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'[EE] LiOn Batteries - damaged if voltage goes too '
2007\10\26@101645 by alan smith

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I'm acting as a "consultant" on a project..ie....I just review whats being done, and give my opinions.  So if you may recall....when I posted about the battery issue...Great Power..etc.  Well...the power supply design was based on just using a regulator for the 3.3V rail, and when the battery reached 3.3V they shut down due to low battery.  I said...ought to use a buck-boost circuit so you can run this thing when the primary cells run below 3.3V.  The battery itself is a two cell in parallel to give the required ampHr capacity.   The engineer working on this came back and said the battery works from 3V to 4.2V, typical output is 3.7V, and the spec on the battery said if the cell drops below 3V, it may damage the battery.  So is this a characteristic of the LiOn chemistry?  I would think you could run them down pretty low before they give up...ie...once you drop into the boost mode, the capacity will drop off quicker but still should be able to pull power from it?
 
 Thoughts?
 

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2007\10\26@103839 by Brendan Gillatt

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alan smith wrote:
> I'm acting as a "consultant" on a project..ie....I just review whats being done, and give my opinions.  So if you may recall....when I posted about the battery issue...Great Power..etc.  Well...the power supply design was based on just using a regulator for the 3.3V rail, and when the battery reached 3.3V they shut down due to low battery.  I said...ought to use a buck-boost circuit so you can run this thing when the primary cells run below 3.3V.  The battery itself is a two cell in parallel to give the required ampHr capacity.   The engineer working on this came back and said the battery works from 3V to 4.2V, typical output is 3.7V, and the spec on the battery said if the cell drops below 3V, it may damage the battery.  So is this a characteristic of the LiOn chemistry?  I would think you could run them down pretty low before they give up...ie...once you drop into the boost mode, the capacity will drop off quicker but still should be able to pull power from it?
>    
>   Thoughts?

Li-Ion batteries loose capacity if they are charged from a low voltage.
They actually _like_ being top-up charged (as opposed to nickel
batteries). Li-Ion batteries will loose a fair bit of capacity over long
usage anyway (not that much of a problem in consumer goods which are
often discarded after a year or two).

See:
http://www.batteryuniversity.com/parttwo-34.htm

- --
Brendan Gillatt
brendan {at} brendangillatt {dot} co {dot} uk
http://www.brendangillatt.co.uk
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2007\10\26@104129 by Marco Genovesi

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>>the spec on the battery said if the cell drops below 3V, >>it may damage the battery.  So is this a characteristic of >>the LiOn chemistry?



This guy speaks about a max discharge voltage of 2.5V, but a bit safe higher voltage seems more safe..

http://www.batteryuniversity.com/parttwo-34.htm


regards
Marco




2007\10\26@110720 by Michael Rigby-Jones

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{Quote hidden}

2.7 volts is a common cut off voltage, but it varies between manufacturers.  Deep discharging these cells damages them, to the point where trying to recover them from very low voltage is actualy somewhat risky.  This is why laptop batteries will blow an internal fuse once the cell voltages have dropped too far, to prevent it being re-charged.

Regards

Mike

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2007\10\26@110904 by alan smith

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Actually...now that I've done some research....I can answer...yes, they can be damaged, unlike NiMh types.  What about LiPo ? I haven't yet found anything that says they can be damaged if you bring the cell too low.

alan smith <micro_eng2spamKILLspamyahoo.com> wrote:  I'm acting as a "consultant" on a project..ie....I just review whats being done, and give my opinions. So if you may recall....when I posted about the battery issue...Great Power..etc. Well...the power supply design was based on just using a regulator for the 3.3V rail, and when the battery reached 3.3V they shut down due to low battery. I said...ought to use a buck-boost circuit so you can run this thing when the primary cells run below 3.3V. The battery itself is a two cell in parallel to give the required ampHr capacity. The engineer working on this came back and said the battery works from 3V to 4.2V, typical output is 3.7V, and the spec on the battery said if the cell drops below 3V, it may damage the battery. So is this a characteristic of the LiOn chemistry? I would think you could run them down pretty low before they give up...ie...once you drop into the boost mode, the capacity will drop off quicker but still should be able
to pull power from it?

Thoughts?


