Maybe you’re not one of those e-bike nerds (which I am) that owns a fancy-dancy e-bike battery charger that let’s you adjust the charging current and percent charge (which I do). So the question is, if I don’t really want to charge my e-bike battery to 100% every time (which is bad – for your battery), how do I know how long to keep my battery on the charger if I only want to charge to 80%, for example? Great question, and I’m glad you asked!
For the answer, I’m going to discuss the ‘easy’ way to do this, which will get you in the ball-park. For this example, I’m going to use my 48 Volt (V) 18 Amp-hour (Ah) Hailong e-bike battery recently purchased from Amazon.
Let’s begin by calculating the total Watt-hours (Wh) for this battery: Ah x V=Wh
18Ah x 48V = 864Wh
So, when I finished my last ride, the display was showing 2 bars on a 5 bar scale, so I know I have approximately 40% SOC (state of charge) remaining in the battery. (note: each bar on a 5-bar scale represents about 20% SOC)
Now I need to know what my charger’s output is in Watts (without looking at the back of the charger where it’s posted). This is easy: Volts x Amps = Watts
And since I know my battery is 48 Volts and the charger is rated at 3 Amps (yes, you might actually have to look for this number on the back of your charger), then I can calculate this charger’s Wattage to be 144 Watts.
Then by dividing my battery’s Watt-hours by my charger’s Watts , I can figure the total time it would likely take to fully charge my battery from 0-100% (which is never a good idea). In my case, dividing 864Wh by 144W says this operation would take approximately 6 hours. But if I still have 40% SOC in my battery and only want to charge to 80% SOC, I’m going to have to do a little more math.
So, if 40% of 864 is 346, and 80% of 864 is 691, and the difference between the two is 345Wh. Divide this number by 144W and you’ll get something like 2.4 hours to charge to 80% from a 40% SOC.
Now that wasn’t really that difficult, was it? And we all know that always charging our e-bike batteries to 100% is just not a good practice if one cares at all about battery longevity (which I do). So putting a bit of effort into running these calculations for your battery a) is a good brain exercise, and b) should improve your battery’s longevity significantly! I often use the following chart in my e-bike classes to demonstrate this point:
Useful (charge/discharge) cycles is a measure of battery longevity, and using this image as a guide, your battery could have a useful life 3+ times longer by charging only to 80% versus 100%. And this is a good thing for the planet and for your pocketbook!
** Update **
To simplify the math for y’all, I created this very cool Excel spreadsheet. Go to this link to download it:
https://ratelectricbike.com/wp-content/uploads/2024/09/Battery-Chg-Calc.xlsx
When you download the spreadsheet, you’ll be able change any of the relevant parameters to recalculate ‘time-to-charge’ in hours:
In this example, I’m using my 48 volt 18 amp-hour battery with the factory 3 amp charger. Change any or all of these values and the time-to-charge will change. Enjoy!