# [EVDL] Balancing Stubborn LiFePO4 Cells with Bench Power Supply



## EVDL List (Jul 27, 2007)

On Sat, Sep 05, 2009 at 01:04:26PM +0100, martin winlow wrote:
> 
> I have read about people using bench power supplies with constant current
> and/or voltage outputs to charge lithium cells individually and wonder if I
> could use my 30V, three amp power supply to bring the three cells up either
> together or individually to speed up the balancing process. If it is
> possible how exactly should I connect the PSU to the cells and what is the
> procedure?

The GREAT attraction to using a ~4v power supply is that you can do no 
harm due to inattention. If you do several cells in series, you risk 
one cell going too high.
> 
> Secondly, am I likely to be doing any damage to the cells by leaving them at
> 3.85V for prolonged periods for balancing purposes?

I don't thinks so. Rumor is that the LFP cells have longest shelf life 
about half charged. For that reason, I don't charge my pack at every 
opportunity.

-- 
Willie, ONWARD! Through the fog!
http://counter.li.org Linux registered user #228836 since 1995
Debian3.1/GNU/Linux system uptime 630 days 1 hours 31 minutes

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## EVDL List (Jul 27, 2007)

I used a bench supply to charge up my LiFePO4 cells. It was a 'battery 
eliminator' supply (capable of up to 30A at 14.7V). I charged 4 at a 
time and babysat them to watch for overvoltage. I used a big series 
diode since some power supplies will react badly to a battery load. I 
then adjusted the voltage to get the current I wanted. As one cell 
reached 3.8V I would see the current back off due to the voltage limit. 
At that point I would do the three remaining cells (adjusting the 
voltage) and on down to single cell. It worked well, but as others 
warned me the voltage rises quickly once you get past 3.6 or so.

- SteveS



> martin winlow wrote:
> >
> > I have read about people using bench power supplies with constant current
> > and/or voltage outputs to charge lithium cells individually and wonder if I
> ...


----------



## EVDL List (Jul 27, 2007)

Martin,

Yes - the first three cells may be 40 Ah lower than the rest,
so it requires *at least* 2 full days (48 hours) long of slow
(0.9 Amp) charging with you BMS that apparently watches the cell
voltage and applies a 1 Amp bypass as soon as the cells reaches
3.85V so after two full days of this balance-charging, the three
cells of each sub-pack should have had a chance to catch up.
NOTE that your charger must always throttle back to below 1A
before the first cell starts its bypass, else it will be 
overcharged - it was not clear if you built that protection
into your charger.

How long did you already charge? You were too vague when saying
"despite having been left for some time on a low
(less than one amp) charge."
How long is "some time"?

BTW, if you have done 100Ah cycles then you have skirted the
lower batteries close to total discharge if they had lost 40Ah
and you took another 100Ah from the 160Ah cell...
I hope your low voltage discharge protection works well.

Regards,

Cor van de Water
Director HW & Systems Architecture Group
Proxim Wireless Corporation http://www.proxim.com
Email: [email protected] Private: http://www.cvandewater.com
Skype: cor_van_de_water IM: [email protected]
Tel: +1 408 383 7626 magicJack: +1 408 844 3932
Tel: +91 (040)23117400 x109 XoIP: +31877841130

Please consider the environment before printing this e-mail.
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of martin winlow
Sent: Saturday, September 05, 2009 5:34 PM
To: EVDL Post Message
Subject: [EVDL] Balancing Stubborn LiFePO4 Cells with Bench Power Supply

Hi,

I have 38 ThunderSky LFP160 cells in series in my conversion traction
pack.
Each cell has a protection circuit attached which limits the minimum and
maximum voltage. The maximum voltage is controlled by a four ohm power
resistor which bypasses the charging current limiting the maximum
voltage to
3.85V. 

Before installing the pack in my vehicle I had it sat on a bench for
some
months with battery voltage monitoring devices attached which drew about
5
milliamps from the first three cells of each sub pack of seven or eight
cells. One pack which powers the battery monitoring display as well as
the
sensing unit draws about 10 milliamps. Eventually I installed the packs
in
my conversion and have been using it for the last few weeks - I have now
done about 550 miles. I have only recently tried to balance the whole
pack
and have found that the first three cells of the pack which had the 10
milliamp drain have still not balanced using the charger and balancing
system I have installed despite having been left for some time on a low
(less than one amp) charge.

