Lithium batteries have become the golden standard for electric bicycles. Their light weight and long cycle life have made them a great fit for any small electric vehicles where weight and efficiency often take a premium over cost. But when it comes to choosing between all the different types of lithium batteries, most people’s heads start to spin.
Lithium batteries: you’ve got options
There are actually many different types of lithium battery chemistries that are used in electric bicycle. Some are optimized for long life, others for small size, and still others for low cost. As you might imagine, each choice has its own associated advantages and disadvantages. Let’s dive into the many options and make some sense out of it all.
First of all, let’s get some nomenclature out of the way. Lithium ion (li-ion) batteries, which some people assume are a specific type of lithium battery, actually represent a whole class of what we today just call “lithium batteries”. All the different lithium batteries used in electric bicycles today fall under the umbrella term li-ion. Within the class of li-ion batteries, we have a number of options: LiFePO4, LiMn2O4, LiNiMnCoO2 (also abbreviated NMC) and RC LiPos, to name the most common lithium chemistries.
Lithium Iron Phosphate (LiFePO4)
LiFePO4 batteries were one of the first widely used lithium batteries in ebikes. Their chemistry makes this an inherently safe, nearly fireproof lithium battery (a great feature for something that rides between your knees). LiFePO4 batteries also provide the longest cycle life of any common lithium ebike battery. Most LiFePO4 ebike batteries are rated at 2,000 charge cycles or more.
With the exception of expensive A123 battery cells, most LiFePO4 batteries are limited to fairly low discharge rates, so you can’t use them for super high powered ebike applications. They are still great for standard, everyday ebikes – just don’t try to go drag racing with them.
LiFePO4 batteries are some of the largest and heaviest of the lithium batteries. These cells also need a Battery Management System (BMS) to keep the cells from becoming unbalanced during successive charge and discharge cycles.
Lithium Manganese Oxide (LiMn2O4)
LiMn2O4 batteries have some advantages over LiFePO4 batteries. LiMn2O4 is a slightly smaller, lighter and cheaper lithium battery chemistry. It also handles charging and discharging better without becoming unbalanced, though most packs are still sold with BMS units. The downside of LiMn2O4 batteries is that they doesn’t last as long as LiFePO4 batteries, generally only 600-800 charge cycles. This means that after a couple years it will likely be time to replace your battery.
Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC)
NMC batteries are a nice compromise between LiMn2O4 and LiFePO4 batteries. NMC is a safe chemistry that can deliver higher power in a lighter, smaller package than the previous two chemistries. This is one of the newer ebike battery chemistries that started coming into popular use around 2013-2014 and is still continuing to gain market share. The next few years could see NMC lithium batteries become the dominant lithium chemistry in the electric bicycle industry.
RC LiPos (LiCo)
Lithium Cobalt, often referred to as RC LiPo batteries due to their prevalent use in the remote controlled (RC) airplane industry, have gotten a bad rep for their ability to violently explode into a flame-spewing nightmare when over-charged/over-discharged/over-heated/punctured/dropped or basically fooled around with in any non-approved manner.
Now imagine this happening on the bike between your legs
In recent years some improvements have been made, but this is still a lithium chemistry that should only be handled by those with proper understanding of the correct operating procedures for LiPo batteries. The batteries have become popular due to their small size, incredibly low weight, super high power output and low cost. Basically, if they weren’t known to turn into expensive little bombs, they’d be the perfect chemistry.
Oh, that and they only last a couple hundred charge cycles. But hey, you can’t have everything in one lithium chemistry!
The future has a lot in store for ebike batteries
New types of lithium batteries are being introduced all the time, with some finding success only in the lab while others make it into real world products and eventually onto electric bicycles. In the next few years we’ll likely see further improvements that will end up leaving us talking about the above four chemistries with nostalgic charm as we remember the way ebikes used to be. Until then, these are the main types of lithium batteries you’ll see in ebikes today.
Who knows what tomorrow has in store for the battery universe.
photo credit 1
Hi Micah,
Is there a way to know which substance a 18650 cell is using?
It’s usually in the name, like NCR18650, IMR18650 or other. All good reputable cells will have spec sheets from the company like this: http://industrial.panasonic.com/cdbs/www-data/pdf2/ACI4000/ACI4000CE17.pdf
Hi,
Ok, just had a bad experience and I need some serious help so I don’t end up writing another piece from the burn unit of a hospital. I purchased a battery pack with Panasonic 18650 cells. On a test ride, (I was not riding it) the installer of the motor was on a mountain trail going fast and hit a bump, the battery launched from the bike (not held down) tumbled down the hill and caught on fire. Spontaneous combustion of cell by cell ensued, and the fire department had to come and put it out. This make me worry about replacing the battery with another with the same cells. I have a Catrike Recumbent and the only place I could fit the battery pack was right under my left thigh about 3 inches away.
I did this for purposes of keeping the weight balanced on the bike and I welded a really heavy duty battery holder with sides and straps to hold it down, but now I am having some worries about what would happen if this happened right under my leg while I’m sitting an a nylon mesh seat.
I am wondering, and of course you can’t offer me anything more than advice and not be responsible for anything that might happen even if you do, so, do you think there is anything I can use as fireproofing material to keep me safe to allow me to bail out if I had to in the event of a fire. I was thinking of Nomex but I hear there are other things that would work better, but I’m not sure.
I really need some advice and at the least some avenues to pursue to look for materials. I’m also thinking of putting a heat sensor on the battery in the event it rises to a temperature that might be heading in the wrong direction.
I and I’m sure many others would be greatly appreciative of any ideas you might have.
Thank you very much.
Victor
Hi Victor,
That’s quite a story! Glad to hear that you and the rider are Ok though.
First of all, don’t blame Panasonic too badly. They are one of the top cell makers in the world and have really top notch quality cells. But lithium-ion batteries are inherently very energy dense and if they are structurally compromised (such as being thrown down a rocky hill), the case can break and cause a thermal runaway condition, releasing all of that energy at once as heat and ultimately fire.
This is not a normal occurrence, but it can happen when the cells are abused. Generally speaking though, li-ion cells (good quality ones) don’t just spontaneously combust during a normal ride.
Now to your specific situation. I don’t think you need to worry about this scenario too much as long as you don’t get in any terrible wrecks (and I hope you don’t!) but just for the sake of being prepared, let’s look at a few options. Nomex, like you mentioned, would certainly be a good option. I’m not sure how expensive it is as I’ve never tried to fireproof by bike, so you’ll have to explore that. Muffler wrap could be a good alternative. I’m sure it’s cheaper than Nomex.
You could easily add a BBQ thermometer that snakes into your battery case if you wanted to keep an eye on the temp. Many BMS units that come in the battery already have heat sensors that shut off the battery if it gets abnormally hot. They are a step in the right direction, but if you are in a collision that breaks your battery in half, it’s not going to do much in that case.
The best method would probably be to just put a physical barrier out of a heatproof fabric like muffler wrap or Nomex in between your thigh and your battery. But again, this isn’t a problem that I think you’ll have to worry about too much as long as you ride safely and treat your battery carefully.