72v 3000w BLDC motor kit and 36v batteries in series problem

Numanang

New Member
Region
USA
Hello Everyone! I am a new member and glad to be here. I’ll keep my query short...I have a project I am working on using a 72v 3000w BLDC brushless motor kit and matching controller. I have (2) 1000w 36v 20AH battery packs that I am running in series for power. The issue I am having is once all is hooked up I get throttle response for about 5 seconds and then nothing. I swapped controllers and the same thing happens. After that, 0 response and both controllers seem dead in the water. I searched here for a similar issue, but could not find anything. Any thoughts are much appreciated!
 

harryS

Well-Known Member
Are you getting any voltage out of the batteries? Were they good quality packs? Can they deliver 40-45A?

When you hook two batteries in series, there's a failure mode most people don't consider. A pack can reset if the current exceeded the BMS rating. If one of your 36V packs happens to reset and shut off the BMS, you will get 84 volts (if both packs are charged to their 42V max) across the open circuit MOSFETs in the battery that shut off. You want the BMS in those packs to have 100V transistors.

If both your packs are still able to show 42V on the outputs, the BMS probably didn't blow. But now the controllers won;t power up?

Disclaimer. My biggest controller is 25A, so the 3000W life is not something I personally deal with,
 

Numanang

New Member
Region
USA
Thanks, HarryS. That’s something to consider. I did check ea pack after charging and they were both right at 42v. I am going to see what they are putting out in series, but wouldn’t that be correct having 84v and 1000w in series? Would under voltage be a problem for the 3000w motor/controller combo?
 

BigMatt

Member
Thanks, HarryS. That’s something to consider. I did check ea pack after charging and they were both right at 42v. I am going to see what they are putting out in series, but wouldn’t that be correct having 84v and 1000w in series? Would under voltage be a problem for the 3000w motor/controller combo?

I suspect your 36 volt batteries are limited well below the ~42 AMPs required to power 3000 Watts (72V x 41.7 AMPs ~5,000 Watts). While you double your volts running in series, the AMPs are constrained by the minimum AMPs put out by either of the two batteries. I don’t think there are many 36v batteries out there that put out close to anything like 40 AMPs. I’ve seen one that can’t do 30. A 36v x 40 AMP battery could put out 1,500+ Watts, and that’s not a common way to try to get to 1,500 Watts. Usually a 48v or 72v battery would be built for the purpose of doing that. You may need to swap batteries or perhaps run another smaller 72v battery in parallel to increase continuous AMPs to ~42.

I used a 12 (max continuous) AMP 48v battery (designed for a 500W bike) to test a 48v 1,200 W hub one time and got a similar “crap out” after a few seconds of throttle. The 30 AMP 48v battery I had ordered for the hub was delayed in the mail, so I used it temporarily to make sure I had thinks set up right to get the hub rolling.
 

Numanang

New Member
Region
USA
Hello BigMatt, thanks for that. That makes sense. I had the batts and was hoping to get something out of them even if the motor was not receiving juice at full potential. Looks like I will be using the 36v batts for another application and put the 72v motor set up on the backburner for now.