You can see a plot of the cell voltages when I first started balancing
the
cells at the following URL;
http://www.winlow.co.uk/html/ev_conversion.html.
It's the last couple of pictures right at the bottom. You can see the
first
three cells of each sub pack at about 3.2 volts whilst the rest are up
at
the maximum 3.85 volts. The same display now shows all but the first
three
cells at 3.85 V.

Due to the extended time the pack was sitting around before being
installed
in the vehicle it is possible that these three cells may have been
discharged up to 40 amp hours more than the rest of the pack (0.010 x 24
x
28 x 6 - six months). They were not charged at all in this time. The
rest of
the cells all came up to the 3.85V balancing voltage within about 24
hours
of the commencement of the balancing charge. The last three cells still
won't come up despite having been balancing over the last three days or
so
in between a couple of 100Ah or so discharges and recharges.

I have read about people using bench power supplies with constant
current
and/or voltage outputs to charge lithium cells individually and wonder
if I
could use my 30V, three amp power supply to bring the three cells up
either
together or individually to speed up the balancing process. If it is
possible how exactly should I connect the PSU to the cells and what is
the
procedure?

Secondly, am I likely to be doing any damage to the cells by leaving
them at
3.85V for prolonged periods for balancing purposes?

Regards, Martin Winlow

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## EVDL List (Jul 27, 2007)

Cor (et al),

Thanks for your replies.

The last 3 cells came up finally yesterday evening before I had a chance to
try to charge any of the cells individually with the power supply - thanks
anyway.

My controller/charger/DC-DC/BMS system is a fully integrated one supplied by
Belktronix and - after one or two relatively minor teething troubles, some
of my own making, and all quickly and effectively dealt with by Bryan at
Belktronix - all is working nicely now. The over and under charging dangers
are simply and effectively dealt with in the Belktronix system, although
obviously dealing with 4 times the number of cells in a lithium setup than
lead-acid does mean 4 times as much 'spaghetti'. 

In the discharge scenario, when any of the cells reaches minimum voltage the
controller output is cut back limiting the output progressively. I have
operated my conversion twice in this mode and have found that the power
available from the controller slowly falls to the point where the vehicle
will do no more than a walking pace where it is drawing about 10 amps from
batteries. On both occasions I was able to limp home without having to
resort to the ignominy of stopping along the way and asking some startled
member of the public for a recharge (especially at 2 in the morning). I was
able to monitor the voltage of the cell that was causing the controller
throttling (one of the three cells which have been dawdling) and it safely
remained above 2.6 volts the whole time.

At the other end, the charger output is automatically limited to around one
amp when any of the cells reaches the maximum voltage limit which then
allows for a period of balancing.

On balance, I would recommend the Belktronix system for small to medium
(European small to medium, that is) conversions, and if you want to convert
something bigger Bryan supplies an 800 amp version of his controller too
now.

Regards, Martin Winlow, Herts, UK
http://www.evalbum.com/2092
www.winlow.co.uk



-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Cor van de Water
Sent: Saturday, 05 September 2009 21:02
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Balancing Stubborn LiFePO4 Cells with Bench Power Supply

Martin,

Yes - the first three cells may be 40 Ah lower than the rest, so it requires
*at least* 2 full days (48 hours) long of slow
(0.9 Amp) charging with you BMS that apparently watches the cell voltage and
applies a 1 Amp bypass as soon as the cells reaches 3.85V so after two full
days of this balance-charging, the three cells of each sub-pack should have
had a chance to catch up.
NOTE that your charger must always throttle back to below 1A before the
first cell starts its bypass, else it will be overcharged - it was not clear
if you built that protection into your charger.

How long did you already charge? You were too vague when saying "despite
having been left for some time on a low (less than one amp) charge."
How long is "some time"?

