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A123 charging problem

Daniel828

100 mW
Joined
Oct 26, 2007
Messages
37
Location
Monmouth Co. NJ
I've experienced some melt downs while charging my battery packs lately. The first mishap look liked this.
100_1059.jpg

I rebuilt the pack with more cells and the same thing happened.
I'm using a Mastech 30 volt 20 amp cccv power supply. Could the power supply not be switching from cc to cv? Can someone help me please?
Thanks
--Dan
 
what are your settings?

are you watching what it does?
 
Yikes!

Great picture. I'd guess you have it turned too high. You need some way to make sure the voltage on any cell never goes over something like 3.7v.
 
I also have a mastech supply and apparently to set in "CV" mode requires a special initial setting of the knobs. I actually caught my lifepo4 when it mysteriously went from 3.2V to 4.5V. :shock: It turns out I accidentally started it out in constant current mode at 2 amps and... I'd say setting the mastech is not really intuitive.

(I have the 18V 3A model, so it may be a little different than yours. )
 
I usually set the power supply all the way up. The battery packs are 8s1p, there are 3 of them in parallel. The display on the Mastech before the packs are plugged in reads 30 volts and 0 amps. As soon as one is plugged in the volts drop a littleto 27-28 volts and the current comes up full 20 amps. As the charging takes place the volts will rise to pack voltage and the current trails off to near zero.Unfortunaly I was not watching but I was only gone for about 5 min.
The one thing that I didn't mention was the one cell was dead (the one thats ballooned the most) it was reading like .8 volts but I could still charge the pack for awhile.Could that be the problem? but the same thing happened on my brand new pack on the first charge.And all the cells were 3.3 to 3.4 volts except two or three were a little below 3 volts, well with range I think? Very discouraged.
 
30v would be a little high for 8s. I'd use closer to 29.2v.

If one or more cells is much lower than the others, it can allow a much higher voltage on the remaining cells. The batteries blow up when the voltage goes too high.

When building a new pack, it would be a good idea to charge each cell individually at 3.65-3.7v first to get them in balance.

A battery management system that monitors the voltage of each cell and prevents it from going too high or too low is what you really need.
 
i was wondering can the cells be charged by paralleling them then re series ing them once charged so they can be used?

fechter said:
30v would be a little high for 8s. I'd use closer to 29.2v.

If one or more cells is much lower than the others, it can allow a much higher voltage on the remaining cells. The batteries blow up when the voltage goes too high.

When building a new pack, it would be a good idea to charge each cell individually at 3.65-3.7v first to get them in balance.

A battery management system that monitors the voltage of each cell and prevents it from going too high or too low is what you really need.
 
Ejonesss, what you mention is how I charge/use the pack. I charge the packs, 3 in total, in parallel. I discharge in series. The plug maintains the configuration
from series to parallel.I'll post some pics of the plug tonight.
Fechter, thanks for the help. I forgot to mention that I intend to buy some BMS's from Gary. I was reluctant because the BMS adds a little complexity to the set-up but it is a needed piece.
Also, is it that critical to set the power supply .8 volts lower from 30 to 29.2? I don't understand, when the power supply is on but no pack plugged in, it is set at 30 volts. When the packs are plugged in the voltage drops to around 27 volts. Do I have to monitor the charger and decrease the voltage as the packs charge and reach cv? Or can I initially set the power supply to 29.2 volts and let it go from there?
Thanks
--Dan
 
I think the problem is definitely that you are overdischarging the cells, to the point that one, or more, are dropping below 1V. Those cells are definitely permanently damaged, and trying to recharge a pack with damaged cells will be disasterous, as you have seen.

More than the charging protection a BMS would provide, you really need at least a per cell LVC capability, to keep from over discharging your cells. There's nothing more robust than a123 cells, but their Achilles Heel is that you can easily kill them by over-discharging the pack/cells.

-- Gary
 
GGoodrum said:
I think the problem is definitely that you are overdischarging the cells, to the point that one, or more, are dropping below 1V. Those cells are definitely permanently damaged, and trying to recharge a pack with damaged cells will be disasterous, as you have seen.

More than the charging protection a BMS would provide, you really need at least a per cell LVC capability, to keep from over discharging your cells. There's nothing more robust than a123 cells, but their Achilles Heel is that you can easily kill them by over-discharging the pack/cells.

