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Old 25-03-2013, 05:59   #2461
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thank you all for the replies, most informative.

Terry: The bank is composed of 260AH cells so 3P4S implies 780AH capacity and the alternator pushes 160Amp

Daddle: The 3P4S configuration was suggested by Balqon. They said that it reduces the need for balancing across individual cells and therefore reduces failure rate (my interpretation of what was said). This is supported by the installation manual of the MiniBMS. In other way the balancing is done across the 4 3P group and not within the 3P group. If I had used 4S3P I would have 12 cells to balance via BMS.
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Old 25-03-2013, 06:30   #2462
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Doesn't matter how its arranged for the fail-safe issue. So long as there are at least 8 units then a cruiser can continue with reduced capacity by reconfiguring the units. For the BMS function it is handy to parallel the batteries to simplify the design. I suppose a failure of the somewhat complex BMS system can be compensated for with a voltmeter and attentiveness. May be wise to carry some BMS spares.
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Old 25-03-2013, 08:02   #2463
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Float isn't needed, but if it's a function because the motor is still running for other reasons, 13.8v will be ok. the important thing is a warning system that a cell has reached 3.6v, stop charging till it drops below that voltage, any higher on a regular basis will cost you cycle life and speed of recharging.
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In my opinion, this is too high for "float". As I just discovered and posted a page or so back, 3.35V/13.4V held for a 24 hour period was at least 95% SOC. Any more than that and I think you risk overcharging. But my definition of "float" may be different? For me it's a passage (powerboat), where constant charging is available the entire time, or plugged into the marina shore power.
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Old 25-03-2013, 08:51   #2464
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

If the cell voltage is monitored and charging stopped for any period a cell is over 3.6v, over charging can not occur, the cells just remain full. By maintaining the 13.8v, any loads are powered by the charging device, ot the battery, unless the load is greater than the charging current. if you start with full batteries, it's longer before rechargig will be required.
There is no point in keeping a charging device running just to maintain a float condition, but if the engine is already running, it may as well keep the battery fully charged.

As far as the multiple cells in parallel to build capacity rather than single big cells, I was still able to use my battery pack even though I cooked 4 cells in one parallel pack, I just rearranged them so I had 3 cells in parallel and 4 sets in series, and used the 4 cooked cells as spacers, I couldn't have achieved that with big single cells.
The other avantage is the averaging of numbers, in ever batch there will be some with a little more capcity than others, some charge faster than other, by having multiple cells in the one parallel pack the chances of the high cell being match with a low cell and a fast charging cell being match with a slower charging cell is gratly improved, and adds the option for mix and match, the pack stays far better balanced when built up from multiplre smaller cells in parallel than they do with single large cells.
Well that's my experience over the last 2 yrs with the 6 battery packs I have personally built and the numerous packs I've had a hand in building.
If you see a pack dropping lower than the others you can check to see if you have a problem cell and isolate it, you can't do that with large single cells.

T1 Terry
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Old 25-03-2013, 09:34   #2465
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If the cell voltage is monitored and charging stopped for any period a cell is over 3.6v, over charging can not occur, the cells just remain full. By maintaining the 13.8v, any loads are powered by the charging device, ot the battery, unless the load is greater than the charging current. if you start with full batteries, it's longer before rechargig will be required.
There is no point in keeping a charging device running just to maintain a float condition, but if the engine is already running, it may as well keep the battery fully charged.
T1 Terry
I disagree. 3.6V held indefinitely will lead to overcharge and reduced battery life. It's way high. The only formal specs available for LFP use 3.4 or 3.45V for float. See the Valence and A123 cell specs. So perhaps those levels are OK. However, Jack Rickard, who has done more cell testing than anyone I know of claims 3.38V maintains 100% charge. It's also generally accepted that staying away from the top and bottom ends extend cell life. So why push it for the last 5% of capacity?

