Re: EV Digest, Vol 76, Issue 31

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Re: EV Digest, Vol 76, Issue 31

Electric Vehicle Discussion List mailing list
I'm wondering how much capacity the capacitors are adding to the system,
as the voltage of the capacitors can never go below the sag voltage of
the LiIon cells under load, and can never go above the LiIon cells
voltage during regen.


I am not sure this statement is correct.  I am counting on the caps to take higher voltage.
If the regen statement is true maybe under regen the battery pack should be disconnected.  Lawrence Rhodes
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Re: EV Digest, Vol 76, Issue 31

Electric Vehicle Discussion List mailing list
Lawrence Rhodes wrote:

>> I'm wondering how much capacity the capacitors are adding to the system,
>> as the voltage of the capacitors can never go below the sag voltage of
>> the LiIon cells under load, and can never go above the LiIon cells
>> voltage during regen.
>
>
> I am not sure this statement is correct.  I am counting on the caps to
> take higher voltage.
> If the regen statement is true maybe under regen the battery pack should
> be disconnected.  Lawrence Rhodes

It is correct: you are proposing connecting the battery and caps directly in parallel, therefore they must always be at (very nearly) the same voltage.

But, don't worry, this doesn't necessarily prevent them from achieving your goal.  You stated your goal is to try to keep the peak current seen by the battery below 90A despite peak loads of 100A.  When the load changes, current will divide between the caps and battery based on their impedance relative to each other.  That is, suddenly applying a load to the battery causes its voltage to sag (somewhat); however, as soon as it sags even a little, the lower impedance ("stiffer") cap bank supplies more of the load current, and so the battery sags a bit less (and supplies a bit less of the load) than it would without the caps.  The opposite occurs during regen: suddenly applying current to the battery causes its voltage to increase, and if it rises high enough the controller will reduce the regen current (hopefully).  With the caps in parallel, as soon as the battery voltage tries to rise even a bit, less of the current goes to the battery and more of it goes into the caps.  Less current into the battery results in a bit less voltage rise, so the peak battery voltage during regen tends not to spike as high as it would without the caps.

How much reduction in battery current is realized will depend upon how much lower impedance the caps are than the batteries.  The benefit will be more pronounced if the battery has significantly more impedance than the caps.

Cheers,

Roger.

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