Yes, it’s very possible to use an 18v battery in a 14.4v DeWalt drill. It is not important which voltage of battery you are using, what counts is the Ampere- hour.

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There are more than 12volt batteries with 18 amp-hours of energy. You can use these batteries for your DeWalt 14.4v drill. For safety purposes always check the voltage rating of both batteries before starting to use them together. But there are several considerations to think about.

DeWalt 18v batteries are designed to be used only with DeWalt tools. Even if the voltage rating is the same some differences like Ah might affect your Dewalt drill speed performance. Because you would be getting more energy from the battery which would make the tool work harder and wear out sooner. So always check before you buy any new batteries for your Dewalt 14,4v drill that they are manufactured by DeWalt or at least compatible with it. You can use Dewalt 12v battery in Dewalt 14,4 v drill as well because these two voltages are safe enough to use together without damaging anything inside of your charger or tool. Simply put first on first place, second on second place, etc.

Again you need to check if it’s compatible with your charger. Some chargers are designed to charge only 14,4v batteries but not 18v. cheap Dewalt batteries are sometimes incompatible with original DeWalt chargers so usually you can buy better quality batteries from trusted sellers on the internet like eBay for a reasonable price and use them with the original DeWalt charger which is safe.

So finally my answer is yes, using 12 or 18 volt battery with different Ah in the same tool is possible but only if they are made by Dewalt or at least compatible to DeWalt for safety reasons.

If they are not compatible then do not use them together! I think by now you got that point.