Why can’t rescue tool battery packs be used on a PPV fan?

When you peel back the obviously cool, headlining design features of IP67/68 ratings for immersion in water, you find that the two leading brands of rescue tools in the world both use similar battery pack design—electrically speaking. With nominal voltages of 25.2 VDC—or 28 VDC fully charged—both battery packs are built with a similar 7S configuration—that is seven li-Ion battery cells in series—and differing cell specs to achieve various amp-hour (Ah) ratings for runtime and weight considerations. The average power of these top brand rescue tool batteries on the market is 176 Wh.

These batteries are likely perfectly optimized for operating rescue tools, powering micro electrohydraulic pumps to achieve maximum cutting force in very short operation times with the primary mission of extricating a vehicle from around a patient in as short of time as is safely possible.

Extrication typically consists of a series of cuts and spreads, often shared amongst 2-3 tools—each with its own operational specialty. This means that the electrical load is shared across 2-3 battery packs, each of which is operated for <30 seconds at a time and rarely is discharged to the point of low voltage cutoff. This short operational time application is where the major differences exist between rescue tools and PPV fans.

In a period of 30 seconds a battery pack, even if maximizing its discharge capacity (…think of redlining your car’s engine…), does not develop enough heat to worry about—assuming the manufacturer has designed a BMS that regulates the maximum discharge current to within the safe limits of the specific battery cell model used in their pack. With a rescue tool, the cut operation is then stopped to move onto the next step. Cut pillar. Cut hinge. Place cutter down. Grab spreader. Proceed to roll dash forward. At each change of step, the battery has a chance to cool down when not is use.

After turning on a PPV fan, a firefighter expects it to operate at full speed for at least 30 minutes (1,800 seconds, or 60x longer than a typical rescue tool operation). Further, to move more than 12,000 cfm (20.000 m³/hr), a PPV fan needs about 1.9 hp (1.4 kW). To deliver this much power for the expected runtime from a rescue tool’s 176 Wh battery pack, a PPV fan would require five of these packs to hit the mark.

Beyond the power and runtime comparisons, the most often misunderstood factor of battery safety is the importance of proper heat dissipation—something that every firefighter should know about when studying the challenges of EV fire suppression. Batteries must be designed, from the ground up, with proper heat dissipation features specifically for the sustained periods of high discharge currents of PPV applications.

On one extreme of battery design you have a Tesla with a fully water-cooled system to manage the high heat levels seen when dumping battery current to achieve <2.3 second 0-60 mph times. On the other extreme, you have a flashlight battery that has no need for thermal management due to the extremely low discharge current rates required to power an LED.

There is no one size fits all when it comes to battery pack design. History has shown that the risks involved with using li-ion batteries are far outweighed by the utility afforded by the energy-dense lightweight design possibilities of lithium cells. That being said, risks are only mitigated by application-specific pack design that takes into consideration the actual electrical loads and runtimes of the tool that the battery is designed to power.

It takes a lot of experience and know-how to make a battery pack operate safely. When the pack engineers are the same people that designed the fan, you can trust that you are making the best choice for your department.