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

Ed Blackmond wrote:
> You don't need IGBT modules to bypass an empty cell. You need a pair of
> MOSFETs (or a pair of paralled MOSFETs to handle the current) that are
> rated to handle your cell voltage in a half bridge configuration.
> _____________
> | _|
> | | |
> | H | |
> | ___| |__
> ===== |________o A
> === __|
> | | |
> | L | |
> | ___| |_
> |_____________|_________o B

This could work, and indeed, has been done in EVs where relay contacts
replace the MOSFETs. The challenge with MOSFETs is that there are a lot
of them, and they are carrying full pack current. It requires some
exceptionally good MOSFETs to keep the losses (and consequentially the
heat) low.

> This circuit can survive all catastrophic failures...

MOSFETs tend to fail shorted. If one shorts, it will short the battery
the next time the other one turns on. You're going to need fuses in
series with each battery to protect against this failure mode.

When built with switches, the switch was built so it was physically
impossible for it to short the cell.

> For cost reduction, you probably put three or four battery cells in a
> group with a half bridge around them.

Look up the "rectactor" circuit for examples of this. Rather than two
switches per battery, each rectactor block has 2 batteries and 3
switches per module (fewer parts overall). It can wire the batteries in
either series or parallel. Repeating it lets you select series/parallel
combinations to get every possible voltage with fewer switches overall
than the above scheme.

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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