Question: how to tell if your car battery is flat?
Answer: Your dashboard warning light. Your dashboard warning light comes on. ... 
Engine cranks slow or not at all. Your engine cranks slowly or won't start at all. ... 
No radio. Your radio doesn't turn on. ... 
Swollen battery size. ... 
Odd smell from your battery. ... 
Your battery is old. ... 
Check engine light flashes.