Bafang BBS02 inaccurate battery capacity detection

jwong

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Hello, after spending a long time researching I recently decided to purchase and install a Bafang 750W BBS02 kit which I paired with a 48V 17AH battery and L200 Bluetooth display

Almost everything is working great except the unit seems to be very inaccurate when it comes to detecting how much capacity is left in the battery, and gets more inaccurate as voltage goes down.

Basically it thinks there is no power left when the battery has approx 40% capacity remaining - I believe this is the correct number b/c both the battery indicator on the battery pack itself (shows 2/4 bars), as well as manually measuring the voltage on the battery (45.3V), match up with this. I also tested with multiple voltmeters showing the same measurement.

Here's a short clip:

I'm using this 48V battery chart to map from voltage to battery capacity
https://docs.google.com/spreadsheets/d/1E1HCw-kZOfo8pdpjXWLjaK5VaTN5axZFjDMcuTpb_b8/edit?usp=sharing

I understand there is a low voltage cutoff setting on the controller (42V) but I am nowhere near it and afaik that shouldn't cause the battery % remaining reading to be offset so significantly if at all.

I also understand there is both a voltage sag that scales with load and that the voltage can be prematurely lowered and recover a bit after leaving the battery alone. I've experienced both with my setup and behavior is what I'd expect except occuring with this large offset. Ex. near 50% battery capacity (based on manually reading voltage in rested battery), if I try to use PAS at level 4 it feels like the unit's performance seems to downgrade to PAS level 3 after ~1 second, and if I do this around 45% the unit will cut power, saying there is no battery left. Anything lower than that I need to stick to PAS level 1 or 2 until around 40% where it doesn't have enough left to run at all even at lowest PAS.

I've begun tracking the manual battery measurement against what the Bafang unit thinks remains with the following results. For the Bafang % measurement I've used both the bluetooth on the display + manufacturer's app, as well as the Bafang programming cable + Speeed app, both show the exact same numbers so I don't think it's the display that is the issue:

BatteryCapacityDifference.PNG


BatteryMeasurement.PNG
DeruizApp.PNG
Screenshot_20230427-214040.png


You can see how the difference increases as the voltage decreases, and when the Bafang unit thinks there is around 10% remaining, it does not run anymore even at lowest PAS.

I was never expecting to get to use the whole battery capacity, only up to maybe 80-90%, but it feels like I'm currently only getting to use a little over half the full capacity which seems wrong

VoltageChart.PNG


Is this normal and has anyone else experienced this? I really hope I don't need to get a new Bafang unit just for this
 
what happens is the voltage sag after 50%has been depleted will trigger the low voltage cutoff when you get on the throttle
hard. Make sense?

So when you get on the throttle hard after your battery is at or below the 50% mark....
that causes the voltage sag to be around or sometimes right at the low voltage cut off causing the
situation you are experiencing.
Hope that helps.
 
what happens is the voltage sag after 50%has been depleted will trigger the low voltage cutoff when you get on the throttle
hard. Make sense?

So when you get on the throttle hard after your battery is at or below the 50% mark....
that causes the voltage sag to be around or sometimes right at the low voltage cut off causing the
situation you bare experiencing.
to be clear, I'm not throttling hard in that case, I was already reduced to using PAS 1 or 2, and closer to 40% just PAS 1 and on flat pavement - about as low load as I imagine you can get while still riding the bike

even if I were to put on decent load, having the battery unable to handle it when hitting 50% capacity feels unexpected, like I would have expected this to happen at maybe 20%, but please correct me if that's not the case

What is you low voltage cutoff set to ?
42V
(btw I also tried to lower this but it made no difference)
 
So your low voltage cutoff on a 48v battery should be set to 40.6 which is 10% left of the battery charge,
unless you just want to shut it down at 20% left.

Has it done this since you got it? Did you get it brand new?

What is the current set at? (current is amps) 18, 25, 30 ?

Like i've previously stated, when the battery reaches 50% or less (yours seems to be 40%)
the voltage sage is more than it would be if your battery were at 70%, make sense?
 
So your low voltage cutoff on a 48v battery should be set to 40.6 which is 10% left of the battery charge,
unless you just want to shut it down at 20% left.
Ok I'll try to reduce the cutoff again to see if that helps, assuming voltage sag is causing the voltage to go from 45.3V to less than 42V

Has it done this since you got it? Did you get it brand new?
Yes this is a brand new unit purchased recently

What is the current set at? (current is amps) 18, 25, 30 ?
Max current is set to 24A, think it would help if I reduced this and if so how much should I reduce it to?
Pedal start current 40%, keep current 60%
Speeed app settings attached below, let me know if you think I should change anything

Screenshot_20230328-212033.pngScreenshot_20230521-132353.pngScreenshot_20230328-205709.pngScreenshot_20230328-205718.pngScreenshot_20230328-205743.pngScreenshot_20230328-205811.png
 
oh man, I believe @m@Robertson would certainly be able to help with those settings alot better and can give you better explainations than i can.

He has some guides which he has written that are very nicely put together.
 
The workaround (simply set your display to show battery voltage instead of a percentage) is a good workaround. It's unfortunate that Bafang hasn't fixed this "bug". I mean, displaying a percentage that is nonsense... that's pretty bad. It's an important piece of information for the rider.
 
oh man, I belie
oh man, I believe @m@Robertson would certainly be able to help with those settings alot better and can give you better explainations than i can.

He has some guides which he has written that are very nicely put together.

ve @m@Robertson would certainly be able to help with those settings alot better and can give you better explainations than i can.

He has some guides which he has written that are very nicely put together.
 
Thanks. I am new at this, and I sent a private message to m@roberts about 40 minutes ago, and am wondering if I should wait for him to respond, or should I post it here?
 
I ve got the same issue When pas is solicited on a hill (3 out of 9) it is cutting off when the battery is near 40%.
After a pain staking pedaling uphill for 30 minutes unit could come back from the dead but only to be working few kilometers . The threshold seems to be 35% …
The most frustrating is that once I plug it for recharge unit shows instantly 42% charge.
My wife uses an even lower battery, 16 vs 17,5 for me, on a 500 w Tongshen and her bike goes a long way over 90 km,s
 
Have you considered that maybe your battery pack is a bit faulty? It's always a possibility.

I have a BBS02 with a 13aH battery, and despite using PAS level 6/7, it'll give me 80km of range.

One thing I did change was the max current draw - I saw recommendations online to drop it to 20A. I also adjusted the rate at which power comes on, so it's a little bit gradual, rather than spiking straight to maximum.
 
I sent a private message to m@roberts about 40 minutes ago, and am wondering if I should wait for him to respond, or should I post it here?
Did I miss this back in the day (this thread is about a year old)? I have a lot of online discussions and as a result I can't always remember them as is the case here.

Reading this thread for the first time, I concur with others here who say to switch the display to voltage readout. Percentage readings and bar graphics are notoriously bad, and its not just Bafang you can blame for this. In my (not universal) experience its a problem across every mid drive and hub motor I've ever worked with, unless I am reading actual volts and interpreting the number with my brain rather than the bird brain inside the display.

As an aside, I noted the settings chosen by @jwong leave a lot to be desired. 40% PAS startup current (!), 10% throttle startup, throttle mode by display's command jumped out at me. I didn't do a close look at them all but its clear there was a lot of room for improvement.

 
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