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

[ebaugh]

Terry,

Thanks for the comments.

The GBS cells are prismatic, but have an unusual interconnect system. They use 4 smaller screws in a square pattern over each cell instead on one large bolt. The theory is a larger mechanical enforced contact area. I'm not convinced its better, just different. But I will be using their interconnects, which have the hump for vibration.

There are at least 3 slight variations in the lithium phosphate family, at least based on specs, Calb LiFePO4, Old Thundersky, Winston and maybe Sinopoly, LiFeYPO4, and GBS is LiFeMnPO4. Depending on the chemistry, and the age of the specifications, the peak charge voltage ranges from 3.6 to 4.25 V per cell. What's interesting though, is all of them discharge at 3.2 to 3.3 V per cell depending on load for the flat part of the curve. My uneducated conclusion is they are more alike than different and that the original charge spec was set by some engineer who picked that value for testing, which became the specification. And lower is better, so long as you can achieve 90 to 95 percent SOC.

My older inverter/charger is problematic in that it has few settings. But I think I can get the other one to mostly control the process. If not, I will eventually replace it with one I can control better. In the meantime, I become the control.

The BMS marketed by the Elite folks for GBS batteries is a top balancing charger. It shunts at 3.7 V and disconnects at 3.8 V. But it is intended for higher C rate charging. I don't plan on hitting the 3.7 V level normally and certainly not 3.8V per cell.

In my installation, my max charge rate is 200A, but for load sharing on the generator, I usually have it set to more like 150A. So that's just a little over .1C in my case, a very low charge rate. There is little if any data on a charge profile at this rate. For better or worse, I plan to start out trying to set things up based more on amps out equals amps in, basically SOC based on 97 to 98 percent efficiency advertised by the chemistry. I think this is a conservative place to start. But it will be an iterative experimental process getting the voltages and rates so the inverters go to float at the right point.

Bob

[ebaugh]

I have a discussion topic. Many of you don't have a BMS, and don't believe one is required. I see that point of view, in fact I don't ever expect to see a condition where the BMS would activate. But in my case there is a little over 6K investment in batteries, and I want an insurance policy in case I do something stupid. One of the issues of installing a BMS in a boat is what happens when the BMS senses an error condition and pulls the plug. Let's say it's nighttime and I'm navigating an unknown inlet somewhere and all the lights and navigation equipment go dark. This is bad.

On a clean sheet build, there are ways to deal with this, but it's much more complicated for a retrofit. I have essential equipment that depends on both 12V DC and some on 110AC. Presently there is one house bank that powers everything. It is directly connected to the alternators for charging underway. So simply pulling the plug is not an option. But the BMS must have a way to isolate the lithium pack on an error condition. That's defined as less than 2.5V on any cell, more than 3.8V on any cell or over 150 degrees on any cell.

Think about this a little while because it's going to sound a bit crazy at first.

I'm planning to also parallel a retained pair of golf carts with the lithium bank, at a minimum while underway. This keeps the lights on since it keeps things going for awhile even with the lithium offline. Underway, I just have float charging set anyway, about 13.2V which is compatible with both types of batteries.

It will be on a manual switch so I can isolate it if I want to when I'm at anchor. But I predict it won't be necessary. The higher operating voltage of the lithiums means I won't discharge the golf carts much if at all. I figure the lithiums will never go below 3.2V or 12.8V unless discharged below 25% SOC at my normal discharge rates. So the golf carts should retain almost a full charge. On the flip side, there will be a small penalty in current flow from the lithium to the lead acid, but I predict it to be less than 1/2 amp initially, decreasing as the lithium bank discharges. On the next charge cycle, the LA may be a bit overcharged, but I don't think they will mind much for 3 to 4 hours a day.

The only other idea I had was to place an isolation diode only allowing the current flow from the golf carts to the lithium bank. If my predictions are off, I may yet do this. But for alternator charging, and other issues, the 1/2 to 1 volt drop across the diode is problematic. A battery combiner with customized parameters might work as well, but there is no such thing I can find.

