Effect of battery C rating on performance

dario_ds

1 mW
Joined
Oct 7, 2022
Messages
17
Hi, I am trying to understand the effect of the "C" rating of the battery cells on the performance of of the motor.
My queries :
I have a battery pack of 15ahr. My motor draws 12A at full throttle. What would be the effect on top speed if the battery is 0.5C or 1C? The answer I got on google is that 0.5C would discharge in 2 hours with a load of 7.5amps and in 1 hour with 15A load but no answer to my query. Please shed some light.
 
dario_ds said:
Hi, I am trying to understand the effect of the "C" rating of the battery cells on the performance of of the motor.
My queries :
I have a battery pack of 15ahr. My motor draws 12A at full throttle. What would be the effect on top speed if the battery is 0.5C or 1C? The answer I got on google is that 0.5C would discharge in 2 hours with a load of 7.5amps and in 1 hour with 15A load but no answer to my query. Please shed some light.

The C rating is simply another way of describing how much current (amps) a battery, or battery pack, can discharge. The rating is relative to the capacity (Ah) of the battery, or battery pack. So, for your 15Ah battery pack, if the cells were rated at 1C, then that implies the current the pack could provide is15A. If the cells were rated at 0.5C, then the pack could only provide 7.5A. You could just as easily describe the battery or battery pack as being able to provide 7.5A and not refer to the C rating at all.

The C rating won't affect speed, since it's related to current, and voltage is what dictates an electric motor's speed by itself. The current could provide more torque in cases where speed may be limited by wind resistance or terrain/grade where torque is required to overcome that additional load.

Regarding pack capacity, that in your case is 15Ah. By definition, 15Ah means you can discharge that pack at the rate of 15A (1C) for 1 hour (hence 15 Ah) before the pack is depleted. So, at 0.5C, or 7.5A it would take 2 hours.
 
Ebike batteries are not generally rated by a C number. Sellers will usually only give voltage and amp-hours. Rarely do they even mention the current rating of the battery. Good vendors, of course, will tell you this number,

If the battery uses name brand cells, where you can look up the cell AH and C rating, you can figure out the battery's current rating from the number of cells. For example, LG MH1 cells are good cells for an ebike, A 48V14AH battery would use a 13x4 array of these cells, The cells are rated at 3500 mah and rated for 10A, so the C rating is around 3. The battery would have a max current rating of 40A with four cells in a parallel group, but it would be best paired with a 20-25A controller for battery longevity.
 
E-HP said:
dario_ds said:
Hi, I am trying to understand the effect of the "C" rating of the battery cells on the performance of of the motor.
My queries :
I have a battery pack of 15ahr. My motor draws 12A at full throttle. What would be the effect on top speed if the battery is 0.5C or 1C? The answer I got on google is that 0.5C would discharge in 2 hours with a load of 7.5amps and in 1 hour with 15A load but no answer to my query. Please shed some light.

The C rating is simply another way of describing how much current (amps) a battery, or battery pack, can discharge. The rating is relative to the capacity (Ah) of the battery, or battery pack. So, for your 15Ah battery pack, if the cells were rated at 1C, then that implies the current the pack could provide is15A. If the cells were rated at 0.5C, then the pack could only provide 7.5A. You could just as easily describe the battery or battery pack as being able to provide 7.5A and not refer to the C rating at all.

The C rating won't affect speed, since it's related to current, and voltage is what dictates an electric motor's speed by itself. The current could provide more torque in cases where speed may be limited by wind resistance or terrain/grade where torque is required to overcome that additional load.

Regarding pack capacity, that in your case is 15Ah. By definition, 15Ah means you can discharge that pack at the rate of 15A (1C) for 1 hour (hence 15 Ah) before the pack is depleted. So, at 0.5C, or 7.5A it would take 2 hours.

Does that mean that for a 350w motor rated at 36v will give the same top speed for the battery of 7.5ahr and 15ahr, both 1C rated? Theoretically the current consumption for 350w motor rated at 36v is 350/36=9.7Amps. Wouldn't the motor be starving at full throttle on a 7.5ahr battery and thus drop the voltage hence reducing the speed ?
 
dario_ds said:
Does that mean that for a 350w motor rated at 36v will give the same top speed for the battery of 7.5ahr and 15ahr, both 1C rated? Theoretically the current consumption for 350w motor rated at 36v is 350/36=9.7Amps.

Motors aren't rated for volts, they are rated for power and speed. Motor speed characteristics are rated in kV, which is a rating of rpm per volt. At 1C, the 7.5Ah battery can deliver 7.5A and the 15Ah battery can deliver 15A. The voltage is the same. The rated no load top speed is the same. The power characteristics determine how long a motor can run at that power level continuously/forever. You can exceed that 350w power level by a lot, if you don't do it too long, but if you exceed it too long, the heat of the motor goes up, and will eventually overheat, and windings meltdown.

If you lift the wheel off the ground and apply full throttle with either battery, the top speed will be the same (based on the rated kV) and current will be very low. When you apply load/resistance, when riding the bike, then more power is consumed since rolling resistance and wind resistance become a factor, and wind resistance increases with speed, requiring more current/power to overcome. The 15A battery can provide more current to overcome that resistance to attain a higher speed if wind resistance becomes the limiting factor..

dario_ds said:
Wouldn't the motor be starving at full throttle on a 7.5ahr battery and thus drop the voltage hence reducing the speed ?
I don't follow your voltage drop conclusion. Maybe someone else can make sense of it.
 
