The best quick test is another known-good compatible battery. If the drill behaves normally with the second pack, the first battery deserves the blame. If both act the same, keep diagnosing before you spend money.
Do not diagnose the battery from one hard job
A drill that stalls on a large bit may be overloaded, in the wrong gear or using a dull bit. That is why the first step is to compare behavior with a second battery under the same ordinary task.
Look at runtime, charging and consistency
A battery that used to run a long time and now fades very quickly may be worn. A pack that will not complete the normal charge process with the correct charger also deserves attention. Heat, storage conditions, charger condition and battery age can all affect what you see.
Check the charger before buying two things
If no battery will charge correctly on one charger, the charger itself belongs in the diagnosis. Borrowing or using another known-good compatible charger can be just as useful as swapping batteries.
Would a second battery be useful even if the first one is not dead?
Sometimes, yes. If you routinely finish one pack before the job is done, a second battery can keep you working while the other charges. That is a runtime decision, not proof that the original pack is defective.
Read the higher-Ah battery explanation before buying the biggest pack on the shelf.
A three-way test is even better
If you own more than one tool on the same battery platform, test the questionable battery in another compatible tool and test a known-good battery in the drill. That can separate three possibilities: battery, charger or drill.
What not to do with a suspicious lithium-ion battery
Stop using a pack that is swollen, cracked, leaking, unusually hot or physically damaged. Do not open the battery case or try to repair cells yourself. Follow the manufacturer’s disposal and service guidance.
When the replacement decision is really about the battery platform
If the drill is old and a replacement battery costs a large fraction of a modern kit, compare the cost of staying in the old platform with moving to a new one. On the other hand, if you already own several compatible tools and chargers, one new battery may be the cheapest way to keep the whole set useful.
Sources used for this answer
Back to class: Cordless Drill Class.