The battery is the source of energy for an electric bike or electric vehicle. It would be nice if it were like a little gas tank — fill it up and use it. Unfortunately, batteries don’t work that way. They are complex electrochemical systems and there is no such thing as an ideal battery. So, you have to make trade-offs to find a battery that’s right for your needs.
The three most important characteristics of a battery are light weight, low cost, and high capacity. Alas, you only get to pick two out of the three. Or, put another way, high capacity and light weight are going to cost you and that’s just the way it is, though as each new generation of batteries comes along, the cost of light weight and high capacity drops
When deciding on a battery for your Electric Mountain Drive or Mid-Drive, it’s important to think carefully about what your needs are. How much are you going to pedal vs use the assist motor? What range do you expect? How important is battery weight to you?
Energy Capacity and Range
The energy stored in a battery is given by a very simple relation:
ENERGY STORED IN WATT-HOURS equals CAPACITY IN AMP-HOURS times BATTERY VOLTAGE
If you have a 48 volt battery with a capacity of 10 amp-hours then it will store about 480 watt-hours of energy when fully charged. A 36 volt battery with a capacity of 10 amp-hours will store only about 360 watt-hours of energy and so is a smaller battery. We say “about” because there are lots of factors that affect the exact amount of energy stored, but the above equation is good enough for all practical purposes.
How much range you get from a given battery is just how much energy you use per mile, or km, divided into the amount of energy your battery stores. If you use 16 watt-hours per mile and your battery stores 480 watt-hours, you will go 480 watt-hours divided by 16 watt-hours per mile equals 30 miles. How much energy you use per mile depends on many things, including how heavy your bike is, how aerodynamic it is, and how fast you go. The following list shows typical energy use under different riding styles with a typical road bike. Add about 20% for mountain or cargo bikes under similar conditions. Also, when off-road or trail riding subtract about 20%.*
Conservative Riding: 10 watt-hours per mile (6 Wh per km)
Normal around town or moderate speed cruising: 15 watt-hours per mile (9 Wh per km)
Fast cruising: 20 watt-hours per mile (12 Wh per km)
Aggressive, almost full power all the time: 30 watt-hours per mile (19 Wh per km)
So you can see that the amount of energy you will use can vary widely and can even be higher or lower than the above values. The normal number though is a good place to start. What we consider to be normal riding allows liberal use of the assist. In this scenario you would use the assist to climb every hill, accelerate away from every stop, and frequently use the assist to boost your speed, though not often exceeding 20 mph (32 km/h). The conservative riding style still allows use of the assist in hill climbing and for acceleration but doesn’t often use the assist to boost speed.
Once you know how much energy you’ll likely use per mile, and you know how far you want to travel between battery charges, then you multiply those two numbers together to get the battery size you need.
BATTERY SIZE IN WATT-HOURS equals TYPICAL ENERGY USE times DESIRED RANGE
Of course, batteries come in fixed sizes so you usually won’t find the exact size you want and may even want more that the largest size of the battery you’re interested in. The solution to that is ot use more than one battery and we have an accessory that allows you to do that.
We’re always on the lookout for better battery systems. Batteries have always been the weak link in electric vehicles so we always try to use the best batteries we can find or build.
New for 2012 we are offering made in the USA tri-metal batteries. The exact chemistry is Lithium Nickel Manganese Cobalt. These batteries feature high cycle life and power output comparable to the industry standard LiFePO4 chemistry but with the light weight of lithium cobalt batteries commonly used in laptop computers and cell phones. The manufacturer uses an advanced thermal management system to keep the batteries cool under hard use and backs them with an almost unheard of two year warranty.
One of the key features of our system though, is the ability to work with almost any battery. So, if you want something different that what we offer, there are many choices. See the next section on how to choose batteries from other sourcess.
Using other Batteries with your EcoSpeed System
The most important consideration will be battery voltage. With our Electric Mid-Drive series we recommend 48 volt batteries. Electric Mountain Drive systems will perform well at either 36 volts or 48 volts. The only consequence of using too low a battery voltage is that the motor may not be matched perfectly to the gearing and may reach top rpm before you reach maximum cadence. If the battery voltage is too high, there’s no problem as long as it’s within the 18 to 60 volt range of the controller.
The other consideration is how much current the battery can supply. To get full power out of your EcoSpeed system you’ll need a battery that can supply at least 30 Amps continuous, if it is a 48 volt, or 40 Amps continuous if it is a 36 volt.
What Battery Type or Chemistry?
All advanced batteries currently use one of the lithium chemistries. The longest life and highest power potential of any lithium chemistry currently available is lithium tri-metal, which is what we use. Our previous generation batteries used Lithium Iron Phosphate or LiFePO4. There are many sources of quality LiFePO4 batteries.
Other common lithium chemistries you’ll see are lithium manganese or lithium cobalt. You’ll also see the term lithium polymer, or LiPo for short. This is a battery construction technique, not a chemistry, so you’ll see lithium cobalt, lithium manganese, or LiFePO4 chemistries in a lithium polymer type battery.
Lithium cobalt and lithium manganese will give you almost double the amount of energy per unit of battery weight as LiFePO4 and are similar to lithium tri-metal in that regard. They’re also more expensive than LiFePO4 and last half as long, or less, as lithium tri-metal or LiFePO4.
If absolute performance is your goal, then lithium tri-metal is the obvious choice. For a bit of cost savings and the same performance at a bit higher weight, then LiFePO4 is the way to go. Lithium manganese and lithium cobalt are effectively obsolete for e-bikes at this point, but if you find a deal on one, their performance is excellent.
What about Lead-Acid?
Lead-acid batteries, the type used in automobile batteries and many other applications, are obsolete as far as electric vehicles are concerned. LiFePO4 is cheaper in the long run and much lighter and more powerful. Still, lead-acid batteries still have the lowest up front cost. You put together a 36 volt, 10Ah lead-acid battery pack for less than $200 including a charger. That will working perfectly fine in powering your EcoSpeed system and give you about 80% as much range as a 10Ah LiFePO4 battery pack. The downsides are weight — it will weigh almost 3 times as much, and life — a lead-acid system only lasts about a year in heavy use. If you only use your bike infrequently though, say just on weekends, you’ll get several years of life out of lead-acids.
* Off-road speeds tend to be low and most riders pedal except when climing hills or for short, steep trail sections. Under these conditions off-road riding is surprisingly economical. The factors that increase energy consumption of gas powered off-road vehicles don’t apply to electric drive.