Author Topic: Lithium cell balancing  (Read 405 times)

BruceM

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Lithium cell balancing
« on: July 04, 2021, 07:30:42 PM »
I've noticed that LiFePO4 cells direct from China have come down quite a bit.  So I've been researching how to best handle cell balancing for a 120VDC battery bank (39 cells in series operated from 119V to 132V. (3.4 to 3.05V)  I've got a few years to get ready for the switch.

I wondered if Starfire or anyone else might chime in on homebrew designs?  I am familiar with the various commercial  balancing products, but they generate a lot more EMI than I'd like.  The modest current, passive balancers are lowest EMI but still it is connecting a microcontroller with SPI interface to all the cells.

mike90045

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Re: Lithium cell balancing
« Reply #1 on: July 06, 2021, 06:16:34 AM »
I would prefer not to use balancers, but to order some extra batteries with the initial order, and then sort/bin the batteries, so you build a bank with all matched batteries, and top or bottom balance the bank.   (and hold in reserve the low capacity & high capacity cells )
Then just rely on the BMS to monitor cells in case one fails.

BruceM

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Re: Lithium cell balancing
« Reply #2 on: July 06, 2021, 07:48:56 AM »
Some folks do nothing but manual  monitoring and balancing every few weeks, even months for the first year or so.  I think I'd like something automated.  I'm not getting younger.


 I'm now thinking that some passive top balancing (fixed voltage load switch type) and modifying my big linear PV regulator to be fixed voltage might suffice.  The commercial ones are very cheap but the trip voltage is preset too high for my planned scheme.  The ones sold for bike packs are only 35ma of discharge, only when the batteries are full and still on the charger. That's not much time or current.  For my off grid PV system, it would be most of the day every day, so might work out even at 50ma.

I've never used lithium cells at all so lack the practical experience with them despite doing a lot of reading on BMS/balancer designs. 

My experience with my own linear 12V battery regulators is very positive; the batteries last a long time and I can swap in a new battery without trouble.   I could implement the same thing for lithium cells, though it might be much more than is needed, and with 38 or 39 regulators needed, it's quite an expense and build!






BruceM

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Re: Lithium cell balancing
« Reply #4 on: July 07, 2021, 07:58:37 AM »
OK on the basics but he is not mentioning battery balancing hardware, even though the photo of the 4 cell pack clearly shows a balancing cable hooked up for a battery balancer. 

Daly makes a decent 4 cell LiFePO4 basic one for cheap, I just don't care for the added EMI of that type with a microcontroller.  There are also plenty of  balancers with current and low voltage protection as well, for 12, 24, and 48V packs.  It more serious money for a balancer for 39 cells in series, and again comes with EMI issues I don't want.

Because my bank is full every day, most of the day, I think a simple passive top side shunt regulator or top voltage switched load resistor should suffice.  Because of the number (39) required, I'm trying to resist the urge to get fancy.


BruceM

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Re: Lithium cell balancing
« Reply #5 on: July 12, 2021, 11:28:35 PM »
I joined a DIY solar form to solicit experience with LiFePo4 cells.  It seems that claims of 10 year plus are fantasy. One retired EE circuit designer made an impressive 100V series string and balancer, his own inverter.  In three years with new balanced cells and modest DOD, he's had 3 of the 30 cells fail suddenly and without warning, as a complete short (dead), the first in a year. That's 10% failure rate in 3 years. And these were very expensive batteries with a very sophisticated BMS/balancer.   A friend of his who bought a set from the same supplier had the same experience.  Of course, the seller says it's never happened.  Uh huh.  The retired EE is planning on switching to refurbished forklift batteries at 48V.

It seems that LiFePo4 is not quite there yet in durability and reliability, once you get past the new adopter enthusiasm/delusions.


32 coupe

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Re: Lithium cell balancing
« Reply #6 on: July 13, 2021, 04:21:21 AM »

I wondered what was what when you first brought this subject up. I thought I'd wait to comment.

Chinese batterys ? Really ?

I don't think you can buy ANY good batterys anymore. (Yea, I'm sure I'll catch *** for that.)

