Charging a Ni-cad battery

[Ken]

I have a beard trimmer that runs on a Ni-cad battery that I recharge about once a month when the battery runs low. Recently, the battery stopped being able to hold a charge. I charged it for ~12 hours, tested the voltage, and got zero. OK, fine, so I soldered in a new ni-cad, and everything works fine now. The trimmer is only a few years old, so I thought the battery died earlier than I would have expected.

Then I got to thinking about the charger, which is just a simple wall wart. The battery is one AA ni-cad, 1.2 V 600 mAh. The wall wart charger is labeled 2.3 V, and I measured 7.5 V DC actual output. The charger is what came with the trimmer, and molded into the trimmer is something about "use only charger # such-and-such", which is also the number on the charger. So I'm sure that the charger is the one that the manufacturer intented to be used with the trimmer.

Did the higher than expected voltage on the charger lead to the early demise of the ni-cad battery? Should I find a new wall wart that has a voltage closer to 1.2 V? If so, what current output should it have? (I save each and every wall wart from every dead appliance that I have ever owned, so I have a wide selection to choose from a box in the attic, although I think most of them are 5V and up.)

Ken

[Joseph Meehan]

You found that voltages change depending on how you measure them.

The battery is rated at the voltage it can deliver (likely under a specific load).

The charger is rated at the voltage it delivers under a certain load.

The voltage you measured was likely with a modern digital meter that have a very high resistance, resulting in the high voltage recorded.

If you were to measure the voltage when you first put a battery in need of a charge on it and then measure it again when the battery is fully charged you are going to get two more measurements, the last likely near that 2.3V specified for the charger.

Measure the voltage output of that 5V vampire with that same meter and no load and you might get something about 10V.

The moral of the story, stick with the one designed for the use you have.

[Rich256]

Stick with the recommended unit. The open circuit measurement doesn't mean much.

1.325 is the voltage of a fully charged cell.

For a longer cell life don't leave it in the charger all the time.

[m Ransley]

Under charge load it probably is the right charger and voltage of apx

1.36-1.5v. 1.35 is fully charged for a nicad. 1.2v is considered discharged. Likely just a cheap battery as alot of junk is made. Sanyo and Panasonic are tops. Just don`t leave in the charger and overcharge, and over discharging ruins nicads. When it just slows it is dead. 1.2v is discharged

[Stormin Mormon]

I've got a Norelco that's doing a similar thing. I carry it in the truck for when I forget to shave at home.

It's only got one cell. I'm really tempted to drill a hole in the side of the shaver, and wire on a single AA battery holder. Put a high capacity NiMH cell in. Then in two years just pop in another cell. Or use an alkaline (I've got lots of those) and change battery when it gets weak.

[Rich256]

You might want to use at least a C cell.

I wonder if it would happen if it was hooked to 12 volts. DC motors draw very little current unless they stall. The faster they run the more back EMF they create. Still, 12 might be too much for it.

I did something along that line. I had an old rechargeable B&D screwdriver with two sub C cells. They died and the instructions were how to dispose of the whole thing. I opened it up and about three feet of heavy wire out. I found that the charger would charge 4 batteries if left on long enough so I hooked up 4 D cells. It has power now!!

[tnom]

Get rid of the problematic Ni-cads

Go to Wal-Mart and buy the NiMh rechargeables. They are the same type of battery that digital cameras often use. The have no real memory effect and are cheaper than N-cads. You can usually find the batteries sold with a charging unit. I have found a four AA battery charger with four batteries for as low as $10 at Wal-mart.

You can also browse

[Bob S.]

Lead acid battery chargers are voltage dependent (i.e. 13.5v to charge a 12v battery). Ni-Cad battery chargers are current dependent (i.e. ~

500ma charging current). So the voltages you measured are somewhat irrevelant. You can take a 12v battery charger and hook a 12v lightbulb that draws 300-500ma in series with your Ni-Cad battery and it will charge just fine. I've done it for years.

Bob S.

[Joseph Meehan]

If I am guessing right, the OP needs a battery that is soldered in the device. He may not be able to find a NiMh with the proper solder strips attached. Really no need for NiMh as the Ni-cads do just fine.

[clifto]

A fully-charged cell that's in good condition, you mean. As they lose capacity through abuse, the terminal voltage goes up.

