Signalab BMS ok?

[ruds23]

Hi. I have a signalab BMS I have just hooked up to a 24v (8 cell) headway battery. I am not seeing any LEDs lighting up when it has finished charging. Is this a problem? I do see one LED light up that is connected to a cell which I know is dodgy when the pack is under load. Is all as it should be? I have only done 3-4 charges with the BMS hooked up after running the pack with no BMS for a couple of years. The charged voltages of all the cells are 3.34, other than the dodgy one which is 3.32. I would have thought the BMS could handle this difference in voltage ok.

Charger is a QET QQE192-4CH12-B-L. See http://www.docin.com/p-24724064.html

Thanks for the help.

[dogman]

Likely that the charger is not taking the cells high enough to get the led's to light. There may be various settings for signalab bms's , but my signalab bms on a pingbattery is set so the led's don't start lighting up till above 3.65v.

Your dodgy cell may have less capacity, so it's the only one getting full enough to light the led. You may want to adjust your charger to a slightly higher voltage to get all the led's to light.

Or,,, it might be just as good a strategy to have the charger slightly undercharge the cells, but still have the bms as a back up to bring down any really high cells. Your call, If you have no cells that remain significantly undercharged each cycle, you might be fine to just charge to 3.4v per cell or so, have less capacity, but potentialy longer cycle life.

[999zip999]

Some signalab bms take 3.65v-3.8v per cell to fire the bms and balance. I think the bigger the ah cell format the lower it likes to be charged. In general. Just quessing.

[dnmun]

3.33V is not charged. if you used the lifepo4 pack for years with no BMS, the cells will by now be far outa balance and maybe damaged so that may account for the 'dodgy' as you put it, cell.

if it has reduced capacity then it will charge up right away and if it was damaged from overcharging or over discharging during this period then it would have higher internal resistance and it would climb up in voltage ahead of the others. so it would light the led and the others did not because they were still undercharged.

your charger has to put out 29-30V in order to charge all the cells up and balance them. if it is still too low then you can open the charger and we can figure out how to adjust it up so the pack can balance as best it can.

you should measure the voltage on each cell as it is charging, and keep track of how fast each increases in voltage with respect to the others. the one with the lowest capacity will climb in voltage fastest so you may wanna consider doing a capacity test by discharging the pack into a dummy load and see how much capacity each cell has. then replace the ones that burned up before you got the BMS.

then when the cells are close in capacity again it will charge and balance faster and produce more power.

[ruds23]

Thanks for the replies guys. I partially dischaged the pack the other day and measured the voltage on all cells as being 3.28 - 3.29. There were none out any more than 0.1V. I will try discharging the pack further and see if any drop out of balance. And check the voltags as tey are charging.

I have noticed that it seems to take less time to charge the pack now than it used to (I noticed this before the BMS was installed). Just checked the charger by measuring voltage at the charger connection to the BMS (-ve) and to the pack (+ve) while charging - it cut off at 27.8V which seems too low and is probably the problem? Is adjusting the output of the charger a difficult job? I'm pretty new to this electronics stuff - still getting the soldering sorted.

Thanks again.

[ruds23]

By the way, I like your idea of under charging to increase cell life Dogman, but the capacity of the pack is marginal for me at 10Ah, so need to get the most out of the battery I can, then get 15Ah cells when this one is worn out.

[dnmun]

undercharging does not increase cell life. your pack is never charged up and so it is always using power from a partially discharged state. the cells should charge up to 3.65V when balancing. 3.3V is not charged.

if you do not have a wattmeter or cycle analyst then you don't know how much capacity you are using from the pack. you may only be using about 5-6Ah from the bottom of the pack.

[Spacey]

Your pack is so far out of balance that it could take a few days left on the charger to light the BMS up.

[docnjoj]

Ping Signallab BMS batteries usually over charge slightly so they can bleed down. Could go as high as 3.8 to 3.9 per cell so it should go above 31 volts to activate the BMS. Possibly 32 volts. Seems not to harm cells. Not sure if Headways like this type of charge though.
otherDoc

[dnmun]

i don't think raising the voltage that high will solve the problem, it may help, but we don't really know what the problem is because he never measured the cell voltages while charging. until then we can't really help him.

we don't even know what the open voltage of his charger is now. it could even be up over 30V and we would not know.

[docnjoj]

Oh! I just saw the "running the pack for a couple of years without a BMS" Hmmmmm,,,,,,,,,,, dnmun is right again! We need more info but headways could have a problem.
otherDoc