[EVDL List]

Hi Buddy,

I found it was quite easy to get the pack to sag below 2.93vpc. At the
time I had my BMS boards on and that is when they would send a LV
signal. I could have a fully charged pack which had cooled to 0-3=B0C
and a 1-1.5C draw would cause the LV alarm to go off. As best as I
could tell all cells were in the alarm state under this load. I tested
it at night while trying to look over my shoulder at the top of the
pack and I couldn't detect any cell alarming before any other. This is
with a 200Ah 64V nominal pack. TS-LFP100AHA cells made in 11-2009 and
in a 2p20s arrangement.

I had a few long runs where I was pulling ~130A continuously from the
pack for over an hour and the pack temperature would climb into the
mid to high teens. The interesting thing about this is that the pack
sag remained about the same or even decreased even though the SOC was
going down.

[quote] Buddy Mills <xxx@xxx.xxx> wrote:
> David, Curious question. What is your voltage and how much sag did =
you
> see? Inquiring minds would like to know.
>
> TIA
> Buddy Mills
> xxx@xxx.xxx
>
> Look mom, no gas. http://www.evalbum.com/2887
>
> Disclaimer: No animals were harmed or killed in the process of writing=
this
> email. Any stories to the contrary are, for the most part, either fict=
ional
> or greatly exaggerated.
>
>
>
>
> -----Original Message-----
> From: xxx@xxx.xxx.edu [mailto:xxx@xxx.xxx.edu] On Beh=
alf
> Of David Nelson
> Sent: Monday, June 18, 2012 12:33 PM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] Shaky battery maker A123 claims an advance.
>
> The article did say "extreme" temperatures. I know my TS LFP pack sags
> significantly when it is at 0 degrees C.
>



-- =

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

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[EVDL List]

Under modest to high discharge rate, the ohmic heating inside the
cells warms them up. Warmer cells have lower internal resistance.
Also, the chemical diffusion rate goes up as temperature goes up.
Both of these effects reduce the voltage sag. Thus, you tend to get
lower voltage sag later in the discharge.

Quite to opposite of what you get with lead-acid.

Bill D.

>The interesting thing about this is that the pack
>sag remained about the same or even decreased even though the SOC was
>going down.

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[EVDL List]

Huh?
I though that when lead-acids are cold they also are
saggy and I have seen how they stiffen up while they
get warmer, just like the LFP when they are cold and heated
by the discharge.
That is one of the reasons that it is recommended to
not fully charge your lead-acid pack immediately after
you return home from your commute, but instead try to
finish the charging just before you leave in the morning,
especially when it is cold. The heat from the charging
(especially the finishing stage) will warm up the
batteries so they work better in the cold...

Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: xxx@xxx.xxx Private: http://www.cvandewater.com
Skype: cor_van_de_water XoIP: +31877841130
Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203

-----Original Message-----
From: xxx@xxx.xxx.edu [mailto:xxx@xxx.xxx.edu] On
Behalf Of Bill Dube
Sent: Thursday, June 21, 2012 11:54 AM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Sag on TS LFP (RE: Shaky battery maker A123 claims
an advance.)

Under modest to high discharge rate, the ohmic heating inside the cells
warms them up. Warmer cells have lower internal resistance.
Also, the chemical diffusion rate goes up as temperature goes up.
Both of these effects reduce the voltage sag. Thus, you tend to get
lower voltage sag later in the discharge.

Quite to opposite of what you get with lead-acid.

Bill D.

>The interesting thing about this is that the pack sag remained about
>the same or even decreased even though the SOC was going down.

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[EVDL List]

But in my limited experience with just one vehicle using a pack of
T-875s and now LFPs I never saw the pack stiffen as it was driven to
the point that the sag decreased like I have with LFP. Maybe I never
drove far enough on the lead acid's to see it. Likely because they
were dead before they could warm up and the voltage drop vs SOC is so
much steeper than it is for LFP.

Have you ever started with a cold lead acid pack and had the sag at
the end of your drive less than or equal to when you started? I'm
talking about the voltage under a particular load being at or above
what it was under the same load at the start with cold batteries?

[quote] Cor van de Water <xxx@xxx.xxx> wrote:
> Huh?
> I though that when lead-acids are cold they also are
> saggy and I have seen how they stiffen up while they
> get warmer, just like the LFP when they are cold and heated
> by the discharge.


