BMS, Discharge Rate and Controller Question

[knurn]

I built an ebike years ago from a Grubee kit with lead acid batteries. I am now returning to the EV scene with a newer bike and a direct drive 1000 watt Golden Motor that I have had sitting around idle. Choices today are overwhelming - which is actually a good thing
I am starting to look into purchasing a battery pack (lifepo4, 48 V 15 AH). The pack will include a BMS.
I have seen options for the discharge rate as listed below:
20 - 40A (20 continuous discharge current, 40A peak discharge)
30 - 60A
50 - 100A
I understand the motor controller delivers and regulates power to the motor and the BMS protects the battery from operating outside its safe operating area. Is it better to purchase a battery pack with a higher discharge rate or is it necessary to have a match between the BMS, batteries and the controller? I have a direct drive 1000 watt Golden Motor.
Thanks in advance.

 

[dnmun]

they just add more mosfets to increase the current capacity of the BMS. if you only use 1000W then 40A should be ok.

 

[cycborg]

I am starting to look into purchasing a battery pack (lifepo4, 48 V 15 AH). The pack will include a BMS.
I have seen options for the discharge rate as listed below:
20 - 40A (20 continuous discharge current, 40A peak discharge)
30 - 60A
50 - 100A

You've seen all these different specs for 15 Ah LiFePO4 batteries? Something odd there...

Unless the pack is made from A123 cells, you probably want to stick to 1-1.5 C continuous, 2-2.5 C peak, which would be in line with the first set of specs you listed.

The BMS only has a single overcurrent setpoint, which is probably set to something like 3 C. It won't keep you from running the pack at 2.5 C continuously, which will age the pack pretty quickly. So you'll want to be able to monitor your current with a Cycle Analyst or similar, and you may be able to set up your controller to limit battery current if you want safer limits.

If you run the motor at its continuous rating of 1 kW, that's about 20 A from the battery, which is in the safe range. A 35-40 A controller would be a good match for this system.

 

[wesnewell]

For choosing a battery, the only thing that matters is the max amp draw of the controller. Motor wattage doesn't matter one bit. Only the controller draws power from the battery. if you pick a pack with a continuous discharge rating equal to or higher than the max amp draw of the controller, you shouldn't have a problem. That's assuing the the battery specs are true and the maker just didn't put a higher rated bms on cells not really capable of the extra load.

 

[knurn]

Thanks dnum, cycborg and Wes.
Other than needing a charger for a 48v lifePo4 battery pack what other considerations are needed for the charger? How can I determine the maximum amperage to safely charge the battery pack?

 

[knurn]

I will try and answer this myself; please provide feedback.
Since I am looking for a 48v 15 AH LiFePo4 battery pack 1c is 15A so I could safely charge the pack at 15A and the BMS will manage this at the cell level.
Is this correct?

 

[dnmun]

the BMS is not responsible for limiting the rate at which you charge. that is not what they do.

 

[cycborg]

Usual practice is to use a slower rate for charge than discharge. I'd go for 0.5 C max, preferably less. I normally use around 0.2 C. It's possible to do it safely with higher rates, but you need a more sophisticated charge profile than the two-step CC-CV typical of lithium chargers.

 

[wesnewell]

The bms has max charge rates, so it depends on the bms you have. Some limit the charge rate to 5A. Some can go a lot higher. But that doesn't mean it's good for the actual batteries. 5A will charge a 15ah pack in ~3 hours.