Very Basic Rules of House Lifepo4 Simplified - Rule1,Rule 2,Rule3 and Rule4

[Fuss]

Its crazy what goes on here with confusion, complication and more confusion.... and then a few weeks later ...repeat...

I have just been slightly involved in the "lead is dead" thread in electrical, batteries category.
I'm going to get some stick for this, I know...

We need to break it all down into Rules and so for now we have Rule 1 and 2 and 3 and 4, later we can have rule 5.
yes, yes, I know, but anyway....

Simple Rule 1 for Lifepo4...
The BMS must have a higher High Voltage Cutoff (3.6v) than the charging devices such as alternator, generator, chargers (3.4v).

The BMS is last resort, sort of like a fuse, if a charging device completely unexpectantly gets a fault then the BMS disconnects the battery. So this should nearly never happen.

Simple Rule 2 for Lifepo4...
There must be a minimum of 2 batteries for redundancy.

Simple rule 3 for Lifepo4...
Lifepo4 does not need to be fully charged charged so the extremes could be 3.1 and 3.4v. Try to keep between 70% depth of discharge and 30% when out and about.
So size the system accordingly. You can fully charge back at the dock or with your solar occasionally. I imagine most old lead chargers would be ok as they're under BMS High Voltage Cutoff.

Simple Rule 4 for Lifepo4...
Top balancing using the BMS is not that important.
If your batteries regularly go out of balance, there is something wrong and it needs to be fixed. BMS Balancing can be done at the dock or by your solar sometime.

[SaylorMade]

It is natural with confusion when new technology (LFP's are not new, but still "new" in the cruising world) arrives.

I am not sure any of the posters on this forum (with one or two execptions) are in a position to create "rules" for LFP batteries.

It's great to have a sharing of experience among actual LFP users.

[SailingHarmonie]

Originally Posted by Fuss

Simple rule 3 for Lifepo4...
Lifepo4 does not need to be fully charged charged so the extremes could be 3.1 and 3.4v. Try to keep between 70% depth of discharge and 30% when out and about.
So size the system accordingly. You can fully charge back at the dock or with your solar occasionally. I imagine most old lead chargers would be ok as they're under BMS High Voltage Cutoff.

With a “rule” of cycling batteries from 30% to 70% for routine house usage that means I need a Li battery with the same nameplate capacity as the SVLA batteries typically used and cycle from 50% to 90% while “out and about”.

I thought one of the benefits of Li batteries was you had access to much more of the nameplate capacity in normal use, and could use a battery with a smaller rated capacity?

Previous recommendations I have seem recommend a 20% to 80% cycle for a Li installation. Your rule would require a battery capacity essentially equal to the SVLA bank being replaced and 50% bigger than other “rules”.

Who’s right? And why?

[wholybee]

You didn't think you would post that without differing opinion?

1. Agree. The BMS should NEVER be allowed to disconnect.

2. Absolutely not. LFP is Much more reliable than Lead. While it is accepted practice to have 2 banks with lead for redundancy (Start and House at a minimum) that really isn't a rule with LFP. A single LFP bank sized and with an appropriate BMS could be used for both start and house and that is fine. More often, there is still a Lead-Acid battery for starting because that is cheaper. And redundancy is never bad. But there isn't any reason to divide a house bank into 2 batteries. Build a larger bank instead. Use higher quality parts. If you go the budget route, then you get what you pay for, and a second battery is warranted.

3. Partially Agree. LFP does not need to be fully charged. But the negative effects of operating LFP outside 30%-70% are exaggerated. What is detrimental is how long the battery is at rest outside 30%-70%. If you are cycling daily, it is less than 0.5% of the battery life where it will be at rest at 100%. That isn't going to hurt anything. You can cycle LFP from 1% to 100% without any concerns. When you leave the boat, leave it below 70%.

4. Partially Agree. An *initial* top balance is critical. In theory that is already done for drop in batteries. If you build your own bank, you must with 100% certainty top balance before you begin using the battery. I do agree that if the BMS is unable to keep the battery balanced, something is wrong. Most fix that with an active balancer, which is a workaround not a fix.

[rslifkin]

Originally Posted by SailingHarmonie

With a “rule” of cycling batteries from 30% to 70% for routine house usage that means I need a Li battery with the same nameplate capacity as the SVLA batteries typically used and cycle from 50% to 90% while “out and about”.

