the problem with "doesn't do it's job" is that the job changes.
The battery could still start the vehicle under normal circumstances, but the envelope for performance and esp. the reserve capacity decreases.
So say you decide to listen to the radio for an extra 5min to finish a song before you go into the office for the day.
Then suddenly the battery doesn't have enough juice to start when you leave at 5pm.
Now you're stuck for 30min or an hour or however long it takes for you to deal with that situation.
The battery could still start the vehicle under normal circumstances, but the envelope for performance and esp. the reserve capacity decreases.
So say you decide to listen to the radio for an extra 5min to finish a song before you go into the office for the day.
Then suddenly the battery doesn't have enough juice to start when you leave at 5pm.
Now you're stuck for 30min or an hour or however long it takes for you to deal with that situation.
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