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Author Topic: Lithium batteries, the care and feeding of  (Read 2046 times)

starfire

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Lithium batteries, the care and feeding of
« on: October 01, 2019, 07:22:46 AM »
For anyone interested......
Last year my lead acid batteries died, so decided to spend big on Lithium, or LiFePo to be exact, this flavour of chemistry seems the safest and most forgiving of them all. $4500 later, they arrived.
First problem was the placement. Like me, these things dont like low temperatures, we are talking minus something, so I placed them inside the hovel to keep them warm. The generator shed being 30 feet away gave voltage drop issues with the 100 or so amps charging current. This caused havoc with the alternators seeing over 18 volts at their output terminals.
I cured this by running independent sense wires, directly from the battery terminals back to the power shed, allowing the over voltage regulator to work correctly, it now reads the  actual battery voltage and not the alternator output.
The maximum  terminal  charge voltage of the batteries is 14.7 volts, anything higher will damage the chemistry.
Here I simply designed a high voltage cutoff using a LM311 voltage comparator that switches off the alternator field current when the battery voltage reaches 14 volts... plenty, as the Lister is only used as the  backup when the solar fails..
The main charging voltage comes from 800 watts of solar panels. The MPPT controller will switch of at 14.6 volts, this means the bank is full.
Here is the schematic  of the car alternator control when I remember how to do this picture posting thing.....
These old car alternators are modified by removing the internal regulator and connecting one brush to ground, the other to a wire into the controller.
I then wondered about these Battery Management Systems built into these Lithium packs.
After reverse engineering one sample, I could not see how they can "balance" each cell with just a 100 milliamp draw, when each  cell has a capacity of 600 amp hours..... it would have zero effect.
Firstly, I decided to establish if these Lithium cells actually do get out of whack with each other, they cannot be equalised as a lead acid bank can, this would overcook the highest cell. Is this an actual problem, or just an internet theory, steeped in myth and confusion.
I removed the BMS.
After nearly a year, the cells are within 3/10ths of a volt, highest to lowest, so unless I am extremely lucky, very unlikely, the balancing thing appears not to be the issue its made out to be. So, to simplify matters, we leave out the BMS.
It it also important not to  discharge these LiFePos to less than aound 10.7 volts. This apparently causes permanent damage.
This requirement is easily met , almost all inverters will automatically disconnect at around 11 volts.
The difference with Lithium to lead acid is night and day, with a charge absorption efficiency approaching 95 percent, no sulphation or gassing..... no watering, no stress, just set and forget.

mikenash

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Re: Lithium batteries, the care and feeding of
« Reply #1 on: October 01, 2019, 07:37:05 AM »
Mr Starfire.  You're back.  Excellent.  I was just talking about you . . .

I'm interested to hear how the lifepo's will go long-term.  I just killed my (cheap, 2nd-hand) AGM by neglect and absenteeism and am mulling over replacements

Cheers

starfire

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Re: Lithium batteries, the care and feeding of
« Reply #2 on: October 01, 2019, 07:52:55 AM »
Yes Mike,, and many thanks for the kind words......
I can honestly recommend these LiFePo batteries. I used to worry and stress over the lead acids..... how much charge I had to leave in for longevity reasons,  periodic equalising, are they sulphating, much like a woman, you are never sure you are doing the right thing, and the costs of not doing so can be crippling.
Lithium are bloody expensive, but the full capacity can be used, they cannot be damaged by long periods of partial charge, in fact, they will exceed the rated 2000 full cycles if "mistreatred" like this.


mikenash

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Re: Lithium batteries, the care and feeding of
« Reply #3 on: October 01, 2019, 08:26:14 AM »
 Cool.  Thanks

The Bay of Plenty property where I spend a bit of time has lots of sunshine but I'm only there for the odd weekend or long weekend. To save using a noisy generator just for a few watts (lights, electric blanket - to keep that woman happy lol, small fridge - etc) I spent $300 on a panel (200W I thinK?  I forget, AGM 100 A/H battery and a cheapie PWM controller & inverter.

