Battery Information V1.00


[Document Version: 1.07] [Last Updated: 5/95]

1. Contributors

Alan Kilian, Allen Brown, Bill Mayhew, Bill Wallace, Bob Myers, Byron A Jeff, Chris Kaiser, Dave Platt, Dave Wrightson, David Prutchi, Donald J. Miller, George Goble, Gerry O'Rourke, Ian Hawkins, James A. Zaun, Jim Knoll, Joern Yngve Dahl-Stamnes, John Whitmore, Larry Bradley, Mark Forbes, Mike Dewit, Mike Fahlbusch, Paul Opitz, Paul Repacholi, Rick Miller, Roderick Young, Sergei Borodin, Tom Bruhns, and aslt@acad2

2. Can I recharge ZincCarbon/DryCell batteries?

(From Gerry O'Rourke)

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):

  1. It's possible to recharge primary cells up to ten times.

  2. The cells aren't fully rechargable, they charge up to 0.1 volts less than their original terminal voltage.

  3. The cells should be recharged when they discharge to 1.2V.

  4. Cells should be disposed of when they won't fully charge anymore or don't power the device for long.

  5. The charge current is an AC square wave with an approximately 50% duty cycle, about -1 volt for the negative going part and about +3V for the positive part of the square wave. The negative voltage is to stop electrolyte buildup on the electrodes.

  6. The charge current is about 10mA (positive and negative) or the batteries capacity in Ah/100 gives an approximate charge current.
Personally, I wouldn't recharge primary cells because I don't use batteries at all, but if you use volume as in several sets a month it would probably be worthwhile. Using rechargable NiCd's would probably be better as they can be recharged many more times and chargers for them are readily available.

3. Can I bring my NiCd batteries from the dead?

NiCd cells which have developed internal short circuits can sometimes be "zapped" back to life by a high-current pulse, which burns out the metal whiskers which caused the short. This is usually done by charging a good- sized capacitor up to 5 volts or so (from a DC power supply, through a series resistor), and then "sparking" the NiCd cell from the capacitor... the sudden current surge will vaporize the whiskers. The cells should be removed from the battery [pack] before this is done... don't try to "zap" the cells while they're still connected to one another or to any other equipment.

4. How SHOULD I care for/charge my NiCd batteries?

[This one will change a LOT very soon]

[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.

5. Do NiCd batteries have "memory"?

[Definition: a "cell" is a single 1.2-volt device. A "battery" is two or more cells wired up in series/parallel, giving a multiple of 1.2 volts/multiple of current.]

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:

  1. Batteries achieve full overcharge.
  2. Discharge is not exactly the same each cycle - plus or minus 2-3%
  3. Discharge is to less than 1.0 volt per cell.
Remember, the existence of any ONE of these conditions eliminates the possibility of 'memory'. GE has not verified true 'memory' in any field application with the single exception of the satellite application noted above. Lack of empirical evidence notwithstanding, 'memory' is still blamed regularly for poor battery performance that is caused by a number of simple, correctable application problems."

(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':

  1. Cutoff voltage too high - basically, since NiCds have such a flat voltage vs. discharge characteristic, using voltage sensing to determine when the battery is nearly empty can be tricky; an improper setting coupled with a slight voltage depression can cause many products to call a battery "dead" even when nearly the full capacity remains usable (albeit at a slightly reduced voltage).

  2. High temperature conditions - NiCds suffer under high-temp conditions; such environments reduce both the charge that will be accepted by the cells when charging, and the voltage across the battery when charged (and the latter, of course, ties back into the above problem).

  3. Voltage depression due to long-term overcharge - Self-explanatory. NiCds can drop 0.1-0.15 V/cell if exposed to a long-term (i.e., a period of months) overcharge. Such an overcharge is not unheard-of in consumer gear, esp. if the user gets in the habit of leaving the unit in a charger of simplistic design (but which was intended to provide enough current for a relatively rapid charge). As a precaution, I do NOT leave any of my NiCd gear on a charger longer than the recommended time UNLESS the charger is specifically designed for long-term "trickle charging", and explicitly identified as such by the manufacturer.

  4. There are a number of other possible causes listed in a "miscellaneous" category; these include:

To close with one more quote from the GE note:

"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).

6. Do NiMH batteries have "memory"?

[Does anyone know if/how the newer NiHM[sic] batteries are affected by memory effects?]

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.

7. What's the deal with the "Alkaline Rechargers" on TV?

I have done some testing on these batteries for possible use in a portable consumer product, and one thing that I found was that although Rayovac claims that these batteries have a full charge right out of the package, this is not always necessarily so. This may have had an effect on your data.

A Rayovac applications engineer visited our company and told us the following:

  1. Renewal Cells will have a total capacity greater than that of a similar sized NiCd, but somewhat less that a regular Alkaline cell. The capacity of these cells is measured down to the 0.9 volt per cell point.

