Entertainment Center: TVs, Stereos, VCRs and DVDs - Nickel-Cadnium batteries

Does anyone know what the best way is for charging/discharging Ni-Cd batteries are for a cordless phone?
A neighbor has had several, but the newest phone one is 6 months old and allready losing time on how long it stays charged, I blame it on having the handset on the base constantly charging.
Note that he does not use the cordless set frequently.
(Panasonic KX-TG2237S model is probably still available, or updated similar)

I always recalled that rechargeable batteries should fully discharge, or close to it before a recharge. THe batteries have a so-called "memory".
Any ideas?
gj

There's a lot of conflicting ideas on nicads that I've seen.
Old theory specifies completely discharging batteries
before recharging to avoid memory effects. New
batteries say there's no memory effects and they
can be topped off anytime.

What to believe? good question.

I can say one thing for sure... one can prematurely kill
a nicad if it's completely discharged too deeply too often.
The cell will ultimately short itself out or reverse bias itself
a lot sooner than the advertised lifetime.
So complete discharge isn't a good thing to do.

aaron

The new batteries are commonly Lithium Ion (Lion). They don't develop memories.

NiCads can be fully discharged to bring back their full use life - up to a point. Once they're shot, they're shot.

Try using a 12-volt lamp to drain it. A lamp from a car's dome light will take it down in a hurry. Small alligator clamps and a couple of wires is all you need.

I'm going to try draining it to "reset" the NiCd.
gj
I am also questioing the phone transmitter, I may call Panasonic tech support, or swap it with a new unit from the store, I think the handset was dropped when there were some guests staying over one weekend about a month ago.

Ni-Cad developing memory is real. As Rick said, it has to be "fully discharged" before "fully recharging". Let me explain the words, "fully discharge" and "fully charging" here

The word "fully discharge" does not mean discharge until the voltage of each cell in the battery is zero volt. Doing so will damage the cell(s). Here is how.

Each Nicad holds 1.2V, if the phone used 6V, it will consist of 5 cells in series. During use and while the batteries are not exhausted, each of the 5 cell will pass DC current through its positive terminal and through the circuit and back to the negative terminal of the battery.

However, all NiCads are not created equal, they do not held the same amount of charges everytime it is fully charged. So when one of cell is exhaused before the rest of the cells, and the batteries are continued to be in use, the weak cell will have a DC current passes through its negative terminal and then out the positive.

This condition is reversed from its proper charging condition (DC current come in through positive, out through negative). It will create an irreversible chemical reaction (I don't remember what gass it created) which will shorten or damage the cell(s).

So, when you fully discharge nicad battery, it is important that you discharge until one cell is out. In the case of 6V (5 cell), you discharge until you get 4.8V, in the case of 12V (10 cell) you discharge until 10.8V. I hope you get the idea. Do not discharge the battery beyond this point. You wil be killing it.

Now how does recycling the battery can help recondition the battery? This is because when you discharge the cell (properly) and then fully recharge it. You always recharge at the rate of 10% of the capacity of the cell. For example, if the cell is 600 mAh, you then charge at 60mA for 10 nhours. Charging Nicad at this condition does not damange the cell even if you slightly overcharge the pack. By slightly overcharging the pack, it will also make the weak cell in the pack able to catch up to have the capacity equal or close to the rest of the cell. In this case recharging more than 10 hours (12-14 hrs) is healthy.

Ni-cad is the oldest recharging battery technology. NiMH (Nikel Metal Hidride)is the newer technology that has no memory and store more capacity on the seme weight and size. The newest technology is Lithium Ion (3.6V/cell) has much higher capacity per weight and size to the former predecessors. However, NuMH and Li-ion can not be discharged at high dischage rate like NiCad.

Oh well, I'd better stop before confusing you guys more and more.

Just to let everyone know what the end result is:
The phone worked fine the next day after simply leaving it totally disconnected for 24hrs.
It must have reset itself.
gj

Some battery info I came across: http://www.taunton.com/finehomebuilding/pages/h00106.asp

The web site that GregH suggested is helpful to understand the mysteries of the rechargable batteries. I recommend reading it to pick up additional information on Ni-Cad.

One thing I want to point out is the article mentioned about heat which is the enemy of the Ni-cad. This is because the article refer to "Fast Charging" the Ni-cad batteries.

The charging condition at 10% capacity as I mentioned before is "slow charge" which does not create dangerous heat if the pack is slightly overcharged. In most portable tools, they can not afford to wait 12-15 hrs each charge cycle, so they do it at much higher charge current (Fast charge).

Charging with higher current creates heat from faster chemical reaction inside the cell. It is worse when the cell reaches its max charge. If the charging continues, the heat will cause the cell to outgass, then leak fluid and loss capacity. It will deteriorate or destroy the cells.

The charger that comes with the tools have auto cutoff circuit built in. Some measure sharp rising in temperature which indicates fully charge or measure "negative delta voltage". The negative delta voltage is an accuraqte way to measure the cell at fully chare than the temperature.

It is the characteristic of Ni-Cad that its voltage per cell raises during charging. As the cell reaches its peak capacity, the voltage stop rising and then start to drop as the cell continue to be charged. At the place where the voltage start to drop (negative delta voltage), it is the peak capacity of the cell. Most automatic Ni-cad charger terminate the charge at this point.

For Ni-MH, the concept is still applies, however the volteage will not go negative at full charge. The charger terminate the charge at the point where the voltage stop rising and remain constant (Zero-delta voltage).

Hope this helps explain the misteries of rechargable batteries somewhat.