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Old 29-07-2011, 08:32   #16
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Another apology to Terry, you stated 1C load in your post. I need to stop posting on forums before my morning coffee

However, I still need to say that resting voltage is meaningless (with some exceptions) and the fact that cells have same resting voltage tells you virtually nothing about them, as your test clearly reveals. You have one cell with significantly different IR, yet you can never tell it from resting voltage. You also can't tell SOC from resting voltage unless its at either end of SOC range.
You can't judge balance by this test at all since top balance can only be observed at the end of charge, not discharge. Perhaps we need to define meaning of top balance. In my mind, top balance is when all cells in the bank reach upper knee region together, hence raising total bank voltage equally and causing charge current to stop. If one cell reaches top sooner, then others have no chance to get a full charge and first one is risking overcharge. So, top balance is all about lining up voltages at the end of charge, once they reach upper knee where you can have meaningful interpretation of the voltage.

Saying that you cells are all lined up at 3.2V or 3.3V or even 3.4V is meaningless because at that voltage they are not in the upper knee region, hence you can't tell SOC level. The key is to observe end of charge, when voltages start to climb at different rates, which reveals their relative SOC.
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Old 29-07-2011, 17:43   #17
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Hi Electric1,
A couple of points you are missing for some reason,
You can't have resting voltages above 3.4v unless the batteries are fully charged, no matter how high the C charge rate is.

Prismatic cell manufacturers rate their cycle life at 0.5C discharge and recharge

If the charge is voltage controlled then a voltage of 3.45v per cell or 13.8v across a 4 cell battery the current will taper off as the cells reach fully charged. Once 13.8v is reached the current flow stops, it doesn't continue to trickle charge like AGM lead acid batteries, it just stops.

The 3.45v is just at the knee of the charge curve, this gives lots of scope for cells to be out of balance, 0.55v before the 4.0v limit is reached. If one cell is a long way behind the other 3 it's charging cycle will stop before it reaches 3.45v, the other 3 cells will be above 3.45v, as soon as a load is applied the 3 cells above 3.45v will rapidly loose voltage till the 3.4v threshold is reached but the low cell will hold virtually the same voltage till nearly fully discharged.
A theoretical example:
Cell 1, 3 & 4 are closer to fully charged than cell 2, let's do an extreme of 0.5v. Charge stops at 13.8v, cell 1/ 3.5v, cell2/ 3.335v , cell3/ 3.48v, cell4/ 3.485v. When the load is applied the voltage on cell 1 will rapidly drop to 3.4v, the voltage on cell 2 will remain fairly close to 3.3v because it is in the flat part of the curve, cell 3 will rapidly drop to 3.4v and cell 4 will rapidly drop to 3.4v. Now the terminal voltage is 13.5v, as little as 1 ah would have been drawn from the battery, cell 1, 3 & 4 are still fully charged and cell 2 has only lost 1ah. On the next recharge cycle cell 2 is 1ah closer to being at the same SOC as the other 3 cells, as cycling continues this gap will continually reduce.

This self balancing works far better with solar and/or wind/water turbine charging included as part of the charging regime because the charge cycles are multiple while the sun is out or the turbine is spinning, a high output isn’t required as it’s only a balancing current.
I can post another graph to show that even with an available charge voltage as low as 13.8v the cells will fully charge, a little slower granted but they still fully charge.
With alternator charging the charge voltage potential will likely be over 15v for a nom. 12v alternator, pulse width modulation charging allows this 15v pulse to impact the battery until the terminal voltage rises to the cut off point, in my case I'm suggesting 13.8v so charging is still quick, much faster than lead acid.

Another advantage LiFeP04 cells have is they are not harmed by sitting partially charge or rarely being fully charged to the 3.4v point so the fact that not every cell reached the same point on the knee curve isn't really that critical is it.

T1 Terry

PS. The time scale is across the bottom of the charts
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Old 29-07-2011, 19:04   #18
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Terry, I am sorry, but your entire theory is 100% wrong. It simply does not work as you describe. There is no such thing as self balance. If you apply any amount of charge or any amount of load to a number of series connected cells, each cell will contribute exactly the same amount of charge/discharge and they will stay at the same level of imbalance, unless something intervenes on a cell level, like BMS shunting or manual charge/discharge of individual cell to bring it in line with others.