I am in the marine industry and haven't had good luck with any of the major brand batterys.
About 2 years is the new norm for battery life. Wet, mat, gell....they are all junk.

I have owners ask about  lipo's etc. all the time. A while ago I asked a friend who I consider to
be an expert on batterys, chargers, solar etc about them and he says the problem is that their are
not really any  "standards" for most of the newer compounds in batterys and that the charge and
maintenance schedules are all over the place.

Unless one wants to go down the "virgin lead" road throw away batterys are here to stay.

I wish you could buy good batterys at a reasonable price today. I remember ( here we go !)
when the average car battery would last for years. My daily driver truck seems to eat a battery
on a 2 year cycle. Not cheap baterys either. The guys I work with seem to have the exact same
results. The last couple have gone dead suddenly. Start fine for months then BAM dead.

There is so much junk out there today you never know what you are getting. Especially from China.
I have gotten pretty cynical the last few years. Terrible service and no warranty seem to be the new
industry standard. That new battery with the fancy plastic case and bright shiney colors might just
as well be a gold plated t**d. And don't even bring up cost.....thru the roof !

Bruce, I thought you were using "cycled" fork lift batterys ? I'd stick with that and save your money.

Now if you run across those tall batterys used in back ups for com towers and crash carts grab them.
I haven't used them but have seen excellent reviews for them. I remember pricing some for a customer
a couple of years ago and he went with standard wet cells after seeing what those "virgin lead" batterys cost !





OK, rant over. Let the stones fly !

Gary




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BruceM

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Re: Lithium cell balancing
« Reply #7 on: July 13, 2021, 07:07:35 AM »
I'm using wet lead-calcium, "marine deep cycle" batteries, ten in series for 120VDC.  Because my DOD is so low, they last 4.5 to 5 years.
They only need a bit of water once a year, and float current is under 0.1 amp. 

I have zero interest in the fork lift batteries; 100Ah batteries are plenty for me, I'm only using  20AH.

My battery bank replacement cost is now up to $1000., every 4.5 years.  That's not bad, a fraction of what my neighbors with 48V L16 banks pay, and longer lived as well.



 

mikenash

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Re: Lithium cell balancing
« Reply #8 on: July 13, 2021, 08:21:04 PM »
Hey Starfire I wonder if you would like to chime in here?

It's a topic that interests me too as I need to deal with a situation where solar-charged batteries are often unused & unattended for weeks/months, and where they can be charged at sub-zero temps on a crisp, sunny winter's morning

I know you know what you're talking about; and I guess you have a couple of years under your belt using the new LIFEPOs

Cheers, Mike

BruceM

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Re: Lithium cell balancing
« Reply #9 on: July 13, 2021, 09:02:34 PM »
Mike, you just can't charge most LFP/LiFePo4 batteries below 0C.  Most the better BMS hardware automatically disconnects the PV regulator input at that temperature, as it will wreck the batteries.  Another issue is that a BMS failure can also result in the destruction of your battery bank; a concern for unmonitored or remote situations.

So LFP/LiFePo4 wouldn't be a wise choice for your application, besides the cost of the batteries sitting there for the taking while you're away.

I'm not sure why Gary (32coupe) is having such bad battery experience.  New cars tend to eat batteries more today because of so much "always on with ignition off" electronics load.  Perhaps in boats as well.  Heat is another well known battery killer, and virtually no auto or marine battery charge regulators bother to do temperature compensation.  So batteries in hot climates are getting overcharged to death, and just sitting in the heat, the batteries are aging prematurely. 

I've gotten consistently good service on both single AGM and wet Marine type batteries for my big 120V battery.  I get 6.5 years life on the former and about 4 to 5 years on the latter.  I'm on my 3rd AGM and  my 4th of the marines.  My neighbor has the same setup and gets the same service.  I bought my latest set 6 months ago, a bit early at 4 years but he was at 5 years so we got a good bulk buy deal on 20 at our nearest Car Quest.  We both use temperature compensated charge controllers and a battery balancing system on the large battery string.

Bruce



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Re: Lithium cell balancing
« Reply #10 on: July 14, 2021, 05:50:15 PM »

I know heat does play into the life span.
It gets HOT in florida !

 
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