-- Gary

Yeah. I'm thinking an accurate diagnosis is that charging a series pack without some kind of balancer will induce different charging voltages across the cells as there's variation in internal resistances between the cells(imagine a series of resistors with slightly different resistances: there'll be different voltage drops among different resistors); More extreme variation in internal resistances, like accidentally over-discharging a cell/cells at one time, will result in more variation in charging voltages between cells which will likely... put some over the top(the higher resistance ones like the... damaged ones.).

How does internal resistance change as the battery's SOC change(like when it's almost completely charged)? I'm thinking that relationship might also be impacting your charging results.
 
Hi Gary. thanks for responding.
After the first pack (in the pic) melted I built a new on. I used cells I bought on ebay. I checked the voltage on them. Most were at 3.3 or better but two were below 3 volts. About 2.7-2.9. The same thing happened with this pack, it melted. Was the two lower voltage cells the culprit? I thought they would self balance if you charge them a little higher?
The other dilema. I like to use three 8 cells per pack, packs. I charge them in parallel, then connect them in series for roughly an 80 volt working pack. The question is, should I parallel the cells first then in series. So, I use them as 2 cells soldered in parallel, then 8 pairs of those 2. For a total of 16 cells per pack (8s2p).Or, should I have (6) 8 cells in series per pack then hook two in parallel, then the three pairs in series to get 80 volts. Gosh I hope that makes sense.
With out going thru the Care and feeding thread, does your BMS provide a per cell LVC capability? If not what could I do?
Thanks
--Dan
 
Daniel828 said:
With out going thru the Care and feeding thread

I don't get it... why wouldn't you read that thread!? You own some of the best and most expensive cells in the world and aren't bothering to understand their capabilities? :shock: There are very good reasons these cells need "care and feeding" not the least of which is the incredibly fast discharge at the end of a cycle. Man I wish I had your kind of money to be just trashing through these cells like it's no big deal. :wink: :p
 
Trust me pwbset, I don't have the cash to burn thru. The first time it happened, I thought to myself "ok I guess this is my initiation". The second time, which was this weekend, I felt like crying or throwing up. I just saw $500 bucks literally go up in smoke.
It's just that the Care and Feeding thread is long and I've been thru it before but my head is spinning with this melt down problem, I'm just trying to figure it out.
I was getting away with out using the BMS for so long, I guess it's catching up to me.I won't be able to use the MEC soldeless power tube with the BMS, so I'm going to have to figure out a new layout that will incorporate the BMS.I like ggoodrum setup. So clean and simple, but I haven't had any luck coping it. The battery layout that is.
Dan
 
Daniel828 said:
Hi Gary. thanks for responding.
After the first pack (in the pic) melted I built a new on. I used cells I bought on ebay. I checked the voltage on them. Most were at 3.3 or better but two were below 3 volts. About 2.7-2.9. The same thing happened with this pack, it melted. Was the two lower voltage cells the culprit? I thought they would self balance if you charge them a little higher?
The other dilema. I like to use three 8 cells per pack, packs. I charge them in parallel, then connect them in series for roughly an 80 volt working pack. The question is, should I parallel the cells first then in series. So, I use them as 2 cells soldered in parallel, then 8 pairs of those 2. For a total of 16 cells per pack (8s2p).Or, should I have (6) 8 cells in series per pack then hook two in parallel, then the three pairs in series to get 80 volts. Gosh I hope that makes sense.
With out going thru the Care and feeding thread, does your BMS provide a per cell LVC capability? If not what could I do?
Thanks
--Dan

This is a good thread because it shows others exactly what not to do with a123 cells, or any other LiFePO4-based "homemade"/DIY pack. First of all, you need to make sure you start with healthy, fairly matched cells. Cells that down around 2.7-2.9V are not "well within range". You need to first individually charge each cell to about 3.7V (set the Mastech to 3.70V and about 2A...), wait about 1/2 hour, and then check the voltages. Healthy cells will retain a surface charge, and the cells will be somewhere in the range of 3.55-3.70V. Cells that are "stressed", meaning they have been previously damaged due to over discharging them, will have lost about 10-15% of their capacity and althought they will appear to take a full charge, up to 3.70V, after a few minutes the surface charge will bleed off by itself and the voltage will drop to about 3.38-3.45V. Do this test with each of your remaining cells, and with any of your replacements. Only build packs with the ones that pass the surface charge test, that is the ones that will hold the voltage up to something above 3.55-3.60V.

