I have an old LiPo battery, how long is a LiPo battery usually supposed to last? How will one know if it needs to be replaced?
This question is difficult to answer without a few specifics.
By ‘how long does a LiPo last’, if you mean when it’s on a quad, then usually around five minutes on a five inch quad.
If you mean how long before it degrades, then that depends on a lot of things:
How low do you run your batteries during flight? Lowering them to below around 3.3V can damage them and reduce the lifespan.
How quickly do you get your batteries to a storage charge? Leaving LiPos at anything other than 3.7-3.8V for extended periods can lead to damaging them.
Do you balance charge them? Some chargers have the option to either charge or balance charge LiPos, so it’s good to check.
Do you charge up your LiPos to 4.2V? Some racers charge regular LiPo batteries (not LiHV as far as I’m aware) to 4.35V, which gives higher short term performance but degraded the battery.
Over all, there are many aspects towards the lifespan of a LiPo, and in general if you treat them right, they will last longer.
Some signs that you may want to retire a battery are visible puffing, if you feel there is a reduction in performance, and if the internal resistance of the cells rises significantly from when they were new.
How long is a LiPo battery usually supposed to last?
Most LiPo batteries are not rated to last longer than 300 charge cycles. That is the battery fully draining and charging again. However, measuring a LiPo battery’s lifespan may not be practical, as batteries go through varying depths of discharge when they are used.
Many manufacturers have stated that their LiPo batteries will last 2 or 3 years. This is a somewhat realistic approximation for a scenario where a battery is regularly used and charged around 2 or 3 times a week.
How will one know if it needs to be replaced?
After a LiPo battery goes past its lifespan, it can't hold as much charge anymore, so look for signs that it doesn't last as long as it used to on 'full charge'.
Just adding to the excellent answers...
Since the question asks also for a predictor of battery life, generally I'd expect a LiPo battery to have lost approximately 25% of it's fully charged amperage after roughly 200 charge cycles.
Depending on your demands on the battery, a LiPo will last somewhere between 300 and 500 recharge cycles (obviously depending on how you use it). To make it last longer, don't overcharge it; charge at the correct voltage; and store your battery partially decharged. Further reading here.