Discharging NiMH: when to stop ?

This is a newsgroup copy, it will be used for a voltage cutoff point on the robots.

Rinus Luijmes
Oct 12 1999, 2:00 am hide options
Newsgroups: sci.chem.electrochem.battery
From: Rinus Luijmes <digir...@xs4all.nl> - Find messages by this author
Date: 1999/10/12
Subject: Discharging NiMH: when to stop ?
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Hello all,

I made myself a accupack of 10 1,2 V NiMH cells (12 V, 3000 mAh) to
power a 10 W halogen light on my recumbent. Do I need a sort of
protection against over-discharging of this accu ?

To what voltage-level can I safely discharge (under 10 deg C) this
accu? Or don't I need one and can I just switch off when the light
becomes faint? I want this accupack to last as long as possible.

TIA,

--
Rinus Luijmes
N 51° 56.974' E 006° 24.655'
http://www.xs4all.nl/~digirini


Sponge!
Oct 12 1999, 2:00 am show options
Newsgroups: sci.chem.electrochem.battery
From: "Sponge!" <Battw...@sunlink.net> - Find messages by this author
Date: 1999/10/12
Subject: Re: Discharging NiMH: when to stop ?
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discharge at will....... it won't harm them.. but don't take them to the
point that they switch polarity on you 8)

- Hide quoted text -
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Rinus Luijmes wrote:
> Hello all,

> I made myself a accupack of 10 1,2 V NiMH cells (12 V, 3000 mAh) to
> power a 10 W halogen light on my recumbent. Do I need a sort of
> protection against over-discharging of this accu ?

> To what voltage-level can I safely discharge (under 10 deg C) this
> accu? Or don't I need one and can I just switch off when the light
> becomes faint? I want this accupack to last as long as possible.

> TIA,

> --
> Rinus Luijmes
> N 51° 56.974' E 006° 24.655'
> http://www.xs4all.nl/~digirini

--
Sponge!

Battery
Specialist


dave pierson
Oct 12 1999, 2:00 am show options
Newsgroups: sci.chem.electrochem.battery
From: dave pierson <pier...@mail.dec.com> - Find messages by this author
Date: 1999/10/12
Subject: Re: Discharging NiMH: when to stop ?
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Sponge! wrote:

> discharge at will....... it won't harm them.. but don't take them to the
> point that they switch polarity on you 8)

Lacking more exact info, if the pack is 12v nominal, then
when the first cell goes to zero, there will be a sort of step
to (12-1.2) or 10.8. (the point where the weakest cell is
'reversed' by the rest... It should help if discharge were
terminated at this point. 'faint' is not easy to quantify,
a voltmeter (if practical, on the bike), or, more
elaborately an automatic comparator should help...

(in general, mild reversal is not 'instant death', but, ideally
is avoided. The issue, as i understand it, is high current
reversals, eg, a cranking battery, on an engine, run flat...)

> Rinus Luijmes wrote:
> > I made myself a accupack of 10 1,2 V NiMH cells (12 V, 3000 mAh) to
> > power a 10 W halogen light on my recumbent. Do I need a sort of
> > protection against over-discharging of this accu ?
> > To what voltage-level can I safely discharge (under 10 deg C) this
> > accu? Or don't I need one and can I just switch off when the light
> > becomes faint? I want this accupack to last as long as possible.

I wouldn't.

--
thanks
dave pierson |the facts, as accurately as i can manage,
Smart Modular Technology |the opinions, my own.
334 South St |
Shrewsbury, Mass |pier...@mail.dec.com
"He has read everything, and, to his credit, written nothing." A J
Raffles
"Internet: net of a million lies..." after Vernor Vinge


Mickey McInnis
Oct 12 1999, 2:00 am show options
Newsgroups: sci.chem.electrochem.battery
From: mcin...@babe01.austin.ibm.com (Mickey McInnis) - Find messages by this author
Date: 1999/10/12
Subject: Re: Discharging NiMH: when to stop ?
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In article <38039A92.A0031...@sunlink.net>, "Sponge!" <Battw...@sunlink.net> writes:

|> discharge at will....... it won't harm them.. but don't take them to the
|> point that they switch polarity on you 8)

Sorry, but this is wrong for NiCd or NiMH cells.