Running a single string of these only gives you 2.3Ah of capacity. On an ebike, that will not last but a few minutes/miles. The problem is that because a123 cells are so strong, you don't get any indication that the cells are about to dump until it is too late. It can feel just as strong 10 seconds before you are killing cells, which you apparently have done repeatedly, as it does at the beginning of the capacity. I think I remember telling you, or maybe someone else, that you were making a big mistake with this setup, in another thread. You have componded the problem by not even bothering to start with fully charged, matched cells.

If you are trying to run a 24s2p setup, using 8s2p "sub-packs", it would definitely be much better to wire the cells in parallel first, and then the pairs in series. You definitely need to have a 24-channel LVC function, so that each parallel pair of cells can be monitored (once connected in parallel, the cells will self-equalize to the same level, so they can be treated as one, larger cell...). The way the LVC function, which is part of the BMS, works is to trip an opto-coupled output if any one cell drops below 2.1V, under load, or otherwise. These opto outputs are all connected in parallel and then you use this "ganged" output to switch on the brake input on your controller. Think of it as 24 individual pushbutton switches, all connected in parallel to the brake input. Any one circuit trips, and the controller immediately cuts the throttle. Once the load is removed, the voltage will recover up above the cutoff, and so the throttle is re-enabled. If you don't let up on the throttle, the cutout will occur again. It has been my experience with healthy packs that when the cutout first occurs, usually under full throttle, you have about 5-10% of the capacity left. If I back off the throttle, I find I can usually go another couple of miles until the cutouts happen with even the slightest throttle. At that point, if I charge the pack, it will take almost exatly the full rated capacity back in. BTW, a fully drained pack will still have a "resting" voltage that is well above 3.0V per cell, more like 3.10V.

The problem is charging. I'm assuming you still want to use the Mastech to charge the three 8s2p sub-packs in parallel, using the Mastech. I'll have to think about how one of the new 24-channel "tear-away" BMS boards might be configured to allow parallel charging of three sub-packs. Actually, I think this might work. What you would do is break the 24-channel BMS board, shown below, into the 8-channel sections.

24-Cell%20LiFePO4%20BMS-v2.0-PCB.png


The left-most section would act as a "master". The other two boards would have to have the six opto "bus" lines connected together with those on the main board, but the connections between the 8th and 9th cells, and between the 16th and 17th cells would be left cut. Instead, the bottom of the 2nd 8s2p pack would need to be connected to the top of the 1st power resistor on the 2nd board (the 9th...), and the bottom of the third sub-pack, to the 1st power resistor on the 3rd board (the 17th...). On the next version of the board, I will add extra connection holes, next to the existing ones, to make this easier. Anyway, this should allow the 8s2p sub-packs to be connected in parallel, for charging, and in series, for discharging, and still get the full benefits of the BMS, including the LVC function.

-- Gary
 
GGoodrum said:
Cells that are "stressed", meaning they have been previously damaged due to over discharging them, will have lost about 10-15% of their capacity and althought they will appear to take a full charge, up to 3.70V, after a few minutes the surface charge will bleed off by itself and the voltage will drop to about 3.38-3.45V. Do this test with each of your remaining cells, and with any of your replacements. Only build packs with the ones that pass the surface charge test, that is the ones that will hold the voltage up to something above 3.55-3.60V.

I wonder if that's a truism with the LiFePO4 chemistry, in general. My ping cells seem to eventually drain down to 3.38-3.45 V within minutes/hours(I don't know the exact time frame, but it eventually happens). I wonder if that means they've been overly-discharged at one point or another or that might just generally be the case with lower C-rate LiFePO4.
 
swbluto said:
GGoodrum said:
Cells that are "stressed", meaning they have been previously damaged due to over discharging them, will have lost about 10-15% of their capacity and althought they will appear to take a full charge, up to 3.70V, after a few minutes the surface charge will bleed off by itself and the voltage will drop to about 3.38-3.45V. Do this test with each of your remaining cells, and with any of your replacements. Only build packs with the ones that pass the surface charge test, that is the ones that will hold the voltage up to something above 3.55-3.60V.

I wonder if that's a truism with the LiFePO4 chemistry, in general. My ping cells seem to eventually drain down to 3.38-3.45 V within minutes/hours(I don't know the exact time frame, but it eventually happens). I wonder if that means they've been overly-discharged at one point or another or that might just generally be the case with lower C-rate LiFePO4.

