# [evdl] kiss bms



## EVDL List (Jul 27, 2007)

*Re: [EVDL] KISS BMS*

Right now I have a modified version of Lee Hart's Batt Bridge. I have
a post about it on my blog. Naturally this only has a resolution of
half the pack, 10 cells in my case. When I saw that my Black Sheep
Technology BMS boards were shunting only a few mA at most between one
charge and the next and that the order they started shunting at was
not the same from one charge to the next I decided to quit charging to
the shunting voltage of 4.00V. I started charging to a max of 69.3V or
3.465vpc after doing a top balance charge and it took a year for the
cell voltage to have a max spread of 0.1V. Since I noticed that I
still got water in the battery box once in a while and a little dust I
sealed up the battery box more, did another top balance to 4.00vpc and
took the BMS boards off and installed the BattBridge setup. I check
the voltage at the end of charge about every month when the charger is
putting only a few mA into the pack. After 10 months the pack
difference was at its greatest of 0.039V but I expect that it will be
below that on my next measurement. The difference generally bounces up
and down a little.

July will mark the 1 year mark for the no cell level BMS test. I
rarely go beyond 80%DOD so I don't worry about any individual cell
dropping out. While it is entirely possible that I could have both
halves of the pack do the same thing that probability is relatively
low given only a 20 cell pack so I expect that my balance meter would
display it. So far the most out of balance I have seen has been about
0.15V difference under a 1.5C load or so. Under no load the largest
difference I've seen is 0.05V.

At some point I will be sending my Black Sheep Technology BMS boards
to get programmed for a lower shunt voltage. The LEDs only light on
over or under voltage which is likely why they have such a low
parasitic load. I haven't decided if I want 3.6V or 3.5V or so. I
really would only need them if as my pack ages the cell's
characteristics diverge from each other. So far "everyone" says that
these prismatic LiFePO4 cells drift and that I'll murder my pack very
fast without frequently balancing them. My data doesn't support that
and neither does the data from others who are not using a BMS.
Understand, however, that my cells are in one rectangular battery box.
There doesn't seem to be any correlation with where a battery is in
the pack and if it is a high or low cell at the end of charge.

I did a couple of runs one day. One run was with no extra load and the
second run was with an 18% increase in total weight. This included a
down hill start, start and stop driving and a hill climb back home.
The energy use difference was at worst 11%. I have not redone the test
without using any regen but given that it is typically only in the
3-4% range it doesn't account for the entire difference between 18%
and 11%.

On Mon, May 7, 2012 at 11:26 AM, Mark Hanson <[email protected]> wro=
te:
>
> Thanks David,
>
> If you're running without a BMS, I assume you have a battery scanner to k=
now where the cells are at (between 2.6 - 3.6V)?
>
> I got about 330 Whr per mile or 3 miles/kWh measured at the AC outlet (wh=
ere it counts $$), Lithium is better but I just thought that's because it's=
800 lbs lighter, less weight to shove around.
>
> My 36 cell BMS draws 3ma with indicator leds glowing green. At 130ah, =
say 100ah down that's 33k hours to discharge if I forget to charge in a yea=
r or so :-0
>
> Stay charged,
> mark
> www.REEVA.info community service RE & EV's
> "wind, solar & EV's are like a 3-legged stool all working together"
>
> Message: 15
> Date: Fri, 4 May 2012 20:47:43 -0700
> From: David Nelson <[email protected]>
> Subject: Re: [EVDL] Simple Reliable BMS; flooded Lithiums?
> To: Electric Vehicle Discussion List <[email protected]>
> Message-ID:
> <[email protected]>
> Content-Type: text/plain; charset=3DISO-8859-1
>
> On Thu, May 3, 2012 at 12:22 PM, Mark Hanson <[email protected]> w=
rote:
>> What we *really* need is flooded Lithiums that would inherently equalize=
like lead of ni-cads. ?I'm not a chemist but it sure would be nice to have=
a *lightweight* chemistry that didn't have the attracted complexity (and r=
eliability hit) of BMS/regulators/monitors. ?I miss my ni-cads but not the =
weight.
>>
>
> I think you are missing the benefits that far out weigh the "just over
> charge to balance" feature of flooded lead acid and NiCd batteries.
> Stay away from the extreme ends of the charge spectrum on LiFePO4
> cells and they are significantly less work to maintain than the
> flooded cells. Besides, having a charge shuttle reaction in the cell
> can decrease the efficiency of the cell. As an example, with my
> flooded 6V pack I could go only 4 miles/kWh measured from the wall.
> With my LiFePO4 pack and the same driving conditions I can easily go
> over 6 miles/kWh. That, along with other benefits, make LiFePO4 packs
> easier and better to deal with. The BMS doesn't have to be that
> complex, just designed and built well. The BMS boards I used to have
> on my pack (I'm running without them right now) only have a 1mA draw
> so they don't even discharge the pack in a perceptible way.
>
> My experience and that of others with prismatic LiFePO4 cells is that
> the BMS "issues" really are not that big. Don't over charge the cells
> and don't over discharge the cells and they will remain happy for a
> long time. Oh, and don't undersize your pack either.
>
> --
> David D. Nelson
>


-- =

David D. Nelson
http://evalbum.com/1328
http://2003gizmo.blogspot.com

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] KISS BMS*



> David Nelson wrote:
> > Right now I have a modified version of Lee Hart's Batt Bridge...
> > Naturally this only has a resolution of half the pack...
> 
> ...


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