LFP vs LA/AGM bank size

[Don L]

I have 800w of solar and 600ah LFP. I thought I would be net-zero. Ha!

Backing up in the thread:

I have 640W of solar and have 5 years experience of their performance in Florida Keys and Bahamas while 100% at anchor or on a mooring. This time of year if everything is perfect (full sun, panels facing south and not getting any shading) I can count on getting 120-150ah out of the panels (12V system). I norm for planning is about 100ah can normally be counted on. But by Mar/Apr I can count on getting 200ah out of them under same conditions.

Now I only have 1 season of use with the LFPs. It may be I get more power out of the panels on a day that was perfect and the SOC was low enough to take all the power all day the panels can put out if there was a place for it to go (my solar controller logs all this data). I don't know.

The reason I don't know is that I still have too much acid battery experience inferring with my mind. So on cloudy days I have still been running the generator to keep batteries from going below 60%. I don't know if I have have been worried about going low, or if it is just a "I would prefer to listen to the generator for 1 hour/day instead of for 3 hours/3 days.

So much about the whole thing revolves around how you operate your generator runtime for other reasons. Do you run it daily for hot water? Do run it every 3-5 days to run the watermaker (I run my watermaker every 5 days because it seems wasteful to flush it just to run it a few days later). For the powerboats do you run it to power the stove? Plus for me it is still hard to accept not worrying about having a high SOC.

I recently let my LFP get to 15-20% to see what would happen. It was hard on my acid battery trained nerves. But now I know for a fact instead of "on paper" that that was OK. I recommend everyone with LFP let their batteries run down and find out it is OK. When I did my it was cloudy for like 4 days and I barely made any solar and at the end still had to force the charge down by turning on every DC load I could.

Over the years I have learned the happiest cruisers about their batteries are the ones that don't know anything about them other than if the voltage was OK. That is almost where we need to get our minds to for LFP house banks.

 

[rslifkin]

Backing up in the thread:

I have 640W of solar and have 5 years experience of their performance in Florida Keys and Bahamas while 100% at anchor or on a mooring. This time of year if everything is perfect (full sun, panels facing south and not getting any shading) I can count on getting 120-150ah out of the panels (12V system). I norm for planning is about 100ah can normally be counted on. But by Mar/Apr I can count on getting 200ah out of them under same conditions.

Now I only have 1 season of use with the LFPs. It may be I get more power out of the panels on a day that was perfect and the SOC was low enough to take all the power all day the panels can put out if there was a place for it to go (my solar controller logs all this data). I don't know.

The reason I don't know is that I still have too much acid battery experience inferring with my mind. So on cloudy days I have still been running the generator to keep batteries from going below 60%. I don't know if I have have been worried about going low, or if it is just a "I would prefer to listen to the generator for 1 hour/day instead of for 3 hours/3 days.

So much about the whole thing revolves around how you operate your generator runtime for other reasons. Do you run it daily for hot water? Do run it every 3-5 days to run the watermaker (I run my watermaker every 5 days because it seems wasteful to flush it just to run it a few days later). For the powerboats do you run it to power the stove? Plus for me it is still hard to accept not worrying about having a high SOC.

I recently let my LFP get to 15-20% to see what would happen. It was hard on my acid battery trained nerves. But now I know for a fact instead of "on paper" that that was OK. I recommend everyone with LFP let their batteries run down and find out it is OK. When I did my it was cloudy for like 4 days and I barely made any solar and at the end still had to force the charge down by turning on every DC load I could.

Over the years I have learned the happiest cruisers about their batteries are the ones that don't know anything about them other than if the voltage was OK. That is almost where we need to get our minds to for LFP house banks.

I agree, once everything is set up and can be trusted to work, with LFP the ideal would be to just set a low SoC warning at some point. If you're not getting down to the warning level, it can all be pretty much ignored. If you get down to the warning level, then it's time to pay attention and decide if you need to fire up the generator for a bit or something.

Even with my current AGMs I try not to watch too closely as I know the system works well and only really needs extra attention or power if there's a significant shortage of solar input. I just try to make some note of the SoC when I get up in the morning and whether they get to 100% in the afternoon or not. 1 day of not getting topped off is no big deal, but 2+ days and I'll try to do something about it.

 

[Barking Sands]

Backing up in the thread:


Over the years I have learned the happiest cruisers about their batteries are the ones that don't know anything about them other than if the voltage was OK. That is almost where we need to get our minds to for LFP house banks.

I could not agree more. I am in the process of bench testing and proving the 920Ah Lifepo4/ Victron system that will go in my boat. I am not even using it yet and I am obsessing over a wide array of details. The reason, IMO, I am obsessing over a wide array of details is because there is a a wide array of details. The system is all Victron with batteries that have "Victron Comms" and many other fancy things such as NMEA2k data out to the Garmins and other superfluous BS. Typically I love this kind of stuff. However, after spending a considerable amount of time hovering over my bench set up and testing it this way and that, I am beginning to settle in to my typical philosophy that targets the 80/20 rule. Striving for perfection to get the last 1% of optimized performance just because we bought expensive batteries will drive you crazy.

Like most things, I am sure I (we) will fall into a normal routine of operation that surrounds a few fundamental parameters. I am ready to be there now...lol.