Maybe some of you will have a better ideas, but this is so simple, and simple is good.

Bob

[T1 Terry]

You can parallel the lead acid and lithium batteries as 2 seperate banks but the lithium batteries will for ever nursemaid the lead acid batteries. charging lead acid to 14v is well within their operating parameter and the upper safe level for LiFeP04 batteries. The li batteries will hold 13v down to 80% DoD, 12v to 100% DoD, therefore the lead acid batteries never actually do any work but they may share a bit of the load as the Li batteries reach 100% discharge. The down side of this is the lead acid batteries are parisites, they will continually draw current from the li batteries till they are themselves fully discharged, around 10.5v.
If you use a cell logger like the Junsi Cell log 8 you can continually monitor the cell voltages, these cells don't suddenly die, it's a long slow death if you are watching it happen, mths not hrs. In that time you should have figured out what the problem is and fixed it, suprises only happen when the cells are not fully monitored.
The shunt style BMS systems are responsible for killing more cells than any other reason. The shunt is designed to trigger at a set voltage and disconnect at a lower voltage but if they over heat due to the charging current being greater than their shunting capability they often simple fuse in the on condition or even a parial short condition and dragging the cell to 0 volts and hold it there, this is the quickest way to destroy an Li cell. Low voltage of even 1.5v per cell won't kill them, it may reduce their capacity but it won't kill them, a high voltage of 5v has the same effect but hold a cell at 0v or above 3.45v for a sustained length of time and it will be damaged and eventually destroyed.

The 4.2v is conditional, you need to understand where this figure comes from, 4.2v is the point constant current charging of 0.5CA to 3.0CA finishes and constant voltage charging starts. The constant voltage is to 3.6v per cell till a current flow of 0.015CA, then all charging must cease. NOTE, that it is not until the cell will not accept no further charge. This is lab stuff, way beyond the controls available for house power systems. There is no further capacity to be gained above 3.45v so why risk cell damage above that voltage?

T1 Terry

T1 Terry

[electric1]

Originally Posted by T1 Terry

The shunt style BMS systems are responsible for killing more cells than any other reason. The shunt is designed to trigger at a set voltage and disconnect at a lower voltage but if they over heat due to the charging current being greater than their shunting capability they often simple fuse in the on condition or even a parial short condition and dragging the cell to 0 volts and hold it there, this is the quickest way to destroy an Li cell.

You should stop spreading this baloney around.....are you an expert in statistical analysis of cell failures? Where do you get your data to allow yourself to make such bold statements?

What you say is true only for a badly designed and badly implemented BMS, of which there have been a few reports, but its certainly not as bad as you are painting it. Several shunting BMSs have been on the market for years with no reported failures, at least not when its installed properly.

I speak from actual experience, not like you repeating some wild stories you hear on the Internet forums. My company makes miniBMS and we are proud of the quality of our product and its design which includes many safety features, proven by 3+ years of real world use.

I do agree that BMS is not absolutely required on 4S banks if handled properly, but at the same time investing a small extra capital in a simple system which serves as insurance policy, is a wise thing to do.

We have many customers using our HousePower BMS on housebanks and very happy with it.

This post is not an ad for our product, but merely an attempt to stop these silly lies being spread by some overly dramatic members.

[s/v Holiday]

Hi electric1,

I was just looking at your HousePower BMS online last night. Can you tell me does it also use the MiniBMS cell units, or is it contained on the board. I am preparing to install 32 100ah Tenergy LifePO4 Prismatic cells, 8P 4S for 800 ah 12v. Do I need a BMS for each cell or just 4 for the Series strings of cells.

Thanks for your help.

Hi T1 Terry,

Glad to see you back on this thread. I have been studying all the previous posts and really appreciate all the good info you and others have posted here. I am also considering adding the Cell logger so I can be more involved in monitoring my bats but I have a question. Will the logger be able to read the individual cells when they are in Series? Or should I just install one for the 4 Parallel strings of batteries?

