adam_one

Well, that's another detail.
According to some manufacturers you don't damage your NiCads as long as you charge them at C/10 even during long time.
But typically it takes about 14 -16 hours to charge a full discharged battery at C/10.

The delta peak chargers came along to facilitate those who have models with electric propulsion.
Whether its healthier for the battery to be charged at 0.5 or 1C I'm not really sure.
However, it is important not to exceed the charging time when charging at more than 0.1C, otherwise it may damage or shorten the battery life.

RJConnet

Sorry folks, I did not mean to imply that anyone is stupid. It is to myself that I always apply the KISS theory.
Actually , 90 % of my charging is done at Approx. the C/10 rate. I just don't worry too much whether it's C/6 or C/12, I simply adjust the time.

adam_one

Ok RJConnet, I must agree that the KISS theory is a really good one…

raymcm

RJ

My question is how do you define the time if you dont know the exact state of your pack.

Constant charging at 1/10C is not a good idea, although a couple of hours extra will not do much harm.


Adamone

Delta chargers came along before electric flight/cars

I use a supernova for all packs, TX, IC-RX, power packs, cordless drill, radios, cd players, kids toys... everything. The big plus is the discharge and battery analysis features which enables me to accuratly monitor the condition of my packs and decide when to chuck them. I dont recommend everyone buy one of these, but a cheap delta peak will be less than $20 and is miles better than guesswork.

Raymond

adam_one

Well, if one is really keen on keeping the NiCads "healthy" its advisable to discharge each element of the pack down to a limit of 1.1V before start charging them with the constant current method.
Since repeatedly recharging an already fully charged battery or a battery with a large part of its charge remaining will degrade performance.

NiCads stand more "abuse" than the NiMH types.
Fast charges at 1C or even 2C using the delta peak method are common practice nowadays. But it requires having a reliable method of stopping the charge once the battery is fully charged.
There are "negative delta V (-DV)" and "zero delta V (0D)" detectors.
"Change of temperature (dT/dt)" detectors are also commonly used.
Since this depends on how good the electronic design is, many manufacturers use negative or zero delta V together with change of temperature detection, in case one method fails to detect.

Regarding this I would suggest a good/reliable delta peak charger in case one considers the "old" method of constant current charging being too slow or obsolete.


Cheers

raymcm

Adam

Its actually a bit less than 1.1v, but you got the idea. Anyway we will all be moving to Li Poly soon so dont spend too much on a new charger yet

Ladyflyer

My Feedback: (2)

RJC you are very welcome

All constant currrent sources have limitations. The limitations in hobby products are built in. I merely made some additional comments to help clarify ,for some the fact that enough series resistance can and will decrease the current flow ,even in a "constant current" circuit. Of course there is no such thing as a perfect constant current source


adam you are welcome also,IF you thank you is sincere the charger in question IS designed to charge at a constant current ,so your conditional "if" does not validate your statement..
Nearly all the chargers of the type in question are of similar "constant current" design and they cannot raise the voltage sufficiently to compensate for the added resistance. That was and is the point.


I just tried to clear up some ambiguity. Redundancy is nothing new to these threads, and since I added some comments to help some of the people who find electronics not so "easy" no apologies We all come here to learn. What is redundant to one may open the door for another to learn.
I'm glad you were able to learn a bit about the type of charger being discussed .

adam_one

Yes, my thank you was sincere, despite I could feel the irony in your comments about the simplicity concepts…

Unfortunately however, and based upon description of those "interesting" wall warts, I just can't agree with you.
Simply because, with a normal transformer together with some rectifying diodes, you'll get neither a constant voltage nor a constant current source. In such a case, both the output voltage and the output current are pretty much dependent on the actual load.
Which means, assuming you have a high resistance load you'll get a high voltage along with a low current, whereas with a low resistance load you'll get a lower voltage along with a higher current.
Obviously it will depend on your own definition of what a constant current source is, but it's obvious that my own definition greatly differs from yours...
In fact, I'd rather agree with Rodney - Titusville, Florida, who wrote the following:
…most are simply transformers and diodes that depend on the winding resistance to limit the current to about 50 to 70 ma, they do not have constant current circuitry built in.