When running at sea or at dock with people aboard there are DC loads like lights and pumps that discharge the house bank and so it's desirable to maintain charge on the battery. So given today's charging systems, that means float, until someone comes up with something to automate a partial discharge to say 80% and then cycle back to charge. That's just not out there yet that I know of for the myriad of different charging sources you might need to have in a marine install.
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Old 25-03-2013, 12:51   #2466
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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We do not yet know the failure modes for LFP in a cruising boat. And your "mostly" word is worrisome: 51% or 99.999%?. I would think the typical cruiser would use some parallelism to avoid total electrical failure offshore or in remote locations ... LFP cells are not readily available even here in the PI. Most cruisers have some form of parallelism with their LA batteries.

Note that prismatic "cells" are actually a battery of several parallel cells. But very difficult to disable one problematic cell.
The main failure of Li cells, either LICO or LiFe is either through over discharge or over charge. There has been no evidence in either technology of sustained issues involving catastrophic failure. ( Dreamliners not withstanding). Early catastrophic failures where down to over charging issues, particularly lack of pre-qualification, and over-discharge/over charging in elevated temperatures. ( or sub zero). Heat stress knowledge in Li has only recently been widespread in the technical press.

Hence in boats failures are unlikely to be of the type where removal of a battery is necessary.

Parallelism in LA has occurred because its not easy or inexpensive to get very large 2.1 V LA cells. so people parallel 12v solutions by necessity.

NOTE that Prismatic cells do not contain paralleled cells internally , prismatic construction involves a layer flat construction as opposed to cylindric. The prismatic cell is a single cell , just like its cylinder counterpart.


While it may be useful to have a set with some parallelism , purely as a backup or a backup, its far more useful to have a Li friendly charge and discharge setup.

Dave
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Old 25-03-2013, 13:04   #2467
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Quote:
I disagree. 3.6V held indefinitely will lead to overcharge and reduced battery life. It's way high. The only formal specs available for LFP use 3.4 or 3.45V for float. See the Valence and A123 cell specs. So perhaps those levels are OK. However, Jack Rickard, who has done more cell testing than anyone I know of claims 3.38V maintains 100% charge. It's also generally accepted that staying away from the top and bottom ends extend cell life. So why push it for the last 5% of capacity?

I dont think that ANY float charge should be applied to an LI battery set, All my reading has shown that Li is damaged ( shorter life) by applying ANY induced voltage when the battery is already full.

Where that cant be done, ie where you have load sharing ( between battery charger and teh battery), I would reccomend that the battery( cell) never be charged above about 80%, and then floated at 12.8 (3.2v) , and certainly not at 3.6v per cell.

Then at least the charge ( voltage stress) on Li is reduced at 80%, and the battery can handle the float charge.

Far better to stop charging and then only recharge when a lower voltage threshold is triggered. There is no advantage to float charging Li, as the self discharge is incredibly low.


What this brings up is actually the unsuitability of Li for applications involving load sharing. Im grappling with this at the moment in Li based battery backup product Im designing . The quickest way to kill Li, is to leave it on high float for any length of time. Li is great for "charge, discharge , recharge " applications, like drill batteries, phones, laptops. It sucks at load sharing , because you are applying a charge voltage continuously. The only way around that, other then experiencing shorter cycles, is to charge to about 80% , ie very conservative cutoffs.

Its a funny irony , that while we are gaining theoretical additional discharge capacity , the restrictions on various issues with Li , mean in practice we gain about 60% of teh batteries capacity ( ie 80% ->20%) , whereas with LA we have nominally 50%.

If you want high capacity charging , ie to 95-100% , then I would not reccomend any load sharing configurations ( ie when fully charged)

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Old 25-03-2013, 14:43   #2468
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Question, if the charger is adjusted to only output 3 something volts max. doesn't the current flow stop when the cell measures 3 something volts? How would the cell over charge? Do I have to disconnect my solar panels when the cells are full?
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Old 25-03-2013, 14:49   #2469
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Quote:
Question, if the charger is adjusted to only output 3 something volts max. doesn't the current flow stop when the cell measures 3 something volts? How would the cell over charge? Do I have to disconnect my solar panels when the cells are full?
There is a lot of discusssion on how much ion transfer occurs under these conditions, because Li is fundementally differnt that LA in that regards. All I have read suggests that float charges are not a good way to handle Li. The reccomendation is that once the designated cutoff is reached the charger is disconnected ( physically or electronically) from the battery and only re-engaged on a fall in terminal voltage.
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Old 25-03-2013, 16:37   #2470

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

Dave, I am interested in the technology. Either it will come to dominate, or it will go away as another dead end. In the 1960's we were all told by Reddy Kilowatt that we would "soon" have home nuclear piles, the size of hot water heaters, and there would be no more grid. RK was the cartoon spokesman for the entire US nuclear power industry, and they got it wrong.

