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Discussion Starter · #1 ·
I have 2 ev's in my shop now with Li -ion batteries and shunt
regulators . One pack of 90 100 ah cells now in the bed of my pick up
came from a car that only had the bms set up to shut off the charger
, so when a cell went bad the car could still be driven just not
charged ( batteries ranged from 0v to 2.50) when I got it) . I have
weeded out about 10 cells and can pull 60 ah from the cells that are
left. I did the slow charge after a full charge and have also
individually charged certain cells to get to this point ( first cycle
gave 12ah) . What I think may be happening is this . Some of the cells
have a different internal resistance which makes some cells voltage go
up faster that others when being charged ( The cells that have the
most voltage drop under load seen to go up in voltage faster when
being charged). I am thinking that the shunt regulators start
shunting current across these batteries first and that on every cycle
these cell don't get the same ah as the others . The regulators on
this car come on at 3.65 and stay on till the cell come down to 3.32v
, So I'm thinking that the week sells hit 3.65 first , start backing
off the charger while shunting 1/2 an amp and the good cells keep
soaking up the ah getting ahead , at some point charger shuts off and
every body drops back to 3.32v with the good cells having gotten more
ah then the week ones. I also noticed this same thing on the bike
that's here ( same kind of shunt regs) with a 48v pack that had 3 bad
cells . I now can kind of see why people are thinking that a shunt
reg my not be the best and something that shows batteries voltage of
every cell with lowest and highest cells controlling shut down along
with some way to individually charge/ shunt cells might be better. It
may well be that not much is needed until some kind of abuse has
happened and thin it becomes an on going balancing act that shunt
regulators can't really do .
Steve Clunn

--
Tomorrows Ride TODAY !
Visit our shop web page at: www.Greenshedconversions.com

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Discussion Starter · #2 ·
Steve,

You may be right in your analysis but I think something that is
exacerbating the problem is that all the batteries are brought down to
3.32v! If these are LiFePO4 cells this is well into the flat part of
the voltage curve. I charge my cells to 3.485vpc average and they rest
at around 3.37v after sitting for 48 hours or more. I think those
shunt regulators are bleeding off a significant portion of the
capacity and that is the bigger problem.

For the past 150+ charges on my pack I haven't been top balancing them
and they are staying within 0.088 volts of each other. They were 0.051
volts of each other after the first non-top balance charge back in
July '10 and 2600+ miles ago.

Try balancing the pack either at the top or bottom then charging only
to 3.5vpc and see what happens.

--
David D. Nelson
http://evalbum.com/1328

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Discussion Starter · #3 ·
Bingo! And, this we have... just no money to get it to market!!! Arghh!!!

http://www.youtube.com/user/eviesystems#p/u/1/agVRs0dcBxM



Steve Clunn wrote:
> cells . I now can kind of see why people are thinking that a shunt
> reg my not be the best and something that shows batteries voltage of
> every cell with lowest and highest cells controlling shut down along
> with some way to individually charge/ shunt cells might be better. It
> may well be that not much is needed until some kind of abuse has
> happened and thin it becomes an on going balancing act that shunt
> regulators can't really do .
> Steve Clunn

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Discussion Starter · #4 ·
Steve Clunn wrote:
> I now can kind of see why people are thinking that a shunt
> reg my not be the best and something that shows batteries voltage of
> every cell with lowest and highest cells controlling shut down along
> with some way to individually charge/ shunt cells might be better.
It's not a BMS unless it does something when ANY cell is too low or too
high.

The reason those cars failed is they had circuit boards masquerading as
a BMS.

Cory

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Discussion Starter · #5 ·
From: David Nelson

>Try balancing the pack either at the top or bottom then charging only
to 3.5vpc and see what happens.<

I'm going to try this ,, I'll take off the reg's and charge the low
ones separately , and watch the voltage on each on

Mark wrote
>Bingo! And, this we have... just no money to get it to market!!! Arghh!!!

http://www.youtube.com/user/eviesystems#p/u/1/agVRs0dcBxM<

This looks nice , dose it have the abulity to switch a relay when any
cell gets to hi or to low. What was the projected price and how many
cells.

Steve Clunn-
Tomorrows Ride TODAY !
Visit our shop web page at: www.Greenshedconversions.com

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Discussion Starter · #6 ·
The YouTube video shows a battery meter, rather that a BMS (Battery
Management System). If Mark (or anyone else on the list) has a BMS (which
ensures cell balancing without operator intervention) I'd be interested in
receiving info.

Regards,
Ken

ken at www.gaiatransport.com
678-983-8957




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Think Big.
Drive Small.
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View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/li-ion-bms-s-short-comings-tp3215874p3216137.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

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