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Old 07-02-2013, 18:24   #1981
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Originally Posted by CharlieJ View Post
The new release (I have only read a synopsis) of IEC 62133 Ed. 2 "Safety Requirements for Secondary non-LA Batteries" has some very specific shock and vibration qualification testing. This leads to two questions:

1. Are any of the cells that we are using qualified to 62133 Ed. 1 or Ed. 2? I am using CALBs and can find no mention of vibration testing.
2 Should we be concerned about the shock and vibration that the packs of LFPs that we are installing will be subjected to? For example, I am working on a 60' 80,000 pound catamaran that slams very hard when coming off the top of a wave. The size of the vessel is immaterial, these cells will be subjected to large accelerations/decelerations. Should we try and mitigate the shock and vibration?
1. Probably not yet, just read some of the standards for the first time since you posted this.

2. I don't know. Maybe. But we never had these standards for LA, or if we did, no one bought batteries based on shock testing given all the private label batteries? If needed, the answer lies in approving the cells, not mitigating the inherent shocks, beyond reasonable installation security.
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Old 07-02-2013, 19:47   #1982
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Oh boy, you really turning this into "science" fiction Terry. Both points are completely baseless, in both theory and practice. None of this stuff actually happens in real world in real scenarios. I suggest to go back to EE 101, thermodynamics 101, etc. before you continue to fill this forum with "information".

I have 100s of customers using our BMS over past 3 years and NONE of the things you so passionately describe actually happen in real life. How do you explain that?
:lol: Attack is not alway the best form of defence. Step one, gather the facts, if a BMS system truely worked the way the seller/manufacturer thought they worked, simply bolting a BMS onto each cell group in the battery and fitting it into it's operating envirnment would be all that was needed, after all, aren't these things supposed to balance the cells?

The reality is, a BMS system needs the cells balanced first, the claim is they are designed to keep the pack balanced. A balance pack rarely goes out of balance unless a cell fails or over charging causes a cell run away. A BMS can't do anything for a failed cell, it won't even tell you that the cell is failing, only cell monitoring will do that.
That leaves cell run away, if a high cell voltage charging cut is used there will be no cell runaway, so why is the cell balancing required?
The active BMS system can only actually function if the cells are over charged, that is the point they turn on, at an over voltage point. This brings me back to the question I asked before, why over charge the cells in the first place, there is nothing to be gained is there?
I don't have anything to sell, I have no reason to make stuff up.

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Old 07-02-2013, 20:00   #1983
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Terry,

That is the beauty of charging with solar, as you have a big voltage differential that keeps the electrons flowing while the panel might be partially shaded or when the battery being charged is almost full, hence the need for regulation. A 12 volt panel will have 36 cells for an output of 18 volts, unless you buy a so called "self regulated" panel which just uses less cells in series for a lower voltage that will stop charging if partially shaded and even if not shaded takes forever to get the last 25% of the charge into the battery due to low voltage differential.
that was the reason for mentioning the mains charging, a PWM controller being fed by a mains charger will act the same way as solar charging, the controller must be able to handle the max ouput of the charger and the charger must be able to handle the output being switch on and off rapidly.
I guess simply switch the charger off when a cell reaches the high voltage cut is a simple method, unfortunately, that moves the fail safe to the primary control, there is no back up if there is a system failure.
This is the problem with BMS units that control the charger, if something in the BMS stuffs up, it doesn't turn the charger off, boiled cells is the result until one explodes due to the pressure build up. If the venting is good that doesn't happen, but if the cloud of boiled vapour isn't also vented the inevitable spark resulting from a very over voltage system starts the fire.
If the cell logger looses it's negative feed, it simply shuts down, no alarm, no nothing, it just turns off, this is why I don't support fusing the negative wire, just the others.

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Old 07-02-2013, 20:44   #1984
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Quote:
But we never had these standards for LA, or if we did, no one bought batteries based on shock testing given all the private label batteries? If needed, the answer lies in approving the cells, not mitigating the inherent shocks, beyond reasonable installation security.
LA batteries are pretty robust units so I don't think that shock and vibration was, or is, an issue with them.

LFPs, on the other hand, are fairly sophisticated with membranes that can be punctured by dendritic whiskers, copper and aluminum bonds, etc. While the standard may offer some assurance, once the manufacturers start testing to it, the designers and installers still have a responsibility to ensure that the installation is within the acceleration/deceleration limits that the cell was certified for.

In the meantime, what should the early adapters do, if anything?
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Old 07-02-2013, 21:10   #1985
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

From the Winston test information that i have on hand:
Vibration test
Vibrate direction: rack vibration
Vibration frequency: 10^55Hz
Maximum acceleration: 30m/S2
Vibration duration: 2hrs
Discharge: discharge the cell with 1/3 3C (A) current until the voltage reach 2.5V
there should not be significant discharge current transformation, abnormal voltage, case distortion or electrolyte leakage.

