How do you know when it's time to replace the battery?

[Timpo]

I know it is time to replace the battery when the battery is "starting to lose its capacity" or "can't hold the charge" anymore.

But what are the symptoms?

Does this mean that battery will drain faster? or does this mean the battery can't charge up to 100%? (for example, all the way up to 54.6V)

 

[indianajo]

Like a cell phone, the life will be shorter. The range the battery used to have will be less. I measure the voltage after each 30 mile trip to decide whether to charge or not. My battery is still getting better. The more I deep cycle the less voltage drop I get over the same range. The battery is 9 months old. That range improvement will turn around at the end.
Inability to reach full charge is a definite danger symptom. That can happen, get rid of it.

 

[AHicks]

If there's a dead cell, the charger won't be able to complete the balance charge, but it may keep trying.

You will likely notice slightly reduced performance at some point, but mostly a big drop in available range. That's what sends the flag up for most people.

 

[AHicks]

Some have a built in timer that's there as kind of a safety valve, that will shut the charger down after a certain length of time, no matter the charge level. If it were not for a feature like that, sure, it would keep trying to bring up that low cell.