I thought one of the benefits of Li batteries was you had access to much more of the nameplate capacity in normal use, and could use a battery with a smaller rated capacity?

Previous recommendations I have seem recommend a 20% to 80% cycle for a Li installation. Your rule would require a battery capacity essentially equal to the SVLA bank being replaced and 50% bigger than other “rules”.

Who’s right? And why?

If you have the ability to put in a larger bank, doing that and being conservative about how much you use has 2 advantages: longer battery lifespan and more reserve capacity for times when you may need it. Only using the 30 - 70% range normally seems a step too far to me though.

[emilecantin]

I think rather than rules, we can have strategies. Your strategy is basically "always under-charge the batteries". It's a fine strategy I'd probably use for an old externally-regulated alternator, but relying on under-charging (your rules 1 and 3) and then following that with never balancing is a good way to get your BMS to trip because of a cell over-voltage when you least expect it. With this strategy, I'd say top balancing is VERY important. An imbalance is not something that'll happen suddently so you at least have time to see it coming, but you'll need to keep an eye on it. For example, Victron recommends you do a full charge (so the BMS can balance) at least once a month.

[Statistical]

Originally Posted by SailingHarmonie

With a “rule” of cycling batteries from 30% to 70% for routine house usage that means I need a Li battery with the same nameplate capacity as the SVLA batteries typically used and cycle from 50% to 90% while “out and about”.

I thought one of the benefits of Li batteries was you had access to much more of the nameplate capacity in normal use, and could use a battery with a smaller rated capacity?

Previous recommendations I have seem recommend a 20% to 80% cycle for a Li installation. Your rule would require a battery capacity essentially equal to the SVLA bank being replaced and 50% bigger than other “rules”.

Who’s right? And why?

This is why "rule" threads are just pointless. People take their own usage and try to make it a rule.

I had LFP house bank for 6 years I charged it to 100%. The one advantage of LFP is you don't HAVE to charge it to 100%. Some chemistries perform poorly is you do short charging and never get to 100%. If you want or can only charge to 80% that is fine, 70% also fine 92.8378% because that is your lucky number also fine, 100% again fine.

As far as discharge the rated cycled for LFP are based on 80% depth of discharge. Discharging less will be more cycles, discharging more will be less cycle but here is the thing for most people cycles don't matter. Cycle life is usually 2500 to 6000 cycles. If you cycle every single day that is 7 to 16 years. However unless your usage is remarkably consistent and the battery bank is a little undersized it is very unlikely you will draw it down to exactly 20% SoC every single day 365 days a year for 7 to 16 years continuously. Some day you may only draw it down to 30%, some days (lots of motoring) only 50%, some days you want draw it down at all, and yes very rarely you might draw it down to less than 20%. So the lifespan based on rated cycle life is probably 15-20 years under real life conditions. Babying the battery may make it last 20-30 years if there wasn't calendar aging.

However LFP also age due to calendar life as well. Even if you bought an LFP battery and kept it at ideal storage SoC (varies by manufacturer) at around 50% with a dedicated charger in a climate controlled room it is going to lose capacity slowly simply due to time and in 20 years you likely will be replacing them anyways.

So baby LFP is just a good way to waste money. It maybe makes people feel good about it because it is a large investment but it isn't needed. No manufacturer recommends it. No test cycle by any manufacturer involves "babying cycles". EVE for example rated test cycle is charge to 100% at 3.65V per cell (so 14.6V for 12V battery) @ 0.5C amps (i.e. 100A for 200Ah cells) then discharge at 0.5C load (i.e. 100A for 200Ah battery) until 20% SoC. This is quite aggressive and is still rated for 6,000 cycles @ 25 degrees celsius) (and 2,500 cycles in a punishing 45C)

Cycle Life at 45C > 2,500 cycles
---------------------------------
Procedure: Under the 300kgf clamp, after standard
charged and 30mins rest, discharge to 2.5V
cutoff with the current of 0.5C(A) at
(45±2) ℃, and then start the next cycle，end
with the capacity decreasing to 80% of the
initial capacity. The number of cycles is
defined as the cycle life of the batter

Standard charge and discharge
Charge / discharge current 0.5 C/0.5 C
Cut off voltage of charge / discharge 3.65 V/2.5 V

Now if you personally want to limit charging to say 90% SoC and personally as a rule don't discharge below 30% except in emergencies feel free. If it brings you peace of mind it will result in a gentler cycle for your batteries. Is it some requirement? No. If it was the cell manufacturers would require that.

