Regarding battery health indicators, there is a correlation between the battery's output voltage at 100% charge and the capacity the battery can deliver.
See below a typical voltage chart for a 36V ebike battery (Bosch powertube for example).
After fully charging the battery, the voltage measured is one indicator of the battery capacity (%) at any given time. The table shows the max voltage the battery can charge to when charged at 100%, hence giving an estimation of the battery's capacity % in comparison to a new battery.
As an example using the table below, if my battery voltage measured after charging at 100% is 39 volts, then my battery max capacity is down to 75% from its capacity when new. In other words, my battery lost 25% of its original capacity.
If the voltage becomes too low, it's most likely time to get a new battery. If the voltage of my Bosch battery drops below the 60% capacity threshold within the warranty period (~2 years), so under 37.2V, a battery replacement may be possible under warranty. Check battery manufacturers warranty conditions.
Battery voltage (Volt) when charge level is 100% | Battery capacity % when charge level is 100% |
42.0 | 100% |
41.4 | 95% |
40.8 | 90% |
40.2 | 85% |
39.6 | 80% |
39.0 | 75% |
38.4 | 70% |
37.8 | 65% |
37.2 | 60% |
36.0 | 50% |
35.4 | 45% |
34.8 | 40% |
34.2 | 35% |
33.6 | 30% |
33.0 | 25% |
32.4 | 20% |
31.8 | 15% |
31.2 | 10% |
30.6 | 5% |
30.0 | 0% |