PDA

View Full Version : Parallel charging- voltage difference across pack



robo
26 April 2013, 1607
I'm parallel charging my 20 CALB CA60 cells for the first time. I'm using a 10A Chinese regulated bench power supply capable of CC/CV. I have the PS connected to one end of the parallel pack. I set the voltage to 3.560 (using my meter) and I've run the current at 5-10A depending on if I'm around to keep an eye on it (the PS runs hot at 10A). This has been running a for a few days as expected.

The PS has now switched to CV and the current is limited to about 6A.

The PS end cell tests 3.511V and the other end cell is 3.355V. Is this voltage drop down the pack "normal"?

The voltage appears to drop with each cell, so I don't "think" I have a bad connection. Additionally, the cells balance voltage when the PS is not applied.

Skeezmour
26 April 2013, 2006
You should have your positive connected to one end of the parallel string and your negative on the other end.

robo
26 April 2013, 2046
You should have your positive connected to one end of the parallel string and your negative on the other end.

Well, that makes a lot more sense then, doesn't it? Why didn't I think of that? I kinda feel not-so-smart right now.

Thank you sir.

You've earned a Frisko Freeze.

podolefsky
26 April 2013, 2348
Don't feel dumb quite yet...0.156V drop does seem like a lot, even with the PS just at one end.

What size jumpers are you using to connect the cells? Are the connections clean and tight?

You should barely be able to measure any voltage difference between the first and last cells, even with 6A running to the pack.

robo
27 April 2013, 1027
I let the cells balance overnight. They all read 3.344v or 3.345v today. Given the method I used to connect the cells, a bad connection is possible (14awg jumpers w/crimped terminals).

But, the voltage drop across cells was fairly consistent as I moved down the pack. I didn't see any particular suspect junction voltage-wise.

There are probably better ways of making a parallel pack, but here's mine:

4359

podolefsky
27 April 2013, 1044
Oh! Yeah, with 14AWG it's totally possible to get a drop that big. It's just wire resistance. I use the original battery jumpers to parallel my pack, which are about 4 AWG equivalent so almost no measurable drop.

What Skeezmour said is a good idea. As the cells fill up and current drops they'll all balance out.

robo
27 April 2013, 1119
Thanks again to both of you. I should have thought about wire & terminal resistance. Makes sense. Charging has resumed on both ends of the pack.

I had all those extra terminals from a previous project, so I used them here.

Hopefully the 2-3 cells on the end near the PS didn't get abused too badly while being sustained at ~3.5v. I'll continue to monitor.

robo
27 April 2013, 2005
With your insights, I changed the charging leads to 8awg. That lowered the resistance enough to push the 10A at < 3.575V without the PS switching to CV and limiting current (at this earlier point in the charge curve). Thanks again guys!

BaldBruce
28 April 2013, 2100
Just remember to shut it off after it switches over to CC and then falls to 1 amp or so. Li batteries do not like trickle charging.... just ask Boeing how that works out. Your batteries are 100% charged when they each read 3.36 to 3.38V each after resting for 24 hours. (This is the voltage potential based on the chemicals used in your CALB batteries.)

podolefsky
28 April 2013, 2116
Just remember to shut it off after it switches over to CC and then falls to 1 amp or so. Li batteries do not like trickle charging.... just ask Boeing how that works out. Your batteries are 100% charged when they each read 3.36 to 3.38V each after resting for 24 hours. (This is the voltage potential based on the chemicals used in your CALB batteries.)

Yup - in fact, you could probably stop well before that. Pretty much once it switches to CV mode.

With 20 cells in parallel, 1A total is 0.05A per cell. By the time you get there, you will have been trickle charging for quite a while.

robo
28 April 2013, 2334
This morning, the PS was in CV mode and down to 4A of current limiting. From there, the current started dropping fast. Within less than an hour, I watched the current drop to 600mA. I shut off the PS with the cells at 3.575v +/- 0.005. I spent the day out of the house while the cells rested.

I hope I didn't push that too hard. I didn't see BaldBruce's and Noah's posts until now. I did know not to trickle charge.

I've seen LiFePo4 charge curves before with that steep non-linear curve at the end, but I was still surprised how quickly that last curve bit happens. I'm very glad I was watching it. It's easy to get complacent over a couple of days of CC charging. The last of the CV bit can sneak up on you.

I'll check the parallel pack in the morning to see where the cells settled.

Lithium LiFePo4 cells are definitely new to me. I appreciate the help. I can definitely see how one could screw this up.

BaldBruce
29 April 2013, 0632
Good point Noah. Just wanted to give the voltage the batterries should be at when fully charged so he knows when to stop. Actual recipe to charge is a seperate discussion. The takeaway is not to trickle charge a Li battery.....(I know - 10 Amps into that many batterries in parralel is already a trickle.)

robo
29 April 2013, 0752
Good point Noah. Just wanted to give the voltage the batterries should be at when fully charged so he knows when to stop. Actual recipe to charge is a seperate discussion. The takeaway is not to trickle charge a Li battery.....(I know - 10 Amps into that many batterries in parralel is already a trickle.)

