Above: Nicad cell voltage climbs as it is charged.

my system holds 220 Amp-hours; a high capacity D cell holds 4.4 Amp-hours. If we want to discharge (empty) the D cell at a C/5 rate, we divide the capacity (C) by the number of hours to get the current.

Capacity (in Ampere-hours)


We would draw 880 milliAmps for 5 hours (4.4 A-h/5 hours) to empty the battery. Or we could fill (charge) the batteries at a C/20 rate. So 220 Amp-hours/20 hours equals 11 Amps of current for my system. For a D cell, we'd need to use a current of 220 milliAmperes (4.4 Amp-hours/20 hours).

But, again, a battery is not exactly like a bucket. We can work the math and predict when the battery should be full by figuring a constant current for so many hours. But no battery is ideal. For larger lead-acid and nicad batteries, we can skirt the issue and look at the voltage to tell us when the batteries are full. But for small nicad cells, the voltage difference between full and empty is in tenths of a volt. (See nicad voltage vs. charge above). So we use the capacity and charge rate to figure a constant current for a certain number of hours, then add more time to make sure the cells are full. How much more? I've heard anywhere from 15% to 50%. For example, the Panasonic AA cells hold 600 mAmp-hours, and like a standard charge of 60 mAmps for 15 hours (60 mAmps times 15 hours is 900 mAmp-hours — a 50% overcharge). Usually the manufacturer gives a recommended charge — so many milliAmps current for so many hours.

Beginning the test

For my test, I have cells with capacities of 500 mA-h to 1100 mA-h. Some can be rapid charged in 1.5 hours, others require 14-16 hours. All the cells came uncharged, so the first step in using rechargeable cells is to fill them up and discharge them several times. I wouldn't know this except for reading some of the instructions that came with the cells. This deserves a digression.

Charging Tips for Nicads

Confused on how to charge your nickel cadmium and nickel metal hydride cells? Here are some tips:

• Nickel Cadmium (nicad) cells and Nickel Metal Hydride (NiMH) cells are usually sold uncharged. They must be charged and discharged several times before they reach full capacity. This appears to be more a problem with new NiMHs than new nicads.

• Nicads can be overcharged, yes, even by the sun, although this is more difficult to do with the smaller panels. Read any literature that comes with the cells and read the label on the cells carefully for recommended charge rate and amount of time needed to charge.

• Observe the correct polarity (+ vs.-) when inserting your cell in the holder.

• Discharge the nicad cells fully at least most of the time. "Fully" means that the flashlight is dim, or the tape is slow, or the voltage is about 1 Volt. Do not over discharge. The infamous memory effect (wherein the cells "remember" the partial capacity as their full capacity) is caused by repeated (dozens) of partial discharges. Richard has a classic "memory effect" story: folks at KTVL TV station where he worked used rechargeable D cells in video camera lightbelts. They would recharge the cells after every shoot. Although the cells were not discharged fully, the camera people did not want to get caught with dying cells in the middle of a roll of film. The cells would only last six months or so. If the cells are discharged fully every few cycles, your cells will retain their full capacity. (See Q&A this issue for more on this.) NiMH cells have no memory effect.

• These cells lose their charge over time. With nicads, the self-discharge rate is fairly slow. Cells that have sat around for a month will need to be topped off before use. With NiMHs, the self-discharge rate is much faster. After a month, NiMHs may need a full charge again.

• Test the voltage of your cells regularly. Sometimes one cell will be less charged than another, and this can cause problems. During the charge or discharge cycles, the cell will give up its charge to other cells. The bad cell can actually reverse polarity, which ruins it eventually.

• Clean the contacts on your cells, cell holders and/or charger a few times a year. These parts can oxidize and a thin film develops which lessens the electrical connection. C. Crane suggest using the eraser on a common yellow pencil to clean contacts because of its abrasive particles.

• C. Crane also recommends the slower the charge rate the better for the longest cell life. Yes, this includes rapid charge cells.

• Some of the higher capacity cells are too large in diameter to fit in some appliances (MiniMag flashlights, for example). Check that your new cells fit your appliances before you start using the cells.

Some of the cells I bought came without any instructions! Both C. Crane and Real Goods sent charging information, and Brian from Sunelco gave me instructions over the phone when I bought the cells. Three cells (Golden Power, Radio Shack, Gold Peak) have the capacity and recommended charge rate right on the label; the other cells did not. In fact, the capacity of the Millenium cells was not listed on the cell, packaging, or even in the Crutchfield catalog where I bought them! The package states that the cells can be charged in one hour, but I need to know at what current!

I think the assumption is that you don't need to know this information, especially if you use a commercial ac plug-in charger. I have heard that many people return rechargeable cells because they tried charging them (often with solar) and found the cells didn't work or didn't last very long. I am requesting more technical specs from the original cell manufacturers. See sidebar previous page for some tips I garnered from the instructions I did receive.

I've started by charging the cells, eight at a time, with three 12 Volt, 50 milliAmp Kyocera Jetski photovoltaic modules. The Saft cells take 15 hours, the nickel metal hydride will take 33 hours. I've been discharging them using a 10 Watt, 10 Ohm (Q) power resistor. More basics here. What the heck is a power resistor, and why did I choose 10 Q resistance?

Ohm's Law Revisited

I've never met Mr. Ohm, but I've sure been using his "law" a lot. Ohm found a relationship between current (I), voltage (E), and resistance (R) which is I = E/R. (Current is expressed in Amperes, Voltage in Volts and Resistance in Ohms.) When I was studying for my amateur radio license, the book used a trick to help memorize this relationship:

E = Voltage

One AA cell: 11.2 Volts i T00


I = Current

I = Current

E = Voltage

R = Resistance

R = Resistance

If you know two variables and want to know the third, cover the unknown quantity with paper or your finger. If the two variables showing are side by side, multiply them to get the third. If one variable is over the other, then divide.

So let's use Ohm's Law: I wanted to discharge four AA cells that are rated at 700 milli-Amp-hours. I have them in a four cell holder (from Radio Shack). The 4 cells are connected in series, that is, the positive button of one cell is electrically connected to the flat negative end of another (see diagram top right).

i series:

Four AA cells in series:

4.8 Volts

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