Due to some bad planning I let it get down to about 50% and I think it's on the way out - so I need to think about replacement

The thing that bothers me with the lifepo's is the cold.  I THINK it's OK for them to get cold, but not to be charged when it's cold

Will have to do some more homework

Well done on the battery upgrade  :)

starfire

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Re: Lithium batteries, the care and feeding of
« Reply #4 on: October 01, 2019, 09:04:03 AM »
Yes Mike, its the charging issue when cold is the bugaboo. Unless this is another internet myth? Anyway, cause the sun always shines on frosty mornings,making the panels active,  its probably a good plan to prevent, rather than cure. Ive been lucky with lead acids, the scrap price had risen dramatically over their useful lifetime, so the scrap value almost equalled the initial purchase price, but no more unfortunately. Solar panels are about the cheapest they will be now I reckon, 1$ per watt for a quite decent panel.
Until you retire, I would wait.... Lithium is bound to get more affordable with time.

mikenash

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Re: Lithium batteries, the care and feeding of
« Reply #5 on: October 01, 2019, 05:10:23 PM »
Good advice & my thoughts too.  Cheers

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #6 on: October 01, 2019, 05:14:31 PM »
A most interesting report, Starfire, thanks.  The need for a BMS on lithium cells is well documented engineering, though with only 12V you only have 4 series banks of cells to worry about so it's less of an issue while cells are newish. (Parallel cells are regulated as one.) 0.3 volts variation is significant for a single cell and I would expect it to get worse over time.  BMS is often ignored for low voltage strings for portable equipment where service life can be sacrificed for lower initial cost.

I agree it's interesting that just 100 ma of balancing shunt current is the typical unaided current rating of most BMS IC's.  Many BMS IC's allow external power transistors to have much greater balancing current.  A true BMS system will limit charge current based on IC feedback so a newly replaced, fully charged cell would cause the charge for the entire series string of cells to be limited to 100ma once that cell was full.  By balancing on every charge, small balancing currents can keep cells from diverging...so perhaps with matching age and use cells, 100ma is actually useful; or, designers are cutting corners and don't want to add additional hardware for increased shunt current and the heat sinking required. If the BMS system doesn't limit total charge when the cell shunt regulators say they are maxed out, it isn't a true BMS system, and cells will drift out of sync until cell murder is achieved.

I use my own design BMS for wet lead-calcium batteries (10 batteries in a nominal 120V series string) and have up to 3 amps of balancing current available per battery before charge current is throttled.  Current is then limited to whatever provides just under max shunt capability of the fastest charging battery.  In actual service, charge is never limited by my regulator shunt current, as the batteries stay closely matched and while a few individual cells sag a bit over a month as seen via hygrometer, monthly equalization for 5 hrs brings them back to matching.  If I could manage to the individual cell level, equalization could be eliminated. 

Historically,  BMS systems didn't come into play until higher voltage series strings  of AGM batteries (96 volts and higher) were used for early electric cars.  Equalization can't be done regularly with AGMs...so batteries were dying fast.  By adding a BMS of shunt regulators with charge current limited based on regulator feedback and a shunt regulator on each 12V AGM battery in series, batteries were kept in sync, and battery life was greatly extended.

I modeled my system on these AGM designs since I was planning on a 120V series string using AGM batteries.  I used Lead-Calcium batteries initially due to low cost for new hardware checkout, and found that thanks to the BMS, service life was nearly 5 years.  AGM prices never came down enough to justify their use. 

« Last Edit: October 01, 2019, 05:28:48 PM by BruceM »

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #7 on: October 01, 2019, 05:41:34 PM »
Mikenash, if you haven't done it already, I suggest a careful equalization, 15.1V charge on your AGM battery, 3-4 hrs.  That will recover a sulphated cell.   AGM's are murdered by sulphation and also by drying out from regular overcharge.  Natural death in small DOD applications is around 8+ years. 

