LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[CatNewBee]

Yes, but in LFP we are talking about 2..5% and not 20% when voltage rises and tail current goes significantly down.

[newhaul]

Originally Posted by CatNewBee

Yes, but in LFP we are talking about 2..5% and not 20% when voltage rises and tail current goes significantly down.

exactly the problem is so many are still stuck in the lead charging modes

[witzgall]

Originally Posted by Ny-vern

Thanks Chris, No i was asked this by someone. I was doubting a continuos draw of 6000w!!

But if you will please see if i have this right - in principle,

6000w / 24v = 250A. If a bank had to continuously supply 250A for 10 hours it would need to supply 2500Ah.

Plus the 20% - If we consider discharging to 80%. Giving a 3000Ah bank.

(I know its ridiculous but I just want to get my head around calculating bank size).

is that correct ?

Sure, your math is correct. One other issue, there is no 6000w inverter I know of that will put out 6000w continuously. I would suspect a 10000w inverter would be needed to supply 6000w continously.

But truly, you are going about this incorrectly. First, determine the average, and peak loads. Then factor in duration. Then size accordingly.

Chris

[kmacdonald]

Originally Posted by Ny-vern

Hi guys I was hoping to get some advice,

I wanted to know with LiFePO4 how to calculate the bank size needed for 10 hours of power supply from a 6000w 24v inverter. I thought it was a simple calculation but most of what i've found uses the LA 20 amp hour rating which can't obviously be used for Lithium.

Thanks in advance!.
Best,

What possible ill-conceived use scenario do you intend to use this for?

[john61ct]

Originally Posted by Dsanduril

There's oodles and oodles of discussion about how full to charge a LiFePO4 bank. Anywhere from 80% to 100%

Sacrificing a bit of capacity off the top for greater longevity is great, I'm all for it.

But 20% is just insane.

2 different stop-charge setpoints, say at 77°C and a .2C current rate:

A: Absorb / CV set at 3.65Vpc, taper to zero current, no amps flowing at all, call that 100% SoC.

then B: Absorb is set to 3.45Vpc, and charge stops when trailing amps have tapered to .06C

I imagine you may think that SoC is significantly lower?

It is in reality 98.69%

It is only if you perform an accurate **load test** comparing A and B that you see just how much (in fact just how little) you are sacrificing by avoiding the top shoulder.

It is true, using **even an accurate** Ah counting current totalizer, the difference between A and B seems much larger than 1.3%.

However a large portion of those "incremental Ah" are just dissipated, probably mostly as heat, not actually **stored** as usable energy in the bank.

And in fact I contend that "excess activity" producing heat and leading to such lower efficiency at the final charging stages, is exactly **why** pushing higher volts and lower endAmps is harmful to longevity.

The bottom cutoff for load testing should be 2.99Vpc, which I also use as my definition of 0% SoC and the lowest last-ditch LVD cutoff.

The "actual SoC difference" between that and some other lower "industry standard" definition is again, very low single digits.

Obviously, when a LFP bank is not being cycled, it should sit at a low SoC, as long as there is no risk of it getting discharged **below** that 3Vpc.

But in daily use cycling, there are tremendous longevity cycle gains from not dropping to too low an **average** SoC point.

If you are going to "oversize" a bit for that reason alone, or perhaps even more, just to have a decent reserve for suboptimal conditions, that is entirely rational. And up to each owner to make the judgment call, even if it is an "expensive" 40% capacity sacrifice.

As long as the vast majority of that sacrifice is off the bottom end, not the top.

But only charging to 80% SoC? Nothing rational about that at all afaic, **if** you are actually measuring SoC points accurately.

That (very common) "20-80%" cliché is IMO a shibboleth showing past sloppy guesstimations of the relationship between voltage and SoC, or perhaps over-reliance on coulomb-counting after hitting the top voltage shoulder.

[SailRedemption]

No single inverter, but Victron has inverters that can be bused together to make more watts. So it's doable, but not practical. A generator is best for whatever application needs 6000w or 50a services.

[Ny-vern]

Hi All ,

thank you for your feedback. I understand the impracticality of such a large bank and draw - precisely why I wanted to confirm the numbers before relaying the information and suggesting alternatives.

Cheers!!

[CatNewBee]

I can't imagine a scenario, where one needs 6kW continuous output for 10h per day except electric propulsion. Even A/C systems do not run at peak performance all day long, also re-charging the 60kWh may be fun after the 10h use.

