Cordless Reciprocating Saws - Part 2

Understand 18-volt battery technology and alternative power supplies.Subject:  Cordless Reciprocating Saws, Part 2Topic:  Power supply for cordless reciprocationg sawsObjective:  Understand 18-volt battery technology and alternative power...


A DeWalt exclusive is a new battery charger unit with a deep discharge feature incorporated into its design. This process, which DeWalt recommends once every five times you charge battery, takes approximately six hours to recondition the battery fully.

Both brands of charger units have LED lights that signal the status of the battery. DeWalt signals charging with a blinking red light and fully charged status with a steady red light. Milwaukee uses a steady green light to indicate charging and a blinking green to signal the end of the charging process.

Immediately after heavy saw use, a battery will be warm, even hot to the touch. This is normal due to the battery discharging its energy. Both charger units have sensors that measure battery internal temperatures and shut down the charging process if the battery is too hot. When the DeWalt charger for example, receives a battery that is internally hot, it automatically starts its “hot pack” charging delay. This suspends charging until the battery has cooled internally. After cooling, the charger automatically switches to the “Pack Charging” mode.

Longest life and best performance can be obtained when the battery pack is charged in temperatures between 65 and 75 degrees Fahrenheit. Do not charge a battery in an environment that is hotter that 105F or colder than 40F.

Because chargers protect themselves from hot batteries, it is not practical for departments to expect to use a battery and immediately recharge it for use again at the same rescue scene. Batteries must first cool - during our endurance tests, this took at least 15 minutes. Charging took another 50 minutes. If you’re still doing rescue work after 65 minutes of on-scene time, either you haven’t trained at all, you made some serious mistakes or the rescue challenges are so great that you are into extended operations. Fortunately, extended-duration incidents are not the norm.

DeWalt also manufactures a DC-powered battery charger with a cigarette lighter socket adapter. The electronics of this special unit will fully charge an 18-volt reciprocating saw battery using a fire or rescue vehicle’s 12-volt electrical system. The charger unit can also be permanently hard-wired into a vehicle’s electrical system. It is not harmful to the battery to be left in this DC charger when not in use. Frequent running and shutting down of the vehicle will not damage the 18-volt battery. In quarters, an electrical shore line should be provided to trickle charge the system, maintaining the saw battery at full capacity.

With a DC charger on a vehicle, do not charge more than two battery packs without running the vehicle’s engine. With the engine shutoff, the vehicle’s battery will be drained. If you use the cigarette lighter adapter, disconnect the charger from the cigarette lighter socket when the vehicle is not in use. This will prevent the vehicle’s battery from discharging while the engine is off.

Portable Power Source Problems

When the DeWalt charger is used with some portable power sources such as a generator or inverter that converts DC to AC, the charger may temporarily suspend operation. It will flash its red LED light with two fast blinks followed by a pause. This indicates that there is a problem with the power source and that its generation of power is out of acceptable limits.

Vulnerability To Moisture

During one session of field testing, this author found out the hard way that both brands of battery chargers are extremely vulnerable to destruction by moisture.

A brief rain shower sprinkled onto our extrication equipment and the charger units we had set up. Almost instantly, both chargers snapped, crackled and popped as they shorted out. The Milwaukee unit put out more white smoke than a smoke machine at a training academy. The water was instant death for these units. Do not expose 18 volt charger units to moisture from any source, including rain, snow or hose sprays. Keep the charger units in protected environments.

Likewise, avoid placing the charger or battery pack in an unusually warm or hot environment. Exposure to direct sunlight when recharging a battery, for example, will heat the battery just taken off the saw. The charger won’t allow it to charge until it has cooled, thus delaying the recharge process.