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Old 07-02-2013, 07:18   #1966
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Has anyone looked at unit cell chargers. I have a nice digitally controlled SMPS on the CAD system. wondering if it would make a nice multi cell Li charger, with in effect a built in BMS, no wasted balancing energy either. The SMPS, can be paralleled ( it has active current steering), so in theory I can put several together,

The outputs are floating so that handles one aspect.


anyone see any issues here , interested in feedback.

Dave
Dave, this concept, while true in theory, is impractical in reality. In a pack of N cells it created N times more points of failure, its efficiency is horrible since power conversion happens N times instead of one, it does not address LVC, so some form of BMS is still needed, etc etc

It has been tried many times in DIY world, and it never works well enough to be repeatable success story.
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Old 07-02-2013, 07:24   #1967
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Moving the boards away from the battery is not a bad idea, but remember that there will be voltage drop on the cables when that 5w resistor kicks in so if the board is relying on the cable to sense voltage you will end up with rapid switching of the resistor, as the voltage drops under load and then returns to the higher voltage when the load switches off. the other thing to keep in mind, each of those 5 w resistors are producing 5w of heat, that has to go somewhere, you have no air circulation in that box, it will get very hot very quickly inside there if all 4 resistor start burning off current.

T1 Terry
Just because resistor has 5W written on it, does not mean it actually produces 5W of heat. Its rated for 5W, but proper design never runs parts at their peak rating, most common rule of thumb is to run at half of peak rating.

Our BMS boards dissipate 2.5W-3.0W , so up to 12W of heat inside the box, and that is the worst case scenario when all 4 are shunting at full, which does not happen in properly designed system.

12W of heat is easily dissipated without any danger of overheating, even in limited space box. In addition to that, each board has PTC fuse, protecting the circuit from overheat and overcurrent.

Again, please check your facts before making public statements.
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Old 07-02-2013, 07:33   #1968
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Dave, this concept, while true in theory, is impractical in reality. In a pack of N cells it created N times more points of failure, its efficiency is horrible since power conversion happens N times instead of one, it does not address LVC, so some form of BMS is still needed, etc etc

It has been tried many times in DIY world, and it never works well enough to be repeatable success story.

OK lets look at this , because in the LI-Ion business , like PCs etc, its the only way thats used, The ICs monitoring the cells control the process on a cell level, I have several such ICs in front of me.

Power conversion efficiencies in large SMPS are not linear, often large SMPS are very inefficient compared to smaller ones. Anyway for mains powered SMPS the inefficiencies are actually small as a function of total power draw, often losses in battery charging efficiencies are greater then the power conversion.

As to points of failure. You are mistaken, redundant parallel power sources are the norm in high reliability, fault tolerant designs. They are also far more efficient, your PC has a 8 stage parallel power source on board.

For example I can design a SMPS board that does 10 amps very cheaply, I can parallel these no problem to get Nx power output, I get several gains

(a) The units can gradually power off as the charge current decreases, hence my efficiencies are greater then a single SMPS, idling along ( the SM power conversion mechanism is quite poor at low loads)

(b) I have in built failure I can loose chargers without loosing functionality, even in extremes I can rewire the chargers to take over each cell, I only need one to complete a charge cycle.

A large SMPS has failure modes that may or may not exist in smaller models. Hence redundant multiple units generally have a much higher MTBF viwed as a system.

Cost is another matter entirely, as is DIY, Im not a DIYer. I was merely looking at the "technical " issues in unit cell charging


A charger can act as a BMS easily , LVC is just a relay output. teh cell charger can report cell voltages raise alarms etc like any BMS. ( its all just software as the chargers has terminal voltage monitoring and current monitoring ability or can be easily added.

dave
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Old 07-02-2013, 08:56   #1969
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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.
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Old 07-02-2013, 10:57   #1970
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Hi Bob,
Have you tried different charging voltages to see just what te difference is? might be different with mains chargers, but the solar regulator doesn't back off till my 720Ah bank reaches 98%, it takes another 20 mins or so for the last 2 percent. witha 100Ah battery, that's only 2 Ah, would that be critical?

T1 Terry
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.
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Old 07-02-2013, 12:17   #1971
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Electric1,

I'm not sure I would dismiss the idea of 4 separate charging sources so quickly. Yes, there are some problems, but as you note, it depends on the value of N. Your business depends on the EV world, where N is on the order of 100, so naturally your designs reflect that. And frankly, for N = 100, the answer does seem pretty obvious. But for N = 4, maybe not. While your stuff works just fine at 12v, it may not be the only practical approach.

Terry,

I know you don't care for built in balancing, and that's fine, but the argument about shunt MOSFET failures is a straw man. Modern power MOSFETs are robust and cheap. They are so cheap that for a 12v design that needs only 4, and is not highly cost sensitive, you can pick parts that are way over specified. In addition, even though present designs don't seem to do so, it is easy and cheap to include some simple sensing circuitry and software to detect shunt current when it shouldn't be there and sound an alarm. Now of course more circuitry is bad, but a ruined cell due to full discharge is a lot worse.

I know you have experienced shunt problems in the real world, but that doesn't mean it's a hard design problem. Some of the small BMS companies do great design, some are strictly amateur. I would say that failures are probably the result of bad design rather than a flawed concept. The end game is this: if LFP batteries really take off and balancing is needed, the market will supply a robust, low cost design. It's just not that hard. If balancing turns out to be a non issue, the the problem is gone.

