Things You Need to Know Before Buying an E-Bike: Batteries

This is Part II of a larger series on E-bikes. If you want to read part I, click here. 

Most E-bikes run on one or more lithium ion batteries (Li-ion). They're typically found attached to a rear rack, the down tube, or the seat tube of the frame, and they can explode if not treated well.

This article is about how Li-ion batteries work, how they should be treated, and why they sometimes explode – and if there's a better option for e-bikes. I don't cover everything there is to know about e-bike batteries here because, well, I don't have the energy. You can find a more in-depth guide at https://batteryuniversity.com/

•  Here's the basic anatomy of a Li-ion battery: You got a positively charged cathode on one side, a negatively charged anode on the other side, and a separator in the middle. Lithium ions flow between the cathode and anode through a liquid electrolyte mixture. This isn't a method of storing energy, in an exact sense, but a way to convert chemical energy into electrical energy. Li-ion batteries do this better than any other battery type (except lithium polymer) because lithium has the greatest energy density per weight of any other metal. 

• Now take a good look at that little separator in the picture above. It's there because if the anode and cathode interact without it, there will be a runaway heating effect that will eventually lead to a meltdown – and lithium melts at around 1000 degrees, so that's bad. The anode and cathode can exchange lithium ions through the separator, but never shall they meet. Or else...

• The most common cause of Li-ion battery explosions is a failure of the separator, which is usually made of polyethylene or some other slightly porous material. Polyethylene is good because if the battery starts to overheat, the pores will melt a little and automatically seal the cathode and anode off from each other. However, the separator is thin – really thin. On some batteries the separator will be thinner than a pixel on this web page – much thinner. 

• When the battery gets hot it expands, forcing the battery to physically bulge. A high-end battery will be wrapped in durable polymers that stretch and resist heat and breaking, but if that polymer coating is somehow ruptured, and the hot lithium gasses are exposed to oxygen, KABOOM!

• Abuse and manufacturer errors can both cause a separator failure. It is recommended that if you crash your e-bike, DON'T TRY TO CHARGE THE BATTERY! If you see any damage or deformation at all on the outside of the battery case, get a new one. Charging a damaged battery encourages interactions between the cathode and anode, and if the separator is damaged – well, you get it. 

• Another popular method of blowing up a Li-ion battery is over-charging. Lithium batteries, unlike some other battery types, will happily just keep charging until they're so packed with energy that they just can't help but release it to the world. "Venting by fire" is the technical term. 

• Therefore, you need a microprocessor to administer a "trickle charge" and monitor the battery during the last 1% of the charge process. Most e-bike batteries will have that processor in the battery casing itself, but a cheaper battery, or a hobby battery will require a special charger with a built-in temperature gauge and shut-off capability. Even still, it's not fool-proof. I charge my lithium batteries in a fire-proof bag. 
  

• This also means that you can charge a battery really, really fast up until 99% full. The charging rate (or discharge rate) is determined by multiplying the C-rating posted on the side of the battery by its posted Ampere hours rating. So if a battery has an Ampere rating of 2 per hour, and the C-rating is 10, then you can charge or discharge the battery at a constant rate of 20 amps. Of course, this is assuming you can trust the numbers written on the side of your battery, which isn't always the case. 

But is the Li-ion battery the best possible option for an e-bike? 

• Yeah, probably. A lithium polymer (Li-po) battery can store more energy, but they're also more volatile. 
• The other option is to forget batteries and go with a fuel cell. Fuel cells can be made safer, cleaner, and they can (hypothetically) release more energy per weight. The problem is that fuel cell science and manufacturing isn't as sophisticated as their battery equivalents, so we might be a few years away from fuel cell dominance – but I believe it will come. 

Oh one last thing:

• Never ever allow a lithium battery to fully discharge. I would even urge you never to let it fall below 5%. This is because once a lithium battery is fully depleted, it doesn't revive – unlike other battery types, like Ni-Cad batteries. Lithium batteries also have no "memory" so you can charge them up to 100% at any time without risking damage. 
• They do die on their own after a few thousand charge/discharge cycles. Right now there is no good, commonly accessible recycling method for lithium batteries – which is bad. To that end, we shouldn't fool ourselves into thinking we're being environmentally friendly by using batteries instead of fossil fuels. Either way we're making a mess for somebody else to clean up down the road.