So, unlike your nice wall warts, a battery charger based on constant current is designed to keep the same charging current during all the charging time, from completely discharged to a fully charged battery.

As for keeping the things simple:
Yes, I really agree that one should not make things difficult when they can be made simple.
But just note the word when (that's an important word in this context).

Well, I guess it's all for now.
Thanks again for taking the time in giving attention to this matter.

Cheers,

Ladyflyer

My Feedback: (2)

Sure Adam,
I think the question was posted in reference to the Wall Wart type charger.
The wall wart (a slang term for the little transformer package ) is an impedance limited device meant to deliver a relatively constant current . Many of them will put out 50 ma even in a dead short. It is a simple inexpensive method that delivers a useful contant current charger for the intended purpose.
Since they are running at the saturation level an increase in series resistance works for further current limiting.
Not much more current to get but less is easy .

Cheers

nitro joe

My Feedback: (2)

Wow

I started this thread over 2 weeks ago. Thought the darn thing had run it's course and died... guess not

I must say, its been a most entertaining and informative read.
Do we have some E-engineering types on board?

Many more opinions and options than I thought there would be.

Question:

What do the later arrival's think of the links to the LM317 circuits?
Can I assume that some feel it's overkill for the stated application. (parts cost ~ $10 U.S.

nitro joe

adam_one

Ladyflyer:
According to your description the famous wall wart behaves like a current limiter rather than a current constant source.
If you can reduce the wall wart current by introducing a resistor in series with the battery means that the current will fall even further as the battery voltage rises during the charging process.
That means the charging time has to be prolonged in order to compensate for the reduced current, which may result in batteries not being fully charged.

The recommended charge time of 14 hours for totally discharged batteries is therefore no longer valid with such a device.
The formula:
Charge Time = Battery Capacity / Charge Current x 1.4 is only valid assuming a true constant charging current.

Nitro Joe:
The LM317 circuit is pretty close to the ideal constant current source.
It seems to be an interesting and cheap alternative provided you can build it properly and it meets your needs.
There are many different types of NiCad chargers varying in complexity and cost.
Each type has its own pros and cons, and the choice is often a matter of particular needs /requirements.

Low cost CC chargers provide no method of detecting when the battery is fully charged. The user is expected to estimate the charging time based on the constant charging current value and the battery capacity, according to the formula above.
Providing this is done and assuming that the NiCads are discharged to about 1.1V level each time before recharging, this type of charger can be used to achieve a reasonably long battery life.

A lot of work done by battery researchers in the recent years has shown that Nicads respond better to a pulsed charging waveform than to a steady DC current…!
By applying the charge current in one-second pulses with brief "rest" periods between them, ions are able to diffuse over the plate area and the cells are better able to absorb the charge efficiently.

This is particularly true at the higher charge rates used in fast chargers. These chargers have a microprocessor that samples the "rest" periods between the charging pulses to read the battery terminal voltage.
Another paradox discovery is that the charging process actually improves even further if the during the "rest period" between charging pulses, the cells are subject to very brief discharging pulses - with an amplitude of about 2.5 times the charging current, but lasting for only 5mS (milliseconds) or so.

It is claimed that these short discharge pulses actually dislodge oxygen bubbles from the plates and help them diffuse during the "rest period". As a result, the use of these brief discharge pulses is known as "burping" or "burp charging".

Many of the high-end fast pulse chargers for NiCads use a charging current waveform complying with the above mentioned findings.
Tests by both US military and NASA have shown that NiCads charged using fast chargers employing the burped pulse system tend to last up to Twice as long as those charged by traditional CC chargers…

Well, I guess it's all for now.

Cheers,

Ladyflyer

My Feedback: (2)

Like I said before adam, there is no such thing as a perfect constant current source . Any and all "constant current sources " have upper voltage limitations don't they?
If you introduce enough series resistance the current flow will diminish.

