As part of an article entitled, "Better use of Dry Batteries" ( U.K Magazine = Practical Electronics - July 1986) a charger circuit was discussed. There is a circuit diagram of a simple charger, with enough information to build it.
I did build one several years ago, &, though I have not carried out any extensive testing; it does appear to "rejuvenate" batteries.
I built it to charge AA & D cells, I found that the AA cells often leaked & the results obtained varied from manufacturer to manufacturer.
Still, I don't have to buy as much batteries as I used to :).
Recently someone asked about recharging ZnC batteries. According to an article on recharging primary cells which I think appeared in the November 1980 issue of (American) Popular Mechanics and also in (UK) Elektor c1991 (the Elektor article was better as it was more up to date):
[What is the most "practical" to maintain a good battery?]
Don't overcharge it... don't leave it cooking in the charger for days at a time... and don't overdischarge it by running it down with an external resistor or light-bulb or LED.
Nickel-cadmium BATTERIES should NOT be completely discharged. To do so runs a serious risk of damage to the battery.
According to what I've read, in battery-manufacturer literature, it is safe to discharge individual cells all the way down to zero. It's usually unnecessary to do so, but it can be advantageous in some occasional cases... a full discharge of a cell to zero will cure"voltage depression", which can occur if a cell or battery is overcharged (left in the charger for too long).
It is NOT safe to discharge a BATTERY of NiCd cells. The reason is that one of the cells will probably run down before the others do, and the"live" cells will continue to force current through the exhausted cell. This leads to a condition known as over-discharge... it's just as if you had inserted an exhausted NiCd cell into a battery charger with "+" and"-" reversed. Overdischarging a NiCd cell will damage it... the cell develops internal short-circuits which will cause it to run down prematurely in the future, and eventually the cell will no longer take or hold a charge at all.
Healthy NiCd cells have a voltage-vs.-charge-level curve which is quite flat. They deliver very close to 1.2 volts per cell until almost all of their charge has been exhausted... then the voltage level drops off very quickly. By the time the cell voltage drops to 1.1 volts per cell, only a few percent of the original charge level remains. At least one manufacturer (Gates) states that the battery should be considered to be exhausted when this voltage level is reached... the small amount of power remaining in the battery cannot be extracted safely, without running the risk of overdischarging one of the cells and damaging it.
Most well-designed camcorders (and, I infer, the NoteBooks) includes power-management hardware which monitors the battery voltage. When the voltage drops to 1.1 volts per cell, the battery is considered to be exhausted and the machine shuts down.
[Will regular partial discharges with complete recharges limit the charge-life of the battery?]
No. There is something known as the "memory effect", which can limit NiCd battery capacity if the battery is _repeatedly_ discharged to _exactly_ the same partial-discharge level, and then fully recharged, many times in a row. Gates[1] states that the memory effect is almost never seen in practice, because it only occurs if the partial-discharge level is repeated very precisely many times in a row.
There _is_ an effect which can mimic the "memory effect", in the sense that it makes the battery look as if it is losing capacity. This effect, known as "voltage depression", occurs if you over-charge a battery. The battery's output voltage drops from 1.2 to about 1.05 volts partway through the discharge cycle, and this may "spoof" a power-monitoring circuit into believing that the battery is exhausted.
Voltage depression is curable. It can be cured by fully discharging each cell of the battery... INDIVIDUALLY... all the way to zero, and then recharging the battery. You can do this if the battery design allows you to access the individual cells. You can't do it if you can't get to the individual cells, but only to the battery terminals.
Alternatively, you can discharge the entire battery until the total voltage drops to 1.0 volts per cell, and then recharge it... do NOT try to discharge the battery all the way to zero, or you will very probably damage it. This 1.0-volt-per-cell shutoff should be safe (in particular, it leaves a good safely margin for any battery rated at a nominal output voltage of 6.0 or less) and should discharge all of the cells well past the voltage-depression point.
You can avoid overcharging by taking the batteries out of the charger when they've been fully charged. If you need to keep NiCd batteries in a "floating" application... if they must be be kept constantly "topped up" to full charge without human intervention... then you should use a charger which is intelligent enough to switch to a low-rate trickle charge once the battery is full. I believe that a trickle-charge rate of about C/100 or so (e.g. 10 milliampere, for a 1000-milliampere-hour battery) is in the right ballpark - it will compensate for the battery's rate of self-discharge.
[I don't want to have to completely discharge the battery every time I use it if I can help it!]