[Statistical]

Originally Posted by rslifkin

If you have the ability to put in a larger bank, doing that and being conservative about how much you use has 2 advantages: longer battery lifespan and more reserve capacity for times when you may need it. Only using the 30 - 70% range normally seems a step too far to me though.

Reserve capacity is useful although I would argue even with a 20% discharge target you still have quite a bit of emergency capacity. Yes discharging an LFP to even 0% is fine as long as it isn't routinely done.

As for longevity this is largely theoretical. For a year round dedicated cruising the cycle life of LFP under a 100% to 20% cycle is going to be 7 to 15 years. Realistically even if someone discharge target is 20% they won't discharge to 20% every single day for 2,5000 to 6,000 days continuously. Meaning real life will be longer.

Sure babying the battery in theory will make it last even longer maybe 25 years vs 20 years but LFP cells suffer from calender aging and this simply can't be prevented. In 20 years the battery will have less capacity no matter what you do.

I agree with you though that being somewhat conservative can make sense (i.e. 20% to 90%) however the OP mandating as a "rule" 30% to 70% is silly. If the OP wants to do that then feel free but it isn't a requirement. If it was then cell manufacturers would specify that as a requirement and their cycle life test procedures would be a 70% to 30% cycle not a 100% to 20% cycle.

[sailorboy1]

I agree with 1, 3, 4

far as 2 goes, there are small boats with only 1 house battery and LFP doesn't change that

I have been on the forum since the first time I ever even heard about LFP.
I think their current biggest is all the "must do" from the earlier days when all of these installations were a science project for the County Fair

[Fuss]

Settle down settle down!! I had the „lead is dead“ thread completely under control but this amount of questions is giving me indigestion!!

Ok, keeping things simple … 30 70 is needed as once you discover the huge benefits and increased usage capacity, you will buy gadgets…and lots of them…then you will be much more experienced to take your slightly over capacity batteries nearer to the edges…like 90 10 sometimes.

2 batteries are needed for redundancy…I fully agree that the lifepo4 can be used for starting too, I did that for more than a year and created a thread on it eve280 starting a 6l diesel with 3500w starter. (Some on here, said the batteries would explode and the lifepo4 would be ruined in a month…but they learnt a lot and one of them is now one of the most prolific lithium posters on here.)…But I want to do the lifepo4 starting thread seperate from house… even though they are connected…
You need more than one as it’s so easy to get an show stopping electronic fault with only 1 battery.

[leecea]

Settle down settle down!! I had the „lead is dead“ thread completely under control but this amount of questions is giving me indigestion!!

As someone trying to learn, is it safe for me to assume you are now the forum's #1 authority on LFP?

[Fuss]

Well, thankyou, I am honoured, but alas, I feel I am unable to accept such an award at this time...
Leecea, didn’t you yesterday facepalm me in the „lead is dead“ thread.
but I digress.
Perhaps my esteemed forum members, Sailorboy1 or CaptainRivet are more qualified to comment on my suitability.

[PippaB]

Somewhere around half of the battery bank sizing question is around reserve capacity. How long do you want to be able to go without charging (broken alternator, cloudy day, windless day)? 12 hours? One day? Two days?

With an LA system the normal procedure is daily cycling, and by not going below 40-50% DoD on a regular basis you a) extend the life of the battery and b) retain reserve capacity for when charging doesn't happen. You can easily draw an LA system down to 20% SoC, giving you an actual usable capacity of 80% on those occasions when you can't charge. The cost of doing so is a reduction in cycle life, hence the recommendation to not go that deep too often.

If you only charge LFP to 70% on your normal cycle then you are actually worse off than with LA (for the same nominal battery size), where the recommendation is to get full as often as possible. You end up with less reserve capacity.

Not advocating for LA, I run LFP (even do it for a living), the advantages to me are cycle life, PSoC operation, and reduced voltage sag. If you factor in that LA has a realistic 80% usable DoD, and that you don't go that deep because you want the reserve, then operationally LA and LFP are not that different. You can quibble about 90 or 95 or 100%, but if you don't regularly get them close to full, and don't limit your discharge to 50-60% SoC then you are doing away with the reserve capacity that comes along for the ride with the 'typical' LA installation.