It's a bit chinglish but maybe it means some thing to some body

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Old 07-02-2013, 21:14   #1986
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I suppose it is too late to stop calling the prismatic LiFePO4 batteries "cells"? I see even the manufacturers call then that. No way are they cells, the cutaway views clearly shows they are batteries of individual cells.

I'll be quiet now. Sorry for the interruption. Carry on.
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Old 07-02-2013, 21:23   #1987
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by daddle View Post
I suppose it is too late to stop calling the prismatic LiFePO4 batteries "cells"? I see even the manufacturers call then that. No way are they cells, the cutaway views clearly shows they are batteries of individual cells.

I'll be quiet now. Sorry for the interruption. Carry on.
Really? Cells in parallel? Kind of hard to believe.
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Old 08-02-2013, 02:29   #1988
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by daddle View Post
I suppose it is too late to stop calling the prismatic LiFePO4 batteries "cells"? I see even the manufacturers call then that. No way are they cells, the cutaway views clearly shows they are batteries of individual cells.

I'll be quiet now. Sorry for the interruption. Carry on.
Not quite sure what you mean? There are 12v batteries available, a plastic outer box with 4 x 3.2v cells inside to make up a 12v battery, are they the ones you are refering too? the cells themselves are made up of multiple pouches to build capacity, similar to plates in a lead acid cell, but they are still only cells as they are all connected in parallel.

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Old 08-02-2013, 04:01   #1989
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Dave, sorry for being brief, I really don't have much time for forums posting.

In this scenario chargers are not parallel, but in series, each one charging its own cell, so there is no redundancy. One charger fails, its cell does not get charged, whole pack is unusable. So, N times points of failure.

In low voltage world cost is largely driven by Amps, and in this case you need N chargers, each with max amps, so cost is N times. There is not much cost difference between 12V charger and 3.5V charger, assuming they have same current rating, since cost of power stage components is approximately the same within low voltage scale, but you need N times of them.

Physical size and weight is also a factor when compared to laptops and cell phones. Small battery charger can be integrated in one tiny IC as you noted, but not so much when you need above 20A in higher voltage packs, and above 100A in lower voltage packs. So charger would be bulky and heavy.

Efficiency is horrible because switching losses are multiplied, since chargers are in series. Again, I am simplifying this point somewhat since I don't have time to run numbers, but its pretty obvious when you have N power stages compared to just one.

These arguments also depend on what N is. Sure its not as bad in 12V bank where N=4 , but gets much worse with N=8,16,24,32....

When all factors are combined, its just not practical in real world, although there are a few examples where it works, but most of those are experiments where people are trying to prove something, never growing into a larger scale market.

Agree with everything you said, ( in the main) Your main arguments are cost based here and I said from the outset this is not a cost based argument. ( for me the costs are virtually zero).

I agree re N= large numbers, I was confining my arguments to boat battery configs, ie 4 large prismatic cells.

As to redundancy, I was actually looking at modular 5amp units , that could be parallel ganged , efficiencies are very high approaching 98% ( measured) when operating in 80% loaded mode, EMI is low. you say three modules could be ganged together to provide a redundant 15 amp per cell charger.

The units have a RS485 comms bus and current sharing connections.

As to overall efficiencies , I think the argument is moot, large SMPS are in practice very inefficient at low loads ( and at close to saturation) , equally standby currents ( quiescent and other wise) are often much higher then smaller designs.

Of course the argument can pale into insignificance given that mains power is bring used. The batteries can waste more power then the whole battery charger put together.

Yes I agree large numbers of multiple units are difficult to manage. my Concept was to rack them like "blade servers"

Dave
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Old 08-02-2013, 04:13   #1990
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Could I pose another question about BMSs

Is there a convergence of thinking on how to do automatic balancing.

Am I right in thinking that all thats happens is that charge current is bypassed into a resistor to try and prevent the cell that has reached the cutoff voltage is not receiving any charge current ?

Why not just bypass the cell in the series with a switch ( electronic etc).?

I read elsewhere that some BMSs balance by drawing off charge after the effect , this seems impossible in a typical system under load etc. , ?

Thanks, just reading more on BMSs , some info is clearly confused

Im talking about balancing BMS heres not just cell loggers/alarm/cutoff systems.

My experience is that balancing in fractional C environments simply isnt required , but still Id like to understand whats out there.

Thanks

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Old 08-02-2013, 05:42   #1991
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Originally Posted by goboatingnow View Post
Could I pose another question about BMSs

Is there a convergence of thinking on how to do automatic balancing.

Am I right in thinking that all thats happens is that charge current is bypassed into a resistor to try and prevent the cell that has reached the cutoff voltage is not receiving any charge current ?

Why not just bypass the cell in the series with a switch ( electronic etc).?

I read elsewhere that some BMSs balance by drawing off charge after the effect , this seems impossible in a typical system under load etc. , ?

Thanks, just reading more on BMSs , some info is clearly confused

Im talking about balancing BMS heres not just cell loggers/alarm/cutoff systems.

My experience is that balancing in fractional C environments simply isnt required , but still Id like to understand whats out there.