You do not need to. If you have a habit of leaving your battery cooking in the charger for longer than it needs, you might be nudging it into voltage depression. If so, then you might want to reset the battery every couple of months, by leaving it in the PowerBook (with the PowerBook turned on and sleep-mode disabled) until the PowerBook battery manager shuts the machine down due to a low-voltage condition. You shouldn't need to do this more than every few months, if at all.
My understanding is that the total useful life of a NiCd depends to some extent on how deeply the cell/battery is discharged during each cycle. A NiCd might be good for 1000 partial discharges (say, from 100% down to 75%), but for only 500 or less complete discharges (down to the 1.0 volt per cell, 98% discharge level). Therefore: if you deliberately deep-discharge your NiCd cells every time you recharge them, you are actually _wasting_ an appreciable fraction of their use life... it's counterproductive.
Bob Myers writes:
(From General Electric's tech. note regarding memory)
"Among the many users of batteries in both the industrial and consumer sectors, the idea of a memory phenomenon in nickel-cadmium batteries has been widely misused and understood. The term 'memory' has become a catch-all 'buzzword' that is used to describe a raft of application problems, being most often confused with simple voltage depression.
To the well informed, however, 'memory' is a term applied to a specific phenomenon encountered very infrequently in field applications. Specifically, the term 'memory' came from an aerospace nickel-cadmium application in which the cells were repeatedly discharged to 25% of available capacity (plus or minus 1%) by exacting computer control, then recharged to 100% capacity WITHOUT OVERCHARGE [emphasis in the original]. This long term, repetitive cycle regime, with no provisions for overcharge, resulted in a loss of capacity beyond the 25% discharge point. Hence the birth of a "memory" phenomenon, whereby nickel-cadmium batteries purportedly lose capacity if repeatedly discharged to a specific level of capacity.
The 'memory' phenomenon observed in this original aerospace application was eliminated by simply reprogramming the computer to allow for overcharging. [Note that no mention is made of adding an intentional *discharge* to clear the problem - RLM] In fact, 'memory' is always a completely reversible condition; even in those rare cases where 'memory' cannot be avoided, it can easily be erased. Unfortunately, the idea of memory-related loss of capacity has been with us since. Realistically, however, ' memory' cannot exist if any one of the following conditions holds:
(End of quote from GE tech. note)
This note goes on to list the following as the most common causes of application problems wrongly attributed to 'memory':
"To recap, we can say that true 'memory' is exceedingly rare. When we see poor battery performance attributed to 'memory', it is almost always certain to be a correctable application problem. Of the...problems noted above, Voltage Depression is the one most often mistaken for 'memory'.....
This information should dispel many of the myths that exaggerate the idea of a 'memory' phenomenon."
James A. Zaun writes:
Companies are known to lie (or play down) certain negatives in order to sell a product. However, memory is largely myth. Here's why...
Long-term continuous overcharging produces an artificially induced drop in capacity that resembles memory. It can also decrease the overall life of the cell. A deep discharge/charge cycle will recover much of the cell's life but long-term damage is very likely. This is not "true" memory because the cell is not subjected to repeated charge/discharge cycles that the cell eventually remembers. It's simply a decrease in capacity due to overcharging, and yes, it is mostly reversible. It is also not memory because the point at which the cell capacity drops out varies with the rate of discharge. The capacity loss due to long-term continuous overcharg- ing is caused by loss of contact of the cadmium hydroxide particles with the negative plate. Electron microscope pictures show that overcharging causes the particles to grow larger, especially at higher temperatures. This reduces the surface contact with the pores of the negative plate. A deep discharge/charge cycle restores the hydroxide particules to their normally smaller size -- increasing surface contact. Overcharging on the negative plate occurs when all the cadmium hydroxide is converted to cadmium metal. Once that occurs, only hydrogen gas and heat are produced (Oxygen gas is produced at the positive plate at the point that it becomes overcharged.) These gases, especially hydrogen, will eventually vent from the cell if overcharging continues, thus reducing the effectiveness of the electrolyte.
The real meaning of memory effect comes from precisely repeated charge/ discharges (without overcharging) of sintered-plate nickel-cadmium cells where the cell seems to remember the point of discharge depth. The effect is exceedingly difficult to reproduce, especially in lower ampere-hour cells. In one particular test program -- especially designed to induce memory -- no effect was found after more than 700 precisely-controlled charge/discharge cycles. In the program, spirally- wound one-ampere-hour cells were used. In a follow-up program, 20-ampere-hour aerospace-type cells were used on a similar test regime. Memory effects showed up after a few hundred cycles. [Test program conducted by Pensabene and Gould at GE, I believe.] This kind of memory appears to be related to the "efficiency" of the positive plate. It seems that repeated precise charge cycles affects the ability of the cell's active chemicals to charge fully, after which the positive plate begins to produce oxygen (as if being overcharged). Hence, it is possible for both gases and uncharged particules to exist simultaneously. Strangely, if the cell is carried out into overcharge the memory effect largely disappears. Hence, overcharging actually reverses the "true" memory effect.
