LiFePO4 - The future for off-grid battery banks?

[PNjunction]

There's two separate LFP battery packs here.

All sounds good. Like me, you have two different battery packs. To hammer the point home about balance (either way!), take one of your Gen1 cells, and replace a Gen3 cell with it and marry it into your larger Gen3 pack.

Can you top balance it? Yes. Can you bottom balance it? Yes. Ridiculous, and of course the triggering for capacity will be on that Gen1 cell. And, for top-balance, we can see that voltage alone does not really mean balance in capacity. Of course the frankenstein battery will be limited by that Gen1 cell in use.

I actually built this frankenstein just for kicks and it drove home the point that voltage means squat - other than just staying in a safe, comfortable specification for the cell, and trying to spend hours lining up the voltages to .0001v of each other when top-balancing can be done, but in the end, balance really depends on the manufacturer's cell capacity and IR, and small variations can be tolerated. Ie, .01v is about the largest spread among cells before I get concerned.

Anyway, that's my story. Being conservative, keeps it simple, and prevents me from spending too much time top-balancing at high voltages for nothing - since my cells are pretty well matched in capacity and IR. (on the 40ah batt that is - the smaller 20ah is a little all over the place.)

[Sunking]

So are you saying you use them as voltage monitors? That I understand but there are better ways to do that without risking the boards from ever turning on.

I used an Orion Jr in the beginning with my GBS cells. The EV I uilt motor controller was crude and did not have a Coulomb Counter aka Gas Gauge. The Orion had that bell and whistles the controller did not have and can be programmed to do all sorts of task. It can be used to work with a 16S Lithium pack on solar easily. It does have bypass capability that can be programmed to operate at any voltage. It also has the capability to never use it.

Today I am running 31 Leaf cells and my new controller can do what the Orion Jr does except Balance. From the start I intended to Top Balance. Being an engineer I took my peers word for it until I heard of Bottom Balance and many documented cases of Top Balance system failures directly related to Balance Boards. I digress, I learned something. I am BB all the way an djust about 1/2 off the EV guys have now converted. They learned the hard way and destroyed cells with Top Balance.

As I stated earlier you have enough knowledge to use BB. Stay with it, your cells will last longer, and almost impossible to ever over discharge them to destruction.

[CrazyJerry]

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

Hi Sunking,
The devil is in the details and I do not state that I have both top and bottom balance. The pack is bottom balanced but also has vampire boards. The vampire boards provide the hookup to the ribbon cable for data logging by the CPU. Since I run the pack charge voltage way under the point at which the balance boards kick in, they do not balance anything and thus have no affect on my bottom balance.

The older pack was the same in the last year before I removed their boards too. Charging short of their kick in point means they too cannot perform their balancing function. They can only be as you say vampire boards and consume a bit of power as they sit and do nothing in regards to balancing.

These are both examples of how a battery pack have have the older dumb boards, and can escape the default label of "if they have boards they are top balanced - end of story." In general I would agree with you but depending on how the end user has set up their charging regime may indeed change the picture as illustrated in the above two examples.

~CrazyJerry

[CrazyJerry]

Not unless you are top balancing them some other way. As you say, you are only using the boards for data logging - which is exactly what I planned to do with a bottom balanced LFP bank as a convenience, just to prevent me from having to measure each battery with a DVM once a week or so.

How ARE you keeping your LFP batteries in balance? I assume you are but have not been following along.

Living Large,
There's two separate LFP battery packs here.

On the new pack, yes, you are correct that the boards are there for convenience in reporting individual cell voltages as opposed to using the dvm which is slightly less convenient with these new style cells (terminals on both ends of casing).

To answer your question, initially on just the pack itself, I pulled each cell down to 3 volts via a PowerLab6 and then haven't needed to touch them since - they are staying solidly in balance. I only had to touch the ~14.08v mark just to set the display capacity at 100%. Had I opted not to use it, then I would've just charged up to approx 3.45 per cell and called it a day.

Above applies to the new Gen III battery pack, but the other pack (4.5 year Gen I pack) was being balanced with the use of automotive halogen bulbs and celllog8 for monitoring purposes. I pulled all the balance boards on those 32 cells since they are nothing more than boards and don't serve any additional function. Since I've lowered the charging profile in the neighborhood of what PNjunction has suggested, I haven't needed to make any corrections to any of these older cells. Not something I thought would be possible previous to discovering this forum.

You are also off-grid?

~CrazyJerry

[Living Large]

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

Not unless you are top balancing them some other way. As you say, you are only using the boards for data logging - which is exactly what I planned to do with a bottom balanced LFP bank as a convenience, just to prevent me from having to measure each battery with a DVM once a week or so.

How ARE you keeping your LFP batteries in balance? I assume you are but have not been following along.

[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

[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]

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

[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

[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.

[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.

[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

[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

[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]

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