 

[Helmsman]

Here is a product from Victron that purports to protect the system in several ways. Anybody have one of these installed?

https://www.victronenergy.com/upload/documents/Datasheet-BMS-12-200-EN.pdf

 

[Don L]

seems like a lot of marketing to me

I feel people don't seem to understand that is a BMS should NEVER do anything to protect a battery (yes my LFPs have a BMS). LFPs with "standard" BMS units already are way more protected than we have had on our acid batteries all this time

 

[guy with a boat]

seems like a lot of marketing to me

I feel people don't seem to understand that is a BMS should NEVER do anything to protect a battery (yes my LFPs have a BMS). LFPs with "standard" BMS units already are way more protected than we have had on our acid batteries all this time

You don't need an external BMS if your batteries have built-in BMS, but not all batteries do. A BMS of some sort is required for safe operation. I think you are just saying you don't need 2 layers of BMS, because you do need one.


They protect your batteries and system somewhat like circuit breakers. You shouldn't rely on either for your primary control of current, but they are both necessary to have so a malfunction doesn't turn into a disaster.

 

[Mambo42]

It is true that, if a battery has an internal BMS it will make sure it get charged correctly, however, a BMS also protects the battery while discharging. If you have multiple batteries a common BMS will make sure all batterie get charged and discharged at the same rate.
Is it necessary ?
Some say yes, some say no. The No sayers will point out that the voltage is equal, but that does not say anything about the state of charge.
I have 7 x Victron 24 V -200 Ah and they do have a BMS inside, but on top of that I have a common BMS and all the batteries are linked up to this BMS. I also have 2 x battery protect switches of 500 A.
Is it overkill ? Perhaps, but it does not hurt having it onboard and it does function well.
The only problem that I have is keeping the batteries in good condition for the winter time, so while I am now preparing the boat for the upcoming 3 months, I am disconnecting the llithiums, will keep them at around 80 % charge and instead I will connect 2 x 12 V 250 Ah AGM's again. For some reason Lithiums don't seem to like to be stored at 100 % all the time, so was advised to try not to do that. Having no idea how to program the inverter charger to only charge to 80 % I decided for this option. It is a bit of work, but if it helps to save the lithiums then why not ?
If anyone can clarify this with more technical insight I am all ears.

 

[Bmarler]

It is true that, if a battery has an internal BMS it will make sure it get charged correctly, however, a BMS also protects the battery while discharging. If you have multiple batteries a common BMS will make sure all batterie get charged and discharged at the same rate.
Is it necessary ?
Some say yes, some say no. The No sayers will point out that the voltage is equal, but that does not say anything about the state of charge.
I have 7 x Victron 24 V -200 Ah and they do have a BMS inside, but on top of that I have a common BMS and all the batteries are linked up to this BMS. I also have 2 x battery protect switches of 500 A.
Is it overkill ? Perhaps, but it does not hurt having it onboard and it does function well.
The only problem that I have is keeping the batteries in good condition for the winter time, so while I am now preparing the boat for the upcoming 3 months, I am disconnecting the llithiums, will keep them at around 80 % charge and instead I will connect 2 x 12 V 250 Ah AGM's again. For some reason Lithiums don't seem to like to be stored at 100 % all the time, so was advised to try not to do that. Having no idea how to program the inverter charger to only charge to 80 % I decided for this option. It is a bit of work, but if it helps to save the lithiums then why not ?
If anyone can clarify this with more technical insight I am all ears.

Can’t you adjust the charger voltage to a lower float setting for storage?

 

[Don L]

I have 7 x Victron 24 V -200 Ah and they do have a BMS inside, but on top of that I have a common BMS and all the batteries are linked up to this BMS.

Is it overkill ? Perhaps, but it does not hurt having it onboard and it does function well.

See those are expensive LFP and I think the more people pay for the batteries the more they feel need for extra protections. So to me your extra BMS is overkill. It doesn't of course hurt anything and you feel better protecting the money of the expense batteries.

Meanwhile there now people like me using what we feel are "cheap" LFPs of less than $300/100ah 12V units that still believe batteries are consumable items and are ok with them "only" lasting 5 years of full time use (think they are going to last a lot longer).

BTW - I have wasted lots of time reading about the "storage" issue. It is true near as I can tell that there is a lost of capacity by storing at 80-100% SOC. But it is only 2-3% more than storing at 60-80% SOC. So it is mostly meaningless unless you think 3% capacity makes a difference. The best storage seems to be 30%. I think it would be less trouble to just get a small charger that you could set to a low voltage and use that for "storage" than swapping batteries.

 

[TBill36]

You don't need an external BMS if your batteries have built-in BMS, but not all batteries do. A BMS of some sort is required for safe operation. I think you are just saying you don't need 2 layers of BMS, because you do need one.


They protect your batteries and system somewhat like circuit breakers. You shouldn't rely on either for your primary control of current, but they are both necessary to have so a malfunction doesn't turn into a disaster.

What are the consequences if the BMS malfunctions and you know longer have "safe operation", "so a malfuncition doesn't turn into a disaster"? Is this why I've heard that some insurance companies (Markel) are refusing to insure them?

 

[tiltrider1]

What are the consequences if the BMS malfunctions and you know longer have "safe operation", "so a malfuncition doesn't turn into a disaster"? Is this why I've heard that some insurance companies (Markel) are refusing to insure them?

If your BMS fails and you have an overcharge situation, your batteries will expand probably damaging the internal case and becoming non functional. There is also the possibility of a runaway charge system turning into a fire situation. I say possibly, most say only a very low probability others say no possibility. In any case it would take a dual failure.