Another reason memory effect is a myth since all the consumer charger's I've seen actually overcharge until there is a slight voltage drop (due to an increase in resistance from the formation of larger cadmium hydroxide particules that cause contact loss). It's because consumer chargers actually overcharge that you have to give the battery a deep discharge from time to time. It has nothing to do with memory.
And just in case you are wondering what a sintered-plate is, the plate is constructed by sintering [welding without melting] a fine nickel powder with a surface area of about one square meter per gram. This produces a honeycombed structure that is about 80% open pores. The negative plate is then impregnated with cadmium hydroxide. The positive plate is impregnated with nickelous hydroxide (which converts to nickelic hydroxide when charged).
This memory effect thing is mostly an urban myth. Memory only affects one very specific kind of battery: sintered-plate nickel-cadmium designs. Pocket-plate nickel-cadmium batteries are free of the effect, as-well-as, all nickel metal-hydride designs. Nickel metal-hydride batteries share the same positive nickel electrode as its older cousin, but the negative electrode is made from hydrogen-storing metal alloys, such as the lanthanum-based alloys (developed by Philips).
[So how does it affect the battery life, if one frequently takes the notebook off the AC strip, uses it for a short while off the AC, and then plugs it back in (where of course it is being recharged)?]
This has less to do with the battery and more to do with the "smartness" of the charging system. If your charging system forgets that the battery is already charged, and thus overcharges the battery until it detects that the battery is already at full capacity, you will "dry" out the battery. Overcharging causes the electrolyte to disassociate into hydrogen gas at the negative plate and oxygen gas at the positive plate. These gases, especially hydrogen, do not diffuse back through electrolyte separator very easily and can build up enough internal pressure to cause the battery to vent. Many of the newer charging systems maintain a charge/discharge history on the battery and are smart enough to not even attempt to recharge until the battery has been discharged more than 10% or 20%. If your computer employs one of these newer systems you have nothing to worry about. Famous last words.
A Rayovac applications engineer visited our company and told us the following:
It seems that these cells will perform quite well if you avoid deep discharge cycles. If you come back from camping, fishing, or working on the car, don't just throw your flashlight in the corner -- charge the batteries right away. They will serve you longer.
The non "high-cap" cells at RS, are the same junk everybody else peddles, with the D being a C inside.. You can pick up a C and a D and if they weigh the same, you know what is going on. The High-CAP D cells are HEAVY.
Alexander battery sells a cell-phone pack for $42 which has six "A sub F" (like AA, but 0.1 inch larger diameter) which are 1.5 AH! instead of the usual 0.5-0.6 AH.
For now, I suppose one has to buy the pack (CL4038-UC) and "mine" them for the 6 cells. One can also buy a CL4038-SK (it is a little less than 2X the price), and "mine" twelve cells from that pack. 1-800-247-1821 or 1-515-423-8955.
There are some systems available (notably the ICS1700 Ni-Cad Battery Charger Controller from Integrated Circuit Systems) that allow you to charge at a faster rate (the ICS1700 works at .5C, 1C, 2C, and 4C). To do this, however, you must carefully monitor the battery voltage during charge, and turn off the charger (or reduce to a trickle charge) as soon as the charge voltage peaks. Otherwise, you end up cooking the electrolyte out of the battery.
Typical battery capacities are: C/10 rate is: N -- .15 A-H 15 mA AAA -- .18 A-H 18 mA AA -- .45. .65, or .85 A-H 45, 65, or 85 mA 9V -- .065 A-H 6.5 mA C -- 1.6 or 2.0 A-H 160 or 200 mA D -- 1.6 or 4.2 A-H 160 or 420 mA
Type ANSI Capacity C5mAH ------------------------------ AAA 220 1/3AA 110 1/2AA 350 AA 700 3/2AA 800 2/3Af 600 4/5Af 1200 Af 1400 7/5Af 1700 1/2Cs 750 4/5Cs 1200 Cs 2000 5/4Cs 2300 C 2800 1/2D 2300 2/3D 2500 D 5000 F 7000 SF 10000 ------------------------------Note that this is the capacity when the battery is discharged over 5 hours time period.
When you shall charge a battery, you should use a constant current source which give 0.1 C5mAH. If you have a C cell battery, the charge current is 2800 mAh * 0.1 = 280 mA. This will charge your battery in 10 hours (if there is no losses at all). But since some of the energy is lossed, you have to increase the charge time to 14 - 16 hours. This charging method will not harm your batteries, but you should disconnect the batteries after 16 hours.
