2000w controller on mxus 3kT @72v tripping BMS Low voltage

[6moe66]

I've a spare cheap Chinese controller and decided to try it out on my mxus 3KT hub motor but it keeps tripping my BMS due to Low voltage.

My battery is a custom 20S6P 72v with 35E cells and 60A BMS.

The motor runs fine off load and very little voltage drop but as soon as I'm on it, it cuts out after 2-3seconds where the voltage drops really quick.

Any ideas?

 

[markz]

Because your loading up the battery load. The more weight, the more voltage sag especially when you have a weak group of cells.
How old is the battery?
How many charge/discharge cycles does it have on it?
Did you store it out in the freezing cold, or dropped the bike in a lake.

Take a voltage multi meter and open up the battery and see what the voltages are when the battery is empty. Take the bike, lift the wheels up and tape the throttle so it uses up the battery until LVC kicks in and turns off the battery. Measure the battery groups. When battery is low is when the imbalance is most extreme.

Another trick is to hook up some cheap 6S RC lipo voltage checkers with enough wire to see when riding, and start checking each group.

Another trick is to unhook the BMS and see if it does the same thing. But you dont want to kill the already weak battery cell groups. You could easily drain a weak group to well beyond 3.20V and you wouldnt even know it until you measure it.

I've a spare cheap Chinese controller and decided to try it out on my mxus 3KT hub motor but it keeps tripping my BMS due to Low voltage.

My battery is a custom 20S6P 72v with 35E cells and 60A BMS.

The motor runs fine off load and very little voltage drop but as soon as I'm on it, it cuts out after 2-3seconds where the voltage drops really quick.

Any ideas?

 

[E-HP]

My battery is a custom 20S6P 72v with 35E cells and 60A BMS.

Seems like the pack is undersized for your motor, and your BMS is oversized for your pack. If you can only fit 6P, then maybe use 30Q cells.

35E are rated 8A continuous, and the BMS shouldn't be sized greater than 6P can deliver continuously (48A) or you are basically killing your cells. The BMS will also allow a lot more current to flow from the pack in bursts, further killing the cells. A 30A BMS might be better choice. It will then trip from current, and protect your cells.

I have 8P in my pack using those cells. And with a 40A BMS, and the CA has registered 110A, off the line, without tripping, but it will sag a few volts. You have a bigger motor, and with only 6P, you need higher current cells. 30Q or 25R.

 

[6moe66]

Thanks,

I run a Kelly KLS72120s with this set up no problems and without pushing settings on the Kelly, i will see bursts of >6000W so i find it hard to believe that my pack is undersized for a 1500-2000W cheap controller?

My workings are as follows-
72v 21Ah 20s6p -18650 35E cells x120 -60Amp BMS
8Amp continuous discharge cells- 48A
@72v=3456W @84v=4032W
15Amp peak 90A
@72v= 6048W @84v=7560W

I see your point with the BMS and it is a good shout. I was looking at a smart BMS and replacing so would you say a 30A would be more suitable i was more thinking 40A but i am no expert?

It does feel like its all or nothing with the throttle but i am just guessing as that is the squarewave instead of the Kellys smooth sinewave? By Pack has maybe been charged 10 times but is still performing very well with my Kelly setup which is why i find it strange.

 

[E-HP]

It does feel like its all or nothing with the throttle but i am just guessing as that is the squarewave instead of the Kellys smooth sinewave? By Pack has maybe been charged 10 times but is still performing very well with my Kelly setup which is why i find it strange.

This seems to be one of the biggest variables when it comes to how controllers respond at higher voltages. Mine is virtually uncontrollable unless it's run the throttle signal through my cycle analyst to temper it, otherwise it's way too twitchy. That may be the reason you're tripping, since it's allowing way more current to flow almost instantaneously when you apply more throttle.

6P of 35E can provide 6kW in bursts, when the pack is healthy, but not continuously, so the more you do it, the more the pack will degrade. So by undersized, I mean for the longevity of your pack.

 

[6moe66]

It does feel like its all or nothing with the throttle but i am just guessing as that is the squarewave instead of the Kellys smooth sinewave? By Pack has maybe been charged 10 times but is still performing very well with my Kelly setup which is why i find it strange.

This seems to be one of the biggest variables when it comes to how controllers respond at higher voltages. Mine is virtually uncontrollable unless it's run the throttle signal through my cycle analyst to temper it, otherwise it's way too twitchy. That may be the reason you're tripping, since it's allowing way more current to flow almost instantaneously when you apply more throttle.

Ah ok, any thoughts on how to make this smoother (I have No CA) I do have a 750C which i use for the Kelly but dont know if i can interface this to the cheap controller? An in-line resistor on the throttle perhaps to help? The controller does have a 3 speed switch option and i use that with the Kelly but im not sure if that is current or speed limting with this controller? I only want to have this controller as a spare and don't mind it slow or with a current limit, just to put in my bag as a hot swap option if im out and about. I dont need a lot of power from it, just a reliable 'get me home'.

[/quote]
6P of 35E can provide 6kW in bursts, when the pack is healthy, but not continuously, so the more you do it, the more the pack will degrade. So by undersized, I mean for the longevity of your pack.
[/quote]

I limit current on the Kelly and have the 3Speed switch which works well for my mixed style riding. The Kelly has way more power than i can give it with my current battery but once set up its been great and has the potential for use with my other/future builds. My previous builds have all been choked by the controllers and it finally feels good now to not be maxing settings and soldering shunts. Just find this a really weird issue where i have a lesser rated 2000W (27Aish @ 72V which should be well within my batteries capabilities) controller that trips my BMS