I don't pretend to know which way it will go, but either way, it doesn't worry me.

Charlie
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Old 07-02-2013, 12:46   #1972
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Electric1,

I'm not sure I would dismiss the idea of 4 separate charging sources so quickly. Yes, there are some problems, but as you note, it depends on the value of N. Your business depends on the EV world, where N is on the order of 100, so naturally your designs reflect that. And frankly, for N = 100, the answer does seem pretty obvious. But for N = 4, maybe not. While your stuff works just fine at 12v, it may not be the only practical approach.

Charlie
To me its pretty simple choice, 1 charger vs. 4 chargers. 4 times cost/weight/volume, but what is the benefit? Balancing? Its barely needed at all, so why spend 4 times more for something you don't need?

And how do you apply this to alternators and solar? Unless all your sources come with 3.5V outputs you are stuck. Unless you propose cell chargers which take 12V rail as input, in which case you still need all equipment you have and plus 4 more units, which drops efficiency and adds cost and bulk, at virtually no benefits.
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Old 07-02-2013, 13:15   #1973
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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

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Old 07-02-2013, 13:26   #1974
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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
You can buy large voltage displays cheap, but no alarm feature. If well balanced, your preset charging voltage should do fine without you constantly monitoring cell voltages.

It is for RC hobbyist, that is why they are so cheap, but do the job needed and trigger an alarm for low cell voltage during use and high cell voltage during charge. For my use, the internal alarm will suffice, but on a boat just use that signal to close a relay for a larger alarm and leave the logger at the batteries.

I don't expect my cells to lose their balance, but having a low and high voltage alarm will insure I don't hurt the cells.
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Old 07-02-2013, 14:03   #1975
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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You can buy large voltage displays cheap, but no alarm feature. If well balanced, your preset charging voltage should do fine without you constantly monitoring cell voltages.

It is for RC hobbyist, that is why they are so cheap, but do the job needed and trigger an alarm for low cell voltage during use and high cell voltage during charge. For my use, the internal alarm will suffice, but on a boat just use that signal to close a relay for a larger alarm and leave the logger at the batteries.

I don't expect my cells to lose their balance, but having a low and high voltage alarm will insure I don't hurt the cells.

I was thinking of that , but it would be nice if I could get a amp hour meter and a cell logger in one unit with alarms. Somebody must be working on something like that, maybe Victron or Mastervolt. .

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Old 07-02-2013, 14:13   #1976
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I was thinking of that , but it would be nice if I could get a amp hour meter and a cell logger in one unit with alarms. Somebody must be working on something like that, maybe Victron or Mastervolt. .

Regards
When .000002% of the boats on the water currently use LiFePO4, there is hardly a market... I suspect when we get close to 2% there might be an opportunity.....

Heck even in my sample size of customers, over 90% still use flooded LA batteries. Even with the "popularity" of AGM & GEL less than 10% of the market here in Maine is AGM or GEL...

I suspect you could easily take the Junsei apart and get a new front panel made for it making thus it more "marine" looking.. Front Panel Express is who I use for stuff like this..
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Old 07-02-2013, 14:16   #1977
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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When .000002% of the boats on the water currently use LiFePO4, there is hardly a market... I suspect when we get close to 2% there might be an opportunity.....

Heck even in my sample size of customers, over 90% still use flooded LA batteries. Even with the "popularity" of AGM & GEL less than 10% of the market here in Maine is AGM or GEL...

I suspect you could easily take the Junsei apart and get a new front panel made for it making thus it more "marine" looking.. Front Panel Express is who I use for stuff like this..
Ahhh you know what I mean , it would be nice wouldn't it


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Old 07-02-2013, 14:47   #1978
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Most of us take the Internet for granted, but as a "roll your own" DIY'er for most of my life, the Internet makes it so easy to find components and sub assemblies to be able to fabricate what you have in your head.

All this worry about LiFePO4 is unfounded. The reason for BMS is for the appliance operators who don't understand batteries, to protect their investment. LA batteries have arrant cells over their cycle life, and then you just toss them. Since the cycle life of LiFePO4 are greater than 2000, over that period cells will get a bit out of balance. A cell logger with high and low voltage sets for alarms is really all that is needed. To re-balance is easy, and your back on track again.

There are many who are following this thread trying to decide if LiFePO4 is right for them. On cost alone, they are right for everyone, but I'm willing to bet between all of our pontification and the Dreamliner's problems, many are going to hold back for the dust to settle. I've used batteries in so many applications from EV to off grid homes, I can say nothing beats LiFePO4, and their price will continue to fall but even at today's prices, has every other battery beat for house banks on boats, what this thread is all about.

So get the lead out, free yourself from electron storage hassles, and join the LiFePO4 family of early adopters. You will be glad you did.
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Old 07-02-2013, 15:16   #1979
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I was thinking of that , but it would be nice if I could get a amp hour meter and a cell logger in one unit with alarms. Somebody must be working on something like that, maybe Victron or Mastervolt. .

Regards
What you want should be here soon.

This company has a BMS that displays to a VGA screen.
http://elitepowersolutions.com/
ebaugh is using this system on his boat in a custom mounting.
The BMS processor itself has a CAN output, so if they could be convinced to work with someone like GARMIN We could see the display on A GMI-10.

There are others, some are making the data available by bluetooth or wifi so it can be displayed on tablets or phones.

If you have a PC at your Nav station, you can display the data from a CellLog connected by USB.
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Old 07-02-2013, 16:14   #1980
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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?
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