Headway "48v" 10ah pack problem

[redorblack]

I'm hoping I haven't really damaged my pack... it's the 16s battery with headways BMS. It's started cutting out at shorted ranges than expected and the other night the charger started fast flashing at me. I opened the case and the glue holding the cells in had failed so the cells/holders/bms came right out. I figured I'd start by checking connections and found a couple of screws on the tops of cells a bit loose, so tightened them up and put it back on the charger while I went to get a volt meter. Came back and the charger had switched it's green LED on... I figured, "Great!". Except I checked cell voltages and most are around 3.65-3.67v... with one at 3.38 and one at 3.5v. Left the charger on for hours and would check voltage on occasion... I don't have my graphing meter at home so can't 'see' the charger cycling. All the cells came up to 3.67-3.68 except the one at 3.38 only came up to 3.40-3.41. Leaving the battery off the charger for about 6-8 hours and the other cell dropped back down to 3.5. Plugging the charger back in and in the amount of time it's taken to right this so far, the cell at 3.50 has risen back up to a reading of 3.60v while the one at 3.38 has risen to 3.40 (measured with charger plugged in).

Look like I need to replace the two cells, or one, or should I try finding a single cell charger and hitting just the weak ones? Or run it down part way or until lvc and see what cell voltages are at cutoff?

I sure was hoping to get years and years of reliable power out of this unit. I don't know if it's the vibration, or thermal cycling, or just poor assembly that caused the loose screws and if those are a red herring and it's just random cell failure.

Any input from the gurus?

Thanks,
Scott

 

[scoot]

I run naked Headway packs (no BMS), and based on everything Iearned thus far, my gut tells me your cells are just fine. I think they probably got out of balance some with the poor connections, and it is just going to take the poor little BMS a few cycles and a lot of time to bring them back together. A single cell top off of the low ones certainly will speed things up, but then they may be the high ones and you'll need to wait for the others to catch up. :?

In one of my packs I have a truly weak cell that I routinely top off when I don't want to wait around for my balance charger (equivelent to a BMS for balancing purposes) to finish its thing. I have been observing how that cell seems to be improving over time (discharge curve also)... perhaps because I am using a slightly higher than normal single cell charge voltage of 3.80 volts. In fact I am coming to the conclusion that using 3.8 volts per cell is one way to get these Headways to retain their "surface charge" as some call it... I am able to have the sustain a rest voltage of 3.6+ volts for a day or more now, and several days above 3.4 volts. Pretty interesing if you ask me. BTW the 3.4 to 3.6 volts range is the steep "end point" of the charge curve, and that means one cell at 3.4 v is just as full as a cell at 3.6 v from a practical point of view.

 

[dak664]

Loose screws would cause localized heating and less power to the motor, so different cell efficiencies and shorter rides. Since all you had to go by was the green light the imbalance got worse until it became noticeable. I doubt any damage has occurred; that 3.38 cell is just a couple amp-hours low. The problem is the charger sees a low pack voltage and begins to put out an amp or so, but the BMS can only shunt 180ma around the filled cells so shuts it down. The net is probably an amp-second of charge, so several thousand charger cycles might be needed. Adding a 10 ohm series resistor should limit the current enough for continuous charging, then ~20 hours would be needed.

Measuring the voltage between the lug and the battery terminal while charging is a good way to spot bad connections, mine are around 5 millivolts at 1 amp = 5 milliohms. A couple were 100mv until I tightened them some more. If the threads are clean and the screw is the right size, measurement between the lug and the phillips head is enough.

 

[scoot]

.... that 3.38 cell is just a couple amp-hours low. ....

Come to think of it, that is quite possible (between 3.3 and 3.4 is where it is nearly impossible to tell how full a cell is)... in which case a single cell charger would get you back in shape a lot faster. Have done the same with new cells that were as much as a full amphour different from each other to begin with.

 

[redorblack]

Thanks guys... I might stop by and see if a hobby shop has a single cell charger locally that will work.

 

[snowranger]

My headway pack had only 8.6 ah out of 20 ah upon delivery. I thought the pack was no good, but then after single-cell charging all the cells, the capacity was over 19 ah. It remains to be seen if the BMS can keep things in balance over time. I tweaked up the charger to 59.2 and now the charge current tapers to 180 ma instead of zero before this.

I have been running these cells for a few weeks now, and they hold voltage much better than the Ping I had.

 

[redorblack]

My headway pack had only 8.6 ah out of 20 ah upon delivery. I thought the pack was no good, but then after single-cell charging all the cells, the capacity was over 19 ah. It remains to be seen if the BMS can keep things in balance over time. I tweaked up the charger to 59.2 and now the charge current tapers to 180 ma instead of zero before this.

I have been running these cells for a few weeks now, and they hold voltage much better than the Ping I had.

Ok, I'm jealous... 20 Ah should be huge range. That the "48v" pack or something else?

I just found a charger that will do what I need, but it's overkill and not quite up to the task at the same time. $79.99 and it's A/C D/C with capability for all sorts of charging including LiFe. Limit is 9900mah though... but that should be close enough. Also would have to buy leads for it.

Anyone in the Tacoma area have a single cell charger I can borrow or can point me to one that would work good for my Headway cells for a lot less money?

 

[scoot]

VoltPhreaks:

https://p10.secure.hostingprod.com/@www.voltphreaks.com/ssl/catalog/product_info.php?cPath=3&products_id=28

or one from batteryspace.com like this:

http://www.batteryspace.com/smartcharger30afor32vli-fepo4batterypack1cellstandardfemaletamiyaplug38vcut-off.aspx

or google lifepo4 single cell charger

In the mean time, I think you'll be ok running your pack... the BMS should protect from over-discharge if it is functioning ok, or just avoid going past halfway if you can.