If you "discharge at will", you can switch polarity on one of the cells.
This can ruin that cell.

You can only "discharge at will" if you are using a single cell.
(i.e. a 1.2 V battery) Discharging any pack of batteries in series
to too low a voltage can reverse charge a cell, since one cell will
drop to zero voltage first and the other cells will try to push more
current through it to further discharge it below zero.

To be safe, stop using the pack as soon as the voltage drops by the voltage
of one cell. You can probably discharge even further if the cells are
evenly matched. The lower the voltage, the more chance of reverse charging.
You're also probably less likely to damage the battery if you've recently
charged it. (So no one cell can have self-discharged more than the rest.)

It is hard to assess the risk, since if the cells are perfectly matched
and age identically, they'll all drop to zero at the same time, and you
won't do any damage. Unfortunately, you can't count on all cells to
be the same, or to stay the same as they age. Just because you get away
with it on one battery pack doesn't mean you will get away with it on
another.

|>
|> Rinus Luijmes wrote:

|>
|> > Hello all,
|> >
|> > I made myself a accupack of 10 1,2 V NiMH cells (12 V, 3000 mAh) to
|> > power a 10 W halogen light on my recumbent. Do I need a sort of
|> > protection against over-discharging of this accu ?
|> >
|> > To what voltage-level can I safely discharge (under 10 deg C) this
|> > accu? Or don't I need one and can I just switch off when the light
|> > becomes faint? I want this accupack to last as long as possible.
|> >
|> > TIA,
|> >
|> > --
|> > Rinus Luijmes
|> > N 51° 56.974' E 006° 24.655'
|> > http://www.xs4all.nl/~digirini
|>
|> --
|> Sponge!
|>
|> Battery
|> Specialist
|>
|>

--
Mickey McInnis - mcin...@austin.ibm.com
--
All opinions expressed are my own opinions, not my company's opinions.


Dyche Anderson
Oct 13 1999, 2:00 am show options
Newsgroups: sci.chem.electrochem.battery
From: Dyche Anderson <d...@mediaone.net> - Find messages by this author
Date: 1999/10/13
Subject: Re: Discharging NiMH: when to stop ?
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Rinus Luijmes wrote:

> Hello all,

> I made myself a accupack of 10 1,2 V NiMH cells (12 V, 3000 mAh) to
> power a 10 W halogen light on my recumbent. Do I need a sort of
> protection against over-discharging of this accu ?

> To what voltage-level can I safely discharge (under 10 deg C) this
> accu? Or don't I need one and can I just switch off when the light
> becomes faint? I want this accupack to last as long as possible.

In my opinion you do, but it does depend to a point on the type of the load. I would
recommend that you not discharge a cell below 0.8 V/cell. If you go much below this, you
can damage the electrodes - additives can dissolve out and you
may lose some power capability.

Assuming that the light is a resistive load, the current will drop as the
pack voltage drops, so the harm will not be as large. Still, a minimum voltage
on the order of 9 to 10V would be wise, and will promote a longer life to the pack.

If your most important consideration is battery life, then 10V is a better cutoff
voltage. If you only go that deep occasionally, I would suggest 9V.

If you have futher questions, please contact me.

Dyche Anderson


Ryan Shook
Oct 13 1999, 2:00 am show options
Newsgroups: sci.chem.electrochem.battery
From: Ryan Shook <rsh...@home.com> - Find messages by this author
Date: 1999/10/13
Subject: Re: Discharging NiMH: when to stop ?
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As one post says "discharge at will" do that and you will cut cycle life
to around 100. Since you are using 3.0Ah cells I assume you are using
some sort of A or long A tyoe (probably 4/3A) depending on your
discharge rate for maximum cycle life terminate at .9 Volts per cell 1
Volt per cell for longest cycle life but once again this depends upon
discharge rate/ Dont worry about reverse polarity is true but if you
deep discharge these cells it will cause sulfation on the plates

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Dyche Anderson wrote:

> Rinus Luijmes wrote:

> > Hello all,

> > I made myself a accupack of 10 1,2 V NiMH cells (12 V, 3000 mAh) to
> > power a 10 W halogen light on my recumbent. Do I need a sort of
> > protection against over-discharging of this accu ?