Thank you BaldBruce. The cells all measured 3.524v this morning after 24hrs of rest. How badly are these overcharged? Or more to the point, any serious harm done here?

BaldBruce
29 April 2013, 1908
As long as they didn't swell, you are fine. The voltage above nominal is a small amount of surface charge that will dissapear if you put a small discharge or just wait a bit. You can use many different "recipes" for charging a Li battery, but don't forget that the point is to fill them without over filling. I charge up to 3.5V per cell at C/6 in constant current and then quit. No CV mode. Voltage settles out to the 3.33 range indicating nearly full. I give up a small amount of capacity for longer cycle life.

robo
29 April 2013, 2143
BaldBruce, no swelling at all that I can see. I used a 55w halogen bulb to bleed off that surface charge. The pack is now ~3.4v after about an hour with the bulb. Much better.

I see now why the extra CV stage above 3.5v isn't worth the risk...next to no additional power is added there as Noah wrote. Only an hour with a dim 55w bulb bled it off. Sticking with CC up to 3.5v with no CV stage makes a lot of sense risk-wise now...and it's also easier.

My cells are 60Ah and almost all charging was CC at 5-8A and current was never more than 10A, so I was at or below C/6 at all times. I was OK there. Cells were never warm. I know the cells never saw 3.6v or even 3.65v that some have used (!).

I monitored everything like a hawk and I followed the recipe I had...it just wasn't the best recipe.

Thanks again.

podolefsky
29 April 2013, 2340
I just meant for the initial parallel balance charge you can probably skip the CV phase, since after a few charge cycles the BMS will do the last bit of balancing.

3.65 is the typical cutoff voltage for LiFePO4. miniBMS stops at 3.6, but it supposedly still allows for a CV phase. 3.5 will prolong cell life, you just lose more and more capacity the lower you cut off voltage and charge time...maybe as much at 8-10%.

Here's a picture of a Headway 10Ah charge / discharge curve. I highlighted the current during the CV phase in red, and current after 3.5V in orange. Red portion is about 0.75 Ah, which is 7.5% of the cell capacity. If you stop at 3.5V instead of 3.65, you lose about another 1.5%. So, stopping at 3.5V and skipping the CV phase could leave 9% of your pack capacity untapped. Depends on the cell type - this is for Headway, but I think it's pretty typical.

4363

Since range is a huge deal, I charge to 3.65V with a CV phase. I guess I could charge to 3.5 most of the time since I usually don't need the range, and that would probably prolong cell life. But there are times I need every last mile. MY GBS cells are rated for >2000 cycles (100% DOD), charged to 3.65V. At 50 mi per charge, that's 100,000 miles. I'm not too worried about eking out a bit more life from them...hopefully by the time they are 1/2 way through their life, EIG / Kokams will be $0.50 / Wh and I'll be building a new pack anyway.

Just my 2c.

BaldBruce
30 April 2013, 1100
No right or wrong recipe for filling up a Li battery. As long as you understand what the definition of "full" is. You could charge at C/10 for an minute and then wait 24 hours to see if you were full at 3.38 volts. Then charge for another minute and wait a day again. No very practical is it, but this recipe would result in you charging to near 100% with no chance of overcharge. The manufacturers give you an approved and tested recipe. As long as people understand that there are an infinite number of recipes that get you to that full state. There is no magic in 3.5, 3.65 or any other charge termination voltage. You can use 4.2 or any other number in between. (Thunder Sky used to post this as there prefered charge termination voltage.) What you care about is that your recipe brings you close to 100% and then STOPS. I prefer the 3.5V point simply because it reduces the risk of overcharging since it is only marginally above the electrode potential of the materials. Remember it is the Overages that kill batteries. Over Charging, Over Discharging, and Over temperature. (Charging a frozen battery doesn't fit my analogy but sure can damage them also!)

Athlon
30 April 2013, 1634
Is always a matter of trading range for life ,if you use your battery from very full to very empty you will get the max range out of them but you will kill them sooner , if your battery pack is big enough you can leave some buffer on top and bottom to improve your long term life

robo
30 April 2013, 2054
Very helpful. I see any combo of reasonable cutoff voltages, current rates, and durations can be combined to approach full SOC...cells are full at 3.38v at rest. Don't go over that rest voltage.

I get the tradeoff of more cell capacity/range with less charging cycle lifetime vs using less cell capacity/range for more charge cycle lifetime. A zero sum game decided by preference.

The piece I still don't get is the resting voltage drop and what governs the drop magnitude.

Apparently this must be some function of charging C rate and/or voltage and/or time? Maybe cell capacity, current cell SOC, and temp too?

To mitigate all overcharge risk, it would be useful to know what an anticipated resting voltage drop would be given (charge time, voltage, current, temp, capacity, etc).

