enormous loss of max speed / acceleration at around 71 volts.

[rexey64]

Hi, it's my first post here as a registered memeber, even if i've been lurking for a while

setup :
battery : 72v 45 ah ( panasonic 21700 cells, 20s9p) BMS brand unknown, i know it's 100 amps though.
controllers : two 72v/50a 12 mosfets controllers ( those https://fr.aliexpress.com/item/1005001692282834.html?spm=a2g0s.9042311.0.0.c5006c37X7XFib)
motors: two 4000w ( peak i guess, can't find a lot of info on them ) hub motors, on 13 inch tubeless tyres.

i'm having a weird issue that i didn't experience with my last Escooter that had the same specs.
at full battery, the acceleration is actualy insane and the max speed around 75 kph ( idk if that's right for this setup)

now the thing is, i know that as the battery voltage deacreses , less current can be pushed which will decrease acceleration / max speed a bit. that's what i experience until about 73-70 volts IDLE i'd say, but then i get an extremely hard drop in both max speed and acceleration. the acceleration is reduced by a factor of 3 i'd say, and the max speed drops to around 45/50 kph and keep
dropping fast the more i use the battery.

i feel like at this voltage there should be more than enough juice to keep both motors active. in fact that was the case on my last one, the "low battery eco mode" was only activated at something like 3.1v per cell.

maybe it's a controller configuration ? i'm not surre, but it's really anoying.

any ideas ? things i could do to pinpoint the problem ?

edit : posted an update below

 

[spinningmagnets]

I suspect the battery is "sagging". Perhaps one cell has gone bad, and when the rest get to a lower voltage, it's dragging the whole pack down? Hard to say, just a guess...

 

[Manbeer]

what panasonic cells are they? if the ncr21700t aka "tesla" cells, they really cant do much over 10a without sagging and warm up pretty quick, so i would say he could be on to something

 

[rexey64]

Thanks for the answer, I asked the manufacturer which cell it is specifically but no replies yet.

The battery is pretty much brand new.. if there is indeed a bad cell I'd be pissed.

 

[spinningmagnets]

Sparkfun has a decent Digital Multi Meter / DMM for $15. Its good enough when starting out. Having one allows you to check the voltage of each paralleled cell group and then the voltage of each cell in any group that is sagging.

 

[goatman]

could try upping the battery wire gauge and switching to xt90 as

 

[ZeroEm]

As volts go down, amps go up to maintain the same power levels. weak cells don't help and you run in to amp limits. Hate when that happens. You get surging and major power cut backs when hitting amp limits.

 

[E-HP]

i feel like at this voltage there should be more than enough juice to keep both motors active. in fact that was the case on my last one, the "low battery eco mode" was only activated at something like 3.1v per cell.

Is there anything on the display that shows when the eco mode is activated, and does it happen when you have the issue? Sounds like sag like most folks are mentioning.

 

[nicobie]

Seeing as you have the one battery supplying two motor/controller systems, maybe one of the motor/controllers is crapping out upon warm up?

 

[rexey64]

UPDATE : the cells are indeed "tesla" cells, aka those : https://i.ebayimg.com/images/g/KTsAAOSw8~Zf8xZx/s-l400.jpg

after extensive testing, the scooter just loose all power / speed at around 70 volt after i kept pushing it.

here is a video about it : https://imgur.com/a/8kJy4Fh

full throttle, dual motor mode, barely reaching 10 km/h, almost zero acceleration. clearly a sing of very low current, as if i try to spin the wheel freely ( scooter lifted ) they turn very very fast.

however, hitting that low of a current at 70volts isn't normal at all. i owned a 72v scooter with roughtly the same specs ( and same cells ) a while ago, and unless i was at like 3 volts per cell and was super close to the bms cutt off, this never happened.

there is no meaningful voltage sag ( it stays at around 69.5/70v) even full throttle .

does someone have any idea ? could this be caused by a few bad cells hitting a low voltage sonner and triggering something with the bms ?

i know for a fact the controllers are fine since i've tried with another pair. also, the scooter works perfectly at full battery.

 

[j bjork]

Are the controllers programmable?

It sounds like you have a amp limit a few volts before lvc.
There usually are, at least on decent controllers. But it seems like in your case it might be set a little high in the voltage range.
Do you know your lvc? (at what voltage your controllers completely cut off)

 

[j bjork]

No, it cant be your bms or battery if you have stable voltage while accelerating.