Simple LiFePO4 battery bank for DIY?

Dave,
While this is true for currently manufactured packs like the Resu 10 and  DIY LFP 10 kWh banks, it's not a limitation for larger 20 kWh (400AH@51.2V) DIY LFP banks capable of 6700 watts continuous and much higher peak. And 400AH is by no means the largest prismatic LFP cell. But I do believe that the availability of much larger prefabricated banks is not far away.
By the way, my 10 kWh LFP bank will start our 2 ton 2 stage heat pump. The 4000 watt Radian inverter isn't crazy about it since it's an 8000 watt surge, but it does manage it.

Rick

Rick, this may be true, but none of my clients would ever do what you guys are doing. Over 100 and there are none who would do this with my name on it.

I also have been told that surging over the spec is like taking 5 cycles away, or worse. This is why it is measured and a really good BMS will shut down if the inverter has not already. A really good BMS can also have firmware changed to allow certain users to exceed the limits but at reduction in warranty. When they are in the store here and at my supplier, is when "not far away" is here to me. Wiring together a bunch of parallel banks is not what I want to install either.

I do like watching the homebrew side!  Keep it up! 

It does always come down to the shekels, and using them easily. I do fine on 9 kWh, but not many can in a home with people offgrid.
20 kWh is very nice

jonr said:

At C/2, a charge shuffler moving less than C/2 is actually moving current from high cells to the load, not cell to cell.  Ie, it's protecting the weak cells from wear, increasing life of the string.

The active balancers/charge shufflers that I have seen transfer the current to the the adjacent cells. I agree that if this raised the voltage of the adjacent cell by enough to activate that cell's balancer and so on that the current would be transfered directly to the load. As the balancing current is fairly low I doubt it will raise the cell voltage enough so the current will end up going to the next cell until it gets too far out of balance. This will mean the current will end up "hopping" its way through each of the cells or even worse hopping backwards and forwards
I can maybe see this being beneficial in high power applications like a bus or truck. Tesla don't bother with active balancing on their Model S, not sure what they do on their other cars.

My off-grid battery spends most of its time in the top 50% SOC,  the lowest recorded SOC was 9% and is regularly at an SOC in the high 90s. Under these conditions I don't see any advantage trying to keep the battery balanced at both the top and bottom, in fact I only see that the extra shunting of charge between the cells that would be needed to keep it balanced at both the top and bottom would cause extra wear on the battery.

Simon 

jonr said:

I expect that the best argument against charge shuffling may be that it's expensive and isn't required.

I agree, one positive with the charge shuffling balancers is that because they can shunt a larger balancing current than the passive resistive balancers and do it in both directions is that there is less likelihood of any damage in the event of someone buying a number of cells and whacking together a battery without balancing it to start with. Not recommended, but I am sure it does happen!

Simon

Here are the KISS rules again for diy'ers:

1) Buy reputable quality cells from a reputable dealer.  That means each cell will be matched reasonably well for both capacity and internal resistance.  An *exact* match is not strictly necessary.   DO NOT be a cheapskate, which is tempting for diy, and that means used / old stock, or deals too good to be true.  THIS is the cause for most of the failures starting out on the wrong foot.  In the lead-acid world, I don't buy "Brojans" that are "slightly used" for the same reason.

2) Charge each with a CC/CV algo, set to 3.6v absorb, and when the current naturally drops to .025C, stop.  Use a charge source preferably between 0.1 to 0.33C.  If you don't know how to *properly* use a programmable bench-supply, "single cell" 3.2v nominal chargers are available for this purpose.  Shoot for 0.2 - 0.3C current capability.  Don't use the wrong voltage charger designed for non-lfp lithiums, ie 3.7v nominal (4.2v cv) chargers !!!

3) Clean and assemble your "links".  Be SAFE and TAPE your tools from the handle to the head in case you accidentally drop it.  A LIGHT coating of aluminox, penetrox or the like is beneficial.  Dont use wire-brushes for this task.

4) Set your low-voltage pack-level disconnect to 3.1v per cell as a conservative point to trip at.  Because you are top-balancing, you should expect a small amount of "ragged" levels towards the end.  Because you sized your bank properly (0.1 to maybe 0.2C maximum draw), you have the luxury of time to go pack-level.  Not so with EV'ers or RC'ers.  (Beware of helpful hints from those outside our low-current application, as they are not always correct)

5) Your CC/CV charger should have a CV from anywhere between 13.8v to 14.4v.  Lower voltages will take longer for the CV absorb to finish.  There is no "holy grail" voltage - it is up to YOU when to stop.  Stop at a specific voltage?  Stop at a specific absorb charge current?  That's up to you.

6) When your cells reach the upper end of charging, the cells should be between 3.45 to 3.6v.  Ideally, they should be no more than 0.1V apart from each other.  Usually it is much less.

7) Obsessing over getting the cells to match *exactly* in voltage during charge is a fool's game.  It's nice if they do, but less than 0.1v difference is acceptable.  Playing this game with flaky bleeder / shunt boards that are an additional point of failure, introduce the TIME DEGRADATION aspect of holding cells at full charge while others catch up.  Cycle over cycle, this time factor adds up (oxidation of the cells).   TIME spent doing this needless obsessive exact matching is something overlooked by those who want to sell you all sorts of rat's nest gadgetry.  (or needed to babysit TRASH cells - but we warned about that from the start!)

8) Did I say don't buy TRASH?  I'll say it again.  When doing a lead-acid bank made of 2v cells, I don't buy "Brojans" from the back of some guy's truck, or lightly-used "Bolls" batteries that sat in a warehouse for 5 years.  Common battery-sense should apply, from purchasing to safety concerns.

@bear_essentials
I agree with @bill von novak . It is a great price, but I would want to see inside the box first. If it contains larger LFP prismatic cells, then they can be used to set up a battery bank. If it has lots of small cells then I would pass.

Rick 

bear_essentials said:

Hi all,
I've got an existing Outback Radian 8KW power center with 1150AH Gel batteries.  I'm looking to replace these batteries with 700AH LiFePo.  I have a line on some Chinese made batteries from Eastar battery company.  The batteries are rated 1C discharge and 0.5C Charge, which is plenty sufficient for my application.  The batteries come with a BMS.  The picture of the battery case (each case contains 200AH of batteries) shows two sets of terminals; 1 pair for charge and 1 pair for discharge.  My Outback power center only has a single pair of wires to the existing Gel battery bank.
Question: Will this pose a problem connecting the charge and discharge to the Outback?  Will i need to bypass the MFG BMS and connect directly to the batteries?
BTW: the price that I've been quoted is tough to resist.  700AH 48V delivered for $10K.
Thanks for the help.

I agree with @bill von novak that if you can get the individual cells for less do that and organise your own BMS.

Ideally your Outback Radian can be configured so that external inputs to the Radian can be used to stop charging or turn the inverter off in the event of any of the cells being overcharged or over discharged. The overcharge or over discharge outputs from the BMS you choose would be connected directly to the Radian. What is the model number of the Radian you have?

Simon

That's good news. Let us know how it goes with the new bank.

@Ampster,
200 and 400 amp hour CALB LFP cells are available for 119 to 126 and 259 to 249 dollars per cell respectfully, depending on quantity. I know 2 people who have bought and installed them. In fact, I installed one of those, a set of 16 400AH, for a friend. Here's the link if interested.

https://www.alibaba.com/product-detail/3-2V-200Ah-Prismatic-LiFePO4-Battery_60790265531.html?spm=a2700.wholesale.pronpeci14.6.55e33d15pxql5M

Rick