I bought my first cordless impact driver at a time when few people had one, so it got a lot of attention whenever I brought it on site. Since then, this type of tool has become so common it would be hard to find a tradesperson who did not immediately recognize its distinctive chattering sound.
Impact drivers drive with more power than comparable drill/drivers and produce less torque reaction – that is, they're less likely to twist against your hand when under load. They're also less likely to strip out the heads of fasteners, because the impacting action keeps the bit engaged. You can use impact drivers to drill holes, but most people use them for driving fasteners.
For this article my crew and I tested 18 lithium-ion impact-driver kits. Some companies sell the same tool in two kits – each with different batteries – and in such cases we tested both versions. We used all the drivers for several months, trading them around so that everybody had experience with each one.
Since I build decks and do remodeling, I mainly drive smaller screws; it would be unusual for me to deplete a lithium-ion battery in the 30 to 60 minutes it takes to charge the spare. In most cases I can work until lunch, swap batteries in and out of the charger, and then work uninterrupted the rest of the day. If the work you are doing is so intense you outpace the charger – say you're driving big lags and bolts in rapid succession – then it makes sense to go for a tool with great runtime or switch to a corded model.
Bosch, DeWalt, Hilti, and Milwaukee offer two battery options per tool: five-cell compact batteries and 10-cell full-size batteries. The compact batteries are smaller and lighter, but the full-size batteries have greater runtime (roughly twice as much). If I had to settle on one type of battery, I'd go with a full-size model, because it would work better with the larger cordless tools (circular saws and recip saws) in that manufacturer's lineup.
The author tested runtime by driving 3-1/2-inch lags into Parallam material until the tool's batteries were depleted.
We got a pretty good sense of how much work the tools could do per charge by using them in the field. But to put actual numbers on their performance, we also tested the tools in the shop by driving 3-1/2-inch Simpson SDS screws unpiloted into Parallam material until the batteries were depleted. We drove fasteners in groups of 10 so the tools would not overheat, then repeated the test with a new battery.
Since Parallam is very dense, we tested again in Douglas fir with a few of the tools. We found that on average, the impact drivers will drive 25 percent more fasteners in solid lumber than in Parallam. We also measured driving speed, by timing how long it took to sink lags in Parallam material. The results of the runtime and timed driving tests can be found in the tables below.