LiFePO4 - The future for off-grid battery banks?

[Willy T]

Jerry, I was looking at your EMS system. I was wondering how the two statements below work together when your EMS system uses cell balancing ?? Does the system do both top and bottom balancing ?? Is the " New Pack " your older cells ??

The new pack that runs the other side of the house is bottom balance (again thanks to this forum).

Upon initial install, the pack/brain/ needs to be charged to 14.2v to start at the full 100%.

The functions of the Elite system.

The functions of an Energy Management Systems include voltage and temperature monitoring of individual cells, pack current measurement, state of charge (SOC) calculation, alarm interlocks to prevent over-charging and over-discharging, cell balancing, ground fault detection, video output, CAN communication, etc.

[PNjunction]

PNjunction, the display is handy. One of my questions with it, and the likes of, is how accurate is it? Or is it accurate enough for basic needs thus making it useful.

Ironically, I only use resting voltages (12 hours!) as a relative state of SOC, but even then, your metering needs to be quite accurate as the spread between full and dead is only 1 volt! Shirt pocket meters are out. The Optimate TS-126 tester lines up with my Fluke 87, so I felt better. This is what I use:

12.7v / 3.175v dead
12.8v / 3.20v 20% <-- I try to never see a resting voltage below this
12.95v / 3.237v 25%
13.1v / 3.275v 50%
13.275v / 3.318 75%
13.6v / 3.40v 100% <---- gen1's typically rest at 3.38v

Anything *resting* over 3.4v after many hours is some bad overcharge! Discharge immediately.

The 12 volt pack was charged to 13.6 and then held there until the end amps setting (.05c, 10 amps in this case) was reached. At that point, the pack entered the "float" stage at 13.5volts. Once at float, a small current of 1.9amps(charging) is reflected on the screen. Upon initial install, the pack/brain/ needs to be charged to 14.2v to start at the full 100%.

Kinda worries me about float - ideally if you can't disable it, then the float voltage should be so low that no current actually flows.

For me, that is a bit too conservative at 13.6v while under charge. That might be too slow in a solar application - it all depends on you. Even though I run at 14v, with a .05C absorb, if I were to go totally conservative, then perhaps 13.8v or maybe nudge it to 13.7v. You'll have to see what level you are comfortable sitting at taking into account solar time issues.

It's fun fine-tuning LFP to fit one's own needs.

[karrak]

The 12 volt pack was charged to 13.6 and then held there until the end amps setting (.05c, 10 amps in this case) was reached. At that point, the pack entered the "float" stage at 13.5volts. Once at float, a small current of 1.9amps(charging) is reflected on the screen. Upon initial install, the pack/brain/ needs to be charged to 14.2v to start at the full 100%.

Your charge regime fits pretty closely with a number of others who post on the Energy Matters forum. Only difference from my charge regime is that I charge at 3.4 volts/cell (=13.6/4) with an end amp setting of .02C and float at a lower voltage of 3.3 volts/cell (=13.2/4) which basically terminates the charge when the current reaches .02C@3.4 volts/cell. I used to be of the opinion that keeping LFP batteries at a float voltage of around 3.35 volts/cell (=13.4/4) was bad for them, but now think this is probably not the case. Others on the Energy Matters forum, have found that if you do float them at this level after charging at 3.4-3.45 volts/cell, assuming enough sun, you end up with a fully charged battery at the end of the day which is the perfect scenario.

Sun permitting I charge my battery up to ~3.47 volts/cell (=13.9/4) once a week to reset the SOC counter. I would be interested to know whether your system does something like this.

As at two days ago charging my battery to 3.47 volts/cell gave a voltage deviation between the cells of 0.081 volts (3.528-3.447). In March of this year the battery was balanced and that deviation would have been less than 0.010 volts. So my battery is going out of balance. Maybe newer LFP batteries or more expensive ones from three years ago would stay more in balance. Ideally, one would hope that at some point in the future, that all LFP cells from the same manufacturer with the same capacity would remain perfectly matched in capacity and coulomb/current efficiency regardless of the operating environment during their entire life, so we don't have to take precautions against them going out of balance.

Simon

[CrazyJerry]

Jerry, I was looking at your EMS system. I was wondering how the two statements below work together when your EMS system uses cell balancing ?? Does the system do both top and bottom balancing ?? Is the " New Pack " your older cells ??

The functions of the Elite system.