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

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[EVDL List]

I think Bill's point on the difference between lead and Lithium is that
with Lead, the sag increases as you discharge the battery, despite the
effects of self-heating. With Lithium, the sag will stay fairly constant
for most of the discharge, and self-heating will cause decreasing sag as
you discharge.

[quote] Cor van de Water <xxx@xxx.xxx> wrote:

> Huh?
> I though that when lead-acids are cold they also are
> saggy and I have seen how they stiffen up while they
> get warmer, just like the LFP when they are cold and heated
> by the discharge.
> That is one of the reasons that it is recommended to
> not fully charge your lead-acid pack immediately after
> you return home from your commute, but instead try to
> finish the charging just before you leave in the morning,
> especially when it is cold. The heat from the charging
> (especially the finishing stage) will warm up the
> batteries so they work better in the cold...
>
> Cor van de Water
> Chief Scientist
> Proxim Wireless Corporation http://www.proxim.com
> Email: xxx@xxx.xxx Private: http://www.cvandewater.com
> Skype: cor_van_de_water XoIP: +31877841130
> Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203
>
> -----Original Message-----
> From: xxx@xxx.xxx.edu [mailto:xxx@xxx.xxx.edu] On
> Behalf Of Bill Dube
> Sent: Thursday, June 21, 2012 11:54 AM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] Sag on TS LFP (RE: Shaky battery maker A123 claims
> an advance.)
>
> Under modest to high discharge rate, the ohmic heating inside the cells
> warms them up. Warmer cells have lower internal resistance.
> Also, the chemical diffusion rate goes up as temperature goes up.
> Both of these effects reduce the voltage sag. Thus, you tend to get
> lower voltage sag later in the discharge.
>
> Quite to opposite of what you get with lead-acid.
>
> Bill D.
>
> >The interesting thing about this is that the pack sag remained about
> >the same or even decreased even though the SOC was going down.
>
> _______________________________________________
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[EVDL List]

[quote]Morgan LaMoore wrote:
> I think Bill's point on the difference between lead and Lithium is that
> with Lead, the sag increases as you discharge the battery, despite the
> effects of self-heating. With Lithium, the sag will stay fairly constant
> for most of the discharge, and self-heating will cause decreasing sag as
> you discharge.

That is correct under some conditions, though there are many factors
involved. Voltage sag is the combined effect of current, internal
resistance, state of charge, temperature, and chemical reaction rates.
They all interact, and different factors dominate under certain
conditions. Blanket statements are difficult to make; what is true under
one set of conditions is *not* true under other conditions. Many books
and research papers have been written on these complex interactions!

With lead-acid, the acid in the electrolyte gets "used up" as the
battery discharges. This slows the chemical reaction rates, which
increases the apparent internal resistance. In other common chemistries
(nicad, nimh, lithium), the electrolyte is *not* one of the reactants.
So this particular factor doesn't change significantly as the battery
discharges. There are still chemical reaction rate limitations, but they
are much less noticeable.

Let's put numbers on this so you can see what I mean:

state of charge 0% 25% 50% 75% 100%
lead-acid: 100 10 5 4 3 milliohms
lithium: 50 5 3 3 4 milliohms

When fully charged, lead-acid can have *less* voltage sag than lithium.
But as it discharges, the lead-acid's sag gets increasingly worse, while
the lithium stays relatively constant until it is almost dead.

Higher temperatures increase the chemical reaction rates, which lowers
its contribution to internal resistance. However, higher temperatures
increase the mechanical resistance of the conductors. Which factor
"wins" depends heavily on exactly how the battery is constructed.

If the source of the heat comes from discharging the battery itself,
then what you gain from the higher temperature may not be as much as you
lose from the lower state of charge. This is particularly true for
lead-acid, where lower state of charge always increases internal resistance.

But even there, there are special cases. If a lead-acid battery is very
cold (subzero), the chemical reaction rate can be so low that its
internal resistance doesn't allow it to deliver much current even if
fully charged (you can't start your ICE with it, for instance). But if
you *short* the battery for a few seconds, nearly 100% of the heat is
produced *inside* the battery. This lowers its state of charge (from say
100% to 80%), but also increases its cold cranking amps (from say 100a
which won't crank your engine, to 300a which will).

Note 1: This is a dangerous practice! Don't do it above 0 deg.F or
you're likely to wreck the battery! My grandfather would short the
battery with a big wrench, and jerk it off as soon as it started to get
hot. But my brother tried around 30 deg.F and exploded the battery!