I don't see a point of discussing ways to better use imbalanced pack, by intentionally reducing charge limit, when you can simply balance it and get on with your life, fully using the battery. Once initial balance is done it tends to stay very well balanced, especially in low C rate application like house bank. Initial balance is so simple that I don't see why we are discussing ways around it.
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Old 30-07-2011, 16:32   #19
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

I guess time will tell, at the moment I am fitting up a house battery pack with daily cell monitoring that records back to a hard drive so the self balancing question will be answered one way or the other. This unit runs a 240Vac 250ltr fridge, a 2500w inverter, lights, water pump, computer, router, mobile phone chargers along with all the usual house stuff and never connects to shore power so it will be a fair dinkum house battery test rig. Charging is via solar and alternator. These are the same cells as used in the load test, the ones you believe "reveals a sizable mismatch in IR among 4 cells under 1C load" so we will see if miss match is an issue also.

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Old 31-07-2011, 17:49   #20
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Quote:
Originally Posted by sytaniwha View Post
To kick this thread off, below is a copy of a post I made on the other 2 threads describing the system I have set up on our yacht.

Hi,

Below is a description of my 400Ahr LiFePO4 bank using 4 * 400Ahr CALB cells for the house bank of our 46ft cruising yacht, plus a link to an interesting video on LiFePO4 charge/discharge characteristics.

McG you may find this link interesting of a video by a Carnegie Mellon Robotics Institute PhD talking to a group of electric car enthusiasts about LiFePO4 charging and conditioning.

Robotics Institute: Videos

He recommends an initial charge/discharge cycle with your cells all in parallel, and stopping at a well defined curve point (e.g. very near full charge or full discharge), to initially ensure all your cells are at a very similar SOC. This is a step further than the good advice offered to you by Electric1.

Also, some of the earlier posts recommend matching your cells for capacity and internal impedance very closely at the factory, and I also think that is critical.

I purchased 4 * 400AHr cells off CALB direct earlier this year, and asked them to match as closely as possible. When they arrived, with the equipment I have I couldn't measure a difference in capacity or impedance, and parallel charge/discharge showed no imbalance (by monitoring individual cell current).

I was also very impressed with CALB's customer service. The sales person still contacts me every few months to see how the cells are, and answers tech questions very quickly.

I run the cells as a 12V (nominal), 400 Ahr bank. They replaced a 600Ahr Trojan SLA pack, and provide more useful energy per discharge. Another side benefit is that I don't have to worry about self-discharge ruining the cells when I leave the boat for an extended period.

I'm currently charging using a 120A alternator through a Sterling Power regulator, and a Victron 50A galvanically isolated mains charger. Both can have their charge parameters changed to fit the ideal charge curve for whatever brand of LiFePO you're using (I note that different brands have some quite significantly different charging parameters).

I use a Vetus Battery Watch unit which controls low voltage cutoff at the pack level to the consumers. Combined with the high voltage cutoff provided by the charge sources, this produces the sensible separation between charge and discharge as suggested by OceanPlanet (i.e. after a low voltage cutoff, the cells can still be charged, whilst a high voltage cutoff doesn't remove the ability for the consumers to draw power from the battery).

I was originally going to add a cell-level BMS, but agreed with GoBoatingNow's comments back in Mar/Apr 2010 that cell level probably isn't necessary for a relatively low current situation (like a typical cruising house bank), especially when there are only a few closely matched cells (4 in my case). My usage thus far supports that view, so we'll see how it goes in future.

I figured that as worst case I might have to do a parallel charge/discharge cycle manually on an annual basis to realign the cells, but I can't detect any drift thus far, so that may not even be necessary.

Anyway, I hope that this info helps some people.
Sitaniwha,

Some things to double check about your HVC and LVC configuration:
1) Are the loads and the charging sources on two separate busses?

2) If so, does each bus should have it's own relay so the either the loads or the charge sources can be isolated from the battery?

3) From what I understand, you only have a LVC (a relay controlled by the Vetus Battery Watch?), but are depending only on the REGULATION of the charge sources to prevent over charge.

Regulation and a HVC are two very different things. If any of your charge sources (solar, alternator, shore charger, inverter/charger, hydro, whatever) has a regulation problem then you could overcharge your batteries. Unless you have them all on a bus that can be isolated from the battery by a BMS (or whatever) that is monitoring cell (not just pack) voltage.

My 2 cents.
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Old 31-07-2011, 18:38   #21
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

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Originally Posted by OceanPlanet View Post
Sitaniwha,
Quote:
Originally Posted by OceanPlanet View Post

Some things to double check about your HVC and LVC configuration:
1) Are the loads and the charging sources on two separate busses?

2) If so, does each bus should have it's own relay so the either the loads or the charge sources can be isolated from the battery?

3) From what I understand, you only have a LVC (a relay controlled by the Vetus Battery Watch?), but are depending only on the REGULATION of the charge sources to prevent over charge.

