Whether you’re shopping for a turn key commercially available electric bike, or trying to find or build a good battery for an e-bike conversion, finding the right battery for an electric bike is a difficult task. The right battery pack is the most difficult part of the e-bike equation. Keep in mind that even if your buying a turn key electric bike, the lithium battery is more than likely the most expensive component in it, and not all lithium batteries are created equal so you should know what you are getting before you buy the ebike.
Lithium electric bike batteries are not cheap, not perfect and not readily available. Some OEMS such as BionX sell a moderately sized lithium e-bike battery pack for $1000 plus. Optibike sells their touring LiPo battery as an add-on accessory for their bike for a gasping $2500 It is surprisingly difficult to find a ready to plug in LiPo battery pack for sale on the internet by any real company. The reason is simply product liability.
Lithium Battery technology in a bicycle is still experimental and you have to use a lot of common sense when using these batteries. However with a little education and some end user carefulness, you can use lithium batteries safely, the same as you can safely deal with putting gasoline in your riding lawn mower.
Lithium batteries and hazardous shipping
Recently the federal goverment has been cracking down on the shipping of lithium batteries. For the vendor, it means that they must have Hazardous Materials (hazmat) shipping and pay hazmat charges, and only can ship a hazmat officially tested battery. This adds considerably to the cost of lithium batteries, and makes it even harder to find an ebike dealer which will sale you a lithium battery pack.
Ready Made Lithium Packs
For people new to the hobby, ready made lithium packs are the way to go. Several manufacturers offer ready to go Lithium packs with a built in Battery Management System (BMS) at affordable prices direct from China.
You can buy LiFePO4 straight from China from Ping Battery. These batteries are a little bulky, but are safe and will last over 1000 charges. You can also buy ready made Lithium Iron Phoshate (LiFePO4) or Lithium Polymer (LiPo) packs from BMS battery.
A high quality USA battery manufacturer by the name of “Allcell” constructs packs consisting of 18650 cells (cylindrical cells that are 18mm diameter and 65mm long) and uses high tech packing materials to spread out the cells and thus the pack gets a longer life. This is the pack of choice in many high end commercially available ebikes including the Optibike, the Picycle, and the Hanebrink. (click on each to see article on that electric bike). The 18650 battery cell format is mass-produced for laptop computers and cordless tools.
When you buy a ready made pack they have a BMS which balances each cell automatically and they can be charged with a purpose made bulk charger such as this:
Buy a ready made LiPo battery, or build your own?
Lipo batteries are currently the “hottest” battery choice for electric bike enthusiasts. LiPo batteries are the most power dense type of battery available to electric bike riders today. The problem is that LiPo battery packs for e-bikes are hard to find, especially one with high output if you are building a racing bike for riding off road.
If you want a LiPo battery pack, one of your better choices if you want to save money and have a lot of output amps is build one of your own from a Hobby King packs. This requires a lot of time and knowledge, not only in building the pack but also in managing it. LiPo batteries can be extremely dangerous and prone to burst into fire if not assembled with a lot of precautions (BMS) and cared for properly.
Essential for either building or buying a ready made pack is deciding how large of pack you want and how much you are willing to spend on that pack in terms of weight and price.
Most of the price involved these days in building an e-bike or buying a ready to go e-bike is the size and chemistry of the battery pack. For the consumer its important to understand the difference between a 24V, 36V, and 48V pack. Also know what a 10-Ah pack is compared to a 5-Ah pack.
A 36 volt 1o-Ah pack contains 360 watt hours. Approximate range on average riding conditions 12 miles
A 48 volt 10-Ah pack contains 480 watt hours. Approximate range is 15 miles
A 48 volt 20-Ah pack contains 960 watt hours and once you get close to 1000 watt hours you are getting serious commuting range which most e-bike manufacturers promise but do not deliver…think 30 miles.
When you are comparing packs of different voltages calculate watt hours to see how much you are getting for your money…especially when buying a turn key e-bike.
How big is the battery I am buying?
Your battery pack size is based on voltage and amp hours. The higher the voltage and the higher the amp hours of your battery the more range your battery will give you. A 48v 10-Ah pack gives you 480 watt hour (48 X 10). This gives you an easy way to determine exactly how much battery you are buying. The wattage of a battery is the only accurate determinant to judge what range your finished ebike will have.
How powerful is the battery?
Different batteries have different amperage capacities. Most cheap lithium batteries are not capable of putting out much amperage. If you have a 48 volt bike putting out 25 amp hours, you are going to want a 48 volt battery that has close to a 25 amps output or more. If you want to eventually hot rod your ebike (read our hot rod hub motor primer here), you may want to invest now in a high amperage battery.
The other thing to consider is that if you have one 48 volt 10ah battery putting out a measly 20 amps, you can add a second of the same battery, wire them together in parallel, and you will have a 20ah pack with a 40 amp capacity, thus effectively doubling your range and doubling your amp output.
