Battery Charged Indicator........

[Janitor Tzap]

I have two Cordless Phones that use 3.6V 700ma Nicad Battery packs
They both work fine. But they burn up battery packs in about a year or less.

The problem is mainly charging.
You put the phone back into the charging base and it shows that it's charging.
But not the condition of the battieries.

I came a cross a 12V 7ah lead acid battery with a interesting charger.
The charger has two LED's; One RED Charge LED, the other Green Status LED.
When the battery is charging only the Red LED is lit.
When the battery is fully charged the Green LED is lit.
{I couldn't take the charger apart, it's case was completely sealed!}

If I could find a circuit that will monitor the charge on the Nicad Battery Packs.
Indicating if they were fully charged, or not.

I could then pull the battery pack off the charging base when charged.
Instead of letting them sit there, and over charge.

Anyone seen something like that in a Charger for Nicad Batteries?

[Chris Smith]

There are a million ways to do this.
The best is to have the voltage or heat sensed, [Thermo or Zener cut off or switch] then switch everything to a part time charger that can trickle charge, pulse charge the battery slightly, or just top up the battery with a small charge upon a sense circuit that does the work.

A lot of "Green Lights" don’t really sense or tell you of the battery condition, [other than this transition] while others will flip back if the battery starts to go dead.

I have forgotten batteries in the charger only to have them go dead rather quickly even though the Green Light said different?

[MrAl]

Hi there,


To add to what Chris said....


There are two main charging techniques used with NiCd cells.
One is to sense the voltage, the other is to sense the case
temperature.

The problem with sensing the voltage is that you can not use any
old charger and measure the voltage... you have to use a power
supply that puts out a constant current roughly around 1C. For
your cells rated at 700mAh this would be 700ma, but even 500ma
would be good enough. This is, this has to be a constant current,
not a wall wart or something like that.
The other problem is you have to be able to measure very small
voltages, on the order of 10mv, because when the voltage FALLS
by 20mv that means the cell is done charging. This is true regardless
of the previous charge state of the cell so you always know it's charged
correctly.

The problem with sensing the temperature is that you have to have
a sensing device (such as thermistor) mounted just right so that
it makes good contact with the case. You look for a sharp rise
in temerature, and that means the cell is fully charged and the
current is now only being used to generate heat within the cell.
This method doesnt require a precision current supply like you need
with sensing voltage.
You might even be able to use one of those low cost temperature
meters you buy at WalMart and tape it to the cells. Keep an eye on
the temperature, which will rise little by little, then all of a sudden
will start to rise much faster...that's how you know they are charged..
then remove the pack from the charger.

[Janitor Tzap]

I see I forgot too mention a few things......

The Battery Packs are in the Phones themselves when they are sitting on the charging base.
So a Quote:"Temperature meters you buy at WalMart and tape it to the cells."
Is out of the question.

I have found that when then packs are fully charged, they are about 4Vdc - 4.1Vdc.
Isn't there a circuit that will light a Green LED when the voltage reachs that point?
These phones are only worth like $15 a piece when I bought them.
So I don't want to spend alot of $$$$ on them.

[Chris Smith]

If the system is real cheap like you say, just cut one of the charger feed wires and add in a 5 to 25 ohm small resistor [try a pot?] into the wire feeding the battery.

This way you will limit and cap just a small amount of energy, possibly raising the life span of the cells.

[You can meter all this action prior to soldering]

Other than this, you really have to start tearing things apart, and its probably not worth it?

[MrAl]

Hello again,

Well, measuring the voltage like that doesnt work very well because
the cell voltage depends on quantities that change over time, and are
not very well predictable.

If you are looking for something simple, then how about this...
Measure the charge current the way it is now and record this
reading (probably in ma's).
Calculate the time it takes to charge the cells when they are fully
depleted...Since your cells are 700mAh, and let's say you measure
100ma from the charger, multiply 700ma times 1.4 to make up for
the charge efficiency of NiCds, which gives us 980mAh, then divide
by the charge current (which we guess is 100ma but you need to
measure this). For our example, 980/100 is 9.8, which is roughly
10 hours to charge.
Now you also buy a timer at a local store, which can go up to at
least 12 hours. You set the timer for 10 hours and when it
rings you turn off the charger or whatever. This way the most
charge it gets will be 10 hours.
You can try charging once per week if you dont use the phone too
much, or maybe once every other day if you do.

