72V Lipo Build 70C with 21S?

Show off your E-bike creation here.
Post Reply
nop   10 µW

10 µW
Posts: 5
Joined: Jun 15 2020 6:10am

72V Lipo Build 70C with 21S?

Post by nop » Jun 29 2020 4:07pm

I've decided to give up looking for the more common 4and 5S packs that are pretty much unavailable at rational prices. I plan a 2P using 3-6S with 1-3S which are more available and either disconnect or remove one of the cells for 20 in total. Or, is it possible to run 21S without damaging the BAC 855 controller for the CYC X1. Suggestions please.

User avatar
amberwolf   100 GW

100 GW
Posts: 30469
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: 72V Lipo Build 70C with 21S?

Post by amberwolf » Jun 29 2020 5:20pm

nop wrote:
Jun 29 2020 4:07pm
Or, is it possible to run 21S without damaging the BAC 855 controller for the CYC X1.
What is the voltage you will charge the pack to?

What is the max operating voltage of the controller?

That's what you need to know to answer the question.

If you use 4.2v/cell then 21s is 88.2v. If you use 20s, then that's 84v. If you use a lower max per cell voltage, then it'll be lower total voltage. So then you just need to know the controller spec. ;)

nop   10 µW

10 µW
Posts: 5
Joined: Jun 15 2020 6:10am

Re: 72V Lipo Build 70C with 21S?

Post by nop » Jun 29 2020 8:56pm

The controller is rated for 36-72V. So if I went with the 21s the max charge would be 4V per cell... seems too low of a charge. Probably better to stay at 20?

User avatar
amberwolf   100 GW

100 GW
Posts: 30469
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: 72V Lipo Build 70C with 21S?

Post by amberwolf » Jun 30 2020 5:38am

nop wrote:
Jun 29 2020 8:56pm
The controller is rated for 36-72V.
So 72v is the max voltage? If so, then you can't even use 20s. 72v / 4.2v = 17.something, so 17s is the max you can use for a controller with an absolute max of 72v. ;)

Remember, the question is:
What is the max operating voltage of the controller?
You need to know the actual number for this to be able to answer your original question. Knowing what basic pack range it's made for doesn't help answer your question:
is it possible to run 21S without damaging the BAC 855 controller

Tony01   10 W

10 W
Posts: 98
Joined: Nov 12 2015 2:24am
Location: San Jose, CA

Re: 72V Lipo Build 70C with 21S?

Post by Tony01 » Jul 08 2020 12:08am

nop wrote:
Jun 29 2020 4:07pm
I've decided to give up looking for the more common 4and 5S packs that are pretty much unavailable at rational prices. I plan a 2P using 3-6S with 1-3S which are more available and either disconnect or remove one of the cells for 20 in total. Or, is it possible to run 21S without damaging the BAC 855 controller for the CYC X1. Suggestions please.
You won't damage it but if you charge to 88v and don't login to the controller to adjust it then it will permanently fault into overvoltage, ie your bike will not run until you discharge the battery back down to 84v or whatever the cutoff is set at.

20s is best. I'm going back from 21s myself.

Also you don't need no 70c discharge batteries. Those types of cells are for drag racing. For actual riding you are looking for 5-15c. With the compromises in lithium cells you will get a low capacity high discharge pack that you will never use more than 15c of.
I don't always listen to Pantera.... but when I do, SO DO THE NEIGHBORS :D

john61ct   100 GW

100 GW
Posts: 5442
Joined: Dec 18 2018 2:06pm

Re: 72V Lipo Build 70C with 21S?

Post by john61ct » Jul 09 2020 9:13am


Tony01 wrote:is it possible to run 21S without damaging the BAC 855 controller for the CYC X1?
You won't damage it but if you charge to 88v[/quote]What do you base that on?

Do you know the internal components are rated to go that high, IOW the 72V max rating is just nominal? Even then wouldn't that target 20S?


User avatar
E-HP   100 kW

100 kW
Posts: 1244
Joined: Nov 01 2018 9:20pm

Re: 72V Lipo Build 70C with 21S?

Post by E-HP » Jul 09 2020 10:09am

nop wrote:
Jun 29 2020 8:56pm
The controller is rated for 36-72V. So if I went with the 21s the max charge would be 4V per cell... seems too low of a charge. Probably better to stay at 20?
That doesn't sound like the actual rating, since there are no electronic components that are rated for 72V (caps, FETs). It's more likely that the controller is designed to work with 36V-72V packs/systems, which would assume an 84V full charge, or 27V-30V on the low end (for a 36V pack).

Post Reply