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Re: [EVDL] Individual Cell Cutoff was Re: Phd in Materials Science on LiFePO4 batteri

Cory Cross wrote:

> Ed Blackmond wrote:
> > Agreed, but these MOSFTEs don't have to switch the full pack voltage.
> > Such MOSFETs with single digit mOhm Rdson are pretty inexpensive and when
> > paralleled to get to the required current, reduce Rdson even more. This
> > doesn't make any one MOSFET cooler, but the combination is cooler than a
> > single MOSFET with a larger Rdson. The copper connection between them can
> > also double as a heat sink.
> >
> 1milliOhm is 40W of heat at 200A, 250W at 500A. The size you would need
> is probably 10"x4"x1" just for the power electronics themselves, unless
> you want to undersize them. Then add on cooling apparatus, which would
> probably be another couple inches of thickness.

Yes, that is a lot of heat. It probably makes sense not to design a
system that requires 200A and forget 500A.

The first goal of this design is monitoring and protection of the battery
cells. Everything is there to monitor and protect the electronics too.
Including a fuse that shuts it down if the current is too high or the
MOSFETs get too hot is required, but the system should be designed to
never need that fuse.

In addition to the battery cell monitoring and protection, the system is a
variable frequency 3-phase drive. Consider that a single phase consists
of 64 modules that are 14.4V each. At full speed, 200A (if it could be
cooled and ignoring the considerable losses) would be about 275KW. For
many, that is considerable overkill for a generic on-road vehicle. But if
cost was no object, MOSFETs parallel pretty well, so it might be possible
to add enough of them that a cooling system could be designed for it.

100KW would get you closer to performance of a typical family sedan today.
This takes about 75A. That is about 6W, and it is split relatively evenly
between the high and the low side of the bridge. Cooling that is
reasonable at least for a brief period of time.

Suppose it takes about 20KW to maintain highway speed. This is about
0.4W lost in the MOSFETs. Now, that 0.4W is multiplied by the 64 modules
per phase and there are three phases. This is about 60W total or about
0.3% of the 20KW output. That seems reasonable to me. Its not going
to win any drag races, but it will get to and from work, the grocery
store, and soccer games.

>
> > Once I had a module that could ether be zero volts or V volts, the
> > idea of
> > a multilevel converter made sense. The BMS can be used to generate a darn
> > good sine wave. Make three strings and operate them 120 degrees out of
> > phase and the BMS becomes a variable frequency 3-phase drive. The
> > multilevel converter makes a good charger too either from regen or the
> > utility.
> >
> This is bad because it pulses high current into the batteries instead of
> DC. My previous investigation into this showed the losses in this system
> were unacceptable (I don't quite remember, I'm guessing 70-80% efficient
> versus a controller at 96-98%).

I appreciate this feedback. I don't have my own data here yet. It may
turn out that using the multi-level converter as a charger won't work.
Although the literature shows that there are a lot of people who think it
does.

Ed

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