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Recondition cordless batteries? Dan_Musicant 04-22-2007
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Posted by Jim Yanik on May 12, 2007, 2:18 pm

>
>>
>> :Chris Lewis wrote:
>> :> As I understand it, if you're going to discharge this way, you
>> :> want to stop discharge just as the voltage across each cell is
>> :> around 1V.
>> :
>> :As I understand it, if you're going to discharge this way, you want
>> :to stop discharge just as the voltage across *the lowest cell* is
>> :around 1V. By the time you discharge until the average cell is
>> :around 1V, or until the pack voltage is about 1V per cell, or worse,
>> :until all cells but one are below 1V, chances are good that you will
>> :be able to measure reversed polarity across the weakest cells.
>> :That's an indication that those cells are no longer cells.
>>
>> I agree.
>
> Amen! Been there, done that.
>
> As I read the literature, good multiple cell batteries are made up
> from uniform cells all from the same production batch. Then you hope
> they stay uniform.
> Best to quit using the battery as soon as you detect a slowing speed.
> But my wife would take a half discharged device and run it till it
> stopped. Hard to recover from that.
>
> I now keep small appliances like hand vacs and grass clippers on
> chargers modified to trickle charge (added resistance). For
> frequently used tools, the slow charge might not be at all convenient
> but for occasional use devices, the resulting full charge adds a large
> margin of safety. My old Dustbuster has 10-year old cells used this
> way and still does its job.
>
> SJF
>
>
>
>

the cells in the Dustbuster are designed to be left on charge continuously.
digi-Key sell s NiCd cells expressly designed for continuous charge.
And the transformer already trickle-charges the cells.(C/10 rate)

My first DB pack lasted 10 yrs,then had the pack replaced at a Dewalt
service center.

--
Jim Yanik
jyanik
at
kua.net

Electric Radiant Heat 468x60
Posted by Dave Martindale on April 26, 2007, 12:16 am


>Chris Lewis wrote:
>> As I understand it, if you're going to discharge this way, you want
>> to stop discharge just as the voltage across each cell is around
>> 1V.

>As I understand it, if you're going to discharge this way, you want
>to stop discharge just as the voltage across *the lowest cell* is around
>1V. By the time you discharge until the average cell is around 1V, or
>until the pack voltage is about 1V per cell, or worse, until all cells
>but one are below 1V, chances are good that you will be able to measure
>reversed polarity across the weakest cells. That's an indication that
>those cells are no longer cells.

As I understand it, you can safely discharge NiCd down to zero volts,
but no further. If you had some way of measuring each cell's voltage
individually in a battery pack, you could discharge the pack until one
cell reached 0 V. But you generally can't do this, so the "1 V per
cell" rule of thumb is an approximate way of finding a point where one
cell's voltage is diving fast.

For example, a 4-cell pack is 4.8 V (4 * 1.2 V) nominal. When its
voltage drops to 4 V, there are *probably* still 3 cells putting out
almost 1.2 V, while the weak cell has dropped to 0.4 V. Time to stop
discharging.

Unfortunately, this doesn't work so well with larger packs. When a
10-cell 12 V pack drops to 10 V, you may have 9 cells still at 1.2 V,
and one unlucky cell already reverse-charged to -0.8 V. So pack
manufacturers try to match cell capacity within a pack so you don't get
one cell significantly weaker than the rest.

Also, NiMH cells don't like being taken all the way to 0 V.

        Dave

Posted by clifto on April 27, 2007, 1:40 am


Dave Martindale wrote:
>>As I understand it, if you're going to discharge this way, you want
>>to stop discharge just as the voltage across *the lowest cell* is around
>>1V.
>
> As I understand it, you can safely discharge NiCd down to zero volts,
> but no further. If you had some way of measuring each cell's voltage
> individually in a battery pack, you could discharge the pack until one
> cell reached 0 V. But you generally can't do this, so the "1 V per
> cell" rule of thumb is an approximate way of finding a point where one
> cell's voltage is diving fast.

I've had exactly zero luck with any cell that ever went as low as 0.8V
for more than a second or two. I've read articles by others who've had
the same experience you claim, but I've been completely and utterly
unable to duplicate it under any circumstances.

> For example, a 4-cell pack is 4.8 V (4 * 1.2 V) nominal. When its
> voltage drops to 4 V, there are *probably* still 3 cells putting out
> almost 1.2 V, while the weak cell has dropped to 0.4 V. Time to stop
> discharging.

Safest way I've found with packs is to watch for a sudden small drop in
voltage. When the weakest cell gives way, it'll drop from 1.1V or
thereabouts over a few seconds to less than a volt. That'd take the
10-cell 12V pack to, say, 11.6V in a few seconds.

--
Music hath charms to soothe the savage breast.
That's why stereo has two channels.

Posted by Jim Yanik on April 23, 2007, 7:41 pm


clewis@nortelnetworks.com (Chris Lewis) wrote in

>
>> Most drill/driver battery packs do not have terminals one can clip
>> onto,they have flat contacts recessed to prevent accidental shorting.
>> One would have to build a socket for the pack to plug into,with
>> contacts to mate with the pack's contacts. Or cannabalize an
>> old,spare charger for the socket/contacts.
>
> If you're just trying to discharge it a few times, you can get away
> with masking, duct or electrical tape putting pressure on a chunk of
> wire against the contacts.

Good way to start a fire. NiCds can source a LOT of current in a short
time. NiMH too.

>
> The Dewalts I'm familiar with have spade lugs that are easy to clip
> to.

On the outside of the pack? Or do you have to open it up first?

the DeWalts I've seen have the same flat contacts as other brands,that
prevent accidental shorting



--
Jim Yanik
jyanik
at
kua.net

Posted by Chris Lewis on April 24, 2007, 2:50 pm


> clewis@nortelnetworks.com (Chris Lewis) wrote in
>
> >
> >> Most drill/driver battery packs do not have terminals one can clip
> >> onto,they have flat contacts recessed to prevent accidental shorting.
> >> One would have to build a socket for the pack to plug into,with
> >> contacts to mate with the pack's contacts. Or cannabalize an
> >> old,spare charger for the socket/contacts.
> >
> > If you're just trying to discharge it a few times, you can get away
> > with masking, duct or electrical tape putting pressure on a chunk of
> > wire against the contacts.
>
> Good way to start a fire. NiCds can source a LOT of current in a short
> time. NiMH too.

I wasn't referring to putting a single wire across the contacts,
tho, I can see how it could be read that way.

Oops.

I was trying to say that the ends of the wires connected to the
load (eg: lightbulb) can be taped to the contacts, and it will
be good enough for a couple of hours of discharge as long as you
don't yank on anything. Or rubber bands.

> >
> > The Dewalts I'm familiar with have spade lugs that are easy to clip
> > to.
>
> On the outside of the pack? Or do you have to open it up first?

Outside.

> the DeWalts I've seen have the same flat contacts as other brands,that
> prevent accidental shorting

Take a look at the 12V versions. There's a rectangular opening in the
upper end of the battery housing. Inside the opening, there's two male
spade lugs separated by a plastic divider. When you drop the battery
into the charger or tool, the spade lugs go into a split female
connectors.

You could just about plug those lugs into female automotive spade
connectors. Perhaps slightly more heavy duty ones.

It's easy to connect to the lugs with alligator clips.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.

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