Strange problem cell nr. 9 always low

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MAGICPIE3FOCUSPOWER

10 kW
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Aug 23, 2012
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I have a 16S 15Ah Headway pack with BMS.
The problem is cell nr. 9 is always low voltage after a ride.
I had several times the BMS cutt off (after long ride), because cell nr.9 voltage is too low e.g. 2.5V while other cell voltages are 3.2V.

To make it sure cell nr.9 is (not) bad I have replaced cell nr. 9.

But after replacement and fully charged I went for short a ride.
Checked all cells voltages again, yup cell nr. 9 is still lower then all other cells.


After the short ride I charged all cells separate, all cells "capacity IN" are about 1.3AH while cell nr. 9 "capacity IN" is more then 4Ah.

No wonder why the bms cut off after a long ride.
So my question could the bms be the problem?

What is the purpose of the 10 fets in my bms?

Thanks for help!


BMS
17s26s-24a-max-discharge-current-bms.jpg
 
Have you checked all the cell voltages after charging to ensure the bms is balancing them properly?

I think you first need to establish if the problem is with charging or discharging, as replacing the cell should have eliminated the question of that cells capacity.

If they are all balanced when charged can you do a discharge directly from the main battery leads bypassing the BMS to see if the cells remain balanced after a ride.
 
yes, we need real information. the purpose of the ten large power mosfets is to conduct current from the B- terminal of the battery to the P- terminal of the BMS.

charge the battery for a long time until it no longer cycles and the green led is constant, and while charging, <while charging!>, measure the cell voltages of each of the cells and measure the charger voltage at the same time.

measure to .01V and list the cell voltages here.
 
Thanks a lot for help!

Okay I will try first without bms.
Now I am charging all cells separate and will perform a ride to see how the cells voltages will be.

I hate my charger, when one cell reached 3.9V the charger stop and green led will lit.
So some cells are are still low e.g. 3.3V, so you think the pack is full :evil:

Another question about the fets; those doesn't have any influence for cells unbalance?

Thank you :D
 
nope, the only way to make it balance is to use the BMS and charge to full voltage. by removing the BMS it will just go farther out of balance. tried to help you. nothing i can do. cannot even get a measurement for a clue.
 
When your charger stops and the green light comes on that is when it is starting the balancing stage, leave it plugged into the battery and monitor the cells, it may take days to balance a badly out of sync pack!!

Your habit of taking it off the charger too soon could have created this unbalance.
 
yes, i've been reading various charger data sheets, and red light means stage 1, green means stage 2 starts. NOT FULLY CHARGED! certainly not balanced yet. They don't have an indication when all fully balanced!
Not the ones i've read about. don't know about yours.
 
MAGICPIE3FOCUSPOWER said:
Thanks a lot for help!

Okay I will try first without bms.
Now I am charging all cells separate and will perform a ride to see how the cells voltages will be.

I hate my charger, when one cell reached 3.9V the charger stop and green led will lit.
So some cells are are still low e.g. 3.3V, so you think the pack is full :evil:

Another question about the fets; those doesn't have any influence for cells unbalance?

Thank you :D
Click to expand...

From a quick search you might be using LifePo4 which shouldn't be charged past 3.6v. I wonder if your bms doesn't have functional cell level hvc. Maybe your just bulk charging as it is broke.

A lot does not add up here, I have no new direction.
 
You can speed it up if you have some time to do it. Identify which cells are filling up fastest, and attach a turn signal light bulb to that cell for 10 min or so. It will discharge the highest cells faster, letting the charger turn on again sooner, filling the low cells in less time. This can be done while the bms and charger are in use.

This can really help, if you are so out of balance that the bms will take more than overnight to balance.
 
Aaha thanks!

I have never charger the battery pack over a night :shock:
Always disconnected the battery when green led is lit.

I will also measure all cell voltages.

