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["Wendy" <crazygardens AT nationi.net>]

  >The batteries used on cordless tools are dry cells.  If they are charged
  >at too high a voltage they heat up.  If there is a malfunction they can
  >heat to the point of ignition.

  Another thing the guy at the DeWalt store said was on that issue of heat. 
I
  said that future purchases would be 18 volt rather than my present 14.4. 
He
  said I should reconsider because the smaller batteries have fewer cells 
and
  cool down faster after load stops, which increases battery life. The big
  die young.




  For a better explanation see this:

  http://www.taunton.com/finehomebuilding/pages/h00106.asp

  Understanding Cordless-Tool Batteries

  Misused batteries die young. Replacements are costly. Here's how to get 
the longest life from your batteries.

  by Gary M. Katz

  Three months ago, I carried a boxful of one-year-old 14.4v batteries into 
my local tool dealer's service center. The service technician tested the 
batteries and confirmed what I knew: Those batteries would no longer take a 
charge. He couldn't tell me why they lasted for just a year, which only 
added to the aggravation of buying six new batteries for $60 each.

  The batteries in my old 9.6v tools lasted for years, driving thousands of 
screws. However, I've noticed that 12v, 14.4v and 18v batteries degrade 
faster. After about one year, they don't deliver their original run time or 
torque, regardless of the claimed amp-hour rating. (Amp-hour is the 
industry's standard measurement of the electricity that a battery can store 
and release on demand.) Other carpenters had similar complaints.

  To learn what was going on, I spoke with representatives from Makita, 
DeWalt, Porter-Cable, Bosch, Milwaukee and Hitachi, as well as Sanyo and 
Panasonic, which manufacture most cordless-tool batteries.

  The manufacturers confirmed what I suspected: Higher-voltage batteries 
have shorter lives. Manufacturers estimate battery life along these lines: 
1,000 to 1,300 charges for a 9.6v battery; 800 to 1,000 charges for a 12v 
battery; 650 to 800 charges for 14.4v battery; and 500 to 800 charges for an 
18v battery.

  Battery life isn't wholly dependent on voltage ratings, though. How a 
battery is used also affects its life. What follows is a summary of my 
conversations with the manufacturers.

  Heat is a battery's worst enemy
  My crews use today's high-voltage cordless tools for harder tasks than the 
light, occasional duty we expected of 9.6v tools. Now, cordless tools run 
large spade bits, hole saws, auger bits, 6-in. circular saws and even 
reciprocating saws. High-voltage cordless tools are often our primary tools 
for tasks that two years ago would have required a corded tool. 
Unfortunately, cutting and other high-torque applications result in a fast 
discharge rate. This is bad. Because of the internal resistance of battery 
cells, fast discharges create more heat. Heat degrades the material within 
cells that stores and releases electricity.

  All cordless-tool batteries, regardless of total voltage, are packages of 
individual 1.2v battery cells (photo above). There are eight cells in a 9.6v 
battery, 12 in a 14.4v battery and 15 in an 18v battery. Higher-voltage 
batteries are more vulnerable to heat than 9.6v batteries because with more 
cells joined together, higher-voltage batteries have less surface area per 
cell to disperse heat. So not only do we use higher-voltage batteries in 
ways that create more heat, but also because of these batteries' 
construction, they are less able to dissipate that heat than are 9.6v 
batteries.

  New battery technology isn't necessarily better
  Nickel-cadmium (Ni-Cd) batteries are the standard. They powered all 
cordless tools until about one year ago, when nickel-metal hydride (Ni-Mh) 
batteries were introduced as an environmentally friendly alternative. 
Worn-out Ni-Cd batteries are a hazardous waste and must be recycled. Ni-Mh 
batteries are still free from that mandate.

  Ni-Mh batteries also have a higher amp-hour potential than Ni-Cds, 
potentially as high as 3 amp-hours for a standard cell. The current ceiling 
for the Ni-Cds used in cordless tools is about 2.2.

