QEX

come to about 80%. However, that cell also had the worst performance on the twenty-fifth discharge, further demonstrating that there is significant variation from cell to cell. We were not surprised when the manufacturer revised the application note, showing new discharge time curves. The initial discharge times were 9.8 and 9.6 hours for D and C cells, respectively, for continuous discharge at the loads we were using. The new times were 9.5 and 7.8 hours for D and C cells, respectively. Most cells came reasonably close to the revised data.

Battery Lifetime

The manufacturer claims that the batteries can be reused twenty-five times or more. Clearly, this is not supported for moderately deep discharges down to 0.9 V. Unfortunately, as much fun as this project was, we finally had to move on to other things and did not have time to fully investigate other discharges points such as 1.1 V.

Standard Alkaline Batteries

The application note states that the capacity of Renewal cells during initial discharge is similar to that of standard alkaline batteries. It gives a chart that summarizes the rechargeable-battery performance versus that of a standard alkaline, using a SEGA game as the load. (The SEGA game uses AA cells.) They claim 4.5 hours of use for the standard alkaline battery and 4.1 hours of initial-cycle use for the rechargeable. (Of course, the rechargeable battery can be used again and again, for a claimed 59 hours of operation.)

These claims did not compare to the results we found using the controlled load of the test fixture, however, at least not for D cells. Mike hiked down to a local department store and obtained a pack of alkaline D cells. He put one in the fixture and set it to the identical trip point and load conditions as the Renewal cells. Fortunately, it was an Eveready battery, so we can safely say it kept "going and going and going," giving us over 20 hours of life (compared to approximately 10 hours of initial discharge time for the rechargeable D cell). This was repeated for several cells. (See Fig 18 for a typical example of the performance.) The initial discharge of a typical newer-formulation D-cell alkaline rechargeable battery is also shown on the graph for comparison. We note that the performance of the standard alkaline cells is surprisingly consistent from cell to cell (although we did see one cell that came up noticeably short).

NiCd Batteries

For comparison, we did a single monitored discharge of a NiCd "D-sized" cell (see the text a bit later in this section). The resulting graph appears in Fig 19. The classic shape to this curve is readily apparent. Note how the voltage levels off and remains constant relative to other types of cells. Once the charge is depleted, the voltage drops off suddenly. This can be an advantage if near-constant voltage is required for the application being considered, or a disadvantage in that little warning is given to signal a nearly dead battery.

One might rightfully ask the question: "Why would rechargeable alkaline batteries be a better choice than NiCd batteries?" At first gl ance, the NiCds have one distinct advantage: Properly used and discharged, they are good for hundreds of cycles. This can make them economical batteries for many applications. They also are able to deliver a relatively high current compared to alkaline batteries, making them excellent choices for high-power applications with a low duty cycle, such as in power tools.

NiCds, however, typically develop around 1.2 V per cell, possibly rendering them unsuitable for direct battery replacements in some devices. They also typically lose about 1% of their charge per day, so they usually need to be charged at the time of purchase. This also reduces the "shelf-life" of a charge to about three months or so. In addition, as an inspection of the graphs quickly shows, the total energy from a fully charged typical "D" cell is considerably less than an alkaline cell. In the case of D cells, this is probably not an entirely fair comparison, but more about this later.

Alkaline cells maintain their charge for years, but, except for the Renewals, are not rechargeable. They also have a higher voltage per cell—typically around 1.5 V—and are perhaps more environmentally friendly in that they do not contain cadmium. But NiCds, when properly used, can last many years, delivering hundreds of cycles worth of service. Yes, they do contain cadmium, but fewer discarded cells will be generated for a given amount of service. Fewer cells headed for the local landfill at least partially offsets the environmental unfriendliness of the NiCd. The much-dreaded memory effect is also more of a myth than a reality. (Both memory effect and voltage depression are discussed in some detail in, "ANiCd Never Forgets, Or Does It?" found in the November 1994 issue of QST.)

As we previously alluded, we discov ered one surprising and unexpected "feature" of NiCd D cells. Many D cells appear to be actually C cells in disguise! Yes, internally they are both the same cell! As an example, Fig 20 shows a photograph of a sample of C- and D-cell NiCd batteries, from different manufacturers. Apparently, the manufacturers use the same internal cells for both C- and D-size cases. We don't know if this applies to all NiCd batteries, but we assume that any two batteries that have about the same weight probably have the same capacity. Another dead giveaway is the rated charge capacity of the cells, if the information can be found on the battery or its package. If a manufacturer specifies the same capacity for both C- and D-sized cells, it's a safe bet they really are the same cell! We did find at least one "heavy-duty D cell" that is apparently a full-size D cell, sold by Radio Shack and shown in the photo. It could not be disassembled.

Lead-Acid Batteries

As we said, our initial introduction to battery testing involved gel-cells. We had obtained four sample products for Lab evaluation, ranging in size from a large HT to a lunch box. Ideal as a portable power source to supplement an HT NiCd battery pack, they can add literally hours to operating time between charges. This can be especially useful to long-winded operators, such as one of the authors has been known to be on occasion. (We'll leave it to you to guess to which of us this applies!) The largest gel-cell in the group was actually touted as being able to emergency start a car! While we never actually hooked it up to one of our vehicles and tried it, neither one of us would be surprised if it worked.

For testing, we initially ran a monitored 1-A discharge on all four batteries, down to 10.5 V. From this, we estimated what load would have resulted in a ten-hour discharge time to the same voltage. We then monitored a discharge for the new load and compared the results. Although the total energy dissipated during each discharge would vary with the battery size, the curve shapes for all four batteries were remarkably consistent. They tend to be more convex in shape than the other battery types we tested. (See Fig 21 for a typical example.) We also noted the total energy delivered per discharge seems remarkably proportional to the weight of the battery, somewhat surprising considering there were four different manufacturers involved.

The"Buddy"

No article on rechargeable batteries would be complete without that TV infomercial special—the "regular" alkaline-battery recharger. Although every alkaline-cell manufacturer offers a stern warning never to recharge alkaline cells, we found one company that claimed to have the technology to accomplish just that, according to their ads and promotional literature. We just couldn't resist; we bought one of these from a consumer mail-order company (complete with the "As Seen On TV" banner in their catalog and a celebrity testimonial on the box).

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