QUESTION: I just bought my first ebike, which came with a lithium-ion battery. I read that these batteries will last through more charging cycles and have less reduction in range over time if I charge it to only 80% each time instead of 100%. Leaving it on the charger until the indicator light goes from red to green tells me when it has reached 100%. But how long should it be on the charger if I want only 80%? —Henry C.
RBR’S STAN PURDUM REPLIES: You are right that charging to only 80% is easier on the battery and will slow its gradual degradation. It’s a good practice if 80% will give you enough range for the ride length you are attempting. But if you’ve got a longer ride in mind, you should go ahead and charge it to or nearer 100%. The battery should serve you and not the other way around. Just don’t try to keep or store the battery at 100%.
But assuming 80% works for you, there are three ways I know of to charge your battery to 80%.
The first method relies on technology.
Acquire a charger that allows you to set the percentage of charge. I’ve seen 48V chargers that let you select 80, 90 or 100%. Here is one from Luna and one from Bafang. There may be something similar for batteries of other voltages, and from other e-battery suppliers. Check with the maker of your ebike battery.
A variation of this method is to make your own percentage charger by adding a charge-control module between your existing charger and your battery. Here’s a description of the parts and the process from a do-it-yourselfer. I have not attempted this myself so I cannot attest to how well it works, and I take no responsibility for your outcome. Maybe wait until your battery is no longer under warranty before attempting this.
The second method relies on record-keeping.
But only until you learn how much charging you need to hit the 80% target from various starting points.
If, for example, at the completion of a ride, your control unit shows that you have 45% of your battery charge remaining, plug the charger into the battery and using a timer, set it for an hour. At the end of the hour, see what percentage of charge your control unit now shows. If it’s gotten the battery to, say, 60% — a 15 percentage point increase — then it’s reasonable to guess that another hour of charging would give you a similar increase and land you at about 75%, and slightly more will take you to 80%. (The same assumptions may not apply as you near 100%, as the final few percentage points take more time to charge.)
Keep a record of how long you needed to charge to get from 45% to 80%. Keep similar track when starting from whatever percentages of charge you have remaining — 49%, 53%, 68%, etc. — and after a while, you’ll likely be able to estimate with reasonable accuracy the charging times needed.
The third method relies on mathematics.
But my cycling buddy Mike Battershell, who knows a lot more about all things electrical than I do, pointed me toward the new ChatGPT AI chatbot to come up with the formula. (I’ve heard that ChatGPT isn’t always good with math, so take this what it is worth. If any of you have the math and electrical knowledge to verify or improve on this, please note it in the comments section below.)
So I asked ChatGPT: “If I have a 48-volt lithium ion battery with 13-Amp-hour capacity, with 45% charge remaining, and have a 2-amp charger, how long do I have to charge the battery to reach 80% charge?”
ChatGPT came up with this formula:
(Battery capacity [Ah] x (100% – remaining charge [%]) / Charge rate amps [A] = Charging time [hours]
Plugging the values from my question into the formula, you get:
(13 Ah x (100% – 45%)) / 2 A = Charging time (hours)
= (13 Ah x 55%) / 2 A
= 7.15 Ah / 2 A
= 3.575 hours
ChatGPT summed up: “So it will take approximately 3.58 hours to charge the battery from 45% to 80% using a 2 Amp charger. Please note that this is just a rough estimate, and the actual charging time may vary slightly due to factors such as the battery’s internal resistance and the accuracy of the charge level indicator.”
Personally, I’d call it 3½ hours, but who am I to quibble with AI?
But I noticed that ChatGPT’s equation didn’t mention the nominal voltage of the battery — 48 volts in this case. So I asked ChatGPT about that.
It responded: “The nominal voltage of the battery does not have any direct bearing on the equation that is used to calculate the charging time. The nominal voltage of a battery is simply a measure of the average voltage that the battery is capable of producing under normal operating conditions. It is not directly related to the amount of charge that the battery can hold or the rate at which it can be charged. … It is only the capacity, charging rate, starting charge level, and target charge level that are used in the calculation.”
By the way, for those ebike riders who set their control unit to show the remaining charge in voltage rather than percentage, consult this li-ion chart page to translate your remaining charge to percentage. There are charts for 36-, 48- 52-, 60- and 72-volt batteries. (These are the nominal voltages; nominal voltage is the value assigned to a system or circuit of a given voltage class for the purpose of convenient designation. The actual voltage may vary above or below this value. The 48-volt ebike battery, for example, is charged to 54.6 volts when at 100%.)
Stan Purdum has ridden several long-distance bike trips, including an across-America ride recounted in his book Roll Around Heaven All Day, and a trek on U.S. 62, from Niagara Falls, New York, to El Paso, Texas, the subject of his book Playing in Traffic. Stan, a freelance writer and editor, lives in Ohio. See more at www.StanPurdum.com.