Note 2: No, just turning on your headlights or some other external load
won't do it. This old wives tale is wrong. The heat has to be produced
*inside* the battery, or you're just running it down to no purpose.
--
Ingenuity gets you through times of no money better than money
will get you through times of no ingenuity. -- Terry Pratchett
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net

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[EVDL List]

Lee,

Since the energy needs to be generated inside the battery
until it is able to generate enough current, I think that
prolonged "starting" (with the starter motor *shorting* the
battery until it increases enough in temp to create enough
amps and volts under load to turn the starter motor) will
achieve this in a safer way than shorting the battery with
a wrench and it automatically gives you the desired action
if the battery can self-heat enough from the short created
by the stalled starter motor...

Regards,

Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: xxx@xxx.xxx Private: http://www.cvandewater.com
Skype: cor_van_de_water XoIP: +31877841130
Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203

-----Original Message-----
From: xxx@xxx.xxx.edu [mailto:xxx@xxx.xxx.edu] On
Behalf Of Lee Hart
Sent: Friday, June 22, 2012 10:43 AM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Sag on TS LFP (RE: Shaky battery maker A123 claims
an advance.)

[quote]Morgan LaMoore wrote:
> I think Bill's point on the difference between lead and Lithium is
> that with Lead, the sag increases as you discharge the battery,
> despite the effects of self-heating. With Lithium, the sag will stay
> fairly constant for most of the discharge, and self-heating will cause

> decreasing sag as you discharge.

That is correct under some conditions, though there are many factors
involved. Voltage sag is the combined effect of current, internal
resistance, state of charge, temperature, and chemical reaction rates.
They all interact, and different factors dominate under certain
conditions. Blanket statements are difficult to make; what is true under
one set of conditions is *not* true under other conditions. Many books
and research papers have been written on these complex interactions!

With lead-acid, the acid in the electrolyte gets "used up" as the
battery discharges. This slows the chemical reaction rates, which
increases the apparent internal resistance. In other common chemistries
(nicad, nimh, lithium), the electrolyte is *not* one of the reactants.
So this particular factor doesn't change significantly as the battery
discharges. There are still chemical reaction rate limitations, but they
are much less noticeable.

Let's put numbers on this so you can see what I mean:

state of charge 0% 25% 50% 75% 100%
lead-acid: 100 10 5 4 3 milliohms
lithium: 50 5 3 3 4 milliohms

When fully charged, lead-acid can have *less* voltage sag than lithium.
But as it discharges, the lead-acid's sag gets increasingly worse, while
the lithium stays relatively constant until it is almost dead.

Higher temperatures increase the chemical reaction rates, which lowers
its contribution to internal resistance. However, higher temperatures
increase the mechanical resistance of the conductors. Which factor
"wins" depends heavily on exactly how the battery is constructed.

If the source of the heat comes from discharging the battery itself,
then what you gain from the higher temperature may not be as much as you
lose from the lower state of charge. This is particularly true for
lead-acid, where lower state of charge always increases internal
resistance.

But even there, there are special cases. If a lead-acid battery is very
cold (subzero), the chemical reaction rate can be so low that its
internal resistance doesn't allow it to deliver much current even if
fully charged (you can't start your ICE with it, for instance). But if
you *short* the battery for a few seconds, nearly 100% of the heat is
produced *inside* the battery. This lowers its state of charge (from say
100% to 80%), but also increases its cold cranking amps (from say 100a
which won't crank your engine, to 300a which will).

Note 1: This is a dangerous practice! Don't do it above 0 deg.F or
you're likely to wreck the battery! My grandfather would short the
battery with a big wrench, and jerk it off as soon as it started to get
hot. But my brother tried around 30 deg.F and exploded the battery!

Note 2: No, just turning on your headlights or some other external load
won't do it. This old wives tale is wrong. The heat has to be produced
*inside* the battery, or you're just running it down to no purpose.
--
Ingenuity gets you through times of no money better than money will get
you through times of no ingenuity. -- Terry Pratchett
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net

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[EVDL List]

[quote]Lee wrote:
> It's hard for me to think of any place in CA that gets cold enough for
this trick to ever be needed.

Inside one of the many environmental test chambers that we (and many
others) test their products for operation in Alaska as well as in desert
climates.
Though I don't usually park my car in one ;-)

Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: xxx@xxx.xxx Private: http://www.cvandewater.com
Skype: cor_van_de_water XoIP: +31877841130
Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203

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