As far as measuring the absolute battery voltage, today it might be
4.00 volts but a month or maybe three months from now it might
be 4.200 volts to a full charge. See? There is no way to really be
sure by simply measuring the voltage itself. This is why NiCd
chargers are built either very simple where they run all the time,
or with chips that sense what is called the "minus delta of the voltage",
and as said before you need a precision current source and a meter
that can measure a 20mv drop in cell voltage.
If you like, you can build a fairly cheap constant current source and
measure the voltage with an ordinary digital meter. You have to
keep an eye on the meter and wait for that 20mv drop, then hault
the charging. Let me know if you want to try this. The parts would
run maybe 4 dollars and add a dc wall wart that you might already
have laying around. You also need a digital meter however.

[cato]

The charger might be over charging your batteries, but I doubt it.

Your NiCads are dieing because you don't let them drain to dead.

Nicads want to be drained.

Silly you, want to have your phone operational at all times , so you keep them charged. Thats whats killing them. They need to be deep cycled.

[MrAl]

Hello again,

I forgot to comment about the Lead Acid batteries...

Lead acid batteries are more predictable when measuring
their absolute terminal voltage. For example, in room
temperature i charge one of my LA batteries up using about
500ma from a wall wart until my meter reads 7.500 volts,
and then i know it's fully charged. With NiMH and NiCd you
cant really do this. You might get an approximation to this
however if you charge for a long time (say 10 hours) and
then read the terminal voltage and use that as your guideline,
but then repeat the test 6 months later to see if this full charge
voltage has changed. Oh yeah, you must do it at the same
temperature too. If you are indoors you probably will get this
as room temperature is usually around 72 degrees, but keep
an eye on it anyway.

One additional note, NiMH are even harder to charge accurately
becuase their response voltage curve changes less than that of
NiCd cells while they are being charged. Course, they do make chips
for this already and im pretty sure they are available on DigiKey.

I almost forgot to mention...
The nice thing about making your own charger is you will learn more
about how these things are charged, and what is good to do and what
isnt good to do.
Also, if you build your own you can build one charger or you can build
100 chargers and they wont cost you as much as if you go out and buy
them.
Also, if you get a new cell phone in the future with a different kind of
battery you can build a new charger to fit that one too!
Nice, huh?

[Will]

Some years ago I made a Battery charger control for the wheelchair of an old lady who lived in the same village. Her wheelchair battery was fairly crude in that it furnished a fairly high open circuit voltage to produce a high charge rate so that the battery could charge quickly when required. The problem with it was that, when she got back from the pub at night she would put the chair on charge but the rate was so high that, once the battery was re-charged after two or three hours, it would gas off furiously which is, of course, dangerous - so she had to wait for it to gas off, then turn it off, before she could go to sleep. She was a little nervous about me interfering with the chair wiring because, if the chair had to be serviced under warranty, that might void the warranty. So! i designed a circuit with a 25A germanium transistor (MOS hadn't even been heard of then) which simply connected between the charger output and the battery. If it became necessary then all they had to do was to disconnect my unit and re-connect the charger output back to the battery. Much to my surprise - it all worked perfectly.
What I did was to arrange some logic together with a 555 (Or 556 ?) timer voltage/trigger level inputs so that, whenever the device sensed a voltage less than about 12.2 volts(I think) (Set by adjusting a pot) it would switch on the Ge transistor for 30 minutes, after thirty minutes the 555 would switch off the Ge, wait one minute or so and then check the battery voltage. If it was less than 12.2 volts it switched the charger on again for 30 minutes. If it was greater than 12.2 volts then it left the charger off for 30 minutes and then went through that logic again. The reason for the one minute sampling delay was to give the battery time to de-polarize. I never found out whether or not one minute was long enough to wait for that but the thing seemd to work OK. In practice waht this meant was that, when the battery was relatively new it might get a half hour charge say once every 12 - 24 hours if that was necessary but, as the battery condition deteriorated, if it became necessary to have a 30 minute charge every 2 or 3 hours then that's what it would get. I don't have any details of the circuit I built (About 1971 time - 36 years ago in England) but I feel certain that, from the above description, many of the guys on this forum could produce a better design than I did. If any more explanation is required, I will be happy to furnish it.
Epilogue - After the thing worked for some eighteen months, I left the village and went to work in Iraq. When I got back after about a year, the old lady gave me back my device. During my absence, something had gone wrong with her wheelchair so she had an friend, (Not of mine) who was a radio operator in the merchant marine, look at it. He came to the conclusion that it was my device that had screwed up the operation so he removed it; they got a new battery and went back to the old system of waiting for the gassing off to start. Since he held more sway with her than I I could'nt convince her to replace the device. I had a spare battery in my own garage that I liked to keep charged so I connected the device to my spare battery and, for about the next nine years, it kept the battery charged for months at a time without any problem.

