What is cadmium migration and how do I prevent it?

Red Scholefield replies:

It has been accepted as dogma that today’s Ni-Cd batteries are not harmed by continuous overcharge as long as the temperature in overcharge is maintained at or near room temperature. Those knowledgeable about batteries however, are beginning to espouse a somewhat different story. To maximize battery life it has been recommended that the overcharge period or float charge of the battery take on an entirely different character than continuous constant current overcharge that has been the “acceptable” method. This “alternate” technique pulse charges the battery on a relatively long duty cycle. A battery is maintained at a full state of charge once it is completely charged and in overcharge (as is evidenced by a finite temperature rise or after an input of 160% of rated capacity), by a pulse of charge current in the C/10to C rate range and sufficient to replace energy lost by self discharge. This pulse of charge current is only applied to the cell for 1 hour out of 24 in the case of C/10 rate or in the 5 to 6 minute range each 24 hours for a C rate pulse. The theory and countless “casual” laboratory observations support this pulse technique over the “less expensive” constant current overcharge presently popular in the consumer product charging regimes. There is overwhelming evidence that constant current, even at very low “sustaining” rates, contributes to “cadmium migration” that forms a conductive bridge through the separator medium from the negative to the positive plate.

There is overwhelming evidence that constant current, even at very low “sustaining” rates, contributes to “cadmium migration” that forms a conductive bridge through the separator medium from the negative to the positive plate.

The rapidity of the reaction forming this cadmium bridge is a function of the current density and time. There is evidence to suggest that the conditions existing at a C/10 to C/20 overcharge rate contribute to the speed of the cadmium migration more than higher overcharge rates. The higher temperatures produced in higher overcharge rates cannot be ignored as playing a role in seemingly slowing the cadmium migration while at the same reducing battery life by increasing separator degradation. Where the optimum balance point of these opposing actions falls is left as yet one more challenge to our testing programs. It appears however, reasonable to assume at this point in time, that pulse charges at the C rate that will result in a “net” charge input equivalent to a continuous C/10 are much less conducive to cadmium migration than a continuous C/10 overcharge rate. While the beneficial impact of this pulse overcharge has not been quantified to any degree, the observations of battery testing personnel and more sophisticated battery users cannot be ignored. The creation of an economical means for pulse overcharge will be the challenge faced by the engineer wishing to design a product for optimum service life.

Questions and answers from Red Scholefield, Battery Engineer. Reprinted with Mr. Scholefield’s permission.

The post NiCd Cadmium Migration, Ni-Cd Battery Charging appeared first on Amicell - Vencon.

Does reflex charging really work?

Red Scholefield replies:

The concept of applying a short discharge pulse during the charge cycle sometimes referred to as “reflex charging” or “burp charging”, has been with late 60s with patents by W. Burkett & J. Bigbee [3,597,673″ Rapid charging of batteries and W. Burkett & R. Jackson [3,614,583 “Rapid charging of batteries”] and assigned to the McCulloch Corporation.

Burkett, an individual with great drive and somewhat uninhibited by the lack of any test substantive test data, enlisted the help of a Professor at Stanford to come up with a reason why the negative pulse charge technique did what Burkett claimed. This individual, striving for academic elegance, came up with the hypothesis that the negative pulse may have stripped away the gas bubbles on the plates and thereby enhanced the charge efficiency and reduced the temperature and pressure build up. He stated it was like burping a baby. Burkett liked the sound of this and it became his theme in promoting the concept. The fact that he was a prolific writer did not detract him from his quest, as he had his concept published in numerous trade magazines and technical journals hungry for a charging break through in the emerging market of cordless products.

After the patent was awarded he took it to General Electric, then the leading Ni-Cd manufacturer in the US, where it was analyzed in detail. General Electric disappointed Burkett when, after extensive testing, they could find no conclusive evidence that the negative pulse offered any advantage. Burkett then proceeded to find other interested parties that would be less critical, and take his word for the phenomena. He sustained the venture for several years mostly by obtaining government contracts to further study the effect of the negative pulse technique for both sealed and vented Ni-Cd systems.

With the expiration of the patents many saw the opportunity to make a great deal of money from the ignorance of battery users and thus it has proliferated in many variations and forms. General Electric, confronted by battery customers who had bought into the Burkett scheme of charging, tested and re-tested the concept as each new variation was presented. The results were the same in each instance. It has never been demonstrated to have any advantage over conventional charging, either on charge efficiency, the performance or the life of the battery.

It has never been demonstrated to have any advantage over conventional charging, either on charge efficiency, the performance or the life of the battery

While many claims have been attributed to this technique, none have ever been substantiated in the laboratory. Fortunately it does not harm the battery in any way and since the concept makes for a rather elegant marketing technifact, it has been adopted as a way to promote the sale of charging systems by numerous companies in which marketing dominates technology.

The reflex chargers are for the customer that cannot separate marketing from sound engineering and feels compelled to perpetuate this hoax while providing a healthy income for its proponents. If reflex charging had any merits that would enhance the performance of batteries the battery manufactures would be supporting it with vigor as would the major suppliers of battery powered products. Since is does no harm to the battery, the battery manufacturers are reluctant to focus on the pointlessness of some customers that insist on using it and risk a technical confrontation that would embarrass the proponents and jeopardize sales.

The post Pulse Charging or “Reflex Charging” appeared first on Amicell - Vencon.

Just sorting out my radio gear for my Pica 1/5 scale Mustang and discovered that I have two 600 mAh packs but not a 1200mAh pack like I want. Anyway I found some extra Y connectors and viola, a light went off. Why not connect the two 600mAh packs in parallel with the Y and get a 1200mAh pack?

At the same time I could be doubling my reliability, unless of course something happens to one of the packs that drags the other down to nothing.