[Edwin Pawlowski]

From my experience in using ni-cads for the past 30 years or so, you did good. they do not last forever.

No it is probably just fine. Some chargers give a high reading under no load. Others can expain it better than I can. Stick with what you have.

[Jim Yanik]

"Joseph Meehan" wrote in news:jrrne.12213$ [e-mail]:

The NiCd battery in his device is probably not a standard AA size,either. It could be a 2/3A,N,or smaller NiCd.("A" cells are slightly larger in diameter than a AA cell.)

[Jim Yanik]

clifto wrote in news: [e-mail]:

ISTR that it takes 1.55v per cell for charging a NiCd.(loaded V) You need more voltage than what the cell is rated for.

[Jim Yanik]

"Edwin Pawlowski" wrote in news:ZLrne.13152$ [e-mail]:

I have a Dustbuster that the first pack lasted for 10 years,the cells are designed to be charged 24/7/365,while most NiCds are not. The Dewalt repair center charged less to replace the pack than for what I could have bought new cells from Digi-Key(of the same charge type).

[Olaf]

There are a couple pros to using ni-cd over NiMH if the info I read on the net was actually correct.

Ni-Cd hold a charge for longer when not in use and I believe will endure more charge cycles then NiMH. Also, NiMH is rather nasty if it is heated to above a certain (rather high) temperature.

I usually tape the trigger on when my Ni-Cd tools run low and let it completely drain the battery before I charge it. They hold a good charge for quite a while. I've found the tool wears out before the battery does... but I'm a little rough.

[DJ]

It is generally accepted that you can charge sealed NiCads at 1/10th capacity on a continual basis. ( 500mA battery on a 50mA charger)

DJ

[Gideon]

Olaf,

Good advice, but I don't agree with taping the trigger to fully discharge the cells. That is overkill and all of my electrical engineering buddies have advised me that 100% discharge to avoid memory effects is not the way to go. When you notice that your drill is struggling to operate, then recharge at that time. You'll get much better life from the cells.

Gideon

===============

There are a couple pros to using ni-cd over NiMH if the info I read on the net was actually correct.

Ni-Cd hold a charge for longer when not in use and I believe will endure more charge cycles then NiMH. Also, NiMH is rather nasty if it is heated to above a certain (rather high) temperature.

I usually tape the trigger on when my Ni-Cd tools run low and let it completely drain the battery before I charge it. They hold a good charge for quite a while. I've found the tool wears out before the battery does... but I'm a little rough.

[mabelmapleleaf]

Andy writes

I disagree. It is generally accepted that the 1/10 rate is the desired rate to charge a NiCad without overheating. Once the cell is fully charged, you can THEN overcharge it if you leave it there..... So 1/10 for about 15 hours is a good rule of thumb to get to full charge...... If you want to "float" it, drop the charging current to 1 or 2 ma to just equal the self-discharge rate.

Andy

[Gideon]

DJ wrote: It is generally accepted that you can charge sealed NiCads at 1/10th capacity on a continual basis. ( 500mA battery on a 50mA charger)

==========

DJ,

Yep - that is the 16 hour charge rate and it is the basis for many/most dumb chargers. Unfortunately, many of us are in a hurry and some manufacturers have put out equally dumb quick chargers which will begin toasting battery packs once they have reached full charge. Sadly, the general public doesn't realize why their cordless items need new battery packs every year or so.

Gideon

[mabelmapleleaf]

Since the best way to charge a NiCad is with a constant charging current, it looks like the wall wart has been design to do that. It probably has a rectifier inside and a series resistor set to approximately:

7.5 - 1.2 ---------------- ohms or around 1200 ohms .05

in which case it is a very inefficient supply, but will provide a fairly constant charging current of, in this case, 50 ma.

This is a cheap way to do it, but it is foolproof , works very well, and doesn't have a failure mode that results in a battery explosion....

You can verify this by reading the open circuit voltage (which you said was 7.5 volts, and shorting the output to ground and reading the short circuit current. That gives you the internal Thevinen resistance. If it is anywhere in the vicinity of 1K-2K, you know that is what they are doing...... I doubt that there are any electronics inside the wall wart other than a rectifier and a resistor, and probably not even a filter capacitor since it isn't needed......

If this is the case, Meehan is correct when he says you should not substitute another wall wart since it will not have the current limiting resistor.

Andy