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PICList Thread
'Battery short circuit protection [OT]'
1998\01\21@054803 by David Lions

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Hi,

May I draw on the eternal wisdom of the list?  I have a problem with
battery protection.

Our product uses a 12V 3AHr NiMh battery pack (and 3 PIC's).  The
battery pack is designed for use in rough environments, and for this and
other reasons, the terminals have been left relatively well exposed.
This is a problem if the battery is short circuited.  The batteries are
a small but expensive part of the unit.

Can anyone suggest a way of short circuit proofing the batteries?  The
method has to :

1. Be user-resettable after a short circuit, or self-resetting.

2. Respond very quickly after the current limit is exceeded (batteries
are quickly damaged).  Ideally the current limit will hardly be exceeded
at all, even for an instant.

3. Draw very little or preferably no quiescent current (NiMH
self-discharge automatically :-).

Some "resettable" fuses that we've tried are way too slow.  The current
limit by the way is 9A.

Any ideas?


Thanks.
David Lions
spam_OUTbjlionsTakeThisOuTspammagna.com.au.NOSPAM

1998\01\21@073102 by Najemy, Daniel

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Daniel Najemy - Numaliine Power Systems

{Quote hidden}

1998\01\21@073724 by Najemy, Daniel

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Here's a handful of  ideas:

1. Check out Raychems polyswitches - although the SRP series is rated
too low for you application, you may
   be able to use the RUE series (assuming you can put this outside of
the battery pack).

2. Put a mosfet in series with the battery voltage, sense the battery
voltage with a micro/low power voltage
   comparator, and then turn the mosfet off when the voltage begins to
collapse - or sense the current w/ a
   low power shunt and do the same.  You would probably us a p-channel
fet and pull the gate low during normal
   operation, this way you won't need a charge pump to drive the
n-channel above the battery voltage.

3. Unitrode/Linear tech have some ic's that do something similar to 2. I
use the LTC1153 in a similar application.

There aren't too many more solutions out there - I think 2 above is the
slickest way to do it.

Daniel Najemy - Data General Corporation,  Numaliine Power Systems

> {Original Message removed}

1998\01\21@082513 by mike

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We use Polyfuses on our power supply.

When we first got samples to play with we connected them directly across
a car battery to see what would happen. It was remarkably unspectacular.


Regards,


Mike Watson

In message  <EraseME34C5D398.4B33E1ACspam_OUTspamTakeThisOuTmagna.com.au> PICLISTspamspam_OUTMITVMA.MIT.EDU writes:
{Quote hidden}

--
Denison Mayes Group

1998\01\21@132032 by wwl

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On Wed, 21 Jan 1998 21:53:12 +1100, you wrote:

>Hi,
>
>May I draw on the eternal wisdom of the list?  I have a problem with
>battery protection.
>
>Our product uses a 12V 3AHr NiMh battery pack (and 3 PIC's).  The
>battery pack is designed for use in rough environments, and for this and
>other reasons, the terminals have been left relatively well exposed.
>This is a problem if the battery is short circuited.  The batteries are
>a small but expensive part of the unit.
>
>Can anyone suggest a way of short circuit proofing the batteries?  The
>method has to :
>
>1. Be user-resettable after a short circuit, or self-resetting.
>
>2. Respond very quickly after the current limit is exceeded (batteries
>are quickly damaged).  Ideally the current limit will hardly be exceeded
>at all, even for an instant.
How quickly is 'very' ? 1S?, 1mS?, 1uS?
>3. Draw very little or preferably no quiescent current (NiMH
>self-discharge automatically :-).
If they self-discharge anyway, a little extra for a protection cct
won't hurt much, would it?
>Some "resettable" fuses that we've tried are way too slow.  The current
>limit by the way is 9A.
I'd use a polywitch (Raychem/Bourns) - small, reliable & cheap.
Although slow at marginal overloads, they trip fast on larger
overloads, which in this application would be what you'd normally have
(i.e. dead short rather than, say 50% overcurrent). I'd be surprised
if there would be much risk of battery damage within the trip time.
If your batteries are shorted so frequently that you'd be worried
about damage, then change the mechanical design!
Even with current-limit, there may be safety issues - 12V at 9A could
easily set fire to things, or burn someone if shorted. I'm not sure if
any of the electrical safety standards say anything about short
circuit current hazards - it's probably worth a look.

The only other sensible choice would be a mosfet & current sense
circuit, but this would probably be more expensive.
You don't say what the ratio of normal to fault current is - this
would make a difference to how precise the sensing cct needed to be.



    ____                                                           ____
  _/ L_/  Mike Harrison / White Wing Logic / KILLspamwwlKILLspamspamnetcomuk.co.uk  _/ L_/
_/ W_/  Hardware & Software design / PCB Design / Consultancy  _/ W_/
/_W_/  Industrial / Computer Peripherals / Hazardous Area      /_W_/

1998\01\21@132037 by wwl

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On Wed, 21 Jan 1998 07:35:12 -0500, you wrote:

>Here's a handful of  ideas:
>
>1. Check out Raychems polyswitches - although the SRP series is rated
>too low for you application, you may
>    be able to use the RUE series (assuming you can put this outside of
>the battery pack).
>
>2. Put a mosfet in series with the battery voltage, sense the battery
>voltage with a micro/low power voltage
>    comparator, and then turn the mosfet off when the voltage begins to
>collapse - or sense the current w/ a
>    low power shunt and do the same.  You would probably us a p-channel
>fet and pull the gate low during normal
>    operation, this way you won't need a charge pump to drive the
>n-channel above the battery voltage.
No need for P channel or charge pumps - remember that in a battery
pack you could switch the negative rail.

    ____                                                           ____
  _/ L_/  Mike Harrison / White Wing Logic / RemoveMEwwlTakeThisOuTspamnetcomuk.co.uk  _/ L_/
_/ W_/  Hardware & Software design / PCB Design / Consultancy  _/ W_/
/_W_/  Industrial / Computer Peripherals / Hazardous Area      /_W_/

1998\01\25@204128 by Larry G. Nelson Sr.

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Take a look at the Raychem Polyswitches. The battery should be able to take
anything they will allow to pass before they trip and after the short is
removed they reset themselves. I have used them on power busses for medical
equipment. The only complaint is they cause a slight voltage drop that you
need to take into account.


At 09:53 PM 1/21/98 +1100, you wrote:
{Quote hidden}

Larry G. Nelson Sr.
TakeThisOuTL.NelsonEraseMEspamspam_OUTieee.org
http://www.ultranet.com/~nr

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