For years, the electric bicycle market used legacy designations for power system voltages, like 24V, 36V and 48V. This was because the earliest systems used the common 12V Sealed Lead Acid (SLA) batteries, connected in series (SLAs were found on electric wheelchairs from decades ago). The truth of todays lithium battery power systems is a little murkier, and I will try to explain what you will be seeing in ebike catalogs over the next year or two, and why.
Fully-charged voltage? or average “Nominal” voltage?
Any form of Lead Acid battery is usually listed as its fully charged voltage, and then as it is used to drive the ebike…the voltage goes down at a steady rate. Lithium is…different. The first couple minutes of the ride, the lithium battery voltage will drop fairly rapidly down to its “nominal” voltage, which is near its average voltage for the entire ride. Then…the voltage will stay very close to that average voltage for most of the ride. Near the end, the voltage will drop off fairly rapidly.
The adjustments on the charger will determine the precise voltage that your lithium battery ends up at, and the Low Voltage Cutoff (LVC) inside the controller will determine the pack voltage when the power is cut off.
SLA’s are very cheap to purchase, but…they are bulky, heavy, and short-lived for the amount of watt-hours they hold, compared to even the cheapest lithium packs. When you factor in a 1,000 cycle life of a quality lithium pack, its is cheaper to buy lithium, because a SLA pack would have to be replaced several times during that amount of time. Also, during that entire time, the lithium pack would be lighter and smaller.
Hopefully, I have convinced you to pay a little extra for a lithium pack that will last many years, but…which lithium?
It is still possible to buy a Lithium Iron Phosphate pack (LiFePO4). For some reason involving chemistry, they have a nominal voltage of 3.2V per cell (sometimes listed as 3.3V), so a LiFePO4 pack that is listed as a 48V pack will typically be using 16 cells in series (16 X 3.2V = 51.2V). That is not the fully charged voltage, or the LVC, just the average. However, LiFePO4 is becoming less common, and the best-performing and most affordable batteries for now (and for the near future) using NCM and NCA.
18650 Lithium Cells with NCM and NCA
The big players in the cordless tool market are Panasonic, Samsung, and LG, and…those are where you get the high-current cells that you will be finding inside the best and most cost-effective battery packs for ebikes. There are several rechargeable cell shapes and sizes used in that industry, but the cylindrical 18650 format (18mm diameter, 65mm long) is now by far the most popular. The entire outer metal shell is the negative anode (underneath the plastic wrap), so they are inherently robust, even before they are configured inside a battery housing.
Two years ago we wrote about the new NCM and NCA chemistries, and last year we wrote about high current cells with these chemistries. This spring we wrote about the new higher capacity cells. All of this means that we are living in an exciting new age for electric bikes. Both of these chemistries have a nominal voltage of 3.7V per cell. Many companies have their chargers use a charging protocol that fully charges the packs to 4.2V per cell In order to get slightly more range, but if you only charge the packs to 4.1V per cell (or even better, 4.05V per cell)…there can be a dramatic increase in the packs life-cycle count.
For the purposes of this article, I am recommending charging to 4.1V per cell, so…a “so called” 48V lithium battery pack that uses 13 cells in series will have a fully-charged voltage of 13 X 4.1V = 53.3V when topped off. I recommend an LVC of 3.0V per cell for max life, so a pack made from 13 cells in series (13S), would have an LVC of 39V. Sooo…the “48V battery pack” would be 53.3V when fully charged, and 39V when it is so low that the controller cuts the battery off (13 cells times the “nominal” 3.7V per cell = 48.1V)
13 Cells in Series:
53.3V fully charged (4.1V per cell)
48.1V Nominal (3.7V per cell)
39.0V LVC (3.0V per cell)
More Volts, Less Amps?
Five years ago, ebike kit-builders did not have high-current battery packs readily available. The common custom upgrade was to build a DIY battery with higher volts (requiring more cell-strings in series), using cells harvested out of cordless tool batteries. But, to even have the the most basic amount of range (from the paralleled connections), these DIY batteries were often very large.