Is the resting voltage drop possible to know analytically ahead of time? If it requires knowing current SOC...that would be tough.

podolefsky
01 May 2013, 0839
Somewhat long explanation here:

The battery voltage is determined by an oxidation/reduction reaction (http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrode.html) (or redox). Different redox reactions give you different potentials (or voltages). For an LiFePO4 cell, it is about 3.3V, give or take.

In a chemical reaction, you can think of it as a bunch of particles bouncing around and sometimes colliding. It is totally random, so the more particles you have the more often they collide and react. As you add particles to the system, more of them can react and the voltage goes up (and vice versa). When you get to a certain point, you are adding so many particles to the system that you are saturated, and the voltage goes up way beyond what the redox reaction would give you - that's the sudden huge increase around 3.5V. On the other hand, when you pull out too many particles, you actually run out of reactants and the voltage drops below the redox reaction potential. It becomes "starved" of ions. (The internal resistance also goes up quickly, so you get heating and can literally boil the electrolyte).

Now, a redox reaction actually runs in both directions. Negative ions go one way, positive ions go the other. When you over charge, since you are saturating you can cause a new chemical reaction that permanently pulls particles out of the system. Same thing for over discharge, just in the opposite direction. You can think of it as "plating" the anode or cathode with the ions, and those ions are no longer free to move and react.

In between the two extremes, the voltage changes just slightly - when charging, you are adding energy that the redox reaction can use to run, so you get a slight voltage rise. How much voltage changes with SOC depends on the chemistry - LiFePO4 is pretty flat, lipo is more "slopey", lead acid changes quite a bit. But even then, every LiFePO4 is a little different. Some are slightly different chemistry (like LiFeMnPO4), and some has to do with cell construction. It also depends on temperature, age of the cells...lots of things.

In practice, what you do is pick a voltage and decide if you are going to do the CV phase. One way to think about it is that there is only so much energy that the cell will accept - once you set a safe voltage (usually < 3.65V) the cell will get to a point where it won't accept any more energy. That's why current drops during CV phase. You basically let the cell decide when it's full.

robo
01 May 2013, 2052
This is fascinating Noah. Thanks. I've wondered how the voltage potential worked. After reading your link and many others, it's interesting that discharged LiFePO4 and charged FePO4 have similar crystal structures. The lithium ions and electrons just move around during the redox charge/discharge as you described:

books.google.com/books?id=COAn3DsOtD0C&pg=PA54&lpg=PA54&dq=lifepo4+redox+reaction&source=bl&ots=svStT9Qmef&sig=VbVRV07pwjvsOngsxJTCCvxKefg&hl=en&sa=X&ei=jNSBUdSCDZP7yAH41YEg&ved=0CFQQ6AEwCQ

Thanks to you and BaldBruce, I realize I can now answer my own question. A given voltage drop after charging relates to the difference between the charging voltage and how "full' the cell is after that charging stops. Knowing that V drop ahead of time would require knowing SOC...hard to do without something like coulomb counting from a known starting SOC reference point. I know this is exactly what BMS designers like Davide at Elithion wrestle with...knowing SOC is a known challenging problem. Anyway...

Bottom line, I over trickle charged my cells a bit. Even though I knew not to trickle charge abstractly, I didn't realize I was in fact doing exactly that because I had an incorrect C value in my head. Luckily my situation does not appear severe.

I did some RTFM in the CALB booklet supplied with my cells. Better late than never. For the record, CALB's values are:

CC/CV stages are at 3.6V. Charging should be stopped when CV current limits to 0.05 C. Max charging voltage...cut off if cell reaches 3.9V. 3.1V is DOD >= 85%. 3.0V is DOD >= 90%. Individual cell discharging cutoff voltage (while discharging at 0.3C) is 2.5V. Absolute minimum discharge voltage is 2.0V. One should never be here. They recommend a BMS require charging at (at least) 3.0V.

CALB "stongly recommends shallow charge and discharge of the cell". So, it pays to play more conservatively than the numbers above.

Thanks for everyone's help. I think I'm starting to get it. Apologies for this long post. I'm grateful to all those who figured this out the hard way before me.

Back to welding.

podolefsky
02 May 2013, 0908
Batteries are simple on the outside, really complicated on the inside.

This video is very informative - I've watched it a few times to absorb what he's saying.

http://www.ri.cmu.edu/video_view.html?video_id=60&menu_id=387

teddillard
02 May 2013, 1207
The aforementioned "FM" that you should maybe "R" for CALB packs, in case yours didn't come with one is here:
http://www.manzanitamicro.com/downloads/category/19-batteries?download=80%3Acalb
(via Manzanita Micro)

robo
02 May 2013, 2100
Awesome video Noah. Thanks. It definitely will take a few viewings to digest all that info. His chemistry and intercalation explanation was informative.

Ted, thanks for the link. Looks like my booklet is an updated version for the gray cells...no pictures of a guy wrenching on the terminal of a blue cell. Mostly the same text though.