18650 Configuration - 24S20P or 20S24P?

[mitchm33]

I am about to build a pack for a E motorcycle build. I have 500 Samsung 30Q's and a Zeva BMS capable of 24S. I was just about to start laying out a 20S24P configuration when it hit me that I may want to switch it. Both my motor and controller can handle the voltage/current with either configuration (72-10V).

Is my understanding correct that in either configuration, the overall power is the same but the only real advantage of a higher voltage would be a need for less current in the system? And the sacrifice would be a smaller Ah pack?

Thanks in advance

 

[e-beach]

Depends on your overall system, but in general volts = speed and amps = torque. So if you ride on a flat terrain, with little wind and little stop and go, then going with a higher voltage might be advantageous. If your terrain is hilly or maybe more windy, torque might be what you want.

Where do you live and what are you normal riding conditions?

 

[mitchm33]

Okay good to know. I live in Nashville and am converting a 1970 Honda cb450 for a city bike to get me to and from work. I'm looking for it to be as quick as possible at low speeds considering most of my riding will be sub 50MPH. Sounds like I want to stick with my original plan, 20S24P. Thanks for the comments!

 

[e-beach]

Getting an electric vehicle moving is where you use your highest amps unless you are going up ah hill that challenges your maximum
amperage you controller and battery pack can deliver. So if you are in mostly city/stop-and-go traffic it is advantages to have enough amps in your controller and battery pack to get your bike going as quickly as possible to get moving. So if 72v is enough voltage for your motor to get you moving fast enough, and, you can pull a lot of amps, you will get up to speed more quickly.

 

[Smoke]

Going by the above, if volts=speed and amps=torque, you just need to figure out if you are more likely to run out of speed or torque first.

If your batteries will do 5 amps output (a conservative number hopefully), you are choosing between 100 or 120 amps which is quite a bit. If your batteries will do more, you will likely have more than your motor can use.

With those choices, my guess is that 24s20p will give you plenty of torque and more speed but it depends on your motor, controller and gearing as well, if one of those is too far out of whack, you will leave performance on the table.

 

[mitchm33]

Motor is ME1616 which can handle 250A continuous and controller is Seven Gen4 size 6 which can handle 225A continuous. Cells are Samsung 30Q which can handle 15A continuous. So I am looking at 300A or 360A continuous. The problem I am running into is this is my first build so I'm not sure how to figure out if I'm going to run out of speed (volts) or torque (amps) first.. I understand the general system well but motors specs tend to confuse me.

 

[Smoke]

Well, 300 amps and 96 volts is roughly 20 hp, that's motorcycle territory so I doubt you will have trouble with torque off the line unless your gearing is way too steep.

If I was doing this, I would pick a top speed, figure out what gearing it would take with 24s and then see if it has enough torque at that rpm to get you there. After that look at zero rpm to see if you will have enough amps for good acceleration. Do it again for 20s, then play around with gearing if you want to improve performance by finding a nice trade-off of top speed, cruising efficiency and acceleration.

From the sound of it, I think you will have a fast bike.

 

[kcuf]

24s full charge very close to most component maximums

likely small component headroom @ 100v full charge

20s may leave more component tolerance headroom

many of us prefer such headroom

motor speed/torque really depends on winding used wheel diameter etc

 

[e-beach]

The amp draw doesn't change with a normal voltage drop. As you battery's get used (usage, age, heat,etc) you will start to notice more sag, but that will take a while if all your cells are new. So the answer is the voltage in a new battery will drop but not the amp draw. In other words, your battery pack will be near LVC but the discharge current will be the same.

 

[mitchm33]

Thanks for all of the responses guys. Its true that 100V is right at my component maxims and I agree that this bike should be fast in either configuration, I just want to try to make the optimal pack if I'm going through the trouble

E-beach: I understand what you are saying but even if you have 20S or 24S each of these configurations will have the same percentage of voltage sag right? What exactly are you suggesting?

 

[e-beach]

....E-beach: I understand what you are saying but even if you have 20S or 24S each of these configurations will have the same percentage of voltage sag right? What exactly are you suggesting?

Your cells new will be able to deliver a certain amount of current, say, 15a continuous. As they get used over time, age, get hot (or not) etc, they will be able do deliver less current. At some point you will feel the difference. As a percentage, who knows, except to say that a smaller battery will sag more then a larger battery under the same load provided the cells are all equivalent.

As far as running out of volts or amps first, your cells will not run out of their 15a continuous rating for a while. That will remain constant. You will have a voltage drop in your battery pack as you use it.

Are you clear on the difference between the C rating on a cell and the amount of amps hours (watt hours) in a battery pack?