Over time, the situation has dramatically improved. High performance still costs more, but decent speed and range are now available at reasonable prices for anyone who wants that, and…you also need it to fit into an average bike frame.
Now that we live in a world where the average ebiker can have higher volts and also higher amps in a more fittable package, what are some good rules-of-thumb to use when planning out a kit ebike build?
Raising the volts in a system (without changing the Kv of the motor), is the common way to raise the top speed of the motor slightly, and it will also raise the power (watts) without a significant increase in system heat. This is because the majority of the heat that is generated in your ebike system is from the amps. It’s been a long-time rule of thumb that whenever it’s possible, you should:
“…raise the volts, lower the amps…”
How high can we raise the volts?
The first things you must consider are the limitations of your controller. If your controller cannot handle high volts then you have to change the controller, or…make do with the maximum voltage of your controller.
We’ve established that the fully-charged volts of a battery are higher than its “nominal” rating. Batteries are rated like this because…that’s what customers have wanted for years! If you want our advice, here’s what the average ebiker should really want:
A) Lithium-based battery, made from NCM or NCA chemistry
B) Your battery pack uses 18650 cylindrical cells for safety, performance, and affordability
C) Charge pack to 4.1V per cell for max life (instead of 4.15V, or 4.2V)
Now, if each step in adding a series-string to a pack design is a step of 4.1V, and a common 13S pack (nominal 48.1V) is 53.3V when fully charged. Then, one step up to a 14S pack would be:
51.8V nominal (3.7V X 14), and 57.4V when charged (4.1V X 14), so…for the purposes of this conversation, lets call a 14S pack a 52V pack.
If 14S is good, is 15S better?
This article is about how to get the most out of the common mass-produced products, that give the ebike builder the best bang for their buck.
Many 14S chargers are adjusted to charge to 4.2V per cell, so a 14S pack (that we are now calling 52V) will have a fully-charged voltage of 58.8V. Using this as our guide, a common charger at 4.2V per cell would charge a 15S pack to 63V, and that is the magic number to remember…
Using efficient components in the controller
The lower the max voltage of capacitors and FETs, the more efficient they are. Now, whenever I hear the word “efficiency”, I immediately think that the slightly more efficient version of a component will provide me with an extra mile or two of range. I always lean towards having a bigger battery than I need, so lower price is actually usually more of an influence on my purchasing, instead of a slightly higher efficiency.
However, we should all care about efficiency, and here’s why: HEAT!
If you have an inefficient system (which you chose because it was slightly cheaper), and you put our recommended 1,200 watts of power into it…some of those watts will be converted into waste-heat, instead of driving the wheel. A more efficient system would run cooler and have more power at the wheel.
There are plenty of expensive ebike components that will provide as much as 100V plus 100A of power all day long (10,000W!!), but…we already know that most new ebikers end up being happy with around 1200W (usually on their second ebike). So, what are the most affordable mass-produced components that will provide that?
The most common capacitors found in the millions of 36V and 48V controllers will have a max voltage of 63V. The absolute cheapest controllers have a fixed system voltage (and a fixed LVC) but, the most popular controllers among kit builders have features that are one step up, and they have programmable voltage capabilities. You could special-order a controller that has higher-voltage capacitors and FETs, but…higher-voltage components will be less efficient, and will run hotter at the same amp-flow.
FET Voltage, and More Numbers…
A MOSFET is a Metal Oxide Semiconductor (what it’s made from) Field Effect Transistor (what it does). It is an ON/OFF switch that ebike controllers use to energize the motor-phases at the precise moments needed for the motor to run. These devices are what flow the amps to the coils, and they are the part that gets hot.
If you want an adjustable voltage controller (whether you program it in your garage, or have the retailer program it for you before shipping), the 4110 FETs are popular for controllers that can be adjusted to run between 60V and 100V (the 4115 will run between 83V to 132V). For up to 100V? also consider the CSD1953KCS, the AOT290L, or the IRFP4468.
