I am considering a regenerative braking system implemented with a AC permeant magnet motor with an efficiently of ~96%. It is easy to determine via energy equations the kinetic energy of the car and therefore how much energy is possible at max efficiently but regenerative systems also rely on the brakes of the car to slow it down to a stop. Is there a particular rule or reference to how much conventional braking is required for a given scenario? I wonder if it is possible to determine how much the motor would actually decelerate the car and then work out energies from there. Could anyone offer some advice?
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Some things to take note of.
Battery maximum allowable charge rate is generally much lower than maximum allowable discharge rate. By about 5:1 for batteries I am familiar with. This likely means that you will not be able to decelerate (using regen) at the same rate you can accelerate because the batteries cannot be charged that quickly. If you have a very large battery pack (hours not minutes), then this may not apply. But you should consider it.
Even if you have a large battery pack, the motor is probably not capable of exerting the same reverse torque as a mechanical brake. This means that maximum performance braking will also require mechanical brakes. Emergency stops may not result in much regeneration.
Four wheeled vehicles typically have four-wheel braking. But not all four wheels are usually connected to motors. So, again, this will limit stopping capability under regen (as compared with mechanical brakes). This may not be a concern in normal driving conditions.
When the battery is fully charged, it is not safe to use regen because it will overcharge the battery, unless the power is dumped somewhere besides the battery. (A brake resistor or similar). This could come into play if a journey begins with a full battery at the top of a hill with a winding road leading down.
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\$\begingroup\$ Why would the size of the battery pack affect how quick you can decelerate? (i.e. why would having a large battery pack mean you could decelerate at the same rate as acceleration?) \$\endgroup\$ Commented Mar 18, 2017 at 21:57
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\$\begingroup\$ Let's just take an example that is partially made up, but realistic. Maximum discharge rate of the battery is 10C, but maximum charge rate is 2C. If you use the minimum size battery pack so that it discharges at 10C, then you will only be able to regen at 2C, which is much less. But if you make your battery pack 5x larger, so that the maximum discharge is the same as before, it will now be 2C due to the bigger pack. And then it will be symmetrical, and under the limit. \$\endgroup\$ Commented Mar 18, 2017 at 23:31