Anyway it seems like a good idea, and if its such a good idea how come I’ve never heard of anyone else doing it? Is there a gottcha that I’m overlooking here?

Red Scholefield replies:

No problem with running the two packs in parallel, but you must charge each one separately. This technique is frequently used by a number of people flying expensive airplanes and wanting the added reliability of dual power systems.

You would be better off from the reliability standpoint if you ran separate switch harnesses all the way from each battery to the receiver. Just plug the other switch harness into any unused channel of the receiver. This will allow you to check each battery separately prior to flight. Switch one on and check, then switch it off and the other on and check, then switch both on and fly. Sort of like the old mag checks on big airplanes. This also has the added advantage of allowing you access to both batteries for individual charging.

Disclaimer: Some feel that one pack will discharge into the other pack. This is better discussed between those that do not understand battery technology and therefore will not be addressed by the author of this post unless the query is to better understand why one battery will not charge another when both start at the same state of charge. The subject is not negotiable.

The post Combining NiCd Battery Packs appeared first on Amicell - Vencon.

I purchased a Futaba FM fp-4NBF in July 95. I charged it to see how it worked and haven’t touched it since then (I was busy with school and just graduated this month). I only had it on to check the transmitter and servos for probably less than 10 minutes. What should I do to store it until needed (probably less than a year)? Did I mess it up by charging it up for the first time and not discharging it for 5 months?

Red Scholefield replies:

You have done nothing to harm the batteries assuming you didn’t store the transmitter in the trunk of your car in Arizona. I would recommend 1 cycle on the packs before you go to fly if to no more than to get a base line on the capacity. For long term storage I would recommend that the pack be removed and stored in the refrige. Not real easy on the 4NBF as you have to remove the back and unplug the pack from the jack next to the switch and then snake out the wires past the charge jack.

The post Storing NiCds appeared first on Amicell - Vencon.

I have had similar experiences with two packs. Both were healthy packs (TX & RX) that were cycling to their factory rated capacity….until I left the switch on overnight. They wouldn’t even take a charge after that!

Red Scholefield replies:

Let’s have a little battery talk. Bill’s comment “wouldn’t take a charge” is used commonly to describe a battery problem. Let’s examine what it really says. It says essentially, “after I pass current through the battery for a period of time the battery could not deliver current at a usable voltage level”. In order for the battery “not to take a charge” it would have to be open circuit (broken lead or so dried out that there is a very high resistance) so that no current is flowing. Neither of which could have been caused by leaving the switch on for a day or so.

In the case mentioned where the packs were left on a load for some time the battery terminal voltage approached zero volts. This would mean that one or more cells were probably reversed since the pack voltage = 0 and not every cell has identical capacity, some would read 1.1 volts and others might read -.5 volts to get to the zero volts seen. Reversed cells have a rather poor charge acceptance on the next recharge and require a longer time to reach a full state of charge. Since cells cannot store any significant energy when reversed they are at essentially zero state of charge, a point where if a short is going to manifest itself it does.

With these suspect packs, you will find, after they have been allowed to stand for a few days, that most of the cells will have a voltage greater than 0.9 volts. One will be at a very low voltage, or zero. If the low cell doesn’t recover and hold a voltage after passing current (normal charge rate C/10)through it in the charge direction this cell is shorted.

Driving a pack to zero volts by leaving it on a load isn’t a good practice. But these things happen. I left one on for six months once and had to replace the whole wiring harness as the cells leaked. A day or two on the other hand will not significantly damage a pack in most cases. A good 48 hour slow charge will usually bring them back.
OK, class dismissed. Lets go flying.

The post Overdischarging a Cell appeared first on Amicell - Vencon.

I’ve repeatedly heard over the years that it could damage a cell to discharge it to less than 1V per cell and are you recommending this as a routine test for modelers to find weak cells?

Red Scholefield replies:

You cannot damage a cell by discharging it to zero volts. Where the problem arises is that when you try to discharge a multicell pack to a low voltage one or more cells will can be driven into reverse due to the differences in individual capacity of the cells. Reversing the cell is essentially charging it backwards and causes venting and if done long enough can damage the cell. When a cell is allowed to discharge through its own individual load and reaches zero volts, current ceases. There is no charge in the cell. Since there is no charge in the cell it is much easier for a short to develop and maintain itself. Because of this, some may associate driving the cell to zero with causing the cell to fail. The weakness was there all the time but did not show up since the current source (the charged material in the cell) would not allow it to manifest itself.

Usually you can decide if a pack is going bad without resorting to this test. Look for higher than normal self discharge (high resistance short is developing). Look for significant drop off in capacity, (High resistance short is shunting charge current and cell is not getting fully charged). Look for lower voltage after full charge (one cell is already shorted). Good news, I have seldom seen a cell short when it’s fully charged so you should get ample warning from the symptoms above. The short down – spring back test only confirms that you do have a short or allows you to get a high degree of confidence that a cell is good (from the separator integrity standpoint).

The post Damage from Discharging NiCd/NiMH cells to Less than 1V appeared first on Amicell - Vencon.

I have a battery pack that is not charging up to full capacity. I know the problem is one (or more) bad cells. What would be a good way to tell which of the cells are bad?

Red Scholefield replies:

Depending on the age of your battery pack I would be wary of just replacing one or two cells, kind of like replacing one or two spark plugs or tires. If it’s a fairly new pack that you suspect a cell has gone bad you can probe the sleeve with straight pins and measure each cell. Any cell reading over 1.0 volt is probably OK. Have you run a capacity check on the pack? If you pack is over 4 years old, replace it.

The post Testing Individual Cells of a Battery Pack appeared first on Amicell - Vencon.