Thanks for your help.

CJ

[electric1]

Originally Posted by s/v Holiday

Hi electric1,

I was just looking at your HousePower BMS online last night. Can you tell me does it also use the MiniBMS cell units, or is it contained on the board. I am preparing to install 32 100ah Tenergy LifePO4 Prismatic cells, 8P 4S for 800 ah 12v. Do I need a BMS for each cell or just 4 for the Series strings of cells.

HousePower BMS integrates everything on a single board ( 2 boards for 24V banks ). Any 4S system, regardless of how many cells are in P , needs just 4 BMS channels, as long as you wire cells in P first, then 4 P groups in a single series string, which is most proper way to wire batteries of cells, you will only need one BMS system like our HousePower system. MiniBMS cell modules are integrated, so its already part of the board.

Next version of HousePower BMS will introduce 2 separate contactors, one for charging circuits and one for load circuits, to differentiate HVC and LVC events, so you don't loose your loads in case of HVC event and don't loose solar trickle charge in case of LVC event. It will also have some logic improvements like advanced alerts before LVC cutoff, to give ample time to start the genset or plug the charger.

I expect next version to be available in a few weeks.

[s/v Holiday]

Originally Posted by electric1

HousePower BMS integrates everything on a single board ( 2 boards for 24V banks ). Any 4S system, regardless of how many cells are in P , needs just 4 BMS channels, as long as you wire cells in P first, then 4 P groups in a single series string, which is most proper way to wire batteries of cells, you will only need one BMS system like our HousePower system. MiniBMS cell modules are integrated, so its already part of the board.

Next version of HousePower BMS will introduce 2 separate contactors, one for charging circuits and one for load circuits, to differentiate HVC and LVC events, so you don't loose your loads in case of HVC event and don't loose solar trickle charge in case of LVC event. It will also have some logic improvements like advanced alerts before LVC cutoff, to give ample time to start the genset or plug the charger.

I expect next version to be available in a few weeks.

Great! This sounds like what I need. Could I get in line for one?

Thanks

CJ

[ebaugh]

Terry,

I've read about those issues with balance boards overheating in other places. They should be designed so they won't shunt more current than they can dissipate heat. But of course, that does not mean they all do that. I spoke to Electric1 about his product and liked the engineering. But I wanted SOC and display of cell voltages integrated with the BMS, and those are features he does not have yet.

One of the safety features of the Elite BMS is the cell boards have to report voltage and temperature to the central CPU, or it pulls the plug when it does not find the correct number of managed cells reporting. If a board failed big time over heat, it would probably stop reporting. It also pulls the plug if it sees over 150 degrees F.

Of course I don't ever expect to get use the balancers normally since that's 3.7V per cell. I guess the 3.45V is what everyone has found to provide good charging at lower C rates. I don't have solar or wind yet, so I'm not planning long periods at charging voltage. It's a matter of finding the right values to trigger the charging stages on my charger. The profile I have to follow is charge to V1 at max current A1, hold V2 for T1 minutes after reaching V1 with reducing current, then float at V3 forever. On my good charger I can set V1, V2, V3, A1 and T1 to anything. The other one is similar, but less programmable.

I would like to use 14.4 in the first stage because it will work better with the other charger, and is within the ordinary operating range of these batteries. At the battery, this should be a little less than that. My supplier tells me to expect 85 to 90 percent charge if I stop there. I figured I'd look at SOC to make a determination of how long to keep charging at a reducing rate before floating. If at all. But SOC as kept in the BMS has no direct control over the charge process. It's just a tool to help establish the profile.

What is the suggested float voltage? One that you could use at a marina for a month or longer?

I understand the LA will be parasites. I'm just hoping its an insignificant amount. Worst case is I only parallel them underway for redundancy. If the BMS trips at anchor for LVC, there would be little charge left anyway.

