Magic pie edge 36v turns off under load

[Malo87]

Hello,
I've got the Magic Pie edge kit from a european reseller since August, and I'm experiencing problems riding the bicycle uphill. After a couple of rides, with the battery gauge completely full, the motors shut down and the display goes off. If I keep on charging the battery every 2 days (10 miles) everything works, but if I forgot it dies (gauge still displaying full 5 bars)

Shall I complain with the seller that the battery is faulty? Fully charged(36v) I can see the voltage dropping in the log to 31v. Could it be that the EMS shut down everything because the amperage is above the limit, due to the voltage drop? I'm using the PAS without the throttle. I've attached logs and configuration.

[Bikemad]

Hi Daniel,

Without knowing which 36V battery you have, its chemistry, capacity (Ah) and also its stated continuous/maximum power output figures, it is difficult to say what is causing your cutting out problem.

The cutting out under load can be a result of any of the following:

• The controller is getting too hot, which is causing the temperature safety cut out in the controller to be triggered.
• The current being drawn by the motor is too much for the battery, and the battery's BMS is shutting down the power.
• The battery voltage is simply too low, and the low voltage cut off in the BMS is activated to prevent further discharge of the battery pack.
• The voltage of one of the groups of cells is falling too low, and the power cut off in the BMS is activated to prevent damage by further discharging the lowest voltage cells.

As the cutting out only occurs when you forget to recharge the battery, I feel it is unlikely to be the controller overheating.

After looking at your controller settings, I recommend that you increase your Undervoltage protection value (V) from 27V to 32V, as this should allow the controller to reduce power gradually when the battery voltage reaches 32V instead of the battery's BMS simply cutting out completely.
You might even be able to set it as low as 31V (or perhaps even 30V) without the BMS cutting out, but you would need to try it and see what happens.

Fully discharging a battery pack (or keeping it in a fully charged state for long periods) will substantially reduce its lifespan, so it would be much better for the battery if you charged it before each use, rather than running it lower and charging it every 2 days.

Regarding your "Fully charged(36v)", I would have expected this voltage to be a bit higher, as the typical charging voltage for a 36V LiMn battery pack is 42V, and for a LiFePO₄ battery pack it should be 43.2~43.8V.

Out of interest, I checked the voltage reading on my MP Edge Bluetooth display and it shows 0.5~0.6V higher than the actual battery voltage, and the battery gauge shows 2 bars remaining @ 30.5V but instantly switches from 2 bars to zero bars @ 30.4V.

Alan
 

[Malo87]

Hi Alan,
thanks for the reply. The battery is 36v 10.6ah, there is more technical details under the Additional Info tab

I lower the Undervoltage protection value (V) param to 27v trying to solve the issue, because I thought the controller was switchin everything off due to low voltage.

I'll try to take the fully charge voltage without opening the battery case with a multimeter, to check the max voltage without load. I'm quite sure that the app shows just above 36v when fully charged. It's just a pain to use cause it crashes continuously

What baffles me is that the battery gauge is always full, it never goes below 5 bars Does it need to be calibrated?
As well, I never let the battery go flat. If I turn off/on the key after it shuts down, the gauge still shows 5 bars. Starts pedalling a bit uphill and shuts down again.

I assume that the battery has 10 cell of nominally 3.75v each

[Bikemad]

Hi Daniel,

The description for your 36V battery is obviously not 100% correct, as it states that it comes with 48V 2A charger. 
The Additional Information tab relates to the individual 5.3Ah cells used to make up the battery, but from the size and style of the cell pictured, I would say each of those 5.3Ah cells (modules) actually contain two 2.65Ah 18650 cells side by side connected in parallel.
This means that your pack contains a total of 40 3.65V cells connected in 10S4P configuration (or 20 modules in 10S2P configuration) to provide the total 36.5V and 10.6Ah capacity.

The fully charged voltage of this pack should therefore be 42V, and I would expect your charger to be clearly marked with this voltage.
If you measure the voltage coming from the charger's connector plug while it is disconnected from the battery, it should read ~42V with the charger plugged into the mains and turned on. 

If your pack is not reaching ~42V at completion of the charge, there must be a problem with either the battery charger or the battery pack.
If the pack voltage is only 37V, this is equivalent to ~29% of its 10.6Ah capacity (or ~3Ah), and it will not be able to properly balance the individual cells if it is not reaching the required 42V.  This means that you could theoretically have a lot less than 29% capacity available at the end of a full charge. 

The Max continuous discharge rate stated is only 13A, but this appears to relate to the individual cell modules, not the actual battery pack itself.
This means that the battery should theoretically be capable of supplying 26A continuously from fully charge to completely empty,but only if the BMS will allow it, but they do not supply any information whatsoever on the BMS's maximum continuous/peak power output or its minimum output voltage before the low voltage cutoff is activated. 

Regarding your comment on the battery gauge on the BAC-601 display, "it never goes below 5 bars", I am wondering whether these were originally designed for use with lead acid batteries (like the throttle mounted LED gauges) in which case the battery display will be totally inaccurate and will always read way too high with a lithium battery.
Unfortunately, I don't think there is any easy way of calibrating the gauge to allow it to operate more accurately.
Placing a suitable resistor into the battery feed wire going to the BAC-601 display might improve its accuracy, but I suspect that the battery gauge might then read much lower while the backlight was turned on.
The LED gauges on the 48V GM throttles will remain fully lit for ~95% of a lithium battery's discharge cycle, and the Green and Amber LEDs will eventually begin to go off in quick succession, just before the battery's BMS cuts the power off completely.

I suspect that your battery's BMS is simply cutting the power off under load because the battery voltage is dropping too low, even though the LCD gauge indicates that your battery is still full. 

Alan
 

[Malo87]

Thanks Alan.
I returned the battery and the charger to the seller and I've put a link to this discussion in my email to them. I'll update the thread as soon as I know more.