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  • From: "Wendy" <crazygardens AT nationi.net>
  • To: <homestead AT lists.ibiblio.org>
  • Subject: Re: [Homestead] Homestead tools
  • Date: Fri, 19 Nov 2004 21:51:04 -0500



>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.





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