So if a batch of pitbulls snapping at each other and each saying they make the only worthwhile technology all tell me a similar story about how THEIR product is going to dominate, you'll pardon me for saying I've heard that one before.

And I'm not trying to confuse things with the mention of Yttrium. I came across that notice of the chemistry being changed while looking at what Balqon and Sinopoly were saying in their spat. For that matter, just as ThunderSky was establishing a reputation, again, POOF and the name is gone.

Well, Esso and RCA screwed up that way too, lost the tiger and Nipper and took a while to bring them back.<G>

Balqon says the operating voltage for their "12" volt battery is 11-16 volts.
Sinopoly says 2.8-3.8 per cell, 3.2 nominal. That's 11.2-15.2 volts, 12.8 nominal. Nowhere near 13.9.

OK, all those different numbers make no difference? If the manufacturer says 12.8, you're going to charge to 13.9? Or tell me these aren't different products and the chemistry we're discussing here hasn't been obsoleted already? The Yttrium is what, just a distraction to put coffin nails in a patent dispute?

And the shift to Yttrium doping (which Sinopoly seems to be implementing across only portions of their line) makes no difference in the charging recommendations, just in the performance?
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Old 25-03-2013, 16:40   #2471
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

A quick bit of math will tell you that a 13.8v float will not hold 4 cells at 3.6v. Generally, if a cell group reaches 3.6v and the charging is stopped for a period while the load pulls the voltage back down, then resumed, the pack will gradually balance.
Some understanding and application of that understanding is needed. A cell is 100% full with a rested voltage of 3.4v, you can't have 105% or 110% full can you? There is no added capacity above 3.4v rested, but charging a to 3.4v will not achieve a 3.4v rested voltage reading, think about why this would happen and you are part way to understanding lithium ferrous battery charging requirement, particularly if a return to full capacity is required.
Next, if yo can't have 105% or 110% full, any voltage over 3.4v is surface voltage, it has no capacity as far as additional Ah, it requires Ah in to get that elevated voltage, but you don't gain any increase in capacity.
Let's take a theoretical figure of 1 Ah is required to lift the cell voltage from 3.4v to 3.6v. When the load is applied, that 1 extra Ah isn't there, but it's 1 more Ah the other cell stored away. the charging stops and the load uses 0.2Ah, the cells that are below 3.4v so below 100% gained 1Ah but lost 0.2Ah, an over all gain of 0.8Ah, the cell that reached 3.6v lost the 1 ah it couldn't store, plus 0.2Ah, even though the cell only lost 0.2Ah, 1.2ah were wasted. Now repeat this cycle, what will happen? The cells at a lower capacity and therefore lower voltage will fill by 0.8Ah each cycle, after a while, the total voltage from all 4 cells will equal 13.8v, before any cell reaches 3.6v.
You can move this down 1v, but the difference between 3.4v and 3.5v is very little, as soon as the chargign stops and a load is still applied the cell will drop to less than 3.4v, so a larger part of 0.2Ah each cycle is being used, it will take a long time for the pack to self balance.
I can't understand why, if a charging source is running, you would want to power the electrical loads from the battery, maybe some one could explain how this would be an advantage.