The wall wart is a rudimentary constant current source for the range it was intended to cover.ALL CONSTANT CURRENT SOURCES have range limitations.
The wall wart was designed to provide for constant current charging . And does .

The transmitter and receiver side are nearly identical. The higher voltage on the transmitter side will usually still limit the charge very close to 55 mA when connected to a lower voltage receiver pack . If you happen to connect the receiver side of the charger to a transmitter battery there will be insufficient voltage to cause a current flow. Of course the impedance mismatch would probably ,in time damage the transmitter charger harnessed to the lower voltage receiver bat.
If you introduce enough series resistance the current flow will diminish in ANY PRACTICAL constant current supplied circuit .

Without getting redundant I also covered the SLIGHT difference in charge current in my first post here. I will repeat it for ease of reference:
"A series resistor would also drop the current but the regulation would become poorer as the charge current is decreased. Probably not enough to be of concern with the small decrease you are seeking. "

Actually the difference in current flow from start of charge to termination of charge is negligible for the currents we have been discussing. Less than 5 ma in most cases. Far from enough to cause thermal runaway And well within the latitude afforded in the C/xx charge regime. At the C/10 rate most manufacturers relate little concern over any damage being caused by a SLIGHT overcharge .
I have many batteries that have given trouble free service in heavy duty Zenon flash equipment for over 15 years. They have all been charged using the C/10 method using a simple timer and impedance limited constant current charging. .


I see we also agree on the LM 317 from your response which was nearly identical to my own about a dozen posts back.

I think any further banter may defeat the purpose of the thread by adding confusion . Back to SIMPLE for now

At least until we have the need to learn the requirements of the batteries now on the horizon.

Cheers, ELLEFF

adam_one

There are circuits designed with enough accuracy to meet the requirements for a so-called constant current source within pre-established limits.

The constant current source concept has in fact been widely used in most industrial plants to send control and measurement signals to several instruments, which are serially linked and located far away from each other.
One can actually add and/or remove many instruments from the serial instrument chain without any measurable change in the actual constant current source signal.

If the current in a given circuit falls significantly by introducing a resistor in series, that circuit can no longer be called for a current constant source as far as definition is concerned.

Cheers,

nitro joe

My Feedback: (2)

Pass the popcorn...

Ladyflyer

My Feedback: (2)

Adam ,
EVERY and ANY of the constant current sources you mention will show a drop in current when sufficient series resistance is introduced Even in your high accuracy data transfer devices.
They ALL have limits and they all have tolerances.
Like I said there are no PERFECT contstant current sources .

The charge regimes for the nicads we use also have tolerances. The simple answer to the man's question is YES he can use a series resistor to drop the current from his constant current charger . The current will have sufficient regulation to perform the task at hand . It is done successfully each and every day . It is a simple answer for a simple situation. No need to complicate it.


Adam quote ;
"There are circuits designed with enough accuracy to meet the requirements for a so-called constant current source within pre-established limits. "

And that describes the charger in question .

KISS

adam_one

If the current of the initially supposed constant current source of say 55mA (such as the actual wall warts) falls as much as 45% down to 30mA just by putting a series resistor, means that the circuit is functioning outside the range it was previously designed for.

Thus, it was put as far from a constant current source as it could be.

That's simply a matter of definitions.

If there's not such a thing as ideal car, ideal house, ideal life, ideal man, ideal woman, so why do we still have such definitions as "car", "house", "life", "woman" and "man"?

Well, there are obvious limits when a previous car is no longer a car, a previous house is no longer a house and so on, and so on…
As well as when a previous constant current source is no longer a constant current source.

Cheers,

JimTrainor

Northern Canada and Sweden
Same latitude
Different wavelength

Ladyflyer

My Feedback: (2)

Da !



No need to make the simple question at hand complicated

KISS

SamD

Yikes!

And here I was, fat, dumb and happy with my little wall wart charging my batteries after a day of flying!!