BTW, if you have done 100Ah cycles then you have skirted the lower batteries
close to total discharge if they had lost 40Ah and you took another 100Ah
from the 160Ah cell...
I hope your low voltage discharge protection works well.

Regards,

Cor van de Water

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Usage guidelines: http://evdl.org/help/index.html#conv
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## EVDL List (Jul 27, 2007)

> On 6 Sep 2009 at 11:33, martin winlow wrote:
> 
> > obviously dealing with 4 times the number of cells in a lithium setup than
> > lead-acid does mean 4 times as much 'spaghetti'.
> ...


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## EVDL List (Jul 27, 2007)

David - You may be right about "module".
The established, and commonly understood terms are "cell", "battery", and
"battery pack" ( or, just "pack"). The use of "module" on this loop, it
seems to me, is an invitation to confusion .

Phil Marino
Rochester, NY



> EVDL Administrator <[email protected]> wrote:
> 
> > On 6 Sep 2009 at 11:33, martin winlow wrote:
> >
> ...


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## EVDL List (Jul 27, 2007)

> On 7 Sep 2009 at 5:49, Phil Marino wrote:
> 
> > The established, and commonly understood terms are "cell", "battery", and
> > "battery pack" ( or, just "pack").
> ...


----------



## EVDL List (Jul 27, 2007)

David - I apologize for being US- centric ( especially to our European
ev-brethren) .
I've seen "module" used here before but it never occurred to me that it was
a commonly used term in Europe.
We may just have to live with an occasional bit of confusion here.

Phil Marino



> EVDL Administrator <[email protected]> wrote:
> 
> > On 7 Sep 2009 at 5:49, Phil Marino wrote:
> >
> ...


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## EVDL List (Jul 27, 2007)

>


> > n 7 Sep 2009 at 5:49, Phil Marino wrote:
> >
> >
> >> > The established, and commonly understood terms are "cell", "battery", and
> ...


----------



## EVDL List (Jul 27, 2007)

In the battery industry all batteries are made up of cells (12v lead has
6 cells)
In 12v lead, 6 cells make a battery. In flashlights cells are batteries.

In OEM EV's cells combine into modules and modules combine into
sub-packs, packs or A Traction Battery. 
So batteries and modules can be considered the same, but you are talking
to an OEM Engineer, the battery is 12 volts and made of lead, modules
are used in a Traction Batteries or Packs, and made up of cells.

This seems to hold up across the ocean, the Pacific Ocean anyway. 

I know this is as clear as mud
Rick 


-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Jeff Shanab
Sent: Tuesday, September 08, 2009 6:04 PM
To: [email protected]
Subject: Re: [EVDL] Balancing Stubborn LiFePO4 Cells with Bench Power
Supply

>


> > n 7 Sep 2009 at 5:49, Phil Marino wrote:
> >
> >
> >> > The established, and commonly understood terms are "cell",
> ...


----------



## EVDL List (Jul 27, 2007)

Ah.. my original post appears to have been hi-jacked ...

Going back to my question... assuming I wanted to bring up a string of 4
contiguous cells whilst still connected to the rest of the pack, do I just
connect the bench power supply accross the four (+ to +, - to -), set the
constant currant value (say 3A), crank up the volts to get 3A flowing and
then sit back and watch to make sure none go over 4.2 (or whatever) volts? 
Do I have to take any precautions - like disconnecting the charger from the
mains (not just turning it off - ie disconnecting earth connection too) etc?

MW

-- 
View this message in context: http://n4.nabble.com/Balancing-Stubborn-LiFePO4-Cells-with-Bench-Power-Supply-tp462295p1461723.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

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## EVDL List (Jul 27, 2007)

That is what I did except I had my BMS modules attached to make sure
none went over 4V. I also kept the current below the shunting ability
of the BMS modules. I did this without the bulk charger attached to
the pack and I only hooked up one power supply at a time or else I
split the pack to do two parts of it at the same time. I wasn't sure
how my dual power supply would handle being put in some sort of series
mode.



> martinwinlow <[email protected]> wrote:
> >
> > Ah.. my original post appears to have been hi-jacked ...
> >
> ...


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