Hi Willy T,
The two statements below should have been clarified on my end a bit more. So, the initial install of this particular pack followed the bottom balance approach first. This is not a function of the EMS - it is something I initiated by all the very convincing posts I read on this forum. From a common sense point of view in a "sub-C" environment (as PNjunction has labeled it), it makes good sense as a starting point.

The EMS is setup to do a top balance at ~3.55 per cell. The pdf for this EMS can be viewed online here:

http://elitepowersolutions.com/docs/User%20Manual%20for%20EMS%20System.pdf

It does a decent job explaining what it does and more or less how it does it. (See pages 17 & 18)

~CrazyJerry

[CrazyJerry]

Ironically, I only use resting voltages (12 hours!) as a relative state of SOC, but even then, your metering needs to be quite accurate as the spread between full and dead is only 1 volt! Shirt pocket meters are out. The Optimate TS-126 tester lines up with my Fluke 87, so I felt better. This is what I use:

12.7v / 3.175v dead
12.8v / 3.20v 20% <-- I try to never see a resting voltage below this
12.95v / 3.237v 25%
13.1v / 3.275v 50%
13.275v / 3.318 75%
13.6v / 3.40v 100% <---- gen1's typically rest at 3.38v

Anything *resting* over 3.4v after many hours is some bad overcharge! Discharge immediately.



Kinda worries me about float - ideally if you can't disable it, then the float voltage should be so low that no current actually flows.

For me, that is a bit too conservative at 13.6v while under charge. That might be too slow in a solar application - it all depends on you. Even though I run at 14v, with a .05C absorb, if I were to go totally conservative, then perhaps 13.8v or maybe nudge it to 13.7v. You'll have to see what level you are comfortable sitting at taking into account solar time issues.

It's fun fine-tuning LFP to fit one's own needs.

PNjunction,

You are correct that "it's fine-tuning LFP to fit one's needs". You've been pretty up front about your charging values and why. My own questions fall into the "what if scenario", in this case, what if I went just a tad lower, etc? Somewhere within all the data supplied by all the folks willing to share should give us a reasonable set of guidelines that yield performance and longevity that is repeatable. Looking back to the 2011 literature I have on the 20ah cells now reveals that the parameters were not optimal - hence changes we see today. Combine that with Sunking's statement that we are in many ways dealing with an immature technology from manufacturers that have changed their chemistry formulations several times in a relatively short time-frame and this leaves me to try to err on the side of conservative/caution this time (even though those little 20 cells are still surviving just fine right now.)

To your point about disabling float - yes, I can set it lower so essentially there is none but at this point in the season I may be wishing to take advantage of any available solar input. I may be incorrect to do this float setting for now but I do want to track any impact this has in relation to heat with these cells - if there is any measurable amount. I'm still not having luck sifting through studies looking for answers to a couple of questions I feel are important to this new setup. One being heat dissipation in the GBS 200ah cell with opposing terminal ends. I understand about activity dispersal uniformly through the cell but I'd like to find any type of data related to heat with this design. I was reading about distances traveled in LFP cells and as I look at these 200ah cells there appears to be some real estate from one end to the other. Whether that translates to more real estate between the anode and cathode is not something I can easily find either.

One thing that concerned me (that I believe you mentioned some posts ago) with the 14v and .05C absorb was the scenario of not enough solar input during the day to reach that point - or even if that was the case for days - never reaching that set-point, but instead just hovering near it for hours and hours. This would (in my opinion) be less desirable than the float setting I'm using (mentioned above) but I could be wrong, or this could be splitting hairs.. To potentially address this issue - maybe the even more conservative approach I'm taking (albeit with the float setting you're not 100% in love with) may be a good compromise. Time will tell and your posts are great at sparking some thinking - a bit of a challenge. Let's continue to exchange ideas and outcomes

~CrazyJerry

[CrazyJerry]

Your charge regime fits pretty closely with a number of others who post on the Energy Matters forum. Only difference from my charge regime is that I charge at 3.4 volts/cell (=13.6/4) with an end amp setting of .02C and float at a lower voltage of 3.3 volts/cell (=13.2/4) which basically terminates the charge when the current reaches .02C@3.4 volts/cell. I used to be of the opinion that keeping LFP batteries at a float voltage of around 3.35 volts/cell (=13.4/4) was bad for them, but now think this is probably not the case. Others on the Energy Matters forum, have found that if you do float them at this level after charging at 3.4-3.45 volts/cell, assuming enough sun, you end up with a fully charged battery at the end of the day which is the perfect scenario.