[Will]

Sorry Guys ! - In the third line after 'bettery' it should say 'charger' i.e the battery charger was crude, not the battery.

[MrAl]

Hi Will,

Did you know there is an edit function on here? You can go back
and re open your post and edit it, then post new.

Also, Lead Acid batteries can be charged by bringing the battery up
to it's float charge voltage with the right amount of current (not to
exceed about C/4 rate). When i charge my LA battery i use about
a C/10 rate (approximately) and simply keep the charger connected
until the battery reaches it's "float charge" voltage. This works
pretty well with Lead Acid batteries.
Automobile battery chargers work about the same way, like the
alternator/regulator in the car. They limit current and cut back
the current as the float voltage is approached. They use
about 2.4v per cell as the float voltage.

[Chris Smith]

Car alternators and batteries are self regulated by the battery and the whole circuit in balance. [Ohms law]

A car alternator can produce all of its amps when needed by the battery, [55 amps to 100 amps or more?] at the given voltage and only the battery decides what is needed.

There are no special circuits in the system to decide for you or the battery. There is a over all voltage regulator and nothing else.

[only generators needed a amp regulation]

The system is a balance between the maximum voltage of the battery and the ohms of the system, and the required amps that are missing from the circuit battery.

If the battery is low, the voltage in the system will rise to its maximum, then the amperage flow will follow to the maximum over all on the way to the battery and the circuit will charge at its fastest.

When the voltage and the current is filling up and then its full, the voltage based on the overall ohms resistance of the system from the battery will slow down the amperage flow inwards all on its own, and the battery will be topped up or off to its max,... and no further current will flow because the two systems are then equal and thus no flow will occur.

Like a large pipe and tank, one is empty, it then fills, then it finds equilibrium on its own.

The value of the battery is the start and stop points of this system.

The charge light of old was on the similar basis having one battery positive and one alternator lead upon the start of the car. The alternator during its rest [no spin] was just a ground wire.

When the system is started, the bulb now feeds this coil circuit to start the alternator system, then the alternator takes off and starts to charge on its own [energized] and now when two positives are at the light bulb,... no further current can flow,.... and thus the light goes off and you know the system is charging.

The best way to think of a car system is “balancedâ€

[Janitor Tzap]

UPDATE:

Well, I took a part the charging base for one of the phones and made a horrible discovery.

The contact points of the charger where the phone sits on the charger base has 6 VAC on it!
Which means the AC Voltage is rectified in the hand set!
And there is no room in the hand set to place extra circuitry into it.

Well, this shoots me down.

But, thanks for all the help.

[Robert Reed]

Actually. the battery is floated once the charging system comes into play. You wouldn't even need the battery at this point if you could provide adequate filtering (capacitors). this holds true wheather the system is automotive or a telco terminal or many other similar situations. Once charging is enabled , the charger supplys all the load current and the battery is just along for the ride (hence the term "floating"). However, as mentioned,it does provide tremendous filtering in lieu of capacitors and is being trickle charged all the while being readied for those times it has to stand alone and provide power (startup,etc.).

[Chris Smith]

Do what I said, drop a small amount of current for this cheap circuit.

It works as best as possible.