Simple LiFePO4 battery bank for DIY?

Raj174 said:

This is a sample LFP life cycle vs capacity graph

The sad thing about the balancer boards, is failures.  Any way to check on their track record? 

And they can only bypass 6A.  If you are charging with 20A, you still have 14 going into the battery and you have to rely on the balancer to trip the Charger Stop Command and stop the charging.  And you have one cell at Full charge voltage, and the rest of the pack is still behind. I suppose you could rig up another 5 A  charger and get all the cells topped off,

Not saying you can't use them but that they are not magic and can fail, just like the rooftop microinverters are doing

DaveB said:

I'm one of those people who use and wrote up exactly what you are talking about here.

- Where are people getting new 3.2V LiFePO4 cells from?  My understanding is I should be looking for 1C or less cells.  Are some brands preferable then others for a renewable energy environment?

You can use any "C" you like, but for our application, solar storage, properly sized in capacity means that you don't need high-C cells if your system is designed for 0.1C discharge or less so you have adequate runtime and for many that means extra days of no sun!  So the "high-C" rate cells are basically a waste of money if you are only discharging at say .1C or less right?

For a DIY'er, I've always said to keep it simple with 4 prismatics for a typical experimental 12v battery.  I find that the 40ah cells are a good budgetary limit in case you make a total mistake!  Ie, don't bite off more than you can chew, but hey, if you wanted to start out with 200ah cells, and have the budget and risk factor taken into account - go for it.

The cells we deal with appropriate for low-rate discharge compared to ah sizing, are not high-rate cells.  1C continuous, and 3C for short <30 second bursts.  Still, that is waaaay overkill.  I mean, who designs a solar system with lead-acid for 1C, or even .5C discharge?  Nobody.  So the so-called lower-performance cells like GBS, Winston, CALB etc suit our purposes just fine.  We're not starting a diesel locomotive, but running lights off inverters, etc.

GBS (aqua cells with purple cell covers) works for me, and the reason I recommend it for DIY'ers starting out is *safety*.  Only GBS provides the cell covers so you don't have to also DIY your own terminal cover system.  In your haste to get things running, the cell covers with other brands become a "get a round to it" kind of thing, and then you drop a wrench and ruins your day.  ElitePowerSolutions is just one vendor.

- Does it matter if the bank has aluminum strapping around it that I've seen on some banks that is supposed to prevent deformation if the battery gets hot?  I assumed in a RE situation this would not be an issue unless something is seriously wrong.

The strapping is to help force the vents to open, rather than let the cases expand if you system goes haywire.  Often overlooked, the need for strapping is to prevent terminal-torque twisting (side by side or up and down), which will kill the battery.  Somebody backed into your battery box?  Vibration from a neaby air-compressor hitting the box/cells?  etc etc.  Because the cells are so light, accidental movement which twists the terminal connectors the wrong way is a bad thing. 

- There have been numerous mentions of different Battery Management Systems.  Is there currently a reliable BMS that is simple, and has all the safety features? (balancing, low voltage disconnect, high voltage disconnect) The "simplest" BMS I've seen and has been mentioned here is the one from evparts.com.au but requires importing from Australia.  Does anyone have experience using that one?

Talked to death on most forums.  Something I touted early on was the KISS principle, which we can get away with *due to our low-current discharge application*.  Even the hard core marine guys have gotten in on the act, many running lower voltages than they used to, disabling the parasitic balancing (mosfets failing closed, and UNbalancing a perfectly balanced bank), etc.

Some BMS companies have gone out of business, or to another product, and replacement parts are hard to find.  Once again, the KISS principle works here (nothing more than top balancing initially by charging each cell individually first), and setting a conservative low-voltage disconnect.  It is a catch-22 situation - the reason I need individual cell monitoring is because the bleeder/balancer mosfet failed, draining one of my cells!  And sadly, for those balancing systems that do work correctly, underneath there may be bad quality cells that DO need this kind of babysitting - and the time needed to do the balance hurts the cells in the long term.

- There was concern about disconnecting the charge controller from the battery bank while PV is still connected to charge controller.  Is this an issue I need to be concerned about?  You already have a circuit breaker between the CC and battery bank that can be turned off manually even when PV is active.

Just treat your LFP bank with the same concerns externally as a PB bank.  *HOWEVER*, because lithium cells will give you everything they have until they are nearly depleted without much voltage drop, FAST fuses, and quality dc disconnects cannot be skimped upon.  Ie, no car-stereo stuff.

- The evparts.com.au has a 240A latching relay to disconnect loads.  Would this not work for disconnecting an inverter?  I realize this is a fail-safe and the Outback inverter should be programmed to disconnect below a certain voltage already.

