Battery BMS or Motor Controller?

[ronaldvidnphoto]

Hello all!

I built my bike a year ago and never really did a committed range test. About a week ago I decided to take the bike out to Central Park and loop it throttle only until the battery died. It ended up being a much shorter test than I thought it would be. Any input on fixing my issues would be greatly appreciated.

In short: My battery won't charge past 56.6v and the bike cuts off at 54.5-.9v. (roughly 10 miles, throttle only). The bbs02 controller LVC is set to 41v.

Long version:
1.The first thing I noticed was the battery only charged to 56.6v when it used to charge to a full 58v. To view the volts with my display (sw102) you have to go into the menu, so this isn't something I regularly checked. I'm not sure when the drop started to happen.

2.I started my first run and the battery ended up dying after only about 10 miles (~1.5 laps). I came to this form and saw mention of battery imbalance and decided to try for a second lap after a long charge.

3. I put the battery back on the charger for 15 total hours. 12 straight and then two smaller stints when I was around. The battery still only got to 56.6v. I also ordered a programming cable to see what the controller specs were set at. The controller was factory set pretty well based on guides I found online, so that ended up not being the problem.

4. Today I went for my second ride. It was much cooler and even got a bit of rain. The bike went a tad further but ultimately died again at ~10miles. I did a quick 2min jump with the charger so I could get the display back on and the battery was still at about ~54.5-.9v. I didn't check this the first time, but all things considered, I'm assuming it was the same. In the past, I've had it cut off at around 52v.

Exact parts:
52v 15ah Samsung 30Q Lithium-ion E-Bike Battery
Link: https://bicyclemotorworks.com/product/52v-15ah-samsung-30q-lithium-ion-e-bike-battery/

Bafang BBS02B 48V 750W w/ SW102 display.
Controller info: Manufacturer HZXT, Model SZZ9, Nominal Voltage 48-52v, Max Amp 25amps, Hardware V1.1, Firmware V2.0.1.1
https://www.aliexpress.com/item/32852257691.html?spm=a2g0s.9042311.0.0.603f4c4dopcsOQ

Remote on/off Solid State Switch (placed between battery and controller)
https://lunacycle.com/remote-on-off-solid-state-switch/

Currently, the bike is back on the charger, waiting for my next run at the park.

 

[JackFlorey]

1.The first thing I noticed was the battery only charged to 56.6v when it used to charge to a full 58v. To view the volts with my display (sw102) you have to go into the menu, so this isn't something I regularly checked. I'm not sure when the drop started to happen.

For this measure it with a meter while it is charging. You should be able to measure it right at the XT60. Before charge the charger should be close to 58.8* (that's the max charge voltage for a 52V pack) and as soon as you connect it it should drop to 52ish volts if the battery is discharged. It will then slowly rise until it plateaus at its CV setpoint. It can be as low as 57.5ish or so; that would be better for the battery's life but will result in slightly less range. For an average conservative charger I'd expect just over 58 volts.

If it's not getting that high it's a charger problem. If it starts off at 52 volts, then slowly rises to 54 volts, then suddenly jumps to 58 volts - then the battery BMS is cutting in to protect a cell from overcharge. If you see that you might have to open the pack and look at individual cell voltages. If one is much higher than the rest you can manually discharge it with a load (a large 2ish ohm resistor would work for that) and see if that fixes it. But if the problem returns then one or more cells are bad; they may be lower capacity (and thus hitting peak voltage sooner) or there may be an open in an interconnect that takes a cell out of the circuit.

(* - you will see the voltage on most chargers. Some don't output any voltage until they sense a voltage on the charge pins; if that's the case you'll have to connect the charger to see any voltage at all.)

 

[docw009]

Sounds like an unbalanced battery. Your advertising link claims that the battery has a balance BMS. That's good. It probably will take a few days of charging to get it from 56.6 up close to 58.6V. Like they say in the ad, "do it in a place where you wouldn't mind a fire." I'm surprised the ad said that.

 

[ronaldvidnphoto]

Sounds like an unbalanced battery. Your advertising link claims that the battery has a balance BMS. That's good. It probably will take a few days of charging to get it from 56.6 up close to 58.6V. Like they say in the ad, "do it in a place where you wouldn't mind a fire." I'm surprised the ad said that.

Thank you for the input. Do you think it's a problem with a cell getting too low, making the pack cut off prematurely? I'm working on getting a meter now, so in the meantime, I've put the battery back on the charger. I had it hooked up for ~8 hours yesterday, It should get close to 12 today. Hopefully, that'll help it in some way.

 

[ronaldvidnphoto]

For this measure it with a meter while it is charging. You should be able to measure it right at the XT60. Before charge the charger should be close to 58.8* (that's the max charge voltage for a 52V pack) and as soon as you connect it it should drop to 52ish volts if the battery is discharged. It will then slowly rise until it plateaus at its CV setpoint. It can be as low as 57.5ish or so; that would be better for the battery's life but will result in slightly less range. For an average conservative charger I'd expect just over 58 volts.

If it's not getting that high it's a charger problem. If it starts off at 52 volts, then slowly rises to 54 volts, then suddenly jumps to 58 volts - then the battery BMS is cutting in to protect a cell from overcharge. If you see that you might have to open the pack and look at individual cell voltages. If one is much higher than the rest you can manually discharge it with a load (a large 2ish ohm resistor would work for that) and see if that fixes it. But if the problem returns then one or more cells are bad; they may be lower capacity (and thus hitting peak voltage sooner) or there may be an open in an interconnect that takes a cell out of the circuit.

(* - you will see the voltage on most chargers. Some don't output any voltage until they sense a voltage on the charge pins; if that's the case you'll have to connect the charger to see any voltage at all.)

Thank you for your input. I have a meter on the way and I'll go through these checks when it arrives. At the moment I have the battery on the charger, hopefully, there's some form of self-balancing that happens in the meantime. I'll report back with what I find.

 

[ronaldvidnphoto]

For this measure it with a meter while it is charging. You should be able to measure it right at the XT60. Before charge the charger should be close to 58.8* (that's the max charge voltage for a 52V pack) and as soon as you connect it it should drop to 52ish volts if the battery is discharged. It will then slowly rise until it plateaus at its CV setpoint. It can be as low as 57.5ish or so; that would be better for the battery's life but will result in slightly less range. For an average conservative charger I'd expect just over 58 volts.

If it's not getting that high it's a charger problem. If it starts off at 52 volts, then slowly rises to 54 volts, then suddenly jumps to 58 volts - then the battery BMS is cutting in to protect a cell from overcharge. If you see that you might have to open the pack and look at individual cell voltages.

I'm back with numbers. I couldn't check how the numbers might fluctuate during charge, but this shows the current levels with a fully charged battery. All numbers were taken with a Fluke 87V and showed steady; ie. no fluctuations of figures. Also, the battery has spent well over 24hrs on the charger to give time to balance cells.

bike sw102 display: 56.7v
Battery charge port (not connect to charger): 57.12v
Battery main output port (not connect to charger): 56.97v . I'm expecting this difference to come from using my on/off switch**

Charger alone: 58.49v
Battery w/ charger : 58.5v
BBS02 controller only (no battery connected): 7.35v. I thought this was interesting. I wonder how this might affect ideal storage.

**https://lunacycle.com/remote-on-off-solid-state-switch/

This weekend I'll do another run till it dies and bring the meter along for the ride to check vitals as soon as it dies.

 

[JackFlorey]

bike sw102 display: 56.7v
Battery charge port (not connect to charger): 57.12v
Battery main output port (not connect to charger): 56.97v . I'm expecting this difference to come from using my on/off switch**

Charger alone: 58.49v
Battery w/ charger : 58.5v
BBS02 controller only (no battery connected): 7.35v. I thought this was interesting. I wonder how this might affect ideal storage.

OK. 57V is a little low (4.08 volts per cell)_but they may be being very careful with the pack. Lower voltages=less capacity but longer life.

The next time you charge you may want to connect the meter so you can watch it and verify that it really does rise fairly smoothly. If there's a jump, then the BMS may be cutting in to protect an unbalanced cell. You can also watch current; it should start out at the charger's CC limit (a few amps) then taper off to close to zero. Again if there's a sudden drop to zero you may be seeing the BMS cutting off charge.

That's about all you can do without opening the pack.

 

[E-HP]

Battery charge port (not connect to charger): 57.12v
Battery main output port (not connect to charger): 56.97v . I'm expecting this difference to come from using my on/off switch**

Charger alone: 58.49v
Battery w/ charger : 58.5v

Your battery might be way out of balance. If your charger is only putting out 58.49V, then the battery may never balance, since the cells (at least one group) would need to hit 58.8V before it starts the process. You can adjust the voltage on most chargers (unless it's the brick kind), adjusting a potentiometer inside, so set it so it outputs the correct voltage. You may need to leave the battery on the charger for a long time to get it balanced.

 

[ronaldvidnphoto]

Your battery might be way out of balance. If your charger is only putting out 58.49V, then the battery may never balance, since the cells (at least one group) would need to hit 58.8V before it starts the process. You can adjust the voltage on most chargers (unless it's the brick kind), adjusting a potentiometer inside, so set it so it outputs the correct voltage. You may need to leave the battery on the charger for a long time to get it balanced.

Thanks for the input. I just reached out to the builder to see if the BMS requires a full cell charge to trigger the balance. Hopefully, this is the issue and I can just get a new charger; the charger I have now is a brick.

 

[ronaldvidnphoto]

OK. 57V is a little low (4.08 volts per cell)_but they may be being very careful with the pack. Lower voltages=less capacity but longer life.

The next time you charge you may want to connect the meter so you can watch it and verify that it really does rise fairly smoothly. If there's a jump, then the BMS may be cutting in to protect an unbalanced cell. You can also watch current; it should start out at the charger's CC limit (a few amps) then taper off to close to zero. Again if there's a sudden drop to zero you may be seeing the BMS cutting off charge.

That's about all you can do without opening the pack.

Thanks again for the input. I just reached out to the builder to see if the BMS requires a full cell charge to trigger the balance as @E-HP noted my charger might not be pushing the cells high enough to trigger the BMS' balancing feature. I'm still planning to ride this weekend and will be able to monitor the charge cycle.