  2. The claimed 25 charges is a rough number. Each charge cycle results in a reduction of capacity. When the charge lifetime is short enough to get on your nerves, you throw them away.

  3. If you don't discharge the batteries all the way down to 0.9 volts per cell, you will get MORE charge cycles. If you consistently run them below the 0.9 volt point, you will get less. These batteries love to be topped off. (ongoing testing that I am running confirms this)

  4. These cells contain no mercury, so they are "green".

  5. The principle reason regular alkaline cells can not be re-charged is because of excess hydrogen gas that is built up within the cell. Renewal batteries are possible because of a tweak in their chemistry that eliminates this problem.

  6. The battery charger (power station) has a special ASIC that measures the internal battery impedance of an inserted cell to make sure that it does not charge an inserted NiCd. The recharger will refuse to try to charge a NiCd.

  7. The Power Station will not attempt to charge a regular alkaline or a zinc-carbon cell because of the way that the charger makes electrical contact with the positive end of the cell. If you look closely at the top (nub) end of the cell, you will notice that the outside insulation jacket does not QUITE go to the top of the cell. This is where the positive charging contact touches the cell. On a regular cell, this point is insulated. If you scrape the insulation away from a regular cell, the exposed surface is the NEGATIVE side of the cell, not the positive.

  8. The charging of each cell must be closely managed. This is why a special ASIC in the power station is employed. Unfortunately, this makes it difficult to use these in OEM applications where they are sealed in a multi-cell pack. It could conceivably be done if taps were brought out from each cell contact. This is probably enough to make it too expensive to design a Renewal Pack.
The above pretty much constitutes a complete dump of what I know about these cells, so I am the wrong source for followup questions. If someone wants to research this further and generate a FAQ, they are welcome to use this article as a starting point.

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.

8. What are the different types of batteries?

The Radio Shack "High-CAP" D cells are great (4AH). Back in '87 I built a portable terminal, which used 18 of the RS High-CAP D's.. It has a Zenith Z-181 Laptop, a Telebit trailblazer, and a cell phone. Runs great.. Still on the original cells since '87. They only get cycled 4-5 times/year... holds the charge for months.

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.

Charge rates are defined by the amp-hour capacity of the battery. If you want a safe charge, but one that takes 12-14 hours, charge at a C/10 rate, where C is the batteries amp-hour rating. For example, most AA batteries now are 0.65 AH batteries. So, to safely charge them, you would apply 65 milliamps. At a C/10 rate, most batteries can dissipate the heat and recombine the oxygen generated during overcharge without venting electrolyte, so leaving them on the charger for longer periods is not typically a problem.

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.

The book has a formula which leads to these minimum recommended discharge voltages:

     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

9. How do I determine which battery to use?

"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. 1992
I stopped getting the Powertechnics mag. in 91. You may find other articles in the more recent issues.

A typical "AA" size 1.5 volt alkaline cell has a capacity of about 500 mAH. This is a rough approximation of usable service life (at least for this discussion) for average load conditions. For instance, one should be able to expect a usable service life of about 100 hours for a load that draws 5 mA from the battery.

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.

10. Are there any chips that will simplify charging/charger design?

Maxim recently released Max712/Max713 NiCd/NiMH "Battery Fast-Charge Controllers". The ICs provide fast charge at rates from C/4 to 4C and C/16 trickle charge rate as well.

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:

  1. ICs require few external components.
  2. Available as free samples from at Maxim (1-800-998-8800).
Linear Technology also produces fast-charge controllers, but unlike generous Maxim, samples are not available for universities (I was designing a HV power supply and ran across one LT's PS controller, called LT for a sample (actually we needed a hundred of HV PSs, but still I wanted to test the IC before starting a production) and got an answer that LT's policy is not to deal with universities :(. So - stick with Maxim.

The unanimous reply to my query last week about single chip rechargers for gel cell batteries was to recommend the Unitrode UC3906. A construction article using this cell appears in the June 1987 issue of QST magazine, pages 26-29. Kits are listed as available from:

A & A Engineering
2521 West La Palma, Unit K
Anaheim, CA  92801
Tel: 1-714-952-2114

If you want to build a circuit that takes good care of the sealed lead- acid battery charger, you can use Unitrode's UC2906 or UC3906 "sealed lead- acid battery charger IC". Both the data sheet and one of their applications sheets will guide you in the design.

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.

Several people on the net were looking for a microprocessor-controlled NiCad battery charger; as I mentioned yesterday, a friend makes these - I have found out a bit more about them.

