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

[greg_kulosa]

Originally Posted by diugo

In my RV I use a low-cost Arduino-like microcontroller to monitor cell voltages, handle charging, and (very rarely) trigger a shunt-trip circuit breaker to disconnect the batteries if the voltages get out of range.

Is this something that you put together yourself, or do you have a link to the website?

Thanks.

[jram]

I run my own system for a 3P4S bank of 540Ah. Its run for over a year now in its prototype form and I'm pretty happy with it.

Its designed to work with latching or conventional relays for HVC and LVC. Also has 2 other programmable output channels (at Bank voltage) for controlling other accessories. Temperature monitoring, alarm etc.

Reports via WiFi.

I'm just about to install the latest version (new board - https://photos.app.goo.gl/6tBB5gJdmB7ZH3nn7).

Originally Posted by greg_kulosa

A while back in this thread (Ha! - Maybe a year or two?) there were several discussions from folks that were designing their own Battery Protection Systems.

Did anything ever come from that?

My first system used a Cellog in a RV travel trailer, and it's worked great for several years.

I'm about to build a larger system for a Class-A motorhome, and I was wondering what the current recommended system is for a DIY type application.

I am NOT interested in a system with cell balancing, and NOT interested in any "drop-in" type system that arranges the cells in series before paralleling.

I'm probably aiming for 400-ish AH system, depending on what cells I can get to fit in the space available.

And yes, cost is a factor, plus it's more fun to build it myself.

Thanks.

[OceanSeaSpray]

Originally Posted by greg_kulosa

A while back in this thread (Ha! - Maybe a year or two?) there were several discussions from folks that were designing their own Battery Protection Systems.

Did anything ever come from that?

Yes, other projects have kept me very busy for a couple of years, but this is now getting much closer as I have been able to get back to it almost full-time lately.

It is quite a significant undertaking and there have been several iterations of the hardware. It is really nice gear, the concept is maximising the value delivered for building marine systems. I just built a new board last week as I was running out of memory with the previous one and there is a little testing and development to go into some new features.

[tanglewood]

I have built a BMS using an industrial PLC. The goal was flexibility, reliability, and off-the-shelf replaceable parts.


Adventures of Tanglewood: Building a Battery Management System (BMS)

[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,

[witzgall]

10 hours at what power level? 6000w continuous? I don't think you actually mean that?

If this is for a boat, just do the math. We have a load calculator here that should help: Selecting a Charging System | Balmar


It is a safe bet to consider 80% of LFP capacity as the usable part. This would potentially degrade a few % every year.

Chris

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,

[Delfin]

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,

Assuming you mean around 250 Ah @ 24 vdc as the power requirement between charge cycles, 'twere me, I would size it for a 70% DoD, so around 350 - 400 Ah bank size. As Mr. Witzgall indicates, many would use 80% DoD, but the depth of discharge does affect longevity so I'd err on the side of slightly larger sized bank rather than the minimum.

[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 ?

[CatNewBee]

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 ?

Yes.
thats right.

[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,

You need a generator. It will work much better for that kind of load and be a lot cheaper. A 6000W inverter to run continuously for 10 hours just isn't remotely plausible.

[Dsanduril]

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 ?

Just a small quibble, the maths are right, but most LiFePO4 systems don't recharge to 100% SOC. So you have to decide what your actual recharge level will be. Pretty common is 20% low and 80% high, which means you actually only have 60% of nominal capacity available.

=> 2500Ah/0.60 = 4,167Ah bank size

So, you can see that not just your bottom end but also your top end SOC decision plays a key role in this bank sizing exercise.

[CatNewBee]

LiFePO/ charge easy and stright to 100%, no problem in high load applications to go that high.

I see more issues with the inverter.

Beside propulsion, what application would require continous 6kW power for 10h?

[Dsanduril]

There's oodles and oodles of discussion about how full to charge a LiFePO4 bank. Anywhere from 80% to 100% depending on the theory(ies) to which you subscribe and the goals you have for the system. Assuming you'll get to 100% on recharge is also lead-acid thinking. If you choose 100% SOC at recharge that's fine, it's just a decision that has to go into the equation.

[CatNewBee]

no problem using the full range when cycling, it is only harmful to stay all the time at 100% SOC on storage....

[Dsanduril]

Originally Posted by CatNewBee

no problem using the full range when cycling, it is only harmful to stay all the time at 100% SOC on storage....

Not everyone agrees with the statement. With lead-acid (Firefly Carbon possible exception) consensus it that 100% SOC on a very regular basis is a necessity for long battery life. And getting there on an off-the-dock boat isn't always easy with the long, low tail currents.

You still have a tail current with LiFePO4, just not nearly the same time restrictions as LA. But it does mean that last few percent (pick your number) are slow. You also don't need to get there for battery health (or at least it so appears). So you can have high-current short-time charging scenarios that get you close (again, pick your number) but not quite there. In LA this would be death.

That leaves you with deciding how you want to run your pack, and thus deciding the 'normal' top end you'll use. And most of the systems I've seen don't use 100% SOC.