Regulation and a HVC are two very different things. If any of your charge sources (solar, alternator, shore charger, inverter/charger, hydro, whatever) has a regulation problem then you could overcharge your batteries. Unless you have them all on a bus that can be isolated from the battery by a BMS (or whatever) that is monitoring cell (not just pack) voltage.

My 2 cents.
Good point, the cell logger I am using has an alarm setting for individual cell LVC, HVC and cell differential so they can't drift too far apart. The problem is that it's only one alarm output. To safely protect cell over voltage separately from cell under voltage 2 Cell Logger unit would be required, each controlling a relay. I guess only one unit needs to be data logging so the cheaper unit that is just a monitor would do the job, I think they were around AU$12. I have wired the cells via a DB9 female 9 pin data plug at the battery pack. That way any 9 pin data cable will plug in and the Cell Logger can be mounted in a convenient position for easy viewing and cable replacement is an off the shelf item. A piggy back plug or splitter cable would make connecting 2 units easy enough with a separate relay connected to each unit. The alarm setting can be toggled on or off on each unit so they could be set up so one monitors cell LVC and cell differential and the other HVC and cell differential. A passive BMS for under $50 plus relays.
Itís a work in progress so a few more mods to the current set up isnít a biggy, thanks for that

T1 Terry
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Old 01-08-2011, 07:10   #22
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

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Good point, the cell logger I am using has an alarm setting for individual cell LVC, HVC and cell differential so they can't drift too far apart. The problem is that it's only one alarm output. To safely protect cell over voltage separately from cell under voltage 2 Cell Logger unit would be required, each controlling a relay. I guess only one unit needs to be data logging so the cheaper unit that is just a monitor would do the job, I think they were around AU$12. I have wired the cells via a DB9 female 9 pin data plug at the battery pack. That way any 9 pin data cable will plug in and the Cell Logger can be mounted in a convenient position for easy viewing and cable replacement is an off the shelf item. A piggy back plug or splitter cable would make connecting 2 units easy enough with a separate relay connected to each unit. The alarm setting can be toggled on or off on each unit so they could be set up so one monitors cell LVC and cell differential and the other HVC and cell differential. A passive BMS for under $50 plus relays.
It’s a work in progress so a few more mods to the current set up isn’t a biggy, thanks for that

T1 Terry
One thing to keep in mind: My housebus solar controller was wired by the bus builder. He connected the 12v load and solar charge cables into the same lug and bolted it to the battery + terminal. All good when nothing goes wrong. I had no idea that he did it this way so I installed the BMS with the same lug now connected to the relay. With the first cell HVC, the relay opened and the solar power pumped straight into the load cable frying my Espar heater controller and almost set my bus on fire . I suggest having one on the load and one on the solar side and only cut the load on LVC.
On HVC, only cut the Solar.

Also, thus far I tend to agree with Electric1 on the SOC.
No matter what the SOC of the cell is, when load is applied, it uses equal amounts of amps from all cells. So the out-of-sync will stay. When charging, equal amounts of amps are sucked up by the cells, thus the imbalance should stay. A great influence seems to be the connections between the parallel pairs. If you have a slightly imperfect connection or inferior (too thin) cable, it can mess up the balance. If the cables get warm while charging, get thicker ones. This is just my humble opinion thru experience thus far and I'm open to correction if I'm talking bolony - so have a go.

Curious to see the outcome of your experiment though Terry.
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Old 01-08-2011, 08:59   #23
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

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One thing to keep in mind: My housebus solar controller was wired by the bus builder. He connected the 12v load and solar charge cables into the same lug and bolted it to the battery + terminal. All good when nothing goes wrong. I had no idea that he did it this way so I installed the BMS with the same lug now connected to the relay. With the first cell HVC, the relay opened and the solar power pumped straight into the load cable frying my Espar heater controller and almost set my bus on fire . I suggest having one on the load and one on the solar side and only cut the load on LVC.
On HVC, only cut the Solar.

Also, thus far I tend to agree with Electric1 on the SOC.
No matter what the SOC of the cell is, when load is applied, it uses equal amounts of amps from all cells. So the out-of-sync will stay. When charging, equal amounts of amps are sucked up by the cells, thus the imbalance should stay. A great influence seems to be the connections between the parallel pairs. If you have a slightly imperfect connection or inferior (too thin) cable, it can mess up the balance. If the cables get warm while charging, get thicker ones. This is just my humble opinion thru experience thus far and I'm open to correction if I'm talking bolony - so have a go.