> > To what voltage-level can I safely discharge (under 10 deg C) this
> > accu? Or don't I need one and can I just switch off when the light
> > becomes faint? I want this accupack to last as long as possible.

> In my opinion you do, but it does depend to a point on the type of the load. I would
> recommend that you not discharge a cell below 0.8 V/cell. If you go much below this, you
> can damage the electrodes - additives can dissolve out and you
> may lose some power capability.

> Assuming that the light is a resistive load, the current will drop as the
> pack voltage drops, so the harm will not be as large. Still, a minimum voltage
> on the order of 9 to 10V would be wise, and will promote a longer life to the pack.

> If your most important consideration is battery life, then 10V is a better cutoff
> voltage. If you only go that deep occasionally, I would suggest 9V.

> If you have futher questions, please contact me.

> Dyche Anderson



Dyche Anderson
Oct 14 1999, 2:00 am show options
Newsgroups: sci.chem.electrochem.battery
From: Dyche Anderson <d...@mediaone.net> - Find messages by this author
Date: 1999/10/14
Subject: Re: Discharging NiMH: when to stop ?
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If you were to discharge to 0.9 V/cell every cycle your cycle life would
be quite short. I consider 0.9 V/cell as a 100% depth of discharge. You
will probably find that the optimal DOD for life (net amp-hr throughput)
to be on the order of 60-80%. Your cell manufacturer should be able to
give you more info on that.

Obviously, NiMH batteries cannot suffer from "sulfation" because, last I
heard, there was no sulfate in KOH. What happens during overly deep
discharges (<~0.8 V/cell) is that additives (such as cobalt) in the plates
can dissolve out, lowering the conductivity of the plate, and thus your
current capability. It will also reduce life, as the resistance will
go up as well.

Dyche Anderson

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Ryan Shook wrote:

> As one post says "discharge at will" do that and you will cut cycle life
> to around 100. Since you are using 3.0Ah cells I assume you are using
> some sort of A or long A tyoe (probably 4/3A) depending on your
> discharge rate for maximum cycle life terminate at .9 Volts per cell 1
> Volt per cell for longest cycle life but once again this depends upon
> discharge rate/ Dont worry about reverse polarity is true but if you
> deep discharge these cells it will cause sulfation on the plates

> Dyche Anderson wrote:

> > Rinus Luijmes wrote:

> > > Hello all,

> > > I made myself a accupack of 10 1,2 V NiMH cells (12 V, 3000 mAh) to
> > > power a 10 W halogen light on my recumbent. Do I need a sort of
> > > protection against over-discharging of this accu ?

> > > To what voltage-level can I safely discharge (under 10 deg C) this
> > > accu? Or don't I need one and can I just switch off when the light
> > > becomes faint? I want this accupack to last as long as possible.

> > In my opinion you do, but it does depend to a point on the type of the load. I would
> > recommend that you not discharge a cell below 0.8 V/cell. If you go much below this, you
> > can damage the electrodes - additives can dissolve out and you
> > may lose some power capability.

> > Assuming that the light is a resistive load, the current will drop as the
> > pack voltage drops, so the harm will not be as large. Still, a minimum voltage
> > on the order of 9 to 10V would be wise, and will promote a longer life to the pack.

> > If your most important consideration is battery life, then 10V is a better cutoff
> > voltage. If you only go that deep occasionally, I would suggest 9V.

> > If you have futher questions, please contact me.

> > Dyche Anderson



Ryan Shook
Oct 14 1999, 2:00 am show options
Newsgroups: sci.chem.electrochem.battery
From: Ryan Shook <rsh...@home.com> - Find messages by this author
Date: 1999/10/14
Subject: Re: Discharging NiMH: when to stop ?
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Bad choice of wording dendrites will grow on the positive electrode

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Originally Posted by Skylinux @ 2005-12-24 02:52:35

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