AGM replacement with LFP, fudged battery size initially, want to do better 2nd time around.

[Simi 60]

Hi
We are full time cruisers on a boat that is set up to be away from shore power indefinitely.
When we got her the batteries and inverter were dead and on the perhaps false assumption that it was sized correctly, simply replaced with a bit more.
8 batteries replaced 6 and 5000w inverter replaced 3000w.

It seems to have worked well, batteries never get below 80% SOC (20% used) and due to large solar and Genset head start if needed (cloud/winter) have been at 100% by lunchtime every day so says the Victron battery monitor.
Saying that batteries are showing signs of fail after 5 years so I am here to learn more about LFP


Before we go to much further I will give you a rundown on what we have so as to ensure suitability and compatability with an LFP setup.

Main 240v loads come from 3 fridge freezers one 550 litre 2 door, a 120 litre bar fridge and same size freezer all running 24/7.
Occasional use of 240v pressure cooker - once a week for an hour
Very occasional use of 240v microwave - once a month for 10 minutes
2 Lenovo tiny PC running 24/7, screens turned off when not in use.
Daily we manually turn on and off the 240v 180 litre hot water system - we do that either with Genset running or plenty of sun

In the future, I would like to consider a 240v induction cooktop.

Everything else, pumps, lights are 24 or 12v

To power this we currently have.
8 X 220ah AGM @ 12v joined into 24v

Charging is provided by.
9 x 250 watt Jinko solar panels on flat roof running through a Midnite classic 150 mppt
A 7.5 is diesel Genset
And a victron 5000va 120 amp inverter charger

I did plug all that into a sizing calculator and included the hot water system it came back saying I needed about half the battery bank that I currently have, which based on current experience surely ain't right.
Pics of results included

My initial guestimate was we'd be after at least 400ah of LFP, preferably more.
I was considering 8 X 400ah Winston cells but felt that may not be enough
On here reading about the "blue" cells and we could power a planet for similar coin.

Thoughts about sizing?
Am I on the right path?
Worry about cell brand choices latter

 

[iamrich]

Sounds like you use about 9kwh a day, and have 2kw of panels x 4 hours (~8kwh). On a perfect day you are losing the battle to the tune of about 1kwh, so either you need to run the generator to top off at some point or you need to cut back. LFP 400ah at 24v is ~8kwh usable, so I would say if everything was perfect you would need to use the generator every 4-5 days to get you topped off. It won't really matter how big your battery is. If you don't either make more power or use less your battery is going to run out of juice. The size will determine how quickly that happens.

 

[time2roll]

If you are running the battery down only 20% then yes you have more battery than needed. However if I was on a boat full time I would easily want 2x or 4x+ the battery I needed for day to day use. With LFP definitely get 24v batteries to avoid balance issues. Do not get 12v batteries in series. With the current light usage I would get the same rating as you have in AGM. Leave room to add more battery and second inverter for the future induction range.

 

[chrisski]

Since you have the boat and are full time, getting accurate power usage would be very important. I have a Victron Smart shunt attached to the battery bank, and that tells me my KWh usage from the battery bank for the prior day. Between that and my SCC, I know how much power I produce every day and used that to get the correct amount of panels for what I wanted, and then when a cloudy day came around, I adjusted that.

Since you are in the boat full time its such an advantage to have good data.

If your total power usage is 9 kwh, and you wanted to build your own battery bank, you could get away with getting 16 total 280 ah cells and putting all 16 in series for one 48 volt battery, or building two 24 volt batteries with 8 cells in series. Both options come up to 12 kwh total with about 9 kwh to 10 kwh usable.

 

[Simi 60]

Sounds like you use about 9kwh a day, and have 2kw of panels x 4 hours (~8kwh). On a perfect day you are losing the battle to the tune of about 1kwh, so either you need to run the generator to top off at some point or you need to cut back. LFP 400ah at 24v is ~8kwh usable, so I would say if everything was perfect you would need to use the generator every 4-5 days to get you topped off. It won't really matter how big your battery is. If you don't either make more power or use less your battery is going to run out of juice. The size will determine how quickly that happens.

That calculator I used wanted a us postcode to work out sun.
I put in Florida as that's pretty much the same latitude as where we are in Oz and the 4.5 hours was a winter calc. So 1 hour of Genset in the am is about right and what happens near every day in winter.

In summer the sun and amps smashes in and we very rarely use the Genset at all.
Batts usually at 97% by 10am, hot water turned on then and we generally still have + amps coming in.

But did you click on the 2nd pic with batteries?
They were suggesting 400ah for AGM
But only 200ah for LFP
Something very wrong there.

Since you have the boat and are full time, getting accurate power usage would be very important. I have a Victron Smart shunt attached to the battery bank, and that tells me my KWh usage from the battery bank for the prior day. Between that and my SCC, I know how much power I produce every day and used that to get the correct amount of panels for what I wanted, and then when a cloudy day came around, I adjusted that.

Since you are in the boat full time its such an advantage to have good data.

If your total power usage is 9 kwh, and you wanted to build your own battery bank, you could get away with getting 16 total 280 ah cells and putting all 16 in series for one 48 volt battery, or building two 24 volt batteries with 8 cells in series. Both options come up to 12 kwh total with about 9 kwh to 10 kwh usable.

Generally the victron 702 says we use about 180ah @ 24v during the night - average discharge says 0.2Kah.
Online converters say that's 4320 watt hours

I'm guessing during the day it isn't counting usage as the sun is putting in more than we take out.

 

[Simi 60]

If you are running the battery down only 20% then yes you have more battery than needed. However if I was on a boat full time I would easily want 2x or 4x+ the battery I needed for day to day use. With LFP definitely get 24v batteries to avoid balance issues. Do not get 12v batteries in series. With the current light usage I would get the same rating as you have in AGM. Leave room to add more battery and second inverter for the future induction range.

I would like 4x as well but $$$ prohibits that

 

[Aussietrucker]

I recognise that boat, im in the same situation I have the same inverter and my AGM battries just crapped out after 4 years I was planning on building my own lithium pack however im stuck at Magnetic island so I purchased 4 24v 100Ah drop in replacements just waiting for them to arrive from Brissy.

 

[Simi 60]

I recognise that boat, im in the same situation I have the same inverter and my AGM battries just crapped out after 4 years I was planning on building my own lithium pack however im stuck at Magnetic island so I purchased 4 24v 100Ah drop in replacements just waiting for them to arrive from Brissy.

Hi, we are at the Keppels at the moment.
Coming out at GCCM in October and hoping with Genset assist that we can hold it all together until then.
Trying to get LFP sorted now to be at Cleveland in September but it seems pretty bloody daunting for a first timer

 

[Aussietrucker]

Hi, we are at the Keppels at the moment.
Coming out at GCCM in October and hoping with Genset assist that we can hold it all together until then.
Trying to get LFP sorted now to be at Cleveland in September but it seems pretty bloody daunting for a first timer

Lots of good information here it’s not too hard just triple check everything then triple check again if I was closer I’d give you a hand I’m planning on stopping in cairns for the summer and working up here.

 

[Simi 60]

Have fun, stay safe.

 

[Daddy Tanuki]

I love the Spirit... remember BOAT is an acronym for Break Out Another Thousand$ . @Simi 60 what did you work out in the end and how well did it work for you? @Aussietrucker Did you do road trains. That is one for my bucket list to drive a road train cross country down under... if nothing else just as a paid passenger/occasional helper.