Curious to see the outcome of your experiment though Terry.
Egads; what voltage was/is your solar controller set to? Why would it go overvoltage and fry other devices on the load bus? Or perhaps the controller is too slow to handle the voltage spike when a relay opens? Just wondering.
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Old 01-08-2011, 11:27   #24
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

An automated balancer would be easy to build. It would be clipped onto the batteries of an existing bank. (No need to drian them down of connect them in parallel.) Currents would be minimal and over a short-enough time it would balance the batteries.

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Old 01-08-2011, 16:21   #25
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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An automated balancer would be easy to build. It would be clipped onto the batteries of an existing bank. (No need to drian them down of connect them in parallel.) Currents would be minimal and over a short-enough time it would balance the batteries.

Regards,
Brad
Hi Brad, can you give a little more detail please? How will the balancing happen without draining the high cell? Curious to know your view.....
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Old 01-08-2011, 16:28   #26
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

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Egads; what voltage was/is your solar controller set to? Why would it go overvoltage and fry other devices on the load bus? Or perhaps the controller is too slow to handle the voltage spike when a relay opens? Just wondering.
It's an Outback Flexmax 80 set to 14.4V. The Outback went into "limp" and started flashing its display after frying the heater control unit. I also thought it to be strange but obviously the Outback does not like being connected to a batteryless circuit. I think what happened is the controller did its sensing routine to determine SOC. It probably found the circuit to be "uncharged" and opened full throttled bulk charge into the circuit. Does that sound like a possibility?
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Old 01-08-2011, 16:36   #27
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

The main problem with active cell balancing units is have a bad history of turning on to discharge a high voltage cell but not turning back off again because of a component failure. If you are constantly watch you battery pack then I guess you would notice the light on far longer than expected but if you are like most people there are better things to do than watch batteries, isn't that why you bought a BMS in the first place. The problem doesn't show up until the cell is completely flattened because the voltage remains virtually the same between 98% charged and 5% charged if the load is only small. It shows up just at the worst time, when you really need the stored energy in the battery pack, suddenly it's not there and then things can go wrong big time.
A method of monitoring the cell voltages will soon tell you that a particular cell is falling behind the rest indicating further attention is required, this is referred to as passive BMS where you are the intelligence not an electronic circuit.
I don't want to start the great BMS debate over here, it would wreck a good thread and never ends in a clear cut result. Each to their own. I don't sell anything so I have no interests one way or the other, just a user that doesn't want to spend money unnecessarily, I'm yet to be convinced cell balancing is needed in such a small pack.

T1 Terry
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Old 01-08-2011, 16:45   #28
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

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It's an Outback Flexmax 80 set to 14.4V. The Outback went into "limp" and started flashing its display after frying the heater control unit. I also thought it to be strange but obviously the Outback does not like being connected to a batteryless circuit. I think what happened is the controller did its sensing routine to determine SOC. It probably found the circuit to be "uncharged" and opened full throttled bulk charge into the circuit. Does that sound like a possibility?
They are a very nice unit, I agree that the relay for HVC should be in the solar panel to regulator circuit. A quick question, are your panels connected in parallel to give a 12v nom output or in series to give a much higher output? High voltages require special relays to open a loaded circuit because the arc on a dc switch isn't self extinguishing like on an ac switch where the voltage passes through zero thousands of time a sec.

T1 Terry
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Old 02-08-2011, 03:02   #29
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

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They are a very nice unit, I agree that the relay for HVC should be in the solar panel to regulator circuit. A quick question, are your panels connected in parallel to give a 12v nom output or in series to give a much higher output? High voltages require special relays to open a loaded circuit because the arc on a dc switch isn't self extinguishing like on an ac switch where the voltage passes through zero thousands of time a sec.

T1 Terry
My panels are in parallel.
What I did as a safety measure meanwhile is I fitted a manually operated relay switch. So now I can disconnect the output from the controller before disconnecting the battery pack. I need to still change the wiring so that the controller output goes directly to the battery and not to the shunt where the loads are connected. That way it will be safe when the LVC opens the relay to the loads. The BMS does not distinguish between LVC and HVC. It triggers the same relay in both cases, so I will rely on the charger and the Controller and the current state of balance to keep from over voltage although I think it is necessary to have 2 seperate triggers.

Oh BTW, AC does only 50 to 60 zeros per second, not thousands.......
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Old 02-08-2011, 08:13   #30
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Hi Brad, can you give a little more detail please? How will the balancing happen without draining the high cell? Curious to know your view.....
It would drain the high cell(s) very slightly. IMHO (and I'm new to these batteries), that would be the way to go, using voltage comparitors, and only when above the knee.

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