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Re: [EVDL] Phd in Materials Science on LiFePO4 batteries video WAS: Lithium 60ah 144v

You don't need IGBT modules to bypass an empty cell. You need a pair of
MOSFETs (or a pair of parralled MOSFETs to handle the current) that are
rated to handle your cell voltage in a half bridge configuration.

______________
| _|
| | |
| H |
| ___| |__
===== |________o A
=== __|
| | |
| L |
| ___| |_
|____________|_________o B

In this configuration, the terminals A and B are either shorted together
(MOSFET L on MOSFET H off), or connected through the battery cell (MOSFET
H on, MOSFET L off). When the cell monitoring circuit determines that the
cell is no longer operational, it shorts terminals A and B, so the cell is
no longer connected to the circuit. The voltage of the full string is
reduced, but it is still functional.

The intrinsic diodes of the MOSFETs protect against failures, but to be
safe, it makes sense to put a diode from the source to drain of each
MOSFET capable of handling the full current of the string. This circuit
can survive all catastrophic failures and when one is detected, the driver
can be warned that maintenance is necessary now. It is reasonable to
disable the vehicle for a catastriphic failure, but not when one cell out
of hundreds or thousands is weak and can't keep up with the rest.

For cost reduction, you probably put three or four battery cells in a
group with a half bridge around them. The monitoring circuit then
monitors the voltage across, the current through and the temperature of
each cell in the group and switches them all out of the string when there
is a problem. You lose the whole group when one cell expires, but that is
way better than losing the whole pack.

Once you have this module, you have the makings of a multi-level
converter. Using 100 cells in groups of 4, you can make a sine wave with
49 steps peak to peak. Such sine waves are referred to as true sine
waves. Further divide the pack into three such strings of smaller
capacity cells, operate the strings 120 degrees out of phase and you have
the makings of a variable frequency three-phase motor drive. Your BMS
electronics has just replaced the need for a separate motor drive and its
expensive IGBTs.

This controller is also pretty efficient. There are no high frequency
switching losses. The MOSFETS switch at hundreds of Hz rather than tens
of thousands of Hz. The true sine wave effectively eliminates the
harmonics of the fundamental frequency further increasing efficiency.
With no harmonics of the fundamental frequency or of high speed switching
frequencies, the motor runs much more effieiently too. An inexpensive
synchronous industrial motor can be used.

The multi-level converter can also be used to charge the cells by matching
the amplitude and phase of an AC source. Now the BMS electronics has just
replaced the charger. It is a very efficient charger too. It looks like
a resistor to the power company. There are no high speed switching losses
and of course no harmonics of the fundamental frequency.

Ed

Cory Cross wrote:
> Here is how I understood it. Single cell management is the ability to
> remove individual cell from the circuit and/or reroute energy around it,
> such that single cell failure does not stop the whole pack. This
> requires power electronics ( like IGBT modules ) for each cell, to be
> able to route energy around bad/empty cell. Some people proposed such
> BMS schemes, even on this forum, but for anyone familiar with
> electronics involved its 100% clear that such solution is not cost
> effective. I think this is what Jay was referring to, he actually
> mentioned bypassing a cell.
>

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