  What's the downside? Ni-Mh batteries haven't yet reached their full 
amp-hour potential. But more important, Ni-Mh batteries also have a higher 
internal resistance than Ni-Cds. This internal resistance makes the battery 
discharge faster and generate more heat doing so, reducing both run time 
(see my comparison in FHB #119, p. 126) and cycle life. Additionally, Ni-Mh 
batteries are extremely sensitive to temperature changes; heat easily 
damages the cells. Several tool manufacturers explain that Ni-Mh batteries 
are not as "robust" as Ni-Cds. In fact, some manufacturers expect only 500 
charge-discharge cycles from a 14.4v Ni-Mh battery (compared with 650 to 800 
cycles for an Ni-Cd battery).

  New chargers may extend battery life
  Ironically, the power that restores a battery can also degrade it because 
charging a battery also heats it. Cordless-tool manufacturers all agree on 
this. They are all making efforts to reduce charging-generated heat, thereby 
maximizing cycle life.

  Professional-grade chargers operate in stages, first sensing the initial 
temperature of a battery. If the battery is too hot to be charged, the 
charger waits for the battery to cool off. The fast charge then takes 
anywhere from 30 minutes to an hour, after which the battery is near its 
full capacity. Because charging a battery that's too cold also can damage 
it, Porter-Cable's chargers also sense if a battery is too cold. If it is, 
the charger slowly ramps up for several minutes before going into the 
fast-charge stage.

  Professional-grade chargers sense when a battery is near full charge and 
shut down to minimize heat buildup. However, each cell may not get a full 
charge. Each cell takes a fast charge differently, and chargers shut down 
before they damage those cells that reach capacity soonest. After the fast 
charge, one cell may be at 98% capacity and another at only 85%.

  According to the instructions supplied with DeWalt and Porter-Cable tools, 
their chargers follow the fast charge with an equalization, or topping-off, 
stage that balances the cells. A trickle charge that makes up for the power 
the battery would ordinarily lose through nonuse is last. Other 
manufacturers say their trickle charge equalizes the cells. Leaving 
batteries to trickle-charge, even for days at a time, won't hurt them.

  Full discharge kills weak cells
  Heat may be a battery's worst enemy, but unequally charged cells also 
reduce cycle life, run time and torque. With each succeeding fast charge, 
the weakest cell slips farther and farther from its full-charge potential.

  Weakened cells become prone to the irrevocable damage of reversed 
polarity, called reversal, which is comparable to short-circuiting. Reversal 
is caused by using a battery until it no longer operates at all. The weakest 
cells drain first. While the stronger cells continue to operate the tool, 
the weakest cells are emptied of power. When a fully drained cell is called 
on to produce power it no longer has, reverse polarity occurs. Cells that 
have been forced into reversal will never again take a charge. The result is 
that a freshly charged 12-cell battery might operate on only 11 cells. 
Reversals mean shorter run time, less torque and faster degradation of the 
remaining cells.

  DeWalt suggests that a battery's run time and its cycle life can be 
increased (by up to 30%) with proper care. Because DeWalt believes that 
maintaining cell balance is crucial to long-lived batteries, the company 
advises that after a maximum of ten charge-discharge cycles, a battery 
should be left in the charger for about six hours to ensure complete cell 
equalization. Leaving batteries in the charger overnight once a week is even 
better. Other manufacturers weren't as forthcoming as was DeWalt, but my 
conclusion is that the same advice applies to any battery whose charger has 
a topping-off stage.

  Batteries have no significant memory
  If your charger doesn't have a topping-off stage, there are still steps 
you can take to extend battery life. Batteries should not be left in heat or 
cold, or operated or charged while extremely hot or cold. Batteries will not 
usually take a full charge at other than room temperature.

  Batteries like room temperature. Operated there, they run most 
effectively, take the most complete charge and have their longest run time 
and cycle life. Try not to use cordless tools continuously, and let them 
cool down between applications.

  Finally, no matter what contradictory advice you may have heard, never 
fully discharge a battery. For years, carpenters have passed along the myth 
that batteries have memories, and if not fully discharged, they won't fully 
charge. I've even seen carpenters tape a drill's trigger down to extract 
every last electron from a battery. This action reverses cells and makes 
your batteries die young. Use the battery only until you notice a power loss 
that affects performance, and then recharge the battery.

  Gary M. Katz is a carpenter and writer in Reseda, California. He is the 
author of Installing and Hanging Doors.