Ebike Charging; Fast or Slow?

December 20, 2015

Everyone likes a fast charge…the idea of charging your battery pack quickly is an exciting concept and its actually very possible. However, as it is with all of ebiking…high performance comes with significant drawbacks.

So you might think that the higher cost of a faster charger is the only consideration when upgrading chargers, but…it’s not! You will notice that there are not many ebike chargers available on the market that charge at more than 5-amps…and that is because of the dangers of charging an ebike battery too fast. For the purpose of this article we will consider 5-amps as fast charging, and 3 amps or lower to be slow charging.

Pictured below is a standard 5A charger that most ebikers have become acquainted with. Its made from aluminum and not plastic because the aluminum acts like a heat sink to help keep it cool.

ebike charger



Woah, Lets Slow Down!

We know that everyone appreciates a fast charge…the idea of charging an ebike battery in an hour is an exciting concept. Once you get into the ebike math, the first notion is to get as high an amp charger as possible. With a 10-amp charger you can charge a 10-amp battery in one hour…lets buy one! right?


But charging too fast is hard on the battery, and it will cause its life expectancy to go down, and also it can be a fire hazard in extreme cases of charging too fast.

We really recommend that if you are new to ebikes, and new to lithium batteries in general, that you stick with the slow charger that came with your pack, and do not fast charge. This is both safer and healthier for your pack.

Ebike Charger Math

One of the questions you should ask when buying an ebike is how large is the battery in Amp-hours (Ah) and how many amps is the charger it comes with? Then using ebike math you can quickly calculate how long your battery will take to charge. We know based on ebike math that charging a 10-Ah battery will take 10 hours with a 1-amp charger…or 2 hours with a 5-amp charger.

If  you divide the amp-hour rating of your battery by the amp rating of your charger, that is how long it will take you to fully charge your battery (if it is fully discharged). However all good lithium smart chargers dial back the charge at the end of the charge cycle, so they take a little longer than what they are rated for. So, for a 10 amp hour battery, it will take a 5-amp charger a little more than 2 hours to charge your pack.   

Here is a drawing which illustrates how a typical ebike charge cycle works. As you see the amperage of the charge starts to dip once the battery reaches 80 percent charge. This is so it will be as gentle on the battery pack as possible.


A high-amp charger will only actually use high-amps until the battery reaches about 90% of its full maximum possible charge…then it will use low amps to gently “top off” the pack.


BMS Limits and charging fast:

Commercially sold ebike batteries such as the Luna Cycle packs have some kind of BMS (battery management system)  that protects the pack. One of the things the BMS does is limits how fast the pack is allowed to charge. If you plug a 10-Ah battery into a 5-amp charger, and the battery does not charge, the BMS might be refusing the charge.

Most lithium BMS’s have a limit of around 7 amps as the limit on how fast they will charge. Also, the BMS is in charge of balance charging your cells to further ensure you packs safety and longevity.


The Importance of Balance charging

Any good BMS not only protects the safety of your pack during charging and discharging it also ensures each series of cells is balance charged, which means all your cells are at about the same voltage.  Balance charging does not happen fast…in general most ebike  chargers will slow way down at the end of the charge to allow a proper ebike battery with advanced BMS to balance out each line of cells.  


Chargers that Charge Fast AND they can charge Slow


For an advanced ebike charger that can charge both fast or slow, you have two choices as of this writing. The $300 Cycle Satiator that was previously mentioned, or the $100 Luna Advanced Charger which will charge at either 3 amps or 5 amps. Both chargers are very good. The Satiator is more advanced and the Luna Charger is more affordable.



A third good option is if you already have (and know how to use it) is an RC charger with an external power supply. The RC charger must be able to match the voltage of your pack, which is rare in an RC charger to get up to 48 or 52 volt battery, but they can be found. These chargers can be a wiring  mess and their Chinese interface makes them very difficult to operate.


The Size of your Pack Really Does Matter

The larger the battery, the safer and healthier it is to charge at high amps. For example, 5 amps is fast for a 10 amp-hour battery, but not so fast for a 20 amp-hour battery.