Sam

nitro joe

My Feedback: (2)

LMAO ! That was good.

doctor jason

ok try again

KB9STD

My Feedback: (1)

Every RC wall wart I've looked into (admitedly , I haven't taken every one apart) Uses a 36 volt center tapped transformer.That's 18volt on the transmitter side, and 18 volts on the reciever side.
The higher voltage is used so that the voltage potential (the difference in voltage between the charger output and the battery pack) remains relatively constant during the charge cycle(18-4=14/18-6=12, a maximum change of 15%).They utilize the resistance in the transformer winding to create a voltage drop, which is dependant on the charging current (V=I*R).
If the current is lowered (35ma versus 50ma) the voltage drop will also be reduced. What we are doing is adding additional resistance(in series) to produce the same voltage drop at the lower charge current.The circuit works the same as before.

adam_one

KB9STD,

I've never seen a "wall wart" and my comments have only been based on the descriptions I've read here.

Yet the main issue seems to remain:

Which are the requisites for an electric circuit/device to be classified as a constant current source?

Define a constant current source and determine whether or not the "wall wart" falls within such a definition and why.


Thanks

nitro joe

My Feedback: (2)

Simple Charge Rate Limiter for Small Capacity NiCad Batteries.
If you are going to be flying hand launch gliders, you will probably be using small lightweight battery packs. Since these packs can be damaged by the charge rate provided by the wall charger that came with your radio system, you will need a way to adjust down the charge rate from your wall charger. NiCad batteries seem to charge best and last longest when charged at a C/10 rate…that is a rate of one tenth the capacity of the battery. For example, a 150 mAh pack would be best charged at a rate of 15 mA, a 270 mAh pack at 27 mA, and so forth. Because of the inefficiency of the charging process, you will need to charge for 14-15 hours (rather than 10) to restore a fully discharged battery. You can plug your wall charger into a lamp timer to limit the charge time if you like.
The following simple circuits use an adjustable voltage regulator LM317T as a current limiter. The capacitor smoothes the half-wave rectified DC 55mA output from a wall charger, allowing the voltage regulator to work properly, and the LED is a kind of continuity tester, telling you your battery is in fact being charged. The LED also will prevent your battery from discharging if the power is disconnected (you will need a LED or other diode if you use a lamp timer). Don’t use a LED in this way if you plan on charging at a higher rate than 30 mA (or the max mA rating of the LED)—you will blow the LED. If your wall charger has a continuity light, you can leave out the LED altogether as in the simpler circuit. If you use a LED and resistor in parallel across the input leads (instead of in series with one of the input leads as here), you will have no indication of continuity and no clue if you are charging (done that—crashed). One circuit here includes a switch between a 15 mA and a 27 mA rate, but you can select your own values, or skip the switch and just use one resistor (definitely easiest to build).



Choose your own resistors based on the charge rate you want. You can calculate the resistance as follows: R=1.25/I, where I is amperage, R is resistance. For example, for a 15 mA charge rate (0.015 amp) requires 1.25/0.015 = 83.3 ohms. Unfortunately, Radio Shack carries only certain values, and this is not one of them. Fortunately, you need only be approximate, and by measurement of the circuit, a 82 ohm resistor gives 15.2 mA. For a 110 mAh pack, you could be compulsive and use in series a couple of resistors to get the required 114 ohms, or be reasonable with a single 100 ohm resistor (giving a measured 12.5 mA).

Mount the parts on a piece of proto board, or make your own circuit board, solder on your connectors, and you will be set.

Radio Shack part numbers:
276-1778 $1.99 LM317T adjustable voltage regulator
272-956 $0.99 220 uF axial electrolytic capacitor
275-635 $2.99 SPDT switch (or similar)
276-026 $0.99 LED mini red (make sure it has a max mA rating greater than your intended charge rate)
271-312 $7.99 ¼ watt 5% carbon film resistor assortment, 500 pieces (a lifetime supply!)
276-149 $1.19 small project board
Back to the home page.

Rob Crockett 1/97


http://www.ncws.com/rcrock/charger.htm

adam_one

That's it Nitro!

The LM317T is the best constant current source money can buy!

Cheers,