An authentic high-quality 3077 FET will run at voltages up to 73V, and it will do it more efficiently and cooler than a 4110 FET (which is what you’d need if you want a system voltage between 65V to 100V). Due to voltage spikes and voltage ripple, you really should have a 8V safety margin in the system design, so…a 73V max 3077 FET shouldn’t be fed over 65V.
This brings up a question…Couldn’t we order a controller with higher voltage capacitors, and still use the 73V 3077 FETs? (in order to efficiently use a 60V battery instead of 52V?) Its possible, since a 4.2V-per-cell charger on a 16S battery (nominal 60V) would be fully charged at 67.2V (but remember, adding more series strings to raise the pack voltage will make a pack larger).
The 73V FETs (our sweetheart 3077’s) would be OK when using a 60V pack (67V hot off the charger), but…we don’t recommend that, and here’s why…
Voltage, Amps, and Safety
No matter what voltage you use in your ebike system, a high-current pack that can put out a lot of amps can cause VERY SERIOUS BURNS!
ebikers call this KFF (for Kentucky Fried Fingers), but…it is unlikely to kill you from electrocution. The higher the amps, the worse the burn will be (and we definitely enjoy high amps around here), however…how many volts does it take to kill someone (when talking about DC)?
Here’s a story about a car mechanic and a 12V / 80A battery:
“…A friend of mine (many years ago) was changing a battery on a truck…He made two mistakes … First, he left his wedding ring on…second, he used a non-insulated wrench. As he tightened the live battery terminal, the wrench came into contact with his wedding ring…The ring unfortunately came into contact with the bodywork of the truck…
…The ring suddenly became extremely hot, and luckily did not weld itself to the wrench or the bodywork…He was left with a severe ring-shaped burn on his finger, and the ring had to be cut off due to the swelling of the finger…”
OK, that was 80A of DC, but…at low voltage. The skin on the human body is a reasonably effective insulator at the 48V-63V voltages that we are talking about right now, but…if the skin is burned away from high amps? the interior tissue conducts electricity very well.
The AC in your home’s electric supply “alternates” its direction 60 times a second (60 cycle), and this means that 60 times a second, the voltage passes over a “zero volts” reading. DC has constant flow, and if it is flowing through your hand, it can cause the muscles to contract and you can’t let go! The nightmare scenario is if the electricity is flowing from one hand to the other, which passes across your heart, and…you can’t let go!
So what voltage in that situation is less lethal?
According to ISO 60950-1 (electric shock, 0.2.1, page 23), it is safe to touch 60V DC in a SELV circuit, and this standard is used in both USA and Europe.
If we think about this for a while, we can construct a situation where someone can be killed by only 36V. But…lets just say for the sake of this argument that…100V DC will absolutely penetrate human skin…just by touching it (24 cells in series, 24S, in case you were curious). The 58V of a fully charged 14S battery will not penetrate dry human skin under normal circumstances, and if the skin is damaged from high amps, the amount of internal tissue damage will be the result of how long contact is maintained (meaning tissue damage will occur at even 36V if the skin is opened by high amps)
The farther you stay below 60V, the safer you will be. However, the international standards for voltage penetrating the skin are…voltages above 60V.
Luna Cycle Kits, and 52V batteries
It is no secret that ElectricBike.com and Luna Cycle LLC are sister companies. If you look at their catalog of parts that they sell, you will notice that they are one of the few retailers that carry 52V battery packs, so…it would be reasonable for any reader to be skeptical about an article from a “independent opinion magazine” that suddenly starts telling customers that the thing that they just happen to be selling is the best thing since sliced bread.
But, we aren’t writing about 52V batteries because we are selling them, Luna is selling 52V batteries because of the reasons listed here…Luna Cycle can stock anything they want. Why would Luna try to change what customers are buying, why not just sell to the customers what they already seem to be happy enough to buy? (actually, they also sell 48V and 36V batteries , too)
The vast majority of ebikers are not hot-rodders who build a fast ebike from a kit, and then post about it. Most just want to have a reliable product that they use once in a while. The big problem for most people who are interested in maybe buying an ebike, is…there is no place nearby to get a test ride (to see which model they like)
Recent turn-key factory ebikes are wonderful, but they are definitely more expensive than adding a kit to an existing bicycle, and many new ebikers come to the conclusion that a kit is the way that they want to go.