When you charge your battery fast all the time, it will greatly reduce your batteries life expectancy. We recommend you stick to slow charging (4 hours or more) or go with a charger that can switch between fast and slow charges like the Luna Charger.

Now if you have a 10-Ah battery and you have a 5-amp charger…that is a super fast charger for that pack.

High Quality Packs Can be Charged faster

If you have a pack consisting of high amperage cells like the Samsung 25R cell, and your pack is capable of high discharge, but…it will also accept fast charging without degrading your packs life expectancy too much.  The drawback to these cells is they are expensive and they are not as energy dense as some lower performance high capacity cells such as the Panasonic NCR B (3400-mAh, instead of 2500-mAh for the 25R). So how can you tell if you are charging your pack to fast?


The Finger Test


The quickest way to tell if you are charging to fast is to put your finger on your battery pack when it’s charging and see it the pack is getting more than a little warm.

If it gets fairly warm, you are degregating the life of your cells and its time to invest in a slow charger.


Buying Advice

First of all buy the largest battery that you can carry on your bike, made with quality cells. Here are some good options when choosing what amperage of charger to buy:

#1  Buy a fast charger AND a slow charger.

Until recently this is how I did it…I would have a 5-amp and 2-amp charger around for every voltage of battery I owned.  I had a box of chargers..i solved this by investing in one advanced charger.

#2 Just buy a slow charger, and forget fast charging.

This is the safest way to go, the cheapest way to go, and the way that will make your battery last the longest.  

#3  Buy a big ebike battery (20-Ah or more) and charge it at 5-amps

A big battery solves a lot..they are less delicate and can handle a fast charge. Also with a large enough battery you won’t have to worry about charging in the middle of the ride.

#4  Buy a fast charger and just let it rock!

Who cares if you lose a couple hundred cycles in battery life?…you will still get hundreds of rides and that will probably be able to survive years and by then you will want to replace your battery pack anyway with something better and lighter. This is what a lot of hot-rodders do, they buy a new battery pack every spring and beat it to death until it’s cold at the beginning of winter. Next year? they use the old battery as a back-up…

But now just in 2015 there is another option:


#5 Buy an Advanced Variable Amperage Charger

Rather than buy two chargers, the ideal alternative is buy one charger that can either slow charge or fast charge with the flick of a switch. The  $100 Luna Charger and the $300 Cycle Satiator.  Both these chargers can be set for either a fast or a low setting so that you don’t have to have multiple chargers around and thus reduce the wiring clutter that ebikes tend to make (read our story about the ebike  ratsnest)


Home Made Ebike Packs Charging Insanely Fast on the cheap


Here is an example of  well-made home built pack (they usually look a lot worse). All the little white plugs you see coming out are balance plugs (which is a good thing that many garage builders leave out!) .

Some very knowledgeable ebike builders attempt to use RC LiPo packs that are not BMS-protected to save money.  Also, since these packs are not output-limited with a BMS, they can not only put out a lot of amperage discharge, they can also charge very fast. But, charging fast is hard on the pack and can be dangerous.

Generally they use power supplies as a charger, by using units made by companies such as Meanwell that match the voltage of their pack (they can be stacked in series to reach any voltage you want, like 100V! Don’t try this at home, leave the modifications to professionals). You can buy Meanwell chargers in all sizes, some that can even charge at 60-amps!



Also when you charge this fast without a BMS, the cells are not being balance charged which will drastically reduce the life of your pack…and you will have to rig an apparatus for balancing separately if you want to charge safely. Obviously this can become somewhat of a wiring nightmare. (read our article on the ebike wire ratnest)

We really recommend that you go with a professionally-built 18650 pack or you really do serious research on endless-sphere.com before undertaking building your own pack.


Written by Eric, December 2015


Eric has been involved in the electric bike industry since 2002 when he started a 6000 square foot brick and mortar Electric Bike store in downtown San Francisco. He is a true believer that small electric vehicles can change the way we operate and the way we think.


Leave a Reply