Thanks for help all! 8)
 
it is pointless to measure the cell voltages until the battery is fully charged and then you have to measure the cell voltage while it is charging. otherwise we have no information about how your BMS is balancing your pack and if it is not fully charged to 3.65V/cell the BMS cannot balance the cells anyway. so you have to charge it up to 58.4V at least and measure the cell voltages then. while it is charging.
 
It's possible you just randomly got out of balance on 9 both times. You never balance charged ever, if you unplug too soon. You just charged until most of the cells were charged, but not the lower voltage ones. Ironically, the cell that won't fill is the high capacity one. The good one. That's the case though, only if they started out in balance.
 
if i have a cell that loses .01 v per day due to self discharge, it will in a few weeks be the low cell. it will be the bad one if the others dont self discharge. this is why we let cells sit for a month before building a pack. any v loss is a grade B-F cell depending on how fast it dies, and could cause problems. Of course a balance charge lets us use a grade B-C cell by keeping it charged.
#2 is a full discharge defined as down to 2,7 v This, once again shows the weak cell to be the one that First reaches 2.7. the others are higher and have more capacity. The runt cell is helped by the balance charge, but it is still not equal to the others.
#3. dogman will now explain how the low cell is a good cell, equal to the others. I'm sure i missed something!
 
That would only be the case if you weren't self discharging. As cells die of old age they lose capacity, and get higher resistance.

This resistance heating is lost energy that drives that cell lower and lower in the discharge. So at the end of discharge it is the low one. But perversely, it's capacity is so low that it fills up faster too. The end result can be that the bad cells are the ones your bms is discharging, while the low cell is just the good cell that hold the same wh as the other cells, but at a lower voltage.

I don't know if that makes sense. Two cells, one has 50 wh capacity, the other 75. At 50wh of charge, the good cell has a lower voltage. The bad cell is full, with 25 wh less capacity than the other, so it fills first.

It doesn't get that way when you start out balanced, but when you start out unbalanced. Resulting from not leaving it to balance long enough on the charger in some cases. This can be the result if you bottom balanced IMO, or somehow ended up that way.

All I really know for sure, is I have seen things be backwards from how you expected it. Dnmun always says you watch it charge, and this is how you see what IS happening, vs what you assumed was. Real data solves the puzzle.
 
dogman said:
It's possible you just randomly got out of balance on 9 both times. You never balance charged ever, if you unplug too soon. You just charged until most of the cells were charged, but not the lower voltage ones. Ironically, the cell that won't fill is the high capacity one. The good one. That's the case though, only if they started out in balance.
Click to expand...

the dogman is right, the charger goes green when the first high cell hits the 3.9V HVC and it is the one with the least capacity available to charge up. the HVC cell usually has high internal resistance so it stops the other cells from charging up if you turn off the charger when it hits the first HVC. if you had adjusted the charger output voltage down, and only charged to something like 52V and there is one cell at 3.9V then that is 48V over 15 cells, 3.2V/cell.
 
I have found that there are two types of chargers.

One that charges then turns off never to go on again unless the battery voltage goes below a certain threshold. This type of charger never gives the BMS a chance to balance a pack out and with each ride the pack gets more and more out of balance.

The other type of charger is one that goes into a trickle charge mode, the fan stops on the charger and the light goes green to say it is fully charged....but it then puts out anywhere between 15watts and below of trickle current until the the BMS has stopped balancing or just holds at a low 0.2watt (random figure off the top of my head)
 
Some chargers do shut off at top of charge, and will not restart when the bms lowers the HVC cell. If that is the case, it will never allow the bms to fully balance the pack. This is the kind of bulk charger that is good for charging a pack with no bms. Perhaps NiCad or nimh.

My chargers from ping will still show voltage with the green light on, but watts will actually drop to 0w when green. As the bms bleeds down the high charged cells, it will eventually turn red again, but only till the HVC cells fill. Usually just a few seconds. This off for an hour on for 5 seconds cycle will repeat till all cells are fully charged. At that point, it will stop turning red anymore no matter how long it remains plugged in.

So you do need the right type of charger for what you have, a bms or no bms. A charger that continues to trickle just a few watts of current is typically for lead batteries.
 