Num. of Nominal Minimum
Cells Voltage Voltage
======= ======= =======
1 1.20 -0.20 (0)
2 2.40 0.85
4 4.80 2.95
5 6.00 4.00
8 9.60 7.15
10 12.00 9.25
12 14.40 11.35
"Knowing the Basics of Batteries to select and Use Them Properly", By Red Scholefield of Gates Energy Products Electronic Design -June 1989 "Choosing a Secondary Battery Technology", By Al Harville of Panasonic Ind. Co. Powertechnics Magazine -Feb.1991 "Choosing the Right Battery to Power the Portable Product", By John Costello of Duracell Inc. Electronic Products -Dec. 1992I stopped getting the Powertechnics mag. in 91. You may find other articles in the more recent issues.
My battery catalog doesn't list the capacity for 9 volt batteries, but given the physical size, one can probably estimate a capacity of around 100-200 mAH. This is congruent with my experiences with gadgets I've built here in the lab. I recently put together a couple of active filters that are based on a a TL-074 operational amp chip. Since my filter topology only needs three gain stages, I used the 4th stage for a comparator. I use a forward biased 1n914 for an approximate voltage ref and a resistor divider for comparison. I adjusted the values to flip the output at about 7.2 volts. My device draws about 2.5 - 3 mA normally. It lasts about 60 to 70 hours of use before it becomes necessary to chage the battery. The comparator drives one of those self-blinking LEDs for a low battery indicator. I don't care that the LED draws a lot of current, because it is normally OFF until the battery is too low for proper service anyway. I put a small capacitor across one of the legs of the comparator's divider so that the LED would blink one time when the filter is powered up so that one can tell if the battery is totally dead.
Expecting a regular 9 volt transistor battery to supply 5 mA continuously for a year is a wee bit optimistic. 5 mA would be believable.
These devices can charge 1 to 16 series cells. A voltage-slope detecting ADC, a timer, and a temperature window comparator determine charge completion.
Nice points:
A & A Engineering 2521 West La Palma, Unit K Anaheim, CA 92801 Tel: 1-714-952-2114
QST of June 1987 has an article on charging gelled-electrolyte batteries of all sizes:
"A New Chip for Charging Gelled-Electrolyte Batteries", by Warren Dion (N1BBH), Publushed by QST, pages 26-29, June 1987.It is a construction article based around Unitrode's UC3906 chip, and it also presents operational and charging considerations for these batteries.
It has two independent charging outputs, each of which can connect batteries from 225 to 1300 mAh (1700mAh soon), with up to 8 cells per battery. It features continuous cycling (repetitive charge/discharge), time charging (discharge and recharge batteries at some time in the future, e.g. at 8pm next Friday, ready for the weekend). While cycling a battery, the unit will compare characteristics with the last several cycles, enabling you to decide whether or not the battery is stuffed.
It was created for model aircraft enthusiasts, who have as many problems with NiCads dying mysteriously as any user, and more serious consequences than most users!
The unit costs US$150 complete (not $100 as I thought), in box with leads etc. For those who wish to do-it-yourself, the PCB and programmed OTP microprocessor may be available as a kit - haggle with the creator direct if this appeals to you.
Aurium Systems 41 John Davis Rd Mt Roskill Auckland, New Zealand Phone +64 9 627 1430
Reports are that on Page 18 there is a short article announcing a new battery charging IC from Enchip Inc. (201) 328-2049 or (201) 301-0402 FAX that has the following claims:
Contact Enchip Inc for further information:
Enchip Inc. East Hannover NJ Tel: 1-201-328-2049 Tel: 1-201-301-0402 FAX
Maxim Integrated Products 120 San Gabriel Dr. Sunnyvale, CA 94086 Tel: 1-408-737-7600
"Amega Technology" Loddon Business Centre Roentgen Road Daneshill East Bassingstoke Hampshire RG24 0NG ENGLAND Tel: 0256 330301 Fax: 0256 330302There is a small article on p.252 of the March 1993 "Electronics World + Wireless World" about these chips. They have 8 (yes, that's eight) different ways of detecting the end-of-charge point, including thermal, peak detection, inflection point, etc. They use a pulsed, periodic current reversal charge (reflex charging), and after the battery is determined to be fully charged it reverts to a "maintenance mode", which keeps the battery topped up with occasional discharge/charge pulses.
I suggest you Fax the above company if you want more info - I have about 30 pages of stuff, including technical data, description of operation, etc. There are also Demo and Evaluation boards available. Price was around 10 pounds (about US $15 I think) per chip, less for 10 or more - that was quoted in April of this year, it's probably changed by now. From the info I have, it sounds like the "ultimate" nicad charger! At least with current technology...