Most inverters are set for Pb, not LFP low voltage disconnects, such as 10.7 or 11.2v or thereabouts.  For LFP, you would want that to be higher, about 12.7v.  *At our discharge rate*, that is conservative enough.  Still, 10.7v is better than going down to zero volts, provided you bring the bank back up SLOWLY (no more than .05C) until each cell reaches 3.2v, whereupon you can apply the normal charge rate, typically 0.1 to 0.3C or so.

- With a LiFePO4 battery bank I should remove any temp sensors used by the system to control the voltage charge levels since the LiFePO4 is not need voltage levels adjusted based on temp.

YES, *disable* temperature compensation.  One inexpensive controller I had that didn't disable compensation was tricked into doing so by setting the temperature variable to 0 (zero) mv per c.

My specific instance is for replacing a 250Ah 48V AGM battery bank which uses an Outback VFX3648, MX60, and Midnight Classic.

Don't bite off more than you can chew at first.  Ie, would you recommend to a newcomer to assemble a 250ah / 48v lead-acid bank right off the bat, or would you recommend they start out with just one or two batteries and a simpler system to get their feet wet with experience?

Hence, my recommendation for the DIY'er not to spend too much money up front, and prove to themselves that they can take care of a larger bank so they don't blow a hole in their wallet.  It's called the "learner battery", whether Pb or LFP for a reason.

nickdearing88 said:

mike95490 said:

So now I am looking at getting battery balancers such as http://www.electriccarpartscompany.com/3V-1S-Lithium-Lighted-Battery-Balancers, a means to monitor the voltage of each of the 16 cells, and latching relay if voltage it too high or low to disconnect the bank.

The sad thing about the balancer boards, is failures.  Any way to check on their track record? 

And they can only bypass 6A.  If you are charging with 20A, you still have 14 going into the battery and you have to rely on the balancer to trip the Charger Stop Command and stop the charging.  And you have one cell at Full charge voltage, and the rest of the pack is still behind. I suppose you could rig up another 5 A  charger and get all the cells topped off,

Not saying you can't use them but that they are not magic and can fail, just like the rooftop microinverters are doing

I agree but most users have experienced very little imbalance in cells, even after heavy cycling. This means that by the time ANY of the cells are near their high voltage limits, the charge controller should be in Absorb (constant voltage) and the current being "taken" by the batteries should be very low.

It would seem to me that if the cells are so imbalanced that one or more is near the top-end voltages while others are so low that the CC is still in Bulk mode (higher currents), the cells were not well-matched to begin with.

That's my experience so far, but only a couple weeks (and seems to correspond to the longer-term users here). @Raj174 and @karrak , wouldn't you agree?

PNjunction said:

The cells we deal with appropriate for low-rate discharge compared to ah sizing, are not high-rate cells.  1C continuous, and 3C for short <30 second bursts.  Still, that is waaaay overkill.  I mean, who designs a solar system with lead-acid for 1C, or even .5C discharge?  Nobody.  So the so-called lower-performance cells like GBS, Winston, CALB etc suit our purposes just fine.  We're not starting a diesel locomotive, but running lights off inverters, etc.

- There have been numerous mentions of different Battery Management Systems.  Is there currently a reliable BMS that is simple, and has all the safety features? (balancing, low voltage disconnect, high voltage disconnect) The "simplest" BMS I've seen and has been mentioned here is the one from evparts.com.au but requires importing from Australia.  Does anyone have experience using that one?

Talked to death on most forums.  Something I touted early on was the KISS principle, which we can get away with *due to our low-current discharge application*.  Even the hard core marine guys have gotten in on the act, many running lower voltages than they used to, disabling the parasitic balancing (mosfets failing closed, and UNbalancing a perfectly balanced bank), etc.

Some BMS companies have gone out of business, or to another product, and replacement parts are hard to find.  Once again, the KISS principle works here (nothing more than top balancing initially by charging each cell individually first), and setting a conservative low-voltage disconnect.  It is a catch-22 situation - the reason I need individual cell monitoring is because the bleeder/balancer mosfet failed, draining one of my cells!  And sadly, for those balancing systems that do work correctly, underneath there may be bad quality cells that DO need this kind of babysitting - and the time needed to do the balance hurts the cells in the long term.

Most inverters are set for Pb, not LFP low voltage disconnects, such as 10.7 or 11.2v or thereabouts.  For LFP, you would want that to be higher, about 12.7v.  *At our discharge rate*, that is conservative enough.  Still, 10.7v is better than going down to zero volts, provided you bring the bank back up SLOWLY (no more than .05C) until each cell reaches 3.2v, whereupon you can apply the normal charge rate, typically 0.1 to 0.3C or so.