No, this only applies to a123 cells. I should have made that clear. It has something to do with how the electrodes are manufactured. LiFeBatt/PSI cells, which are about the second strongest LiFePO4 cells I've tried, will self-bleed off the surface charge as well, all the way to 3.36V, and will do that in a matter of minutes. I didn't mean to imply that this was an idicator of the quality and/or health of other LiFePO4 cells. It is simply a characteristic of the a123 cells that can be used to roughly determine the relative health of a123 cells.

-- Gary
 
Yes Gary, it was I who you were talking to about with only a single string of batteries. 24s1p. I knew that wasn't enough but the power and speed was so addicting, and I thought I was playing safe by shutting down as soon as I felt the controller "stutter". I guess it was hitting the built in LVC.
Heres what happened on my second attempt. 3 packs of 8s2p. One survived. I thought I knew what i was doing. Apparently not :oops:
100_1071.jpg
100_1070.jpg
100_1072.jpg

And here is the plug I made to make the three packs inseries.
100_1074.jpg
100_1073.jpg

Heres what it all looked like when every thing was happy.
100_1025.jpg

--Dan
 
Daniel828 said:
And here is the plug I made to make the three packs inseries.

Oooo... I'm totally stealing that for my upcoming 2x 37v9ah konion packs that I want to put in series... thanks! +1 good karma for you sir! :mrgreen:
 
i was over at Headway last week and met with Victoria and her husband. I packed up 40 of their new cells with the screw posts and ill tell you what, on a 10p/4s 12v pack, with a 660a draw these little things are amazing. The larger format cells arn't fragile like the a123 when you hit low voltage. Now, i dont have a bike, but call me crazy but i cant see why you guys dont build a companion pack of headways that are stable to lower discharge voltages without damage (or at least they dont explode!) to prevent the damaging "blow a fuse" dump effect of the a123, and still wire in parallel a smallish a123 pack of equivalent voltage (the cells sit exactly the same at rest) The packs can be maintained, charge, balanced independently, but when parallel you should realize the best of the high C batts and the robustness of the headways(or psi cells, i haven't tried them yet) anyway just my 0.02.
 
Daniel828 said:

:shock:

Where do people get these humongous bicycle tires? I swear the thickest I've seen them in real life was 3" and that's only when I happened to stumble upon one specimen in a bike shop. It almost looks like you have 5" tires!
 
Daniel828, that thing is just sick. Did you weld up that frame yourself? Yes the rubber is crazy too. I think i will build a bike this winter. What is the specs on that beast- weight,speed,rannge, motor, controller, ect... thanks
 
i do the same thing with the lead acid batteries currently with the seriesing to run the phoenix brute and paralleling to charge from a 12 volt charger.
 
Either the ping cells are damaged, or they are just low quality. Self discharge rate is a great indicator of cell quality.
 
ohyea said:
i was over at Headway last week and met with Victoria and her husband. I packed up 40 of their new cells with the screw posts and ill tell you what, on a 10p/4s 12v pack, with a 660a draw these little things are amazing. The larger format cells arn't fragile like the a123 when you hit low voltage. Now, i dont have a bike, but call me crazy but i cant see why you guys dont build a companion pack of headways that are stable to lower discharge voltages without damage (or at least they dont explode!) to prevent the damaging "blow a fuse" dump effect of the a123, and still wire in parallel a smallish a123 pack of equivalent voltage (the cells sit exactly the same at rest) The packs can be maintained, charge, balanced independently, but when parallel you should realize the best of the high C batts and the robustness of the headways(or psi cells, i haven't tried them yet) anyway just my 0.02.

You are flat-out wrong! You will kill Headway cells just as easy by over-discharging them, and a123 cells don't "explode" either when over discharged, The voltage just goes to zero. It is when you try and pump 20A into dead cells that the fireworks start. Headway cells are no different in this regard. Also, Headway cells are anemic, when compared to the discharge "robustness" of a123 cells, or even LiFeBatt/PSI cells. In fact, it is this ability to deliver high-current that causes problem. With every other LiFePO4-based cell, they have to be lin a arger format in order to supply anywhere close to the same current, and even then, most would have to be paralleled into larger capacity packs.

You need to get your facts straight.
 
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