When you buy your battery, make sure you know what its maximum amp output is. Remember by multiplying amps and voltage you get the actual current capacity of the battery. For example a 48volt 25 amp pack can put out 1200 watts.
What is the chemistry of the battery you are getting?
Lithium chemistry is considerably more expensive than lead acid chemistry. If you are buying a battery pack or a bike with a battery pack, be familiar with the chemistry your buying. For example its hard to find a good e-bike for under a thousand dollars with a decent sized lithium pack. Lithium is pricey. Be realistic in your expectations on e-bike shopping on how much the electric bike will cost compared to what kind of range, performance, and life expectancy you will get out of a lithium battery pack.
What format are the soft cells? Hard or soft shell? Big or small?
When choosing a battery for your bike not only is weight important but also volume. You want your pack as small as possible so its easy to stow and easy to hide. So therefore you should consider you battery’s volume not just weight. For sure you need to go with a lithium chemistry and not an old school heavy and large Sealed Lead Acid (SLA) or Nickel-Metal Hydride (NiMH) chemistry.
There are two prevalent ideas in pack constructing in these modern days…one is to use larger pouch-like soft cells to construct the pack. The stealthiest battery chemistry by far is LiPo, large cells with power-dense cobalt in the anode chemistry, such as what comes in Hobby King cells. Here is what I mean by “large cell” LiPo. These are soft pouches and large. When you use a pack made of these it will consist of fewer wired together cells than if you use small cylinder cells.
When you buy a hobby king pack it will have a number of these large cell LiPo’s strung together like this 6 cell (6s) pack. The big downsides of this pack is it will only last you in best case 300 charges and is volatile and susceptible to possible fire if not well managed and cared for. When using cobalt-based LiPo it is best to use some kind of BMS, and also charge it in a safe location.
Here is a an example of a large format soft pouch LiPo pack with 13 cells, and a BMS. This pack was built using cobalt lipo soft cells with a BMS from a Chinese factory for an electric bike. You can see the top cell has been squished, causing the cell to fail and the bms to shut down the battery not allowing it to charge or discharge. This pack is small and light (7lbs). This $500 pack is now ruined, but all is not lost since it did not start a fire and take the house with it.
Here is what I refer to “small cells”, the 18650 (cordless tool) type cells which need to be spot welded or soldered together to form a large pack. The big advantage of these cells is they offer better cooling because of the nature of their shape to the LiPo soft pouches, and therefore have the capacity to last longer.
Also, since the negative electrode is the entire bottom and sides of the cell (formed by a metal cylinder) these cells can take some bouncing around. Be aware if you scratch the plastic wrap on the cylinder, the metal shell underneath is energized to the negative electrode so a short may be possible.
How fire safe is the battery?
Lithium batteries are not 100% fire safe. Some batteries are more dangerous than others, depending on the chemistry, whether it has BMS or not, and what kind of casing the battery is in. If the battery is cased in metal its less likely to burn your garage down than if its encased in plastic. Also be aware that all BMS’s are not alike, some are good and others are crap, just like anything else in life.
Have a plan on where you are going to charge your bike before settling on a battery chemistry. If you live in a crowded apartment building and the only place you have to charge is in your living room floor, you may want to decide against any of these lithium technologies.
Sealed Lead Acid (SLA)
This is the old technology for e-bikes that is heavy as bricks and does not have longevity. Lead acid will double the weight of your electric bike. Unless you have to because of money restraints, we advise to steer way clear of lead acid batteries. Your bike will have a completely different feel and range if you spend the money on one of the new lithium technologies.
Nickel Metal Hydride (NiMH)-
An older battery technology that was popular around 10 years ago as replacment for lead acid in some more expensive commercially available e-bikes. Today it has been obsoleted in e-bike applications because of the recent availability of lipo and lifepo4 cells. NiMh is a finicky technology to deal with. The packs do not have long life expectancy, and have to be treated delicately. One big problem for DIYers is that its very hard to safely charge NImh cells that have been soldered together in parallel. Extra care is needed for nimh in both assembling and charging.
Lithium Iron Phosphate is currently the most common lithium battery used in Ebike applications. It is considered the most table lithium battery type available today (low risk of fire) and has a reasonably hgih life expectancy of over 1000 charges.
For a complete write up on LiFePO4 care and trouble shooting read our story here. LiFePO4 cells nominal voltage is generally from 3.0-to 3.2 volts, and generally, lifepo4 is a heavier and less power dense than available LiPo batteries and is not capable of as high of amperage discharge.
LiFePO4 is currently widely available for purchase as e-bike packs complete with BMS on ebay and other online merchants. Mostly it is sold by small companies. Also, most of the commercially available e-bikes powered by lithium batteries are using the LiFePO4 chemistry.
A123 LiFePO4 Cells
A123 is a brand name of lithium ion phosphate battery used in many EV even full car applications.. A123 cells are known to be of high quality and capable of having high-amp discharge rates with long life expectancy of over 1000 charges. They can provide large amounts of power and have been used in racing applications as well as electric car builds. Chrysler has chosen to use A123 cells in their electric cars.