That is true, you can parallel the Victron's for this task, use power assist or buy a 10kVA inverter, so there are various options to get there. Keep in mind, Victron's get warm at full power and degrade output then, so over sizing is key, a 5kVA inverter can output only 4000W continuously, the model number are peak kVA not kW - it is always 20% less the kVA is measured peak voltage at the sine wave * peak current at this point, the kW are the area surface of the curve of Voltage * Amps for a cycle what represents the average output better. Read the data sheets carefully if you want to push it to the edges.

Same applies to the charging, nominal 220A boils down to 180A continuous output because the electronics step back to protect the unit from heat and also takes the voltage drop on the input into account caused by wiring.

[Dsanduril]

Geez, that's why I said it was a "minor quibble" and it was specifically because the poster had mentioned the difference from lead-acid thinking. The point is that LA think assumes that DoD is measured from 100% SoC. And this is because if you don't get to 100% SoC regularly you'll kill the batteries.

With LiFePO4 you don't need to make the same assumption. Doesn't really matter whether you choose 80%, or 90%, or 98.5%, or whatever other number as your 'normal' top-end SoC, you should still include it in the sizing question rather than making the assumption as you would with lead-acid think.

Personally I think one of the big advantages is being able to work at PSoC for a long time. And I normally see being in a sliding window regime where I work between (solely as an example) 60 and 90% SoC most of the time. That way I can charge with slower charge sources (solar), not cut off charging at the end of the day, and otherwise just not worry about SoC nearly as much as I would with LA, or with LiFePO4 if I'm trying to get above 99% SoC with every charge.

The point is, how much SoC you leave on the table is a design decision, much more so than with LA, and since bank sizing is a design question...

[rgleason]

Originally Posted by Dsanduril

With LiFePO4 you don't need to make the same assumption. Doesn't really matter whether you choose 80%, or 90%, or 98.5%, or whatever other number as your 'normal' top-end SoC, you should still include it in the sizing question

I like the sliding window concept, but wonder about what to use for Sizing LFP for Stop Discharge ? I have been using 20%SOC due to the Winston cycles indications, but is it reasonable to go lower?

Also I had guessed at about 95%SOC as being the absolute top.

[CatNewBee]

Some think it is by design, but in reality you simply don't care about SOC, it is what it is, if you have excess energy you stop charging at your 100% SOC point, otherwise you charge until there is no more charging current available and you use what you get / have until the BMS cuts off at some point, what usually never happens when you have enough resilience capacity planned for bad weather.

I do see sometimes in consecutive sunny days a 100% cut off on the charger and float to run the vessel on solar only, but I also see days where I never hit this point.

I use the energy I have, if the battery is full, I run more power hungry gadgets like A/C or hot water boiler or ice maker or the washing machine... The water maker and the fridges run daily, same for the galley appliances.

But I do not disconnect charging to not get the battery full on purpose, I am happy about every Wh I can get for free in the system.

[exMaggieDrum]

I may be remembering wrong and I don't have the time and energy to look back at the older posts, but I think the OP started out planning on a 12v bank but is now looking at 24v since the wiring can be half the size. The other part of that is that a battery bank of 400Ah at 24v is twice the size of a bank at 12v. Given 400Ah cells at 3.2v you need 4 for nominal 12v. You would need 8 for 24v. That makes the cost/benefit and space available issues really important.

[nebster]

Originally Posted by CatNewBee

Some think it is by design, but in reality you simply don't care about SOC, it is what it is, if you have excess energy you stop charging at your 100% SOC point...

But I do not disconnect charging to not get the battery full on purpose, I am happy about every Wh I can get for free in the system.

Let's be clear: that's your prerogative. Some of us choose not to allow our battery to reach 100%, since we believe that will help improve longevity.

In other words, some of us do care about what the SOC is, and we do stop charging early.

[john61ct]

Functional 100% SoC is not some objective state.

You define it as the owner, usually below where you think the "harms longevity" zone begins.

And should have protective gear in place to prevent going any higher.

No matter how conservative you are, there is no need to sacrifice more than a few % of capacity at the top.

And likely sacrificing nothing, compared to the vendor AH capacity rating.

[mrm]

Originally Posted by Ny-vern

Hi All ,

thank you for your feedback. I understand the impracticality of such a large bank and draw - precisely why I wanted to confirm the numbers before relaying the information and suggesting alternatives.

Cheers!!

There is one more factor which you omitted.

If the 6000W you mentioned is the _load_, then inverter efficiency should be considered in calculations. Efficiency will be probably somewhere around 92-95%, so for the 6000W output load the demand by the inverter will be

somewhere between 6000W/0.92 and 6000W/0.95 (6520W - 6315W).