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Two Nicad Battery Pack/Charger Tranformer Questions

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.

  1. While I was messing with this issue, I needed a drill. I went to Freight Harbour and bought a very cheap () 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.

Reply to
ed
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"6 of the cells measured 0 volts"

That'll do it.

N
Reply to
NSM

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. ======

Reply to
Jerry G.

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.

Reply to
James Sweet

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

Reply to
Engineer

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.

--
*A nest isn't empty until all their stuff is out of the attic

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

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. ;-)

--
*Don't sweat the petty things and don't pet the sweaty things.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

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.

Reply to
ed

Hi!

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.

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

Reply to
William R. Walsh

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.

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.

-

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

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