Problem hub motor/battery cuts out under heavy load

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ridingmn

100 mW
Joined
May 22, 2015
Messages
35
Location
Midwest, USA
Hi I'm very new to ebikes and to Endless Sphere and I wanted to see if someone here could help with a problem I am having. I am using a Magic Pie 3 rear hub motor with a 48v LiFePo4 battery from Golden motor Canada. The problem is that when I put the motor under a heavy load (fast acceleration) the motor/battery cutout, all power is off. When I turn the battery off and then back on again I can ride for another block or so before the same thing happens. I recently hooked a cycle analyst up to see what was happening, starting voltage of my battery after a full charge 53.2v (this is leaving it plugged in for over 12 hrs although the charger indicates that it is finished after maybe 40 minutes?). When I watch the voltage under load it may drop down to 45v under heavy load and then jump back to 52v, am I having a low voltage cutoff issue. I have measured the voltage of each individual cell and they are each 3.2v. Also I measured the voltage output of the charger and it is 58.2v. Shouldn't my battery be closer to that after a full charge? I am sure there are a lot of dumb questions in here and perhaps I have left some important details out, please let me know. Thanks in advance
 
put the battery on the charger and measure the cell voltages using the 20V DC scale on the meter. measure while charging to .01V and post up the measurements here in your thread. switch to the 200V scale and measure the pack voltage while it is charging.
 
Thanks for the fast response, here are the numbers

At 20v scale .01
1. 3.46v
2. 3.44
3. 3.42
4. 3.38
5. 3.39
6. 3.39
7. 3.38
8. 3.38
9. 3.38
10. 3.35
11. 3.38
12. 3.36
13. 3.37
14. 3.37
15. 3.28
16. 3.36

At 200v scale .1
All cells except the 15th came in at 3.3, the 15th cell came in at 3.2v
 
Sorry about that, the battery is a 10 Amh 48v LiFePo4 from Golden Motor, I just purchased it a few months back from Golden Motor Canada.
 
i mean for you to measure the voltage on the battery while charging. your charger should push the total pack voltage all the way up to 58V. if the charger is turning off because of the BMS reaching HVC then we need to know which cell is reaching HVC and what the voltage is when the BMS cuts off.
 
ridingmn said:
At 20v scale .01
1. 3.46v
15. 3.28
Click to expand...

There is your problem right there.

The BMS will kill the charge when the 3.46 cell gets to 3.65, leaving the other cell half charged.

The BMS will also kill the battery when the 3.28 cell gets to 3.0v, even though most other cells are still good.

You're going to need to balance the battery. Since the BMS doesn't seem to be doing it for you, best way would be to sacrifice a USB charger (5.0V) and attach it to each cell independently - and watch it carefully using a multimeter, since it will way overcharge your cell if you don't. Once they're all close, the BMS over time should bring it closer and keep it matched unless some of the cells are faulty.

It's actually worse than it looks. Check out this graph:

20130708180128_98412.jpg


The graph isn't that clear, but a cell at 3.46v is almost fully charged. > 95% at least. A cell at 3.28 is only about 65% charged.
 
When I have the charger plugged in the highest voltage I get before the charger stops ( red light charging to green light full charge) is 54.7v.
 
The highest voltage I get is 3.5v on the first cell and and by the time I get to the 4th cell or so the charger turns off.
 
Probably a dumb question but is it possible that it's a faulty BMS. Also is there another way to find the high volt cell, is it just going to hold that voltage for a short period and then drop back to around the same voltage as the others? Thanks for all of the advice.
 
ridingmn said:
is it just going to hold that voltage for a short period and then drop back to around the same voltage as the others?
Click to expand...

That can be one of the indicators of a faulty cell - at rest it looks normal, but sags more than normal during discharge, and spikes in voltage more than normal on charge.

Just a guess, but it could be cell 15. Want to give that a shot?
 
Thanks for the info guys, I have a few other things I need to work on tonight, but I'll check the other cells tomorrow and post what I've found.
 
Your problem is very simple. One cell is too low. When you charge, all the cells charge until the first one reaches HVC, when the BMS is switching off the charging. There's two things you can do. You can chharge up the low ones to balance it or discharge the high ones. The result will be the same.

Hopefully, your problem is only that the battery isn't balanced, but I suspect that cell 15 is faulty. At 3.2v a healthy cell would still be able to give plenty of power, but your excesive sag and cuting out seems to indicate that that cell is dropping to LVC under load.

Without seeing your battery, I can only guess, but if it's an array of small cylindrical cells, check whether any corner ones have been squished.

One other thing. A 10ah LiFePO4 battery wouldn't normally be able to give enough power for your motor/controller. Which battery is it exactly, and what's its discharge rate?
 
Here are the battery specs from Golden Motor: max continues discharge current 30amps 3c, max peak discharge current 50 amps 5c. http://www.goldenmotor.com/LiFePO4Cells.jpg.

I have also included a photo of the battery and I will try to find the voltage of each cell at the time the BMS turns the charger off.
 

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Here are the numbers for the voltage of each cell at the time when the BMS switches the charger off.
Cell 1 is the closest cell to the charging port (I'm not sure if that makes a difference) and obviously cell 16 the furthest.

1. 3.84v
2. 3.74
3. 3.54
4. 3.48
5. 3.50
6. 3.50
7. 3.46
8. 3.47
9. 3.48
10. 3.47
11. 3.46
12. 3.46
13. 3.45
14. 3.45
15. 3.28
16. 3.44

My voltage meter is not fancy so the first cell my be reaching 3.9v also the voltage one cell 1 drops quickly after 2 minutes it's at 3.51v. Do I need to manually rebalance the cells or are there bad cells again I'm very much a novice, any advise thanks.
 
you should be able to measure the 3.9V when it cuts off. but drain charge from that cell with a power resistor or light until it is lower than the others, and do it while charging.
 
I just finished trying to do the drain charge and I got the 1 cell (highest voltage cell) down to the same voltage as the lowest cell #15 3.3v. The BMS had the charger switched off for all but the first few seconds. I also noticed that some of the other cells voltage is higher than other readings I took while charging in the 3.7v range but this was not consistent across all cells.
 
what is the charger voltage when it turns off now. ie, what is the voltage on the pack when it goes green? if that low cell does not climb fast enuff you can drain all the other cells around it with the light. it is only about 20% charged at 3.2V.
 
increased a little bit, keep charging it up and if that low cell stays low then you can drain the other cells together to push it up. just keep it on the charger all the time until it climbs to 58V.
 
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