Thanks

Dave
You are correct about simple balance systems bypassing to a resistor. But unless the BMS and charger can communicate (not yet for us DIY guys), the resistors don't stop charging, just slow down charging for that cell since the rate is likely more than the resistor can shunt.

For discussion purposes, the "stupid" balancing systems start shunting at 3.55V regardless of where the other cells are. More intelligent systems might start balancing at 3.4V, but only if there is at least one cell below 3.4V, essentially balancing to the lowest cell. Usually there is an acceptable range involved say .02V difference needed to invoke the balancing. This has to be done at the high end of the discharge curve where the differences in charge show up as measurable voltage differences.

Can't switch one cell out of the series connection without adjusting the charger to a 3 cell rate. And only then if you didn't need a 12V bank during charge. Not to mention the series connections pass large currents making switches expensive.

Even in multiple C, not fractional C, balancing requirements are still small, if needed based on EV users. They are fractional C charge usually, but are frequent multiple C peak during discharge.

In my 12P, then 4S, 1200Ah bank after >100 cycles charging to 3.4-3.45V per cell, I checked balance at 3.6V which is where an imbalance would show up and found it satisfactory, but not perfect. However if I wanted to "fix" it, I'm pretty sure I could have done so with 20 minutes using my manual 15A cell resistor based on my initial balancing experience. So at the 3.2V cell level, after 120,000 Ah of charge, 120,000 Ah discharge, a mere 5 Ah would be lost to balancing. The point is, even if balancing was desired, the efficiency of how you accomplish it is not important.

Maybe it helps to think of our application more like a UPS? In essence, that's what mine is, except it runs on battery 90% of the time.

I found this site a great source of information on BMS systems:

http://liionbms.com/php/index.php

It's done by the maker of the Elithion BMS. I found the BMS product more suited to EVs, but it could probably be adopted for marine. But either way, his book and white papers are excellent.
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Old 08-02-2013, 05:57   #1992
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Am I right in thinking that all thats happens is that charge current is bypassed into a resistor to try and prevent the cell that has reached the cutoff voltage is not receiving any charge current ?
Essentially correct. This process is called passive balancing and the excess current going to a single cell is bypassed thru a ceramic resistor to allow the other cells in the parallel string to catch up.

Active balancing, on the other hand, requires communication (CAN bus) between the cell modules and the charging source or sources and the charging current is directed to the cells as required.

BTW, the owner of the site that Bob Ebaugh recommended in #1991 has written an excellent, albeit expensive, book on BMS. It is relatively academic and not directly applicable to our xPyS configurations, but very fact filled, nonetheless.
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Old 08-02-2013, 06:00   #1993
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Can't switch one cell out of the series connection without adjusting the charger to a 3 cell rate. And only then if you didn't need a 12V bank during charge. Not to mention the series connections pass large currents making switches expensive.
Yeah I reread my post and saw that was a real "brain fart" of mine, Im handing back my electronics degree in shame.


SO if the BMS shunts the charge current, how do you protect the BMS from excessive shunt current, It has no control over the charger, nor has it any idea of the potentially greatest charge current that could be applied. Surely you could just fry the bypass device

ie I set my set point on my charger a little to high, so there is stiull serious current flowing at the BMS setpoint, it detects imbalance, and "lots of magic smoke escapes", or a fuse goes, but the balancing has failed. It all seems a little haphazard
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Old 08-02-2013, 06:02   #1994
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Essentially correct. This process is called passive balancing and the excess current going to a single cell is bypassed thru a ceramic resistor to allow the other cells in the parallel string to catch up.

Active balancing, on the other hand, requires communication (CAN bus) between the cell modules and the charging source or sources and the charging current is directed to the cells as required.

BTW, the owner of the site that Bob Ebaugh recommended in #1991 has written an excellent, albeit expensive, book on BMS. It is relatively academic and not directly applicable to our xPyS configurations, but very fact filled, nonetheless.

thanks I may splurge out from the company piggy bank.
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Old 08-02-2013, 06:05   #1995
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Im not really crazy about mounting a Cell log 8 at my chart table , nice to keep things neat on a boat. Its for RC hobbyist !! We need someone to build a nice big display that can be mounted and easily read. Just enlarge the cell log by about 4 times , so I won't have to go looking for my glasses every time I charge the batteries .

Someone build one

Regards
Typhoon:

I built a monitoring station for my 48 volt AGM electric propulsion bank. I have it mounted at the helm so I can keep an eye on things while underway. Got a photo of it here:
THE BIANKA LOG BLOG: MID WINTER CHECKUP
It's not permanently installed yet but, I can check each battery's voltage as well as pack voltage and current. The main battery monitor (XBM) had to be moved out of the cockpit because the LCD display could not stand up to the sun:
THE BIANKA LOG BLOG: REPAIRING A XANTREX XBM BATTERY MONITOR DISPLAY
The backup Paktrakr battery monitor was located in the cabin and hard to read without glasses and also took it's power from one of the batteries causing an unbalanced condition. Having all the info at the helm with this monitoring station helps a lot in keeping an eye on the batteries.
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