Rechargable batteries

This issue is less mixing batteries and chargers and more having an understanding of what’s going on in the charging process. You can get the same bad results with batteries and chargers from the same manufacturer. (Moreover, many of these cells sold under various labels are all made by a handful of manufacturers). Local camera techs are nice, but I’ve met very few that really understand battery and charger technology. It just isn’t their forte.

There are two variables at play that affect heat during charging:

the current output of the charger and the capacity of the cell.

A helpful (though not entirely accurate) analogy is that of a bucket and a garden hose. Turn the garden hose on to a trickle and you can fill any size bucket without splashing water on yourself. The larger the bucket, the higher you can turn on the tap without creating a splash.

In this analogy, the “splash” is heat created by current flowing through the battery which the battery’s chemistry cannot aborb. Heat is an evil thing in the charging process. Extended exposure to heat shortens the life span (number of charge cycles) of the cell and reduces the capacity of the cell. Heat also limits the efficiency of the charging process, making the charge cycle last longer.

When a cell gets hot, it is because it is receiving current from the charger. At low current levels (such as those put out by overnight chargers) the increase in heat is easily dissipated by the surface area of the battery — the cell may not even feel slightly warm to the touch. At higher levels, such as those used by rapid chargers, cells may become very warm.

Older, low capacity cells are like shallow buckets. They can’t take high current levels without “splashing” heat. If you have a rapid charger designed for a 1600 mAH cell and you’re putting 700mAH cells in it, those cells WILL get hot. Why? because the current output needed to charge a 1600mAh cell as quickly as possible is like trying to fill an espresso cup with a firehose. It doesn’t matter who made the cell.

“Intelligent” chargers solve a slightly different problem. Early chargers put out a constant, low current. It took all night to charge the cells, but you could leave the cells in the charger indefinitely. As consumers demanded faster charging, manfacturers increased current levels. This charged the cells in a quarter the time, but it also meant that you had to take the batteries out of the charger when they had been in long enough. “Intelligent” chargers detect when the cell is no longer “using” any of the current and reduce the charge to a trickle.

So, how you you apply all this?

1. Know the capacity of your cells. They aren’t all created equal. Some manufacturers (and these seem to be popular in camera stores) don’t put the mAH rating on their cells. I take the view that manufacturers advertise what they proud of. These are probably low capacity cells and should be treated that way.
2. Know the output of your charger. Some chargers may have multiple output levels. If you plan to leave your cells in the charger after they are fully charged, make sure that the trickle charge output level is small enough for the cells you have. If not, don’t ditch the charger: get new high capacity cells. A set costs $10 or less. (I recently forwarded an offer for 16 1550 mAH cells for $25). Then buy an overnight charger for you low capacity cells. These cells are still very useful as backup and to use in external flashes. Overnight chargers are dirt cheap, about $7.
3. It is possible for chargers to malfunction. If your overnight charger is causing batteries to get hot, check the output voltage with a voltmeter. If it exceeds the labelled millimaperage or voltage, it is defective. The same is true with rapid chargers;
   however, it is *normal* for cells in a rapid charger to get warm — even very warm. If they seem to be excessively warm, check the voltage and amperage. If the charger is putting out correct current, you may have a problem with the cells.

Mike Chambers, coolpix990@yahoogroups.com.