eMTB batteries are expensive – some are about $1000, so it makes sense to want them to last for as many charge cycles as possible. The two things that you can do to prolong your battery life is to not fully discharge it, and also, perhaps surprisingly, not store it fully charged. For example, this article shows that if you discharge to 40%, their example cell-phone battery will last 1500 cycles until it has 70% of its original capacity still functional. But if you discharge to 0%, it is only 600 cycles. They give further examples of how if you leave the battery in their example fully charged for a year at a 25c temp, it will lose much more of it’s original design capacity than if stored at a lower percent state of charge.
My 2020 Turbo Levo with the 700Wh battery can last a long time, and so I usually only need to charge it every three rides. For example, a typical ride may use 33% of it’s capacity. Charging it every ride to 100% would be bad, as seen on the charts, because that would result it in being stored 100% charged. And only charging it when it completely dies would also be bad, as seen in the other chart from the linked article. So to make the battery last as long as possible, I should ride until it is at 20-40%, and then only charge it to 60-80%. If a longer ride is expected – it is no problem to charge it to 100% before the ride, because it is not being stored at that charge for a long period of time, so for sure, charge to 100% when you need to.
The Giant eMTB has a charger with a button specifically designed for this safer storage – push the button and it will charge to 60%. But what should a Turbo Levo owner like me do? You can buy a timer like this Stanley from Amazon. When using it, my 700Wh battery went from 42% to 64% in one hour – so, 22% gain per hour. This means that if I do a ride, and my battery is at 40% and I want to charge it to about 80%, then I need should push the “2 hour” button on the timer.
Another way to look at is is that a 2-hour charge will cover any of my normal rides, so I just push the 2-hour button most of the time. If I know I want a full charge, then the 6-hour button will always cover it. If you want to know more, read the linked article, or countless others. Or just only fully charge the battery before you need to use it at full capacity and try not to run it too close to empty.