T1 Terry
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Old 25-03-2013, 19:53   #2472
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by T1 Terry View Post
A quick bit of math will tell you that a 13.8v float will not hold 4 cells at 3.6v. Generally, if a cell group reaches 3.6v and the charging is stopped for a period while the load pulls the voltage back down, then resumed, the pack will gradually balance.
Some understanding and application of that understanding is needed. A cell is 100% full with a rested voltage of 3.4v, you can't have 105% or 110% full can you? There is no added capacity above 3.4v rested, but charging a to 3.4v will not achieve a 3.4v rested voltage reading, think about why this would happen and you are part way to understanding lithium ferrous battery charging requirement, particularly if a return to full capacity is required.
Next, if yo can't have 105% or 110% full, any voltage over 3.4v is surface voltage, it has no capacity as far as additional Ah, it requires Ah in to get that elevated voltage, but you don't gain any increase in capacity.
Let's take a theoretical figure of 1 Ah is required to lift the cell voltage from 3.4v to 3.6v. When the load is applied, that 1 extra Ah isn't there, but it's 1 more Ah the other cell stored away. the charging stops and the load uses 0.2Ah, the cells that are below 3.4v so below 100% gained 1Ah but lost 0.2Ah, an over all gain of 0.8Ah, the cell that reached 3.6v lost the 1 ah it couldn't store, plus 0.2Ah, even though the cell only lost 0.2Ah, 1.2ah were wasted. Now repeat this cycle, what will happen? The cells at a lower capacity and therefore lower voltage will fill by 0.8Ah each cycle, after a while, the total voltage from all 4 cells will equal 13.8v, before any cell reaches 3.6v.
You can move this down 1v, but the difference between 3.4v and 3.5v is very little, as soon as the chargign stops and a load is still applied the cell will drop to less than 3.4v, so a larger part of 0.2Ah each cycle is being used, it will take a long time for the pack to self balance.
I can't understand why, if a charging source is running, you would want to power the electrical loads from the battery, maybe some one could explain how this would be an advantage.

T1 Terry
terry,

It seems to me you answered you're own question.

If you stop go back and read back to yourself you're writing, you will see the light.

Charge cycle/Float/Standby.

It's done everyday.

Lloyd
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Old 25-03-2013, 20:47   #2473
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NOTE that Prismatic cells do not contain paralleled cells internally , prismatic construction involves a layer flat construction as opposed to cylindric. The prismatic cell is a single cell , just like its cylinder counterpart.
With a single series string of large C cells the capacity will be limited by the weakest cell. A common failure mode of LFP is to prematurely lose capacity ... either by defect or abuse. Paralleling batteries would be prudent for cruisers.

This link (from a CF post back around 1250) clearly shows that a LFP prismatic "cell" is actually a "battery" made from many flat pouch cells in parallel. However for a cruiser this does not result in increased reliability because defective cells cannot be easily removed.

http://liionbms.com/php/prismatic_cells.php
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Old 25-03-2013, 21:40   #2474
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I can't understand why, if a charging source is running, you would want to power the electrical loads from the battery, maybe some one could explain how this would be an advantage.
Because it stresses the battery and shortens its longevity.

There is a compromise solution: monitor load current, and only when it exceeds available charge current, automatically switch in the charger to assist. This way the battery is never overcharged.
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Old 25-03-2013, 23:28   #2475
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Because it stresses the battery and shortens its longevity.

There is a compromise solution: monitor load current, and only when it exceeds available charge current, automatically switch in the charger to assist. This way the battery is never overcharged.
When it's truly in stand-by, it's out of float. That means it's a
power supply standing by.

Last year I rewired/restored a 1960 16M Italian Bag. It had a Gentile Giant and an Iron Maiden. The iron maiden is in stand-by until...

Then when ever a 120/220 load is called, the iron maiden gets the call, stands up, and goes to work, then lays back down and goes to sleep as soon as the call is over.

Gentile Giant=any 120/220 load while away from shore side power.

Iron Maiden=the diesel gen with auto-start.

The call is from any ac load whether hair dryer, cook-top, outlet, or light bulb...etc

A series of resistors senses the voltage drop, and energizes a mag switch, which then calls the auto start to the Iron Maiden...just as it senses the load, it senses the no load which calls for the auto shut down.

This is all done with relays, wound resistors... and no transistors of any kind.

Lloyd
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