Two Nicad Battery Pack/Charger Tranformer Questions

[ed]

Hello,

I know little about electrical issues, so I am looking for some help
answering two questions:

1. I have a cordless drill with an 18V nicad pack. At some point, the
charger failed (no voltage measured off the leads). I took the charger
transformer apart, found a tripped thermal fuse (only because it was
outside the coils) and replaced it. The charger worked again. I
started to charge the battery pack, but found that the tranformer was
getting very hot again. I measured the voltage in the battery pack
after 10 minutes of charging (the transformer was very hot by then) and
it was 12V. So I took apart the battery pack. Inside were 15 4/5 Sub
C nicad cells. 9 of the cells measured 1.2-1.3 volts on the voltmeter.
6 of the cells measured 0 volts. Hence the ~12V I guess.

So here's the question: what about this battery pack makes the
transformer heat up to the point it burns out? There doesn't seem to
be a short in the battery pack, just a few bad cells. . .I don't
understand why this gives the transformer such grief.

2. While I was messing with this issue, I needed a drill. I went to
Freight Harbour and bought a very cheap ($30) 18V cordless drill to
tide me over. The battery pack that comes with it is rated at ~19V. I
tested the output from the pack and it was 19.2V. However, the charger
that comes with it is rated at 24V (or it says so on the wall plug).
So I figured they buried a resistor in there somewhere. However, I
tested the output from the charger -- 24.1V. So here's the question:

Will using this 24V charger on the 19.2V pack damage the charger or the
battery pack? I understand that this is a really cheap drill, but how
can they in good conscience provide a mismatched charger/pack? Is it
because 24V transformers are so cheap?

Thanks for those who bother to read all of this.

Ed.

 

[NSM]

So here's the question: what about this battery pack makes the
transformer heat up to the point it burns out?

"6 of the cells measured 0 volts"

That'll do it.

N

 

[Jerry G.]

This is normal. The way the system is designed, the supply voltage will
equalize itself to the right amount under the proper load. Your meter
alone is not properly loading the supply.

As for your old drill, the batteries probably went defective, and thus
also damaged the power pack. Don't try the power pack from the new
drill to the old one. It is possible to do some damage, or encoure a
safety issue.

Jerry G.
======

 

[James Sweet]

So here's the question: what about this battery pack makes the
transformer heat up to the point it burns out? There doesn't seem to
be a short in the battery pack, just a few bad cells. . .I don't
understand why this gives the transformer such grief.

Sounds like a few of the cells are shorted, this is causing the battery pack
to draw more current and overheating the transformer. It's possible the
transformer is damaged now but it may be fine.

2. While I was messing with this issue, I needed a drill. I went to
Freight Harbour and bought a very cheap ($30) 18V cordless drill to
tide me over. The battery pack that comes with it is rated at ~19V. I
tested the output from the pack and it was 19.2V. However, the charger
that comes with it is rated at 24V (or it says so on the wall plug).
So I figured they buried a resistor in there somewhere. However, I
tested the output from the charger -- 24.1V. So here's the question:

Will using this 24V charger on the 19.2V pack damage the charger or the
battery pack? I understand that this is a really cheap drill, but how
can they in good conscience provide a mismatched charger/pack? Is it
because 24V transformers are so cheap?

NiCd batteries are charged with a constant current, the voltage from the
charger needs to be higher than the voltage of the battery in order to
charge them, so the high open circuit voltage of your charger is normal, it
should drop down to the battery voltage when the battery is connected.

 

[Engineer]

Hello,

I know little about electrical issues, so I am looking for some help
answering two questions:

1. I have a cordless drill with an 18V nicad pack. At some point,
the
charger failed (no voltage measured off the leads).

I read the thread to date - agree with all points made.

BTW, not totally O/T but I have given up on Nicad powerd screwdrivers.
In the three I have had, all of them had failed sub-C size NiCads
would have cost more to replace than the whole unit cost in the first
place! I had modest success in the beginning by canibalizing good
sub-C cells from dead drills to keep the other running. I right PITA,
they are welded together. Eventually, I gave up and now I just power
them from a junked PC power supply (the 5 VDC high current output.)
Don't worry obout the higher voltage; the nominally 3.6 volt motor
thrives on 5 volts - lots of torque and still going strong! You do
have to live with a 16 gauge wire lead, though, and plugging in the
P/S at the worksite.

I also have a small B&D drill (6 VDC.) While it's still going
"strognishly", it now seems to need charging more often - I suspect
its going the way of the screwdrivers. I'll have to find a wired P/S
solution again!

Cheers,
Roger

 

[Dave Plowman (News)]

So here's the question: what about this battery pack makes the
transformer heat up to the point it burns out? There doesn't seem to
be a short in the battery pack, just a few bad cells. . .I don't
understand why this gives the transformer such grief.