starfire

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Re: Lithium batteries, the care and feeding of
« Reply #8 on: October 01, 2019, 11:08:52 PM »
Hi Bruce.
Yes, my legacy  system is "stuck" at 12 volts. This is what I started with back in the 80s and it probably will stay that way unfortunately. All the appliances and ancillaries are 12 volt, quite a mission to change to even a 24 volt system.
The way I have understood this cell balancing is that any cell drifting high compared to its neighbours will reach the 3.65 fully charged voltage first, thereby leaving the other at a partial charge if charging stops at that point, which it must if cell damage is not to occur. Looking at the charging curve, the top most fractions of a volt represent such a very small part of the total; capacity of the cells, the loss is minimal. I have wired each battery in parallel, then ran 3 wires between cell junctions, this effectively places all cells in all batteries as one giant series parallel configuration.  As you say, this limits the problem to just 3 junctions.
These are also connected to 4 of those small Chinese digital voltage displays that are taped to the battery giving a constant display of cell condition.
I have since constructed a automatic cell monitor using 2 LM339 quad comparators. One  input is referenced to a seriesed and tapped  resistor string across the battery, the other input referenced to each cell junction. The tapped resistor voltages rise and fall with battery voltage, but remained in a fixed relationship. The cell voltages rise and fall, but are not in a fixed relationship with each other.
This allows the comparators to monitor not the absolute voltage of each cell, but the difference between what they shuld be from the fixed resistor string, and what drift has occurred in the cells,.
 By alternating between  inverting and non inverting inputs, we can have a window comparator with just a few millivolt window. The open collectors are simply paralleled through one LED to indicate a fault. The remaining two comparators detect 14.6v and 10.7v respectively.
Ill post this diagram too if its useful.
To me, this has been a better way to do this.
The other BMS issue that caused me  problems was its ability to turn the seriesed BMS FETs off as the battery voltage got near 14.5v, it would simply totally disconnectthe battery  from the "grid", giving an open circuit to the alternator, high voltage into the house wiring and killing anything that happened to be turned on at the time.  The overvoltage switch to the field works very well, and in my case, much more elegant.
The one strange thing I have noticed, the high cell is never the same one each day, it varies but so far has never exceeded 3/10ths.
To understand the balancing part of these BMS units, I removed the 100 ohm smd resistors from the dump FETs, ran wires to outboard resistor and LED combo to monitor the action. These turn on at cell voltage 3.55. Now, this is at the top of the charging curve where, at a charge rate of even a few amps, this would have little to no effect given that charging at this point is almost finished.
Still, this is my knowledge of Lithium so far.

starfire

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Re: Lithium batteries, the care and feeding of
« Reply #9 on: October 02, 2019, 12:48:12 AM »
And Bruce, yout knowledge appears greater on this topic. I have just run the Lister for several hours and put in around 80 amps constant. The cell voltages every 30 minutes read:.
3.37, 3.34, 3.35, 3.37
3.42, 3.46, 3.41, 3.51
3.43, 3,48, 3.36. 3.57
3.52, 3.44,. 3.54, 5.55

after sitting for 10 minutes

3. 36. 3,38, 3,37, 3,38

This suggests there is some kind of self balancing happening sans BMS,?
Under charge, the chemistry is all over the place and voltage readings are invalid?
This bit I dont understand.... maybe you already know this?

mike90045

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Re: Lithium batteries, the care and feeding of
« Reply #10 on: October 02, 2019, 12:53:20 AM »
BMS with cell balancers (vampire balancers) that only allow 100mA. are not going to work well.  As you suspect, charging a bank with 15A, and one cell fills up, that 0.1A is not going to do a thing to level out cell voltages.

Did the battery/BMS company explain how to perform initial balance by wiring all cells in parallel for 2 days ?
  (If not, i'm not going to take the responsibility of how to explain it)

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #11 on: October 02, 2019, 02:10:42 AM »
I hear you on the 12V legacy; once you're all set up for that, no point in wrecking a good thing.

Your interconnects to parallel the cells is important in that it keeps those parallel cells matching except for minor interconnect resistance.  Without those interconnects, it could get ugly as far as cell management.  Because of your smart setup you only need 4 cell regulators total.

The fancy BMS IC's are really helpful for high voltage systems since so many of the darn things are needed.  They typically have a fan out of 64 units, 4 or 8 cells each, all communicating back to the charge controller by a 2 wire bus. 

I couldn't follow your circuit description without a schematic; but as long as you're keeping on top of cell voltages you should be fine.  If you could find a way to adjust your charge voltage/current downward based on the feedback from your 4 cell shunt regulators, then you'd have a fancy first class BMS.  On my regulators, I'm taking the voltage drop across the load/shunt power resistor, and sending that back as a linear signal through an opto isolator.