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2007\10\26@113529 by alan smith

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Excellent resource.  In fact, I just got off the phone with them, and answers my questions on the Li based batteries in general....don't run them too low.

Brendan Gillatt <.....brendanKILLspamspam.....brendangillatt.co.uk> wrote:  -----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

alan smith wrote:
> I'm acting as a "consultant" on a project..ie....I just review whats being done, and give my opinions. So if you may recall....when I posted about the battery issue...Great Power..etc. Well...the power supply design was based on just using a regulator for the 3.3V rail, and when the battery reached 3.3V they shut down due to low battery. I said...ought to use a buck-boost circuit so you can run this thing when the primary cells run below 3.3V. The battery itself is a two cell in parallel to give the required ampHr capacity. The engineer working on this came back and said the battery works from 3V to 4.2V, typical output is 3.7V, and the spec on the battery said if the cell drops below 3V, it may damage the battery. So is this a characteristic of the LiOn chemistry? I would think you could run them down pretty low before they give up...ie...once you drop into the boost mode, the capacity will drop off quicker but still should be able to pull power from it?
>
> Thoughts?

Li-Ion batteries loose capacity if they are charged from a low voltage.
They actually _like_ being top-up charged (as opposed to nickel
batteries). Li-Ion batteries will loose a fair bit of capacity over long
usage anyway (not that much of a problem in consumer goods which are
often discarded after a year or two).

See:
http://www.batteryuniversity.com/parttwo-34.htm

- --
Brendan Gillatt
brendan {at} brendangillatt {dot} co {dot} uk
http://www.brendangillatt.co.uk
PGP Key: pgp.mit.edu:11371/pks/lookup?op=get&search=0xBACD7433
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2007\10\26@121216 by David VanHorn

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spec on the battery said if the cell drops below 3V, it may damage the
battery.  So is this a characteristic of the LiOn chemistry?  I would
think you could run them down pretty low before they give
up...ie...once you drop into the boost mode, the capacity will drop
off quicker but still should be able to pull power from it?


These cells have a low cutoff voltage that varies from maker to maker,
but you definitely don't want to discharge below that point.
I use Tenergy products from http://www.all-battery.com   They have handy
protection boards that guard against over and under voltage, and
over-current.

2007\10\26@141042 by David VanHorn

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On 10/26/07, alan smith <EraseMEmicro_eng2spam_OUTspamTakeThisOuTyahoo.com> wrote:
> Actually...now that I've done some research....I can answer...yes, they can be damaged, unlike NiMh types.  What about LiPo ? I haven't yet found anything that says they can be damaged if you bring the cell too low.

Same as Li-ion

2007\10\26@155625 by Chris McSweeny

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Personally I tend to cut LiIon cells off at 3V - gives you a bit more margin
for error that way. You can go down to 2.5 but you get very little more out
that way from my testing, as the voltage is dropping almost vertically at 3V
- I'd estimate you get less than 1% extra out. In fact unless you've got
high current draw, the battery is more than 90% used at 3.3V (probably close
to 95% - I've got discharge figures from my testing somewhere, but I'd
assume google might find you some), so I'm really not convinced it's worth
going to the effort of getting that last bit out given the added complexity.
You will also get more battery cycles if you stop discharge at 3.3V.

2007\10\26@161813 by Bob Blick

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--- Chris McSweeny <cpmcsweenyspamspam_OUTgmail.com> wrote:
> Personally I tend to cut LiIon cells off at 3V -
> gives you a bit more margin
> for error that way. You can go down to 2.5 but you
> get very little more out
> that way from my testing, as the voltage is dropping
> almost vertically at 3V
> - I'd estimate you get less than 1% extra out.

On the polymer or graphite cells this is pretty much
the case, but with the carbon anode cells there is
still some capacity left and 2.5 volts is the cutoff
for them. Although there are not many made anymore for
new applications, there are still plenty in service
and also new for the replacement market. Quite often
the only distinction made is the poly and graphite are
listed as 3.7 volts and the carbon as 3.6 volt. The
carbon cells don't have as much ampacity as the
graphite cells so you really get considerably less if
you also cut them off at 3 volts.

I really like lithium-ion batteries. My cheap Harbor
Freight cordless drills really work well now that I
replaced the crappy nicads with old laptop li-on
cells.