Bob

[OceanPlanet]

Quote from Electric1: "Next version of HousePower BMS will introduce 2 separate contactors, one for charging circuits and one for load circuits, to differentiate HVC and LVC events, so you don't loose your loads in case of HVC event and don't loose solar trickle charge in case of LVC event. It will also have some logic improvements like advanced alerts before LVC cutoff, to give ample time to start the genset or plug the charger."

That funny....Genasun has been doing this for years!! Glad you're figuring it out...;-)

[electric1]

Originally Posted by ebaugh

What is the suggested float voltage? One that you could use at a marina for a month or longer?

Float voltage should be at 3.4V per cell, or 13.6V on a 4S bank.

[electric1]

Originally Posted by OceanPlanet

That funny....Genasun has been doing this for years!! Glad you're figuring it out...;-)

Nothing funny here, Genasun is too expensive, there is no point for us to design a system at same price range as Genasun as we would be in direct competition in a very frugal and small market. So we had a simpler and cheaper system to offer, but now that this market is growing and more people are getting into LiFePO4 it makes sense economically for us to offer a more flexible system, but still at very affordable price point.

[ebaugh]

Originally Posted by electric1

Float voltage should be at 3.4V per cell, or 13.6V on a 4S bank.

That's gonna be a little hi for the LA chemistry. Hmmm.

[electric1]

Originally Posted by ebaugh

That's gonna be a little hi for the LA chemistry. Hmmm.

I should have been more clear, float for LiFePO4 should be no more than 3.4V, but it can be a little less, say 3.35V or even 3.3V. Less than 3.3V makes no sense since they are at 3.3V for a good portion of discharge time anyway.

[ebaugh]

Originally Posted by electric1

I should have been more clear, float for LiFePO4 should be no more than 3.4V, but it can be a little less, say 3.35V or even 3.3V. Less than 3.3V makes no sense since they are at 3.3V for a good portion of discharge time anyway.

The discharge curve I have for GBS shows a rapid fall to about 3.27V at 95% SOC at .5C. Then it flattens out. But that is a pretty high rate for this application. Like 600 amps......way too high.

Does anyone have or can email me discharge curves (for any lithium phosphate) at more like .05C?

[T1 Terry]

Originally Posted by electric1

You should stop spreading this baloney around.....are you an expert in statistical analysis of cell failures? Where do you get your data to allow yourself to make such bold statements?

What you say is true only for a badly designed and badly implemented BMS, of which there have been a few reports, but its certainly not as bad as you are painting it. Several shunting BMSs have been on the market for years with no reported failures, at least not when its installed properly.

I speak from actual experience, not like you repeating some wild stories you hear on the Internet forums. My company makes miniBMS and we are proud of the quality of our product and its design which includes many safety features, proven by 3+ years of real world use.

I do agree that BMS is not absolutely required on 4S banks if handled properly, but at the same time investing a small extra capital in a simple system which serves as insurance policy, is a wise thing to do.

We have many customers using our HousePower BMS on housebanks and very happy with it.

This post is not an ad for our product, but merely an attempt to stop these silly lies being spread by some overly dramatic members.

Quote"
I do agree that BMS is not absolutely required on 4S banks if handled properly, but at the same time investing a small extra capital in a simple system which serves as insurance policy, is a wise thing to do."


If you are that confident your BMS will not kill cells but instead save them from damage (isn't that the reason for fitting a BMS in the first place?) then give a full battery bank replacement guarantee with every one of your BMS units fitted. Until you are willing to do that you are practicing and fine tuning your gadgets in the market place at the customers peril. If a BMS is not really needed for a 4 cell series system then why would someone spend the $$ and take the risks?
If a cell shunt fails to the “on” condition, how does your system turn it off? Isn’t it true that any such failed unit must be removed from the cell to stop it continually discharging that cell to the point of cell failure?

T1 Terry