LiFePO4 Battery Pack Balancing

[Mark Brems]

Hello All,

I'm nearing the finish line. I snuck the 914EV out into the neighborhood for a surreptitious test
drive, and everything behaved very well. I'll still need to adjust my shift linkage and other sundry
tasks.

But the biggest issue facing me right now is pack balancing. I am using the MiniBMS, and
before most of the cell modules go into shunt mode, the alarm is tripping and the control
board is shutting the charger down. I thought the whole idea of shunting was to prevent the
cells from overcharging and tripping the alarm. What gives?

I'm looking at the online balancing instructions, and wondering if there are any gotchas I
need to be aware of. I think I am going for the top balancing scheme. I was considering
clipping my Craftsman 13.8V battery charger directly to small groups of 4 cells to bring their
SOC up.

Any thoughts?

Mark

[Jerry Pohorsky]

Hi Mark,

It is good that the charger is shutting off.  At least no cell will get over charged.
One of the drawbacks of the Mini-BMS is that it does not display the cell voltages.

Rather than charging the low cells with your Craftsman battery, you need to find out which cell(s) are generating the alarm.  Use a digital volt meter to measure each cell.

You can add a load resistor to the high cell(s) to bring them in line with the rest.  Choose a value that pulls at least an amp or you will be waiting for a very long time.  I think 3.3 ohms will pull 1 amp at 3.3 volts, so 1.65 ohms will pull 2 amps, etc.  2 amps will be almost 7 watts, so use a resistor with a high enough power rating or you will burn it up.

One of the lessons to be learned here is that it is best to balance the pack before you wire up the cells.  This is done by connecting them all in parallel first so that the high ones charge the low ones.  Eventually they will all equalize and then you can install them in the EV and wire them up.  Probably too late for that now, but something to try next time.

Adios,

Jerry

Mark

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[Mark Brems]

Thanks Jerry. In fact, I did hook them all up in parallel for a couple days. Check the last picture/paragraph in this blog post:

http://914electric.wordpress.com/2012/04/09/battery-goodness/

I will pursue the load resistor scheme first. I assume I do not need to isolate the individual
cells from the pack to do this?

Thanks,
Mark

[Jerry Pohorsky]

Yes,

No need to isolate the cells but do this when the charger is off.  The resistor will dissipate a bit more power when the charger is on because the cell voltage is higher.

Adios,

Jerry

Sent from my iPhone

[Richard Rodriguez]

Sorry Mark, 

I do not have much to contribute here.  I do not have a BMS.

But I do have some questions.

How many cells are you using?  Which type of cells are you using?  What are you charging your cells up to?

Can you access your individual cells to get measurements?

Just curious

Richard

 

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[Mark Brems]

Thanks Jerry. Very helpful!

Richard - I am using 36 CALB 180AH cells. I have an Elcon PFC2500
charger that delivers some percentage over the pack voltage of 115.2 volts
(3.2 volts x 36). I have no way to charge cells individually.

Haven't tried yet, but I assume I can read the voltage across a single cell
without isolating from the pack. Someone correct me if I'm wrong, please.

Mark

[Mark Brems]

Hey Richard,

I was looking at your EV Album page, and took a shine to the flat panel
display you have in your console area. Is that the Andromeda?
I am also running the HPEV AC50 and Curtis controller, so I assume
it would work in my ride also.

Has it worked for you? Do you have any info you could forward?

Thanks,
Mark

On Thu, Sep 13, 2012 at 8:48 PM, Richard Rodriguez <rick...@san.rr.com> wrote:

[Richard Rodriguez]

The display is a prototype by Brian Gallager.  They should be available next year.  They will be better with touch screen, Ah counter and a thinner display.

I have been using it for over a year and it works great.  The new one will be better.

He just needs to get it finished.

I'll forward your message to him.

[Richard Rodriguez]

Mark,

What did you end up doing with your BMS system alarms going off?

Did you do some hand balancing to try to get things better aligned?

On Sep 20, 2012, at 7:02 PM, Mark Brems wrote:

[Mark Brems]

I bought a couple Radio Shack 1 ohm load resistors, rated for 10 watts each,
and put them in series. They pass about 1.6 amps. I fired up my charger last
night and waited for the alarm, but none happened. The charger shut off without
an HVC event. So I started in on the cells that were shunting with the red LED
brightest, and let them each discharge for about 7 mins each. Very time-consuming,
but I did start to see some of the modules that weren't shunting start doing so
when I booted the charger back up.

Curious that the charger shut down with about half of the cells not shunting,
and without tripping the HVC alarm. Makes me think that the charger itself shut
down without any help from the BMS. The instructions don't prepare you for things
like this.

Anybody else have this same experience?

McB

[David Shields]

I'm going nuts trying to figure out what kinds of shocks and springs to use on my 914 converstion. The guys down at the local suspension shop can't say either. They will install any parts I can get a part number for.
 
I'm going with lead/acid batteries, so that's going to add 800 pounds or so to the vehicle.
 
Does anyone have any suggestions? Can you help a brother out?
 
David Shields

[David Hale]

David,

I'm sorry, I don't remember the details of the make or model numbers, but I did replace the rear shocks & coil over springs, as well as the front torsion bars. I don't think they were anything special except they were beefier than the stock parts (which was the whole point of Electro Automotive including them in their kit). I checked my receipt from them but it doesn't list part numbers, only "shocks". In case it's helpful, I've attached a few pictures. You might be able to get this information from Electro Automotive, but I don't know if they're around any more; my emails to them have gone unanswered for over a year now.

There may be a lot of reasons to argue either side of the lead acid and lithium debate, but since lithium costs about $6000 more than lead acid, I doubt lithium would win on expense or effort of changing the suspension. That was one of the easiest parts of my conversion and the rear suspension needed to be replaced on my 40 year old car anyway. I did it myself in a Saturday morning using a spring compression tool I "rented" for free from AutoZone.

Sorry this isn't too much help on the specifics, other than encouragement to do it! :-)

best,

--Dave

http://evalbum.com/2500

[David Hale]

Incidentally, the last photo was to show my old shock cartridge, which is clearly shot, compared to the new one.

I also found this on TimK's blog... http://914ev.blogspot.com/2007/02/suspension-is-here.html
where you can also see the stiffer torsion bars; maybe he remembers the part numbers.

D

[David Shields]

Thank you so much.
 

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Date: Sat, 29 Sep 2012 09:00:58 -0700
Subject: Re: [914ev] Shocks and Springs
From: astrono...@gmail.com
To: 91...@googlegroups.com

[Ben A]

My local Porsche shop recommended the Bilstein shocks because they make it easy to adjust the ride height.  I have yet to drop the money myself so I can't vouch for this based on personal experience.

-Ben

[David Shields]

Thank you so much Ben.
 

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Date: Mon, 1 Oct 2012 15:23:31 -0400

Subject: Re: [914ev] Shocks and Springs

From: bap...@gmail.com
To: 91...@googlegroups.com