Batteries 101
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 that 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.
When deciding on a battery for your Electric 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? The following table is a good place to start.
Range vs Riding Style and Battery Type
10 to 20 miles,
16 to 32 km |
20 to 40 miles,
32 to 64 km |
29 to 58 miles,
47 to 94 km |
35 to 70 miles,
56 to 112 km |
50 to 100 miles,
81 to 162 km |
75 to 150 miles,
120 to 240 km |
4 to 8 miles,
6 to 12 km |
9 to 18 miles,
15 to 30 km |
13 to 26 miles,
21 to 42 km |
17 to 35 miles,
28 to 56 km |
25 to 50 miles,
40 to 80 km |
38 to 75 miles,
60 to 120 km |
| Not Recommended |
4 to 8 miles,
6 to 12 km |
6 to 12 miles,
10 to 20 km |
8 to 16 miles,
13 to 26 km |
15 to 30 miles,
25 to 50 km |
23 to 45 miles,
35 to 70 km |
Conservative is using assist to climb hills and only very occasionally to boost speed. Range assumes start and end points are at roughly the same elevation and that total climb distance is moderate. Figure you'll use about an amp-hour of battery for every 300 vertical ft (100m) of climb. Note that you must derate lead acid batteries in continuous heavy use, such as climbing. Don't count on getting more than half to three-fourths of rated capacity under these conditions.
Normal/Fast Cruise means either typical stop and go operation or steady cruising at around 20 to 25mph (32 to 40 km/h). Normal means assist used on all hills and for accelerating away from stops. Moderate use for increasing speeds. Fast Cruise means pedaling at a steady, relaxed pace equivalent to about 12 mph (20 km/h) on level ground and using the assist continuously to boost speed to 20 mph (32 kn/h) or over.
Aggressive means near full power operation at all times. This consumes battery power very rapidly. It's like using the afterburners on a fighter plane. You'll go very fast, but not very far. Useful in emergencies or when you need to blend with fast auto traffic for a short distance. The EcoSpeed EMD can take this use without overheating, so you're free to run hard until the last electron dribbles out of the battery. The smallest recommended SLA battery can't supply enough energy for this style of riding.
Note, each entry in the table has a high and low number. Expect higher numbers if you're lighter weight, more athletic, or have a very low