The old Manzanita Power system for early EV- AGM batteries used an approach you might consider.  They had the battery shunt regulators be bang-bang controlled by comparator with hysterisis, with a heat sinked load resistor.  They had an LED on the transistor output so you could see when it was "regulating"/shunting.  As the battery reached full charge lots of LED blinking was happening.  An isolated thermister was attached to each heat sink, biased by the controller and bussed together via diodes so that one voltage showed the hottest regulator, and one showed the coldest.  The charge controller (a high voltage buck converter) would reduce charge current if any regulator got too hot.  They do sell a lithium system now but I haven't studied it. All their new designs are microprocessor based.

My latest linear PV charge controller takes a 0-5V DC signal for 0-16 amps of charge at float, bulk or equalize voltage. (nominal 120VDC)  I compute the charge current via an analog op amp integrator circuit, adjusting to keep the minimum and maximum battery regulators satisfied as loads change.  The battery regulator feedback of how much current they are shunting is the driver of the whole deal.  When sufficient sun or generator sourced DC charging is available the battery regulators are always just slightly regulating and the charge controller is constantly and slowly adjusting the required current to achieve that.  When minimum regulator current is present and maximum regulator current is low enough for long enough, the controller kicks down to float. No microcontroller needed for this newer version which I developed for my neighbor and then rolled into my own system.  Long live analog controls.  The new MIcrochip nano power op amps and comparators use about 1/100th the power of the old workhorse IC's like the LM339's, and some have very impressively low offset errors.


mikenash

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Re: Lithium batteries, the care and feeding of
« Reply #12 on: October 02, 2019, 06:54:01 AM »
Mikenash, if you haven't done it already, I suggest a careful equalization, 15.1V charge on your AGM battery, 3-4 hrs.  That will recover a sulphated cell.   AGM's are murdered by sulphation and also by drying out from regular overcharge.  Natural death in small DOD applications is around 8+ years.

Thanks Bruce.  Next time I go up there I'll take the smart charger thingie from work if it has a "high" setting.  But 15.1 seems very specific?  I'd be interested in how the layman achieves that?

I bought the 100A/H battery, PWM cheapie, equally cheap inverter & panel all for (I think) $300 off a bloke who said it had been sitting in the garage "for a while" so history is uncertain . . . I made the mistake of leaving what turned out to be a faulty inverter attached one time I came back to find the display saying something like 10.8V and 55%.  So it may be dead or it may have come back up by now.  We'll see. 

Cheers

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #13 on: October 02, 2019, 05:21:41 PM »
An adjustable power supply capable of at least 3 amps at 15V will do your equalization after the battery has already been charged to full via PV. 

I use this:
https://www.amazon.com/gp/product/B07JCRG6G6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

It can also be used current limited, to charge any battery to a set voltage at a current rate that is safe for that battery.  I use 7-15 AH  sealed batteries in 12 and 6 volt for electronics development work and it's nice for those.  It also works nicely on 120VDC which is handy for me. I have other 12, 24V chargers but none have equalization capability.  Apparently good consumers are supposed to just buy new batteries more often.  I have one AGM battery that is still in service at 12 years.  It was my house/shop 12V battery, demoted at 8 yrs to House of Lister battery, which is low capacity, mostly float service, and it doesn't get annual equalization any more.  It was a 110AH Universal Battery AGM, a bargain on sale.

The percent of charge shown by your controller is utterly bogus.  If it's 10.8V, it's done, 0.
Being left in that state is sure to cause serious sulphation.  A careful equalization charge might save it for a while.



BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #14 on: October 02, 2019, 10:26:49 PM »
Starfire, I think the small variation under 80A charge may be due to variation in interconnect or internal resistance at that high current.  Without a schematic I'm guessing wildly. 

It would seem that due to the averaging effect of so many parallel cells, and so few cells in series (4) you may only need to balance the cells very rarely.

Cell shunt regulators would have to use bipolar transistors as the voltage per cell could be too low to operate even a logic level Mosfet, unless you want to add a transformer isolated 5V DC supplies for each regulator.