Hi Dogman,

going to have to disagree a bit on the last part of your post. In tests where I have set the charger voltage to 43.9v instead of 43.8v and with a Watt meter attached to the charger output; I have found that the charger will output as many watts as the BMS will use in burning off the access cells. Typically a very low watt measurement but still enough to warrant being a trickle charge.

But I agree with you on the term 'trickle charge' is a lead acid charger terminology.

Once a pack is stable at say 43.9v to 44.1v on a 12S Lifepo4 pack I then bring the charger voltage back down to 43.8v. The reason for the initial higher voltage of the charger is to speed up the balancing of an out of balance pack. When adjusted back to 43.8v if the pack is healthy the output watts from the charger does indeed go back to 0
 
Spacey said:
........In tests where I have set the charger voltage to 43.9v instead of 43.8v and with a Watt meter attached to the charger output; I have found that the charger will output as many watts as the BMS will use in burning off the access cells. Typically a very low watt measurement but still enough to warrant being a trickle charge......
Click to expand...

OK, just for reference, let me as this. Are your sure your watt meter is not counting the watts that it is burning itself?

I have an inexpensive blue watt meter that counts the amount of watts that it is burning while also counting the watts used by my load. When I connect it to a power source with no load it counts it's own watt usage. When I connect it to a stand alone power supply with no load it counts it's own watt usage. This watt meter uses about 100 mW per hour. When using this watt meter I need to keep track of the time and subtract 100mW pre hour to get an accurate measurement.
 
Very sure :D Been at this game for a long time now. It's not a cheap watt meter, I watch it pulse on and off when the BMS is fixing one low cell out of a pack...the meter accurately shows the channel with the low cell reading a slight boost in voltage from the charger and at the same time a very low power draw from the charger as the BMS draws power from it to boost that cell.

2 x low cells comes to twice the power draw etc etc...

Put the charging voltage high enough on the charger and the BMS will constantly be topping off the cells drawing power etc...

I'm as sure as I can be 8)
 
Maybe I misunderstand you. When you said trickle, I assumed you meant a lead charger size trickle.

I also have seen pulsing on and off ( green light /red light/ green light) when a bms equipped pack is used with my bulk chargers. The same kingpan charger simply turns off, when it's connected to a pack with no bms. It still reads voltage, but my wattmeter shows 0w when the light is green. For a very short time after the light turns green it will show more than 0w, but it does indeed stop in a few min. I've never overcharged my RC packs once with this charger, and I will admit to forgetting to take the packs off before going to bed once or twice. I feel its not a big danger once the light goes green.

Maybe it is a very tiny trickle, but my batteries wont let it flow in? If you think about it, how could you have voltage on the charger leads with the green light on, if there was 0.00 watts. Wouldn't there have to be a milliwatt or so?


The lead charger never turns completely off, since the lead likes a trickle. I don't have a 48v lead charger to try it on my lico and see what happens. It may reach a state of charge where the trickle stops flowing too, I don't know.

All this applies to the chargers in my garage. Yours could be different. I can't say what yours do.
 
Voltages when charging and end voltage is 58.4V

First part of 8 cells.

IMG_0764.jpg


Second part of 8 cells.
IMG_0765.jpg


End voltage.
IMG_0766.jpg


I will replace the cell nr. 9 with another (never used) spare cell.
This way is maybe better, because I have switched cell nr. 9 with other cell nr. from same pack.
See what happen.
 
OK help me out here. If the first 1-8 are batteries 1-8 then the second 1-8 carries batteries 9-16...Correct?

If the second "1" battery is indeed battery number "9" then it is at 3.62 volts which is, from all my headway specs is only .03 volts less then what it should be......

Please correct me if I am wrong, but aren't all those 3.7+ voltage mean your Headways over charged? :shock:
 
Sorry my mistake pictures are switched.

Also the this fake cell monitor is not accurate, cell nr. 1 reading is always al little lower (but acceptable compared with Fluke meter).
 
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