Sun permitting I charge my battery up to ~3.47 volts/cell (=13.9/4) once a week to reset the SOC counter. I would be interested to know whether your system does something like this.

As at two days ago charging my battery to 3.47 volts/cell gave a voltage deviation between the cells of 0.081 volts (3.528-3.447). In March of this year the battery was balanced and that deviation would have been less than 0.010 volts. So my battery is going out of balance. Maybe newer LFP batteries or more expensive ones from three years ago would stay more in balance. Ideally, one would hope that at some point in the future, that all LFP cells from the same manufacturer with the same capacity would remain perfectly matched in capacity and coulomb/current efficiency regardless of the operating environment during their entire life, so we don't have to take precautions against them going out of balance.

Simon

Hi Simon,

The initial setup of the system requires the pack voltage to raise to 14.2 to set the brain to the 100% reference point.

Not sure what happens over time, but I'm going to watch all the metering as I continue to stay just below that. Page 17 and 18 of the manual is interesting in that it basically explains the method the system is using to calculate soc (they do mention coulomb counting.) The manual is online at:

http://elitepowersolutions.com/docs/User%20Manual%20for%20EMS%20System.pdf

All good info you've provided on what you're doing with respect to your charge settings. What I've tried to do here was to adopt a conservative charge approach based on 80% pack de-rating (as PNjunction has suggested, and stay out of upper knee if possible). I really want the small daily loads to be fueled by solar if the batteries are near-full and there's excess solar, so the float setting @ 13.5 on the 12 volt pack fits in.

In addition, I am watching the temperature on the cells since I do believe that cell temperature plays into the lifespan of these batteries. This new system does report cell temps and I am checking them against my infrared gun.

As of right now these conservative settings require no bleeding from the vampire boards, and I'm seeing a very close values between the cells - no drift to report - - - yet.

We're in a similar scenario of the variability of solar on any given day, and I had the pleasure of watching the controllers deal with a partly sunny day. There is a delay after the point at which a thick cloud passes and the sun shines abruptly. I wanted to take that into consideration when setting the bulk/absorb voltage. There are three separate banks of panels and each has their own controller so I've tried to be a bit creative in staggering their bulk/absorb/end amps settings, however all the float settings are the same.

Thank you for providing your data - I find all of this useful since it is somewhat of a living experiment.

~CrazyJerry

[Willy T]

Hi Willy T,
The two statements below should have been clarified on my end a bit more. So, the initial install of this particular pack followed the bottom balance approach first. This is not a function of the EMS - it is something I initiated by all the very convincing posts I read on this forum. From a common sense point of view in a "sub-C" environment (as PNjunction has labeled it), it makes good sense as a starting point.

The EMS is setup to do a top balance at ~3.55 per cell. The pdf for this EMS can be viewed online here:

http://elitepowersolutions.com/docs/User%20Manual%20for%20EMS%20System.pdf

It does a decent job explaining what it does and more or less how it does it. (See pages 17 & 18)

~CrazyJerry

My math says that 3.55 is 14.2 volts. Cell balancing is automatic. You may not have to go to 14.08v except to reset the soc % once in awhile psoc cycles does have a way of causing a error unless they are calibrated correctly.


EMS Operation Manual Page 11
9. Battery Balancing
When the battery voltage rises during charging to 3.55V or above the red balancing LED will light on the sense board to indicate the cell is balancing. The sense boards will draw 0.5A until the voltage has dropped below 3.55V. It is normal for some cells to balance more than others and some cells to rarely balance.

[CrazyJerry]

Is the " New Pack " your older cells ??

Willy T,
In response to your earlier question the answer is no, the "New Pack" is not the older cells.
I have two distinct battery packs here.
The first pack is a total of thirty-two 20ah cells that were put into service in April 2011.
The second pack is the new pack and it consists of four 200ah cells put into service November 2015.

~CrazyJerry

[karrak]

The initial setup of the system requires the pack voltage to raise to 14.2 to set the brain to the 100% reference point.

