> On Oct 23, 2018, at 2:14 PM, Lee Hart via EV <[email protected]> wrote:
>
> bvgandhi via EV wrote:
>> Hi,
>>
>> I have lately come across a lot of work done on reconfigurable battery packs
>> which uses the concept of scheduling of battery cells. Instead of oversizing
>> the battery pack with too many cells and adding additional failure modes,
>> reconfiguration works on the idea of using switches and scheduling cells as
>> per power requirement of the vehicle. Does anyone here have any opinion or
>> ideas about this this or has seen previous practical implementations ?
>
> In general, if you have the cells, then *use* the cells. Otherwise, they are
> just dead weight.
>
> All types of batteries have internal resistance. This means they are more
> efficient at lower currents (more amphours, less voltage drop). Discharging
> them at higher currents increases the I^2R losses (i.e. less range, more
> voltage drop). So it is better to always draw power from all the cells,
> rather than first from some of the cells, then from different cells, etc.
>
> However, cells have different capacities. If you always draw the same power
> from every cell, then some will go dead sooner than others. So it makes sense
> to have some method to distribute the load between the cells according to
> their ability.
>
> Many old EVs had reconfigurable battery packs. Their controllers were
> basically a set of switches that reconnected the pack in various
> series/parallel combinations to adjust the voltage and thus speed. During the
> times when cells are in parallel, they are forced to the same voltage, which
> automatically balances the state of charge between them. The ones at a lower
> state of charge naturally supplied less current. See the Henney Kilowatt
> controller at <http://sunrise-ev.com/controllers.htm> for an example.
Another way to approach it is to use the switching to build an AC rather than
DC battery pack. By switching between different series/parallel combinations
it is possible to create a multi-level converter. This can create very good
sign waves without a frequency component at a PWM frequency. Create three
strings operating 120 degrees out of phase from each other and the battery
circuit becomes a very nice variable frequency three phase drive. Running the
strings in phase and synchronized to the utility grid and the battery circuit
becomes a charger. With sensors monitoring all the cells, they can be switched
in and out of the circuit keeping all the cells at the same state of charge.
One circuit integrates the function of battery cell protection, motor drive,
and charger.
Ed
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