Lithium battery charge profiles

[Cniemand]

I am surprised at so many people still recommending BMS for Lithium (LiFePo4 Chemistry) batteries. As demonstrated for several years now, via Jack Rickard at EVTV, in electric car conversion usage, as long as you bottom balance the cells initially, they remain quite close balanced by themselves. I suspect that an EV would be much harsher on them than usage in Households.

My own system shall consists of 16 cells (3.2v/ 100Ah each) in series.

I'll let you know if I burn my 'tool-shed' down! Though doubtful as they can comfortably output at 3C (300amps). I'm not sure what I'd be up to pulling that much!

ps. CALB LiFePo4 cells are going for around $1.50 an Ah. Not bad.

[offgridQLD]

Yes I am very familiar with Jacks methods.

  It's a little different charging your car vs powering a house. Charging  your car perhaps once a week at a set predictable amp level on a ac wall charger . Two things I can see with his methods is he tinkers a lot with his car but how much day in day out use his car get is questionable. I doubt he experience is from consistent cycles over many years. At this stage he is getting away with it as its true the cells will remain relatively balanced.

Its a bit like having a hose filling  a big empty swimming pool and jumping up and down in it telling everyone your shorts aren't wet yet. Just give it some time to fill up and your shorts will be wet. Cycle your cells every day over say 10 years and things could be a little different . Small discrepancy's do add up. Remember a BMS isn't just for balancing if and when its needed its there as secondary (last resort) protecting for over discharge and over charge.

That said I have a small 12v 20ah lifpo4 pack that's being charged by a primitive plasmatronics PWM charger and  120w pv. Powering a small compressor fridge for almost a year with no bms . Its set to slightly undercharge the cells so the balance stays in check relatively well by its self. The cells them self are over 5 years old and couldn't be happier with them.

Kurt

[Cniemand]

Hey Kurt. Nice reply.

I have been using my off-grid system (powering my tiny house - 12ftx14ft and 14ft ceilings) for 202 days. My battery bank is 5kwh - 100aH CALB CA series - 16 cells. Typically I use little over 2kwh a day. However, if I am feeling wild, I will watch a bunch of TV and possibly even use the 1500watt electric heater during the sunny Colorado days. Then of course it could be 8KWH/Day! I make sure to turn off the heater early enough to make certain the batteries are charged, but even with the load, they float happily. I have a 2000 watt array and plenty of intense sunlight on the mountain. Plus the heater actually cycles on and off a bit.

I have been pretty diligent about checking out the voltages on each cell in various charge states. In the morning when they get 40 amps at a shot, or when they float, or when they rest at night.

I'm seeing at rest they are 2-4mv apart and in full charging possibly up to 15mv. They are really close.

However, as you noted being familiar with Jack's method, I intentionally undercharge them. I recently bumped up the voltage parameters but for the last couple weeks they have been holding those ranges above with the settings as follows.

No equalize, Bulk @ 55v, Absorb is 15min @ 55, Float @ 54.5v 

Most of the day is spent floating with a few hundred watts coming in to compensate for loads and the tapering of charge.

Once the Charge controller goes into resting, the voltage drops to around 53.4.

One thing I need to add to my setup is a coulomb counter so I can read exactly what I am taking out of the bank in the evenings to see what portion of my daily usage is straight battery without float.

Then I can figure out the typical DOD %. Though at 2-3 per day either way it isn't more than the 80% and 3000 cycles they claim.

[TomW]

One thing I need to add to my setup is a coulomb counter so I can read exactly what I am taking out of the bank in the evenings to see what portion of my daily usage is straight battery without float.

Then I can figure out the typical DOD %. Though at 2-3 per day either way it isn't more than the 80% and 3000 cycles they claim.

Cloud;

Thats a new one on me (coulomb Counter). I believe a Whizz Bang jr would be a good match for that. It connects across the shunt from a Classic and keeps track of the net amps in the system on the Classic.

I was going to find you a link but I cannot find it on their products page? Maybe they are not in full production yet?

Just FYI

Tom

[TomW]

Ryan sent me the link to the Whizz Bang Jr:

http://www.midnitesolar.com/productPhoto.php?product_ID=519&productCatName=Charge%20Controllers%20-%20Classics&productCat_ID=21&sortOrder=9&act=p

Mine works treat and it even includes a State of Charge readout on the Classic and eventually  the mymidnite logging site.

Good luck with your system.

Tom

[Cniemand]

Thanks, Tom. I was seriously looking at it last evening. People seem to be happy with it and for $80 between the shunt and Whiz, it is pretty inexpensive compared to the other meters or House monitors.