Disclaimer: I haven't actually dealt with the above company - my info came via a friend who has dealt with them. Several of us intend to place a bulk order for these chips, but the guy who's in touch with the company has been too busy so far. The above information is given in good faith, but you're on your own...
MAXIM can be reached at
Maxim Integrated Products 120 San Gabriel Drive Sunnyvale, CA 94086 Tel: 1-408-737-7600 Tel: 1-800-998-8800 (literature, app notes, etc.)
An 18V rms transformer will put out 18*sqrt(2) volts peak, so if you rectify the 18V, you'll get something like 25V, WITHOUT A LOAD. This will fall off very fast for a 2A transformer, when you draw enough current to charge a car battery. Basically, if you draw 2A, you'll probably get about 18V out.
My experience with the LM317 is that it can't handle much power. I don't even know if it should be used as a car battery charger. You must be careful that the difference in input and output voltage isn't too high, and that you have a good heat sink. If you have (18-13 = 5) volts across the thing, I wouldn't set my charging current any higher than 200 mA. This is almost a trickle charge. Remember that the heat you generate is current times the voltage across the regulator.
Once, we used an LM317 with a drop of 7V, and 1A going through it. It shut itself down in an unusual manner. It got hot enough to melt the solder, and fell off the board!
If anyone wants info, e-mail and I will reply with his name and address. [My e-mail address is Mike_Dewit@kcbbs.gen.nz]
National: LP2950 through LP2954. 2 fixed 5V (100mA and 250mA),
3 adjustable 1.23V-29V, 100mA and 250mA
Linear Technology: LT1073-5, 1.5V in, 5V out w/internal divider,
LT1073, similar, but out set w/external divider
Both require cap, inductor and diode
LT1173... Step down; typical 9V in 5V out.
Switching, high efficiency.
The switchers would most likely give more energy out at 5V till end of useful battery life for a 9V battery, but for a 6 or 6.25V battery it might swing the other way...
"Rechargeable Batteries Applications Handbook", by Gates Energy Products technical staff. ISBN 0-7506-9228-6, Butterworth, 1992. TK2941.R43 1991 or 621.31242 catalog listings.It'll tell you anything you ever wanted to know about NiCads.
<strong>Rechargeable Batteries Handbook</strong>
Butterworth Publishing (+1 800 366 2665)
Price: [US]$49.95 + sales_tax + $3.50 shipping.
Rechargeable Batteries Application Handbook
by Technical Marketing Staff, of Gates Energy
Products, Inc.
Butterworth Heineman, 1992.
ISBN 0-7506-9227-0
(note - correct ISBN, incorrect one is listed inside book, correct
one on back cover, per Butterworth Heineman)
$49.95 from
Butterworth Heineman
80 Montvale Avenue
Stoneham, MA 02180-2422
Tel: 1-617-438-8464
Fax: 1-617-438-1479
(The clerk at Butterworth *already* has the information about this book memorized due to this morning's calls, so there *must* be at least a little net-interest in it. :-)"Those NiCad Batteries and How to Charge Them !", By Potter DW QST (October 1981) pp. 34-35.The article describes a very simple constant-current charger using a LM317, one resistor and one diode (besides your input supply ...). It also extends this circuit to provide automatic shut-down when end-of-charge (sort-of at least) is reached. I used this idea in a product that I developed for a company (a NiCd is used as backup power for a data-acqui- sition unit), and it has been working well for a few years of constant use.
Rod Cooper, Wireless World, May, june & july 1985. Plus also September from memory?? but not sure. Also include a reasonable good charger, and construction details.
Alexander Batteries OEM division (619) 480-4445 (619) 480-1351 FAX Duracell Inc. (203) 791-3274 (203) 791-3273 FAX Electrochem Industries (719) 759-2828 (719) 759-7390 FAX Gates Energy products (904) 462-3911 (904) 462-4726 FAX Maxell Corp of America (800) 533-2836 Panasonic Industrial (201) 348-7000 Portable energy products (408) 439-5100 (408) 439-5101 FAX Rayovac Corp (608) 275-3340 (608) 275-4577 FAX Sanyo Energy (619) 661 6620 (619) 661-6743 FAX Seiko Insturments (213) 517-7700 (213) 517-7709 FAX Varta Batteries Inc. (914) 592-2500 (914) 592-2667 FAX
The number of the store here in Bloomington is 612-881-0747. You could call them and find out the franchise office number to determine if there is a store soon to open in your area. The manager's name at the store is Bill Criego.