Q. I recently purchased a DeWalt 18-volt cordless tool with lithium-ion batteries and noticed that the new batteries also fit my old DeWalt tools. Is it okay to use lithium-ion batteries in tools that came with nickel-cadmium (nicad) batteries?

A. Senior editor David Frane responds: The motor can’t distinguish between power from a nicad battery and power from a lithium-ion battery. And because cordless tools are designed so they won’t accept incompatible (wrong-voltage) battery packs, you’re safe using any battery pack from the same manufacturer that fits the tool.

The earliest lithium-ion batteries were not backward-compatible with nicad battery tools, but that’s changed over time. Three of the major tool companies — DeWalt, Hitachi, and Ridgid — now make 18-volt batteries that are both forward- and backward-compatible. Makita and Milwaukee 18-volt batteries, on the other hand, are not forward- and backward-compatible. When Makita began making lithium-ion batteries, it changed from a post-mount to a slide-mount design, which means that its lithium-ion packs do not fit earlier tools. Milwaukee had already adopted the slide mount for its nicad tools, but its lithium-ion slide mount has a different shape. DeWalt and Hitachi stayed with post-style mounts when they went to lithium-ion cells, while Ridgid uses the same slide mount for both types of 18-volt batteries.

Most chargers are also backward-compatible with older batteries, provided they have the same kind of mount. Due to advances in battery circuitry, however, chargers are not forward-compatible; the charger that came with a nicad tool will not work with lithium-ion batteries.