Not sure what happens over time, but I'm going to watch all the metering as I continue to stay just below that. Page 17 and 18 of the manual is interesting in that it basically explains the method the system is using to calculate soc (they do mention coulomb counting.) The manual is online at:

http://elitepowersolutions.com/docs/...S%20System.pdf

Hi Jerry,

From what I can see from the manual, the controller is preprogrammed with the battery size and hopefully "battery leakage" data. As WillyT has already stated it resets the 100% voltage point whenever the charging voltage goes above 3.52*4=14.08 volts. If you are only charging to 13.6 volts the Capacity Meter will never be reset so its accuracy will steadily get worse. If Elite Power are not taking into account "battery leakage", accuracy will decrease fairly rapidly. If their software is really smart they could calculate the "battery leakage" from the change in accuracy. Might be worth contacting them to find out how often they think you should take the charge voltage above 14.08. As WillyT has done you could get your Solar Controller to do an Equalise cycle every now and again at say 14.2 volts to reset the Capacity Meter.

Simon

[Willy T]

Here is a concern, It makes no difference to me how someone balances their bank, it's a personal preference. If someone bottom balances and uses a BMS with boards and they ever go to the point ( 3.55v on the weakest cell ) that the cell will shunt and then the bottom balance will be off on that cell. Jerry said he bottom balanced, yet the system he uses requires him to go to 14.2v or at least 14.08v to reset the soc. It would be very easy for the weakest cell to start to balance. Inmichigan had a similar issue when he had boards on his cells where one cell got way out of balance.

Everyone has their own opinions on Balance and thats fine, but if you are unwittingly changing the balance point you can get into trouble fast.

[Sunking]

Here is a concern, It makes no difference to me how someone balances their bank, it's a personal preference. If someone bottom balances and uses a BMS with boards and they ever go to the point ( 3.55v on the weakest cell ) that the cell will shunt and then the bottom balance will be off on that cell.

With BB you do not use Balance Boards. DUH! Defeats the purpose.

[CrazyJerry]

Hi Jerry,

From what I can see from the manual, the controller is preprogrammed with the battery size and hopefully "battery leakage" data. As WillyT has already stated it resets the 100% voltage point whenever the charging voltage goes above 3.52*4=14.08 volts. If you are only charging to 13.6 volts the Capacity Meter will never be reset so its accuracy will steadily get worse. If Elite Power are not taking into account "battery leakage", accuracy will decrease fairly rapidly. If their software is really smart they could calculate the "battery leakage" from the change in accuracy. Might be worth contacting them to find out how often they think you should take the charge voltage above 14.08. As WillyT has done you could get your Solar Controller to do an Equalise cycle every now and again at say 14.2 volts to reset the Capacity Meter.

Simon

Simon,
This was also a question I had when setting up the system - how often would one need to charge up to 3.52 cell (~14.08 pack voltage) to reset the counter for accuracy. I do have my own ideas of what I'll do to check the readout over time and will post the results should anything come out of that. In the meantime, your suggestion of simply asking the vendor for a suggestion is easy enough to do - so next week I'll send out an email.

~CrazyJerry

[CrazyJerry]

Here is a concern, It makes no difference to me how someone balances their bank, it's a personal preference. If someone bottom balances and uses a BMS with boards and they ever go to the point ( 3.55v on the weakest cell ) that the cell will shunt and then the bottom balance will be off on that cell. Jerry said he bottom balanced, yet the system he uses requires him to go to 14.2v or at least 14.08v to reset the soc. It would be very easy for the weakest cell to start to balance. Inmichigan had a similar issue when he had boards on his cells where one cell got way out of balance.

Everyone has their own opinions on Balance and thats fine, but if you are unwittingly changing the balance point you can get into trouble fast.

Willy T,
The balance boards through their daisy chain connections is what the brain uses to determine cell voltage/pack voltage/cell temp, so if I want that display with those features I do have to use the boards.

As far as the top balance from those boards goes: on a good day, using incoming solar (not the limited output AC charger) I can hit and exceed the 14.08v mark pretty quick and I have observed the time at which all the balance boards engage in relation to that set-point (the 100% done charging) is less than 15 seconds mainly because the cells are in close balance. Do you think a fair statement to say is that short amount of time (in this particular case) would have little affect on the balance of the cells?
~CrazyJerry

[Sunking]

As far as the top balance from those boards goes: on a good day, using incoming solar (not the limited output AC charger) I can hit and exceed the 14.08v mark pretty quick and I have observed the time at which all the balance boards engage in relation to that set-point (the 100% done charging) is less than 15 seconds mainly because the cells are in close balance.

If you have Vampire Boards, you are Top Balanced period end of story. You cannot have it both ways.

[CrazyJerry]

If you have Vampire Boards, you are Top Balanced period end of story. You cannot have it both ways.

I have vampire boards for data logging but they do not engage in their bleeding based on my charging profile which is way below their setting. So, am I still top balanced?
~CrazyJerry