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

The Maxim 712/713 chips are 16 pin devices. They have a two stage charge cycle: fast and trickle. The battery temperature and ambient temperature are monitored. Fast charging starts when the cells are connected and the ambient is above some user selected safe minimum. Fast charge terminates on one of three conditions: timeout, cell temperature rise, and cell voltage peaking. The latter is different for NiCd and MH cells. When a NiCd cell is fully charged the cell voltage actually drops off with time as the charge continues (!), for a NiMh cell the rise in cell voltage during charging merely flattens out; hence the two versions, 712 looks for zero dV/dt and the 713 looks for negative dV/dt. After the fast charge finishes the user selectable slow charge is applied. Pass transistors can be used for big cells and batteries of up to 16 cells in series can be charged. Pretty much everything appears to be configurable by the designer. I have just received the evaluation kit. It is set up with battery holder ready to charge two AA cells with the addition of a 7 volt DC wall cube supply.

June 10 1993 EDN magazine.

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:

Details are sketchy and the photographs of the article have yet to be analyzed by experts.

Contact Enchip Inc for further information:

Enchip Inc.
East Hannover NJ
Tel: 1-201-328-2049
Tel: 1-201-301-0402 FAX

The May 27 1993 issue of "Electronic Design" page 89 has an article titled "A Designer's Guide To Battery Charging, Switchover, And Monitoring" by Doug Vargha. It is quite detailed. I am not knowledgeable enough to comment on its technical accuracy. "Electronic Design" is an industry rag, and may not be available in your neighborhood library. But if you are interested in the topic, the article is well worth reading.

Maxim Integrated Products
120 San Gabriel Dr.
Sunnyvale, CA 94086
Tel: 1-408-737-7600

The DSP nicad charger chip is the ICS1700. They also have one called the ICS1720 for Nickel-Metal Hydride batteries. I don't have an address for "Integrated Circuit Systems", however I do have an address for a crowd in the UK that market these chips.

"Amega Technology"
Loddon Business Centre
Roentgen Road
Daneshill East
Hampshire RG24 0NG

Tel: 0256 330301
Fax: 0256 330302
There 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...

In the Oct 89 issue of RCM there is a design for a peak detection charger which is a lot cheaper to build then to buy a peak detection charger. I am currently working on building a couple of them (to make it cheaper for my friends and I.) I plan on keeping two or three for me. One I will use for fast charging C sized batteries, and another for medium speed charging A sized batteries/ fast charging 250 mAh batteries (ie radio and receiver batteries.) The only problem is that it isn't easy to change the current, so I will actually need to fix that when building the circuit. The main parts are: 1 power MOSFET (IRFZ40) and a 74HC4040. There is also a 7905, and an LM324, and a 5.1V zener. The way I plan on building it, will also include a 7812 (12V regulator) and a heat sink so that I can plug it into the car when driving and charge on the way to the field. The only problem is that you need to run it off a battery or a regulated power supply. Thats why I want the 7812 regulator (plus caps etc.)

As someone else mentioned MAXIM makes a chip, the MAX713 that is a complete Nicad charger controller on a chip ... other than a source of DC, it requires a pass transistor a couple of R's and C's. It uses dv/dt sensing to determine end of charge, and can also use temperature ( you need a pair of thermistors ... one for ambient and one in the battery pack. It will charge from 1 to 16 cells, fast charge rate from C/3 to $c (!). Switches to trickle charge (with a selectable charge rate) when charge is complete. It's cheap ( a few bucks, as I recall). There is an app. note and spec sheet available.

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.)

The LM317 is a really handy voltage regulator. I would suggest trying to find someone who has the National Semi general purpose linear devices data book. Under the LM117/217/317, they have suggested applications, and one of them is a circuit for a 12V battery charger (LM317 + 3 resistors). There is also a circuit for a current-limited 6V charger, which is probably better, although you'd have to adapt a few things for 12V.

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!

I have a friend who makes such a charger (microprocessor driven, monitors the charge current etc). It has an LCD screen and adaptors to connect to most batteries. It will cycle batteries (discharge and recharge repetitively) - if you tell it what sort of battery you are charging, it will tell you whether it's any good (based on observation of the battery's characteristics under charging and discharge). It is about 3.5 x 6 x 1.5 inches in size, and he sells them for about US$100.00 complete. It was designed for model aircraft enthusiasts, (he is one) who can't afford the cost of batteries suddenly dying in service - i.e. when their plane is in the air!

If anyone wants info, e-mail and I will reply with his name and address. [My e-mail address is]

11. How should I regulate battery voltage?

There are a number of micropower references and such available, from National Semiconductor (LM10), Harris/Intersil (ICL8069), and other suppliers. Using a reference and op amp, you can build your own regulator of any characteristics you wish (including low-voltage operation). The LM10 includes both a reference AND an op amp, and is specified for operation down to 1.0 volt (i.e. it will work on a single-cell battery).

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...

12. Any good battery-related references/bibliography?

13. What are the phone numbers for some known battery manufacturers?

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

[Where can one get a 90V battery?] You might try "Batteries Plus", a new franchise starting to spring up across the US. We have one here in Bloomington, Minnesota. They have every kind and type of battery imaginable, from camera batteries to marine deep cycle batteries, and under one roof. They also build battery packs to meet the specs of the customer. Sounds like the place you should go if there is one in your area.

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.

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