Cheerful regards,

Bob

2007\10\26@164926 by alan smith

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OK...thats interesting information.  So, since we are not going to boost the battery voltage but really cut it off at 3.3V, is it still a better design to use a buck rather than a LDO to regulate the voltage?  I'm thinking its more effecient since the input will be at the higher voltage longer.  My concern is what happens to a buck when the input voltage equals the output voltage?

Chris McSweeny <@spam@cpmcsweenyKILLspamspamgmail.com> wrote:  Personally I tend to cut LiIon cells off at 3V - gives you a bit more margin
for error that way. You can go down to 2.5 but you get very little more out
that way from my testing, as the voltage is dropping almost vertically at 3V
- I'd estimate you get less than 1% extra out. In fact unless you've got
high current draw, the battery is more than 90% used at 3.3V (probably close
to 95% - I've got discharge figures from my testing somewhere, but I'd
assume google might find you some), so I'm really not convinced it's worth
going to the effort of getting that last bit out given the added complexity.
You will also get more battery cycles if you stop discharge at 3.3V.

2007\10\26@190736 by Chris McSweeny

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I'd be tempted just to go with the LDO - worst efficiency will be 79% at
start of discharge with battery 4.2V, but it doesn't stay there for long.
Most of the discharge you're around 3.7V, where the efficiency is 89%, which
is about as good as you're likely to get a buck convertor to do (and better
than you'll get from a buck/boost), and you will be struggling with the lack
of overhead to get a buck to regulate properly. The input equalling the
output isn't a problem though - at that point a buck just goes direct drive.

On 10/26/07, alan smith <KILLspammicro_eng2KILLspamspamyahoo.com> wrote:
{Quote hidden}

> -

2007\10\26@190910 by Chris McSweeny

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Fair enough - only got experience with the newer cells (I take them from
laptop packs off ebay, though tend to buy brand new ones to get new cells -
have been burnt with buying cheap older packs).

On 10/26/07, Bob Blick <spamBeGonebbblickspamBeGonespamsbcglobal.net> wrote:
{Quote hidden}

> -

2007\10\27@020858 by Jesse Lackey

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Hi, I've done two designs with li-ions, for both I used the same circuit
- linear's LTC3440 buck/boost to generate 3.3V for one and 3.5V for the
other (which then ran thru 3.3V LDOs b/c it was an RF design and noise
mattered) and used the LTC1440 micropower comparator to shutdown the
LTC3440 at ~2.9V.  If you have microcontroller with an A/D you could do
a trick whereby at power-on you read the batt voltage, and if below 3V
flash a LED and turn off.  Of course, you have to be able to shut
everything down, including the dc/dc, a problem left to the designer.
(these were qty 50 designs so I took the more expensive but easier /
faster to finish route.)

J

alan smith wrote:
{Quote hidden}

2007\10\27@153345 by Bob Axtell

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David VanHorn wrote:
> spec on the battery said if the cell drops below 3V, it may damage the
> battery.  So is this a characteristic of the LiOn chemistry?  I would
> think you could run them down pretty low before they give
> up...ie...once you drop into the boost mode, the capacity will drop
> off quicker but still should be able to pull power from it?
>
>
> These cells have a low cutoff voltage that varies from maker to maker,
> but you definitely don't want to discharge below that point.
> I use Tenergy products from http://www.all-battery.com   They have handy
> protection boards that guard against over and under voltage, and
> over-current.
>  
The issue is one of safety. LiIon cells are capable of awesome
destruction if allowed to overheat.
I suggest that you include a thermistor into your battery pack and USE
it to monitor pack temperature.

I have only done ONE lithium Ion battery pack system; it was a dual pack
at 7.2V.

--More later

--Bob

2007\10\27@153553 by Bob Axtell

face picon face
alan smith wrote:
> OK...thats interesting information.  So, since we are not going to boost the battery voltage but really cut it off at 3.3V, is it still a better design to use a buck rather than a LDO to regulate the voltage?  I'm thinking its more effecient since the input will be at the higher voltage longer.  My concern is what happens to a buck when the input voltage equals the output

I love buck switchers. Very efficient.
{Quote hidden}

--Bob

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