How to choose a battery pack.

[wesnewell]

Motor wattage has absolutely nothing to do with choosing a battery pack. So anytime you see an ad that says good for up xxxxW motor, it's just BS. The only thing that matters is device that connects directly to the battery. For any brushless motor that will be the motor controller. Period.

In a brushless motor kit, the rated motor voltage means nothing. A 36V rated motor will happily run on 12V-100V. You need to match the battery pack to the controller you have. To determine what pack to choose, you need to know the controllers max amp draw, max voltage supported, LVC, and HVC if it has one. As an example, I'm going to use a typical controller that comes in a 48V 1000W kit.Max amp draw will usually be 25-30A. Max voltage supported will usually be 63V. LVC will typically be 42V, and if it has a HVC, it will likely be 60V. Assumig a 30A max controller, the pack you choose should be able to provide a minimum of a 30A continuous output, but the more the better as it will strain the battery less. And that's all you really need to know. Be advised that almost all batteries amp rating is likely over stated a lot, so choosing a battery with at least double the max rating is a good idea if you want it to last a long time.
So how does one figure how many amps a battery pack is rated for. Multiply the max discharge C rate times the ah rating of the pack. As an example,a 10ah 20C pack would be rated for 200A.
Questions?

 

[Arethosemyfeet]

Some useful things I didn't know there, thank you. One thing that's making me scratch my head: surely if your battery can only supply up to a certain current, and uses a particular voltage to make that available, then that limits the maximum power of the motor that can be used with it via P=IV? Obviously the practical upper limit would be lower than that for efficiency reasons.

 

[999zip999]

I see a lot of problems from people using a controller that is over matched for the battery pack being used. Like a Ping 48v 20 ah 1,200watt rated pack with a 40amp controller. That's 2,000 watt's on a battery Ping rates at 1,200watts. A battery should have a safety margin and not be used at the rated limit.

 

[dogman dan]

I think a good rule of thumb is to take controller max amps, and then use a battery that is claimed to be able to handle 5-10 amps more than that, or in general 25% more amps.

So a 20- 25 amps controller and 30 amps rated battery is close enough to that,,, or a 30 amps controller with a 40 amps battery.

Amps, not amp hours. Look for the rating, "max continuous discharge" in amps.

REALLY generalizing, for the typical affordable "bike battery" a "1000w" kit which could have a 30 amps controller, needs at least a 15 amp hour size battery. They can usually handle 2c discharge rate, which is 30 amps for a 15 ah pack.

If the controller is only 22 amps, then a 10 ah battery can handle it, but likely not be as long lasting as a larger pack.

 

[kje]

I'm going to make my own battery pack for this motor:
48V  750W BBS02 Bafang Central Motor, Current limit: 25A

How big battery pack is needed?

 

[wesnewell]

Assuming the controller is 25A max draw, then a pack that will match or exceed that. The size will depend on how far you want to go and at what speed. A typical bike will use ~20wh per mile at 20 mph. ~40wh per mile at 30 mph, etc. So if you want to go 30 miles on a charge at 20 mph, you need a minimum of 600wh. That's with no wind and level ground. Increase the size per the terrain you plan to ride. For that distance I wouldn't go less than a 800wh pack. Nominal voltage times AH = WH. IOW's a 10ah 48V pack is 480wh.

 

[LockH]

Watt he said! (above) Can add, that "size"/"capacity" may mean "weight"? Where power required to move any mass go up w/accelerations, hill climbs, headwinds, etc. My distances tend to be "split" into short-medium and long ranges so I would hope to split the pack also. Parallel connect multiple packs (add more weight) when longer distances are desired. Multiple "short range" packs means "super fast charging" by just swapping one spent ("small"/short range-sized) pack for another "freshie"/fully recharged pack. Analogous to multiple batteries for any battery-electric hand tool.

 

[dogman dan]

Whatever size is specified to produce 25% more than 25 amps, or at least some headroom. So look for 30-35 amps continuous discharge in the battery specs.

What size that is, will depend entirely on the cells used. A 3c rated cell could do it with as small as a 10 ah pack. 10 ah at 3c is 30 amps. A 3c rated 15 ah pack would definitely be big enough, with 45 amps continuous discharge rating.

Or, looking at it another way, a 15 ah pack will get you a very usable bike, with little range anxiety on most trips. A 10 ah might be enough, but lead to worrying about it a lot.