Rechargeable Batteries

This page has been shortened with the removal of Rechargeable Alkaline material.

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A while ago I tested a set of 7, 1000mAH Nicad cells (8.4 volts) after cooling them to -20C. I forgot to save the chart, but the voltage started about 1 volt lower and the curve was flat, even rising a bit as the pack warmed up.
The main problem I see with Alkalines is that because the voltage drop is more linear, any equipment that shuts itself off at a pre-determined voltage may not be able to use the full amount of charge in a pack. Similarly a pack used at extremely low temperatures may cause the same problem. The simple solution is to add a cell or two to packs that will be used in extreme cold. The cold doesn't seem to affect the amount of charge, just the voltage.
Another disparity between types is that the Alkalines are not as eager to supply high currents, as is required in transmitting. The voltage drop when drawing 1 amp was more than twice what the nicads gave. Most 5W handheld transceivers draw about 1A to 1.5A on high power transmit, so if the voltage dropped substantially there would probably be problems with distortion in the transmission.

Memory Effect

If you seem to be confused about the memory effect, maybe this explanation will help. What is purported to happen is that the battery gets a memory. This applies only if you discharge a pack to the same point over and over, that's exactly the same point, say 60%, and then recharge from that point each time. The battery will then have a "memory" of that 60% point and will be reluctant to discharge (ie. provide power) below that point.
If you discharge to 60% one day, 30% the next, 100% the next, 90% the next etc, the battery will not "get" a memory, and should perform better. To cure the memory effect you really want the battery to forget it's memory point. Many experts warn that overcharging (overheating) is the cause of battery death. But I have found the main cause of failure of packs is discharging to the point where one particular cell, the weakest, reaches zero volts, and you continue to draw power. which the other cells will provide. The current continues to flow but because one cell (or more) is no longer supplying power, the voltage across this cell reverses and causes further damage to that cell. If you can't monitor the voltage of each individual cell, it is unwise to discharge a pack lower than about 1.1 volts per cell.

Most of the packs I get to repair have two or three bad cells, because when a pack has only one bad cell, it is usually still useful. The following graph is of an old (traded in) Icom BP7 (13.2V 450mAh) battery pack. As the graph shows it's still in good condition.
When a cell fails prematurely, you can easily see the notch in the graph when that cell stops delivering current.

Lately a method of charging has proved much better than the normal trickle charger. Basically it uses pulses of positive current to charge, interleaved with smaller pulses of negative current to discharge. The net result is a charge rate applicable to the rating of the cells in the pack. The charger is simple to build and uses AC through diodes and a switching transistor with a current control to adjust overall charge rate. When the AC voltage is negative during half a cycle, the battery discharges through a diode and LED at about 40mA. During the positive half cycle the current is fed through a diode to a switching transistor which is controlled by a voltage divider for adjustment. I believe it is relatively simple to convert commercial chargers to this method, and will be doing a few myself in the near future. There are plenty of good books on power supplies that should prove helpful to non-electronic minded experimenters.

One thing you have to know when looking at the graphs is that manufacturers quote a battery at different rates of discharge.
I don't know what rate any of my test batteries are quoted at, but just guessing I would say at the 10 hour rate. Which means you take 1/10 of the rating and use that as the load for 10 hours. Some batteries quote a 5 hour rate or even a one hour rate. This means the figures don't look so good, especially on the sales blurb, as a battery will give more power if you discharge it slowly.
Also they fail to specify what the terminal voltage should be. If it is specified as 1 volt it means you discharge the battery to 1 volt per cell, some specifications are to 1.1 volts per cell. The 0.1 volt difference is insignificant.
Most of the suppliers wouldn't have a clue, so I don't usually ask them, I just test and figure it out for myself.

Given the uses and conditions they're subjected to, battery science is as more of an art than science.

Manufacturers of electronic equipment will quote the longest run-time they can, even if they don't exaggerate. This means they may use power save features, or only an hour or two per day, which allows batteries to recover. They may also use batteries that you can't buy over the counter.

Modern electronic equipment often shuts down at a pre-determined voltage which may occur well before the batteries are flat.

Battery manufacturers will quote the maximum capacity available under ideal conditions, but if you got the cheap ones on special they may be the rejects, or way out of date. If you use them at higher discharge rates, or lower temperatures, you will not get the rated capacity from them. You will also not get full performance from batteries unless they are fully charged beforehand. High capacity batteries require higher current chargers. Multi chargers need to be checked for output current when charging in all sockets at once as they are often not up to specifications even if they are supposed to charge "anything".

The only way to be relatively sure of performance is to measure the current drain in field conditions and time the battery pack's response.

These cells are what I am currently using to repack radio batteries. I am also stocking untagged cells for use in GPS units as we are getting 14 hours per charge with no problems (Garmin 12XL). They are also very useful in Digital Cameras.
I made a 9.6 volt pack because the 8 AA cells fit nicely into an Icom BP7 or CM96 battery case, and the extra cell gives you a fraction more output power, or a margin when working at cold temperatures.

1600mAH Nickel Metal Hydride AA cells.

I've just completed a couple of charge/discharge cycles on a set of four with the following graph of the second discharge cycle which was run at the 5 hour rate (as per the manufacturer's specifications) of 350mA load.

You can order these batteries from me for $6.50(AUS) per cell plus postage, (even air mail overseas is cheap). Just add $10.00 to your order for postage and packing and I will send them same day. Don't forget to specify if you want solder tags (for making up packs) or consumer type without tags (for GPS's and Digital Cameras etc.)

I also have a car charger (with cig lighter plug) that charges 4 cells at the correct rate overnight (10-12 hours) for $20.00, or 8 cell version for $28.00 If you want a 240 volt plugpack type charger, I can supply that too for only $19.95

NEW 1-10 cell computer controlled smart charger/discharger that runs off 240 or 12 volts, only $79.95
A must if you have an odd number of cells to charge.

See below to order.

Five hours at 350mA is excellent.

High Current Tests

GP 1600mAH Cells x 4