Ni-Cads should really be charged constant current with a 'simple' charger.
Now if the charger itself was designed as a constant current device, then
increasing the load (with say a short cell) shouldn't worry it too much.

But I've seen so called rapid chargers on cheap power tools (4 hr charge)
where the charger is simply unregulated DC with a series resistor to give
something *vaguely* near constant current.

And of course few carry a stopwatch around to make sure they don't cook
them - as you will, if the four hour limit is exceeded by much.

So I'd have a look inside the charger base to see what it's actually got
in the way of electronics.

 

[Dave Plowman (News)]

BTW, not totally O/T but I have given up on Nicad powerd screwdrivers.
In the three I have had, all of them had failed sub-C size NiCads
would have cost more to replace than the whole unit cost in the first
place! I had modest success in the beginning by canibalizing good
sub-C cells from dead drills to keep the other running. I right PITA,
they are welded together.

Cheap cordless tools use cheap cells and even cheaper chargers.

Replacing those cheap cells with decent ones - and modifying the charger
to an intelligent type, or even just the old 14 hour type, can transform
that tool. Doing this to my cheap 18 volt drill driver made the starting
torque much higher - and easier to control.

Some of the reasons why an Hitachi costs perhaps 10 times more than a shed
special. ;-)

 

[ed]

Sounds like a few of the cells are shorted, this is causing the battery pack
to draw more current and overheating the transformer. It's possible the
transformer is damaged now but it may be fine.




NiCd batteries are charged with a constant current, the voltage from the
charger needs to be higher than the voltage of the battery in order to
charge them, so the high open circuit voltage of your charger is normal, it
should drop down to the battery voltage when the battery is connected.

Thanks to all who replied. This post especially answered both of my
questions on point. I don't know if I will attempt a replacement of
the bad cells in the pack. Probably more trouble than it is worth to
me, since I have poor soldering skills.

Cheers,

Ed.

 

[William R. Walsh]

Hi!

So here's the question: what about this battery pack makes the
transformer heat up to the point it burns out? There doesn't seem to
be a short in the battery pack, just a few bad cells. . .I don't
understand why this gives the transformer such grief.

You do have some shorted cells and the charger is doing its best to charge
them up. It overheats as a result...probably because there is no (or very
little) current limiting taking place.

If you were to try charging the cells outside of the pack, you would
probably find that they get at least a little warm and that they don't hold
a charge for any appreciable length of time.

Will using this 24V charger on the 19.2V pack damage the charger or the
battery pack? I understand that this is a really cheap drill, but how
can they in good conscience provide a mismatched charger/pack? Is it
because 24V transformers are so cheap?

Anything is always possible, but there are certain safety exams any
electrical device must (or at least should) pass in almost every country of
the world. Supplying a mismatched charger would probably violate some of the
standards that any of these agencies have published or require by law.

It would be my guess that either the batteries were made to charge at a
higher voltage or that the regulation is done inside the battery pack
itself. I have a cheap spotlight that works this way. The adapter provided
with it is 12 volt, but the battery inside the unit is a 6 volt. There is a
printboard inside that contains the charger plug, a cut-out to keep the lamp
from being turned on while charging, and a large resistor that gets warm
during the charge process.

William

 

[Jim Adney]

1. I have a cordless drill with an 18V nicad pack. At some point, the
charger failed (no voltage measured off the leads). I took the charger
transformer apart, found a tripped thermal fuse (only because it was
outside the coils) and replaced it. The charger worked again. I
started to charge the battery pack, but found that the tranformer was
getting very hot again. I measured the voltage in the battery pack
after 10 minutes of charging (the transformer was very hot by then) and
it was 12V. So I took apart the battery pack. Inside were 15 4/5 Sub
C nicad cells. 9 of the cells measured 1.2-1.3 volts on the voltmeter.
6 of the cells measured 0 volts. Hence the ~12V I guess.

You have 6 shorted cells, which means that the voltage opposing the
flow of current out of the charger is about 8V lower than the charger
expects. This leads to higher current draw from the charger. A more
elegant charger might cope with this, but clearly this one can't. I
suspect that if you just replace the bad cells, however, everything
will be fine again, until more cells die.

I would buy 6, or 15, new nicad, or 15 NiMH, replacement cells and fix
this pack.

Will using this 24V charger on the 19.2V pack damage the charger or the
battery pack? I understand that this is a really cheap drill, but how
can they in good conscience provide a mismatched charger/pack? Is it
because 24V transformers are so cheap?

It's hard to be sure about this, It might or might not work, but it
certainly won't work if the bad cells are still in there.

I'd still just replace the bad cells and then you have 2 good drills.
There are lots of places who can sell you the cells you need. Get the
cells you need with solder tabs attached and you can do this yourself.

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