Small hard-cased A123 cells (about the size of a “C” battery) have been salvaged out of power drill packs, car battery packs etc, and have made it into the hands of e-bike DIYers who solder them together in series and in parallel to construct a pack big enough and powerful enough to power an e-bike.
A123 cells are 3.3 volts per cells.
Recently A123 has released a large 20-Ah LiFePO4 pouch cell with the EV market in mind.
Lithium Polymer cells, used mostly in the e-bike community to describe soft pack RC like cells, generally lighter weight per watt hour, and they have a high percentage of cobalt in its anode, which makes them very power-dense (lots of amp-hours in a small package) and also capable of very high amps of discharge (for high performance). Single cell LiPos are connected together in series to form a battery pack.
The word “polymer” in its name merely means the electrolyte is a gel instead of a fluid or solid. We use the term LiPo because that is what most catalogs call them when you are shopping for them.
1 cell = 3.7 volts (nominal voltage, the average voltage across the entire ride))
8s (8 cells in series) = 29.6 volts
10S = 37V
20S = 74V
Common lithium cobalt oxide (LiCoO2) batteries can catch fire or explode during loading and even when just stored. The safety depends on user precautions and the quality of the Battery Management System (BMS).
Golden Rules of proper use of Lipo:
NEVER discharge a Lithium Polymer battery below 3.0V
NEVER charge a Lithium Polymer battery above 4.2V
NEVER puncture a Lithium Polymer battery
LiPo homebuilt with BMS
BMS (Battery Management System)
A lot of DIY’ers these days are making the extra effort to install a BMS in their home built batteries. Adding a BMS is the way to go if you want your battery to be fire safe. BMS’s can range from a simple hobby king cell log with an audible alarm if the pack gets too low or too high, to an expensive custom-made BMS complete with pack shut offs.
There is a plug and play BMS available for hobby king packs and can be found here.
LiPo packs that are homebuilt without BMS can be extremely dangerous if you do not approach your battery with a lot of knowledge and care. Be sure to research extensively before building such a pack. Many E-bikers construct packs with no BMS using Turnigy/Zippy packs, acquired cheaply online through the Hobby King company which is based in China. For those who risk running their batteries without BMS they still use sophisticated chargers to balance their packs and constantly monitor the health of their cells.
If you are thinking about building your own LiPo pack, a 48V / 10-Ah battery pack can be made for around $300. However to undertake this project you should research extensively on www.endless-sphere.com on how to build and take care of your pack. Expect hours of reading before you are ready to build a pack of your own.
Electricbike.com will soon be providing articles on how to build and maintain a high performance LiPo pack.
Safe Guarding a LiPo fire with a metal battery enclosure:
One of the simplest safe guards you can take is to contain your LiPo in a metal box so that if it does break into flames the fire will be contained in a metal enclosure.
The only two ebike companies that sell LiPo to the public are Optibike and Pi-cycles, and both contain the battery in a strong metal box which makes up of the frame. The companies fire tested these enclosures and are confident that their frame are effective and safe vessels for LiPo storage.
You too can find a metal box to store your batteries in while riding and charging. Here is an example of a custom built metal box that holds six hobby king packs perfectly (make certain to add a vent so pressure does not build up if a pack goes into flames!):
Most of the problems occur when charging an ebike because they are unsupervised and that is when a LiPo fire can burn down a house etc. Use common sense on where you are going to charge your bike or battery pack, so that if it does burst into flames it does not take your house with you. I have a big steel barbecue grill set up in my entryway which I charge my battery packs in as nice safefuard. This involved taking the battery pack out of the bike after each ride but I am OK with that:
Another option is to simply charge your LiPo battery in your home oven:
Whats the worst can happen with bad lithium battery choice and mis-management:
Check out this video of what happened when the girl in the above picture out on a ride, did not treat her LiPo battery with care and diligence:
The Future of LiPo
In 2012, the future of LiPo in e-bikes looks bright. Most large EV manufacturers are focusing on LiPo as the lithium battery power of the future. LiPo technology is developing fast and becoming safer, more reliable, cheaper, and with a higher life expectancy. Since LiPo cells are being developed to be safer, more efficient and more economical (mostly for main stream products such as the automobile) the electric bicycle industry will be able to piggy-back and utilize the newly available technology at an affordable price. Currently, the latest “best” chemistry involves Maganese Cobalt which is a lot more stable than the cobalt chemistry of the past.
Look for big improvements in LiPo batteries in the near future, and better availability and more affordability. The one big “if” on the future of LiPo is how easy and expensive it will be to ship.
Written by senior editor Eric, March 2012
Edit: if this article was helpful, you may like our newer article on the latest NCM/NCA battery chemistries, and also our article on high-performance batteries that are NOT made from LiPo. If you have narrowed your battery choice down to LiFePO4, make sure to check out Dogmans expert guide to LiFePO4 batteries.