Batteries of the cordless drill are not interchangeable if the manufacturer is different. But if it is the same manufacturer then you can exchange the battery within different cordless tools such as circular saw cordless drill and others. For example, Dewalt is a renowned power tool manufacturer. It produces different power tools such as cordless or corded drills, reciprocating saws, and many others. You can use the battery of a circular saw in a cordless deal.
Therefore I am making it clear that you cannot use the battery of a certain cordless drill for another drill that has been manufactured by another company. If within the same manufacturer, then it might be OK.
Can I use 18 volt battery in 20 volt tool?
Yes, you can. Ideally, you can interchange the battery of an 18-volt cordless drill and a 20-volt drill. In fact, 18 and 20-volt cordless drills are the same. For example, you can swap the batteries of 18 and 20-volt cordless drills without any problem. I remind you again you cannot swap batteries of different brands.
What to consider before interchanging the battery of a cordless drill?
First, we have to confirm that both of the drills are from the same manufacturer. But this is not the only consideration. It is the prime consideration you have to keep in mind.
Secondly, the voltage of both the batteries has to be e the same. All the 20 volts and 18 volts are interchangeable. If the voltage difference is too much then you cannot interchangeably use those batteries. For example, you cannot use a 12-volt battery for a 20-volt cordless drill. Similarly, you cannot use a 24 battery for a 12-volt drill. Even if the manufacturer is the same the voltage has to be matched.
After the voltage is matched you have to consider the amperage of both the batteries. It is not mandatory that the Amp of both the batteries has to be the same. But it is better to interchange with a battery with the same Amp rating. If the difference of Amp of both the battery is too much then avoid interchanging the batteries even if they are from the same manufacturer.
Is a Dewalt battery interchangeable with black and decker?
No black and decker and Dewalt batteries are not interchangeable even if the voltage and amperage are the same. The reason behind this is that although they give the same current and voltage they will not fit each other. Therefore you cannot fit a Dewalt battery into a black and decker cordless drill. The same thing happens vice versa.
Bottom line
Batteries from the same manufacturer may not be interchangeable sometimes. Therefore, you have to keep in mind all other factors that I have discussed above. If possible, keep multiple backup batteries so that you do not have to run out of power at a prime moment.