I suppose Dario is picking up on a scenario where a too-small or under-rated battery will experience relatively greater voltage sag when pressed to its limit, thereby dropping the top speed (because the voltage is lower), or be incapable of delivering all the needed amps.

This scenario does not apply to Dario because Their bike is 350W and probably tops out around 20mph. Let’s say a bike is wound to go down the road at 30mph, and the rider is a larger-framed person. It may take 800 or 1000W to carry that person at 30mph. If the battery can’t give the 20-odd amps continuously, the top speed will be impacted. In this situation, the battery is the wrong fit and is being mega-abused.

On a 350W 36V ebike, you’re looking at 10Amps continuous to get the job done. That’s not hard to come up with, and there are many options that will provide good headroom for that power need.

If a 7.5Ah battery and 15Ah battery can both provide 10Amps continuous, then they will both let a 350W motor reach top speed.
 
From-A-To-B said:
I suppose Dario is picking up on a scenario where a too-small or under-rated battery will experience relatively greater voltage sag when pressed to its limit, thereby dropping the top speed (because the voltage is lower), or be incapable of delivering all the needed amps.

This scenario does not apply to Dario because Their bike is 350W and probably tops out around 20mph. Let’s say a bike is wound to go down the road at 30mph, and the rider is a larger-framed person. It may take 800 or 1000W to carry that person at 30mph. If the battery can’t give the 20-odd amps continuously, the top speed will be impacted. In this situation, the battery is the wrong fit and is being mega-abused.

On a 350W 36V ebike, you’re looking at 10Amps continuous to get the job done. That’s not hard to come up with, and there are many options that will provide good headroom for that power need.

If a 7.5Ah battery and 15Ah battery can both provide 10Amps continuous, then they will both let a 350W motor reach top speed.

Agreed, except to draw any conclusions, you have to fill in the blanks with assumptions for the info not provided. If provided the types of cells comprising the packs, for instance, the discharge curves would allow the sag to be estimated, for one pack pulling 1C, and the other pulling 0.5C for supplying the same current. I'm assuming no sag unless more info indicates otherwise.

If the kV of the motor is such that the bike at 36V tops out at 15mph, for example, then both batteries will perform the same. That assumes that in most cases, on flat ground, one can go about 16-18mph on 250W. Since the smaller 7.5A battery can supply 270W, the current limit of the battery won't limit the bike from achieving its top speed.
 
Agreeed! And thanks for drawing that out further. I was taking a leap and trying to suss out where Dario was coming from.

I think we’ve shown that either the 7.5 or 15Ah battery will likely suffice. I just hope the OP invests money in a quality battery. Talks of a 0.5C-capable ebike battery make me squeamish.
 
E-HP said:
dario_ds said:
Does that mean that for a 350w motor rated at 36v will give the same top speed for the battery of 7.5ahr and 15ahr, both 1C rated? Theoretically the current consumption for 350w motor rated at 36v is 350/36=9.7Amps.

Motors aren't rated for volts, they are rated for power and speed. Motor speed characteristics are rated in kV, which is a rating of rpm per volt. At 1C, the 7.5Ah battery can deliver 7.5A and the 15Ah battery can deliver 15A. The voltage is the same. The rated no load top speed is the same. The power characteristics determine how long a motor can run at that power level continuously/forever. You can exceed that 350w power level by a lot, if you don't do it too long, but if you exceed it too long, the heat of the motor goes up, and will eventually overheat, and windings meltdown.

If you lift the wheel off the ground and apply full throttle with either battery, the top speed will be the same (based on the rated kV) and current will be very low. When you apply load/resistance, when riding the bike, then more power is consumed since rolling resistance and wind resistance become a factor, and wind resistance increases with speed, requiring more current/power to overcome. The 15A battery can provide more current to overcome that resistance to attain a higher speed if wind resistance becomes the limiting factor..

dario_ds said:
Wouldn't the motor be starving at full throttle on a 7.5ahr battery and thus drop the voltage hence reducing the speed ?
I don't follow your voltage drop conclusion. Maybe someone else can make sense of it.
In India, atleast upto where I can reach out to buy a hub motor, the available motors are Chinese ones and the only specifications provided is is wattage and voltage like 250w @36v or 350w @36v. The 250w motor is claimed to provide a top speed of around 25 kmph on 26 inch wheel and 350w around 35 kmph. I have installed a battery pack of 15ahr 36v and the geared hub motor is 350w watts. The top speed on flat road is 26 to 27 kmph. The controller is rated at 17 amps limiting. The low top speed made me think the battery is 0.5c rated. I weight 63 kg and have installed the hub motor in a 700c cycle that weighed 14 kg.
 
To make that go faster typically you'd upgrade voltage. E.g. make sure your controller supports 48V as well as 36V or get a different controller and then use a higher voltage battery. Motors typically can run higher voltage and wattage than specified, although you can check if it is hot after a run to see if you are pushing it.

A small number of controllers support overdrive/field weakening that can also get you a small top speed boost over the normal amount of rpm the motor produces at a set voltage.

Re determining if current matters in your case, I guess you can lift the wheel off the ground and run at full throttle? If speed is higher without load, maybe more current would help? I typically set my controller's current limit to whatever the max current limit for my battery's BMS is, as the BMS cuts off discharge after that or at high temperatures anyway. So I've never played around much with current limits.

If I set the current limit higher than the BMS limit typically the BMS shuts off discharge as soon as I try to ride up a big hill at full throttle. So I consider current limits more for hill climbing.
 
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