The problem we have heard the most often from new ebikers who bought a kit is that, as simple as they may be, many felt it was more difficult than necessary to get everything together and running. We think that the final performance ability of a kit is more important than whether it takes one hour to install it or two, because…it only needs to be installed once. However, we are sensitive to the concerns of new enthusiasts who are sitting on the fence, and undecided as to whether or not to take the plunge into ebiking.
The Bafang BBS02 is probably the easiest mid drive to install, since the controller is integrated inside the motor case. And…The Golden Motor “Magic Pie” is the easiest rear hubmotor to install, because the controller on it is also integrated inside the motor housing. Not only does having the motor and controller together make the install easier, it removes a lot of the wiring clutter from view, for a sleeker look.
What does this have to do with 52V batteries? Well…both of them will run on a 52V battery system, straight from the factory. Plus, if you are getting the desired amount of power from more volts, you can use fewer heat-producing amps. This means that either of these kits on 52V will run cooler compared to using the same power with a lower-voltage pack (which would require more amps to get the same power).
Our polling over the last few years has shown that most new ebikers try to save every possible dollar on their first ebike kit. But then, they almost always end up wanting more power. More power costs more money, so…how much power is the “sweet spot” that achieves a widely agreed upon balance between power and cost? The answer is about 1200W.
If 1200 watts of power is illegal where you live, then…you shouldn’t be reading this article (you naughty boy!), but for those who can legally use 1200W if you ride in a safe manner where you live…you can get that from 52V X 23A =1200W. Neither of the kits listed here should get hot enough to ever be damaged if you are only running 23A, and that means that these kits should run a very long time, giving you many happy years of service.
And, if you agree with our assessment that “most” ebikes would be safely well-served by a 52V pack, but…you want more power than just 1200W, the Leafbike 1500W rear hubmotor is gaining a lot of popularity as a “hot rod” when using 52V X 50A = 2600W (the “1500W” listed name is way under-rated).
We’ve written many articles about ebikes that use more than 2600W, but when you start planning a build for something like that, you will no longer be using affordable and efficient components. The big fun at that level requires big bucks. (Just for the record concerning what the next step up in voltage is, endless-sphere hot-rodder Luke/LFP recently mentioned that the FETs that will run 20S max are more efficient than the FETs for voltages higher than 20S (20 X 4.1V = 82V when fully charged)
Where to get 52V / 14S battery packs
Over time, more retailers will start to carry 14S battery packs and chargers as an option. Right now, there are several suppliers of 52V packs, and if you are in the US and want a local supplier, the first place to check is Luna Cycle LLC.
Recently, in September…I went to the International Interbike convention, where factories show off their newest designs. The very high-end German off-road ebike “Spitzing” proudly advertised that all their new batteries will be 14S, so…it looks like this may be catching on?
There is nothing wrong with a 48V system, or even a 60V, or 72V system. We’ve seen them all. However, if you plan to buy an ebike kit over the next year, we recommend that you consider the benefits of a 52V battery.
Best of luck, and ride safe.
Ebike battery DC to 120V AC Inverters
edit: I just stumbled across one more reason to like 48V and 52V battery packs. There are many off-grid electrical systems where a cabin has solar panels to charge a huge battery pack. Then the batteries DC voltage is converted to 120V AC, in order to run common household components. The most common voltage of those solar panels? 48V. A quick look at the specs on the inverters that convert the DC into 120V AC shows that, the most voltage that you can input to the inverter is…60V max.
This means 48V and 52V ebike battery packs can actually power a common and affordable inverter during a power outage, but…a 60V or higher ebike cannot.
Written by Ron/spinningmagnets, November 2015