[Robin]

Boy, this thread just goes all over the place.
First of all I would like to thank all of you who purchase Classics and keep them in the box. We report very few problems with those units.
The Classic charges Lithium batteries all the time. boB even added a special mode on one of the aux inputs that shuts the charge off when the battery management sends a signal to do so.
I think anybody charging Lithium batteries without a BMS better have the batteries in a fire proof room! Some of the BMS systems aren't so fool proof either. Ask Boeing.
Lithium is getting more competitive, but you won't finding me using it anytime soon. Too scary and still too expensive. We do like those brave enough to do it though. We are supplying a bunch of stuff to Solar City who are using a large Tesla battery for AC coupling. We had to do some special things though to keep from tripping a breaker or fuse in the battery pack. We will also be working with Juice Box battery company. There are numerous companies out there messing around with the Classic and Lithium. Some in the military.
I don't understand the comment about user interface not being up to date? Most people don't fiddle with their controller like some of you all do, but if you want to use a keyboard, why not hook up to the local app?
We should be releasing an app shortly that will allow the local app to be used on your cell phone. It will also have the new state of charge meter.
I might even be tempted to look at my own system once in a while that way?

[offgridQLD]

I think anybody charging Lithium batteries without a BMS better have the batteries in a fire proof room! Some of the BMS systems aren't so fool proof either. Ask Boeing.
Lithium is getting more competitive, but you won't finding me using it anytime soon. Too scary and still too expensive.

If you take a moment to think about it lithium ( there is quite a few different types of lithium chemistry ) there are millions upon millions of lithium powder devices on charge or in used in our homes every day. The laptop I am typing this response on is powered by lithium batteries. The phone in your pocket (inches away from your crown jewels all day ) Is power by lithium batteries. My EV car that is parked down stairs in the garage under my bedroom charging all night is powered by lithium. It's nothing new and just like lead acid it's safe if used and charged properly. ( Boeing didn't charge or use it properly and they used the most unstable form of lithium there is)

Don't tell me you have never heard reports of a lead acid battery exploding or catching fire when abused in some way beyond its specs.

So for now lets talk about the most commonly used form of lithium for offgrid applications. Lifep04 cells usually in large prismatic form. I want to talk about the BMS- Battery - Monitoring- System for lifepo4 cells as its often misunderstood what its there for and if one is needed.

There is only two things that can go wrong with a lithium battery ..over charge and over discharge.  So all you need is a way to control this. A good charge controller like the classic range is reliable enough to cover this need 99.9% of the time I can set a maximum charge voltage a tapered current end amps and a float voltage. That will keep the batteries conservatively within spec. I need some way to disconnect the batteries from the load if the voltage drops bellow a set point. This could be done a number of ways through  a inverters programming or some other component already in the system than can activate a large contactor and break the load based on low voltage. So with the above in mind most of us already have all the control we need for a lithium battery in our old lead acid system. This could be called a BMS its monitoring the battery and acting on it (controlling the change and isolating the load if needed) .

Now we get onto balancing the cells (led acid guys call it EQ) potentially example 16/400A cells in a 48v lifepo4 system could potentially drift away from one another over time. Honestly with the kind of charge/discharge loads a offgrid house puts on them they will stay in balance very well on there own. Particularly even more so when your operating them in the middle of there SOC range (not fully charging them and not fully discharging them).

That said on cell balancing modules are inexpensive and once a multi cell battery pack has been initially (rough balance) usually don by hand using a load like a light globe or large resistance load  and jumper cables to get all the cells close to one another (wiring all the cells in parallel  for a day or two before installing them can do the same thing)  the little on cell shunts can take over and  bleed of small discrepancies between individual cells if and when they arise. Remember the world doesn't stop spinning or battey blow up if you have a small discrepancy between cells it would have to get really out of whack over a long period of time to be a issue. Though one will find after the initial balance the cell top balancers are not often active as there just isn't any imbalance to correct. The same on cell units can also report the individual cell voltage to a device that can trigger that big contactor I mentioned before killing the load and or charge in a (EMERGENCY SITUATION!) Its basically like a fuse for your battery last resort if everything gets so out of whack. (in normal operation the BMS just sits idle doing nothing)

So to sum up a charge controller like the classic is more than capable of keeping a lifpo4 bank in check if you set it up properly. The BMS will not become part of the picture 99.9% of the time its just there as a last resort (say your classic went crazy) The BMS has its back to activate that big contactor. And finally if they both failed its not going to burn your house down with lifepo4 the result wouldn't be any worse than if your charger went crazy on your led acid . You might have some swollen sick cells just as you would if you boiled the crap out of your led acid.

As for the cost you need to consider cycle life, C rating (silly to have 1000AH of lead when you don't need the capacity but you need the size just to cover big loads due to its rubbish C rating) You also need to consider cycle life based on usable capacity you can use 80% of the capacity of a lifepo4 cell and get 3000+ cycles led acid its 10-20% . The final big advantage particularly when charging from PV is lithium batteries don't need to be fully charged each day. Infact they like to be stored long term at 40 - 50% soc . The same cant be said for lead acid.

Lithium is to lead acid what digital cameras were to Kodak!....lead is dead.

Kurt

[mahendra]

i agree. i my self have done some research on lithium to see if it is feasible in the long term, and it is but not right now.In my opinion there is still a little more work/or a lot more , to be done on lithium for large battery banks(like in off grid systems) . I think most folks out there are not lithium literate and definitely not lithium literate about the types of lithium there are.

[offgridQLD]

"In my opinion there is still a little more work/or a lot more , to be done on lithium for large battery banks(like in off grid systems)"

What work is it that you think needs to be done?

Kurt

[dgd]

"In my opinion there is still a little more work/or a lot more , to be done on lithium for large battery banks(like in off grid systems)"

What work is it that you think needs to be done?

Kurt

I was wondering that too. AFAIK the larger capacity Lithium cells are extremely stable and if configured in a single string then cell imbalancing becomes a minor issue.
Earlier large capacity Li banks usually consisted of several to many strings of smallish capacity cells that created some serious issues with string balancing and could lead to individual cells failing.  Hence those important Li BMS systems that monitored and applied charges to individual cells as needed to keep all in balance.
It seems the most important issue with a single string larger capacity LI bank is having some sort of battery monitor so that charging  replaces what is used - since Li cells stay same voltage down to 80% discharged.
I have used a 48v 300Ah LiFeYPo4 single string bank of 16 cells for near a year without any problem.
Sold these and now witing on 48v 400Ah CALB cells to arrive from China.

The Classic with WBjr and SOC processing looks good for Li cell management.

dgd

[mahendra]

well some one answered before me cell balancing mainly and safety to an extent while i understand that all battery technology can be safe/unsafe , i think some more work has to be don't on safety features until regular folks are educated fully about lithium.Just my opinion though i am no expert in the field.

[mahendra]

Oh and the price vs the years of service does not really play out too good even with the advantages of lithium.I would just say it may be great but we are not quite int the position we would like to be with lithium.Prices for lithium need to come down in order for us to get to that position.

[zoneblue]

Well im as enthusiastic as the next guy about better battery technologys, and the next bank will likely be LFP, but id tend to agree on the not ready for prime time.

If you hang around the forums even a few days youll come across a worrying trend of folk who dont understand even the lead acids they have and are treating them absolutely shockingly.

So what can happen with lithium, whats the worse case? Both Pb and Li are capable of thermal runaway, but i would say from my reading that lithium in general is more vulnerable. The combination of a reactive elements and volitile organic electrolytes does have potential for drama.

The way to send a LFP into thermal runaway is to place too high a load on a charged cell, such that the temperature cant escape fast enough.  More temp, more current, more temp etc.
The videos ive seen of  fire department experiments short circuiting CALB cells isnt actually that dramatic, sure theres a big bang, lots of smoke and (poisenous) gases, but no fire as such.

Then theres over discharge. Over discharged cells tend to short circuit, On its own its only going to kill the cell. But a cell parelleled with others as is common with LFP,  will short  its mates and send them into thermal runaway.

Code Select Expand

...photos of the batteries suggest that such thermal runaway is practically designed
into the 787 batteries. That’s because the eight notebook-sized lithium-ion batteries
are packed next to each other in a sealed metal box.

Those batteries are prone to heat up and the 787 battery design makes it difficult
to vent that heat. Boeing told regulators that it had implemented a computer
controlled system to stop such overheating. ...Needless to say, this system did not
work with the flights that prompted regulators to ground all 50 aircraft.
-- http://www.forbes.com/sites/petercohan/2013/02/06/
   thermal-runaway-in-787-dreamliner-batteries-must-be-stopped/


tldr: lots to go wrong

[offgridQLD]

Your not going to use the same cells in your home as they did in that Jet in your offgrid house ( there is more to that story than you first think).

Lifpo4 is what 99% of people will use and it's a different beast altogether. Unless your planning on  running a aluminium smelter in your kitchen. Then Your not going to ever have a load in a off grid house that will send a lifpo4 cell into thermal runaway.

My 16kwh lithium battery in my car gets 50,000w discharge loads! followed by 25,000w recharge from regen and 40,000w when its being fast charged and its a factory OEM setup that's warrantied for 10 years. How many offgrid homes are going to install a 16kwh bank (small by offgrid house standards) and have a AC inverter that can suck 50,000w from it? Or a PV array that can pump 25 - 50,0000w. and even if they did thats fine because that's the kind of input/output that lithium battery's can handle and do it safe in a OEM warrantied car like mine douse every day.

Thermal runaway?  load your 16kwh lead acid bank up with a 50kw load or charge and you will see a great example of thermal runaway 

Pile of mutton lead on the floor.