According to conventional wisdom, the amp-hour rating of a cordless-tool battery is strictly a measure of its capacity. For example, powering the same drill/driver under identical testing conditions, a new 4 amp-hour battery will bore about twice as many holes per charge as a new 2 amp-hour one. After all, the 4 amp-hour pack contains twice as many 3.6-volt cells as the smaller pack and therefore has double the storage capacity.

Some of our recent tool testers, however, swear that higher amp-hour batteries not only have more runtime than their compact counterparts but can also be noticeably more powerful. Reinforcing this opinion, Bosch recommends using its new MXH180 18-volt oscillating multi-tool with its 4 amp-hour pack rather than 2 amp-hour pack for maximum power and runtime.

That’s surprising, so we called Jason Feldner, Bosch product manager for cordless tools, for an explanation. In a nutshell, he said the cell configuration creates less resistance in 4 amp-hour packs than in 2 amp-hour packs, allowing larger packs to deliver more power. How much more? “A powerful tool like a recip saw might see a 20% performance boost,” Feldner says, “but that percentage can vary by pack and drops with less demanding tools and applications.” —Bruce Greenlaw is a contributing editor to JLC.