The average solar insolation is similar, but Scotland has a much higher variability.

While the ability of the battery to deliver high current at reasonable voltages is important ( it obviously has to be adequate for the load), I don’t think this is the limiting factor.

Australia is a large country, but many areas have a reasonably high variation in solar insolation from day to day. I think 200Ahrs of lithium is insufficient to contemplate all electric cooking with a boat that is primarily dependent on solar power.
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