Please help me find the gremlin : /

I'm relatively new to this and finished my first build a few months ago. My bike is acting strange and I need help. I'm relying on my ebike for transportation and it has left me stranded twice now by shutting off suddenly when it shouldn't.

My setup (all bone stock, no mods)
Fat tire mountain bike, 7 speed with 42 tooth bling ring
Bafang BBSHD
48V 20ah triangle pack from Luna
New color display, DPC-14

I ride about 14 miles to work, and pretty much full throttle the whole way from the south bay to century city. Some mild hills but nothing major. So I go to work on a full charge, get there no problem. Top it off at the office and ride home and it just shuts down after 11 miles or so. I was keeping an eye on everything, voltage was decent still, 46.5 and the meter on the battery read 2 of 7 bars left which used to be no problem. Put a meter on the battery and the voltage was true. Well the screen voltage reading starts to drop slowly until it gets to about 39V reading or so. I swapped out the screen in the field and the other reads no juiced either (DP-10, different model even). I let it sit and watched for a few hours and the voltage started to climb back up on the screen slowly.

WTF?

The last time it did this, I limped the damn thing home and immediately put it on the charger to see what would happen. The screed instantly shows the true voltage as when I put a meter on it and the display on the battery.

Thanks for any help in advance.

Brett in El Segundo

rockwalk said:

Well the screen voltage reading starts to drop slowly until it gets to about 39V reading or so.

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This probably means the BMS shut off pack output to protect the pack from damage. Probably the cells are not balanced so a cell dropped below LVC under load.

Leave the pack on the charger for as long as you can (from the moment you can first plug it in until the moment you *must* unplug it to ride off, at home and at your destinations), every time you charge it, and eventually it should recover it's balance.

This sort of problem usually happens from frequently running a pack down to near or at it's LVC (using up most or all of it's capacity), when cells are not all identical and matched (most packs no matter where they are from are probably not built with well-matched cells, and even if they are, as they age they will do so differently for each cell).

The problem can be exacerbated if you are running the pack at a high current output vs it's cells' actual capabilities. You'd have to check with the manufacturer for that info.


If it is not the battery's BMS cutting off at cell-level LVC, then it is the controller cutting off as the pack dips below the controller's LVC under load.

I go to work on a full charge --- Top it off at the office and ride home

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I see three main things that could be different.
The way the charging happens, the outside temperature, or more stop and go traffic on the trip home.

What is not the same between the two trips ?

EDIT:
Add this:
There could also be a near imperceptible gradient on a long commute such that the rider may not realize it's actually downhill one way and uphill the other.

seconded on checking cell balance. is the pack sealed up or can you read the per cell voltage? if you can check per cell, ensure that they are all the same. mine are 4.21 4.20 4.19 4.20, etc, when i'm done charging, and 3.44 3.45 3.43 3.44 etc when I'm done. This is a pretty good sign that your not over stressing the cells and are charging them up till they are balanced. as others suggested, you likely have 3.55, 3.55, 3.1 and that one cell is tripping low voltage cutoff/dragging the pack down.

How man AH is the pack and how many AH do you use on the ride in?

Thanks fellas. The pack is a 48V 20Ah from Luna, only a few months old. The balance cell issue makes much sense. My ride on the way home is actually more downhill but either way I dont think there is enough hills to make a difference anyway. the majority of the ride is along the beach and flat. The pack is sealed so I can't check each cell easily. I'll make sure to charge 100% before I take it out from now on and see if that does it. Really appreciate you guys taking the time. B.

I can't measure the Ah used but it's over half of capacity for sure. I'd guess somewhere around 13 maybe if its truly a 20Ah. I guess I should probably get something to measure that.

That pack should give 28 miles or more. Battery bms tripping early. Best to wtite down voltage right after bms shutting down like.
1. 3.95
2. 4.15
3. 3.88

14. Xxx volts

Charge battery to 58 volts this will start the balancing process. Write down voltage. Share info. Bms should bleed down high cells over time, charger should turn back on to fill up low cells. Then the bms will drain all the next high cell till low cell # 14 is filled. Time on charger should work. Balance is the goal. Voltage and capacity.

What are my options for measuring the cells individually seeing how it's a sealed up triangle pank?

Oh. Also, I have a luna smart charger that will only go to 54.5v so I don't know about getting to 58v. That won't hurt the battery?

rockwalk said:

Oh. Also, I have a luna smart charger that will only go to 54.5v so I don't know about getting to 58v. That won't hurt the battery?

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Zip meant 54.6V, which is correct for a 13s 48V battery like the one you have - don't charge to 58V. Just leave it on the charger overnight (in a safe place) a few times to let the BMS balance the pack.

roger that, thanks again everybody!

hey rockwalk - were you able to solve your issue ? - I'm having same problem

After reading your post I kept my brand new 48V 21A Q battery on charge at 100% for a full 28 hrs in the hope it would re-balance cells ..- I then did another test ride hoping it had sorted itself out but no joy, .. still died again suddenly at 51V after about 10 mins / 5km of easy riding.
My guess is its a dodgy BMS - Im speaking to eric at luna about it and were just doing a couple of other tests to isolate the issue.

Hey Grover, I haven't taken it on the long ride since the last time. Will take it to work tomorrow and report back. I've been hitting it with full charges everyday so we'll see.

14.5 mile ride @ full throttle 30A for 85% of the trip went okay. Low voltage under load at the end was 44.1 or something close and no shutoff.

Giving it a full charge now and won't leave until the charger shuts off (54.5V).

just wanted to get back to this one last time and say that everything looks pretty good now after a week of full charging so it was probably a cell at LVC. BMS not shutting off like before. Thanks to everyone for the help!

The pack only balances at the end of the charge cycle, and not by much. Once the charger shows full, there is no current, so nothing will be happening. If the cells were really unbalanced, it may take many repeated charge cycles to get it balanced again. They can be very shallow discharge cycles to speed things up. Fully charge until the charger shuts off, wait a minute or so, run the motor for a minute or so, then repeat charging.