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.
Stan Purdum makes a number of very good points. However, the topic of charging to 80% is part of a larger picture.
An 80 / 20 rule works well for most rechargeable batteries, including those with lithium-ion chemistry; it’s best to not charge past 80% nor discharge past 20% – Important Note: completely discharging a battery is likely to have a far more negative impact on battery life than 100% charging. However, high-rate, 100% charging shortens battery life, and without battery specific safeguards, can destroy a battery – and in the case of lithium-ion batteries, with spectacular and dangerous results. However, a properly designed charger/battery combination can safely bring the battery close to 100% capacity in reasonable time. There is no need to be obsessive in applying the 80 / 20 rule, but operating a battery within these guidelines pays dividends. Actual battery life is a statical probability, and the probability of good service life can be made more favorable by treating the battery well (Also, don’t overheat or freeze the battery). However, life can present demands that require overriding some of these considerations.
Precisely matched battery cells are one of the most important factors in battery longevity – such precise matching, however, can add to the battery’s cost, (but is usually worth the investment). A quality, properly designed and implemented battery charger, for the specific battery being charged, can and should take battery state and parameters into consideration – the technology and electronics to do this have been available for years, and have become progressively more cost-effective.
Opinion: if an ebike’s manufacturer is not providing a quality charger, they are likely cutting corners elsewhere on the bicycle as well. Also, there is no place for generic chargers when charging lithium-ion batteries. Given the current state of battery and charger technology, there is no need for cyclists to be required to make manual calculations and waste resources supervising the charging process. Furthermore, the bicycle’s operation and control circuitry can and should prevent total battery discharge. Also, in a related comment, the bicycle’s control circuitry should not generate excessive radio frequency interference (RFI) (some illegally do), which can and does interfere with cell phone, computer, WiFi, Bluetooth, and Ant+ operation.
As Stan says above: The battery should serve you and not the other way around. I have a Bosch Powertube 500 watt hour battery on my 2020 Trek Domane +HP. It comes with a Bosch charger designed with what Bosch calls their intelligent battery management system that checks each cell of the battery while charging as not to over heat or over charge the battery. I do not put a heavy load on the battery normally riding in echo or tour mode never in turbo. I do however charge the battery to 100% before a ride and quite often ride it down to 5% and even to it’s automatic shutoff of 3%. When I get home I’ll charge the battery to aprox. 70-80% until the next ride. While doing group rides I try to ride with the assist turned off and turn assist back on when up front pulling or going up hill. I regularly ride the bike 70 to 80 miles and have done 100 miles with 4,00 ft. climbing three times. I’ve had this bike for over 2 1/2 yrs. have over 11,000 miles on it and the battery is still going strong. Hopefully I can get another couple years out of it. If I do I’ll be satisfied and purchase another one and hopefully have as much fun out of it as I have had with this one. BTW, I have a 2020 Trek Rail emtb with a Bosch powerTude 700 wh battery. Not as many miles as the road bike but I use and charge it pretty much the same way and it’s still going strong as well. All that to say don’t warry to much about the battery. Use common sense and enjoy it for what it’s for.
the chatbot formula is wrong
because current is not constant
to REALLY do this you could measure when the charging volts first reach 4.2 or whatever the final voltage is, and stop then
what a li ion charger does is, put as much current as possible into the battery as long as
a. the voltage has not reached ‘terminal’ [usually 4.20 but may vary with charger]
and b. the battery is not too hot
when terminal voltage is reached, the current is reduced so that voltage does not go over terminal, until current reaches a preset minimum, usually 1% of the capacity, ie .02A for a 20AH cell.
it’s called CC/CV charging – constant current for phase 1, constant voltage for phase 2
phase 2 starts at about 80% capacity.
Related question: Touring ebikes don’t seem to be happening. Yet many of us are realizing our age means we could use an electric boost to carry a camping load. Are there good touring e-bikes? I define a touring bike as one that is built to carry 4 panniers, or about 35 pounds of gear (plus the panniers’ weight).
Or, is it sensible to buy a good touring bike, and add the e-tranmission and batter?
Dave,
I haven’t looked into whether there are any touring ebikes made, but the range limitations set by batteries may have dampened that market. However, I’ve had positive experience with electrifying the touring bike I already had. I wrote about it here: https://www.roadbikerider.com/convert-touring-bicycle-ebike/.
Actually, the 80/20 rule is incorrect, the most recent research is now saying 80/35 to 30 at the very most, they’re also saying to charge it after every ride up to 80%, plus do not charge the battery immediately after riding you have to wait for the battery to cool down for an hour; if you charge while the battery is on the bike, the motor and the battery needs to be turned off before you start charging; only charge to 100% if for some reason you need the extra mileage and then do it rarely.
What I don’t understand is why they don’t have a processor built into the charger and/or the battery to stop the charge at 80%, my Ridgid tools charger does that, so if Ridgid can do that anyone can do it. Ridgid takes it a step further, in that they shut off the battery when it gets down to 20%. I bought my tools 16 years ago and I’m still using the original batteries, which makes me wonder about the 30 or 35% instead of the 20%, but maybe going to 30 to 35 means the battery might last another year? You really do need to be mindful of your battery, my tool battery (has a lifetime warranty) but for me to buy a replacement battery is about $180, but your Ebike averages $700, so it’s important to make sure that battery lasts as long as possible.
I think bike companies probably don’t have much electrical engineering expertise, being able to select charge percentage is common on cars and laptops.
I try to keep to the 80-30 rule and so far have 25000km on my 2017 Turbo Vado, the battery is about 85- 90% of original capacity.
Another thing to be aware of is heat, don’t leave your e-bike in the sun, li-ion batteries don’t like to be cooked. For example Nissan Leaf cars are notorious for poor battery lifespan in hot environments because they lack any active cooling for the battery which most electric cars have.
GrinTech up in Canada makes the best multi-voltage chargers they call Satiators that you can program to only charge to 80%.
https://ebikes.ca/satiator-standard-model-8a.html
Could a couple of rectifier diodes in series limit the voltage to approximately 80% charge. The diode drop would be at least 1.2 volts up to 1.8 volts at higher current.. With 10 cells in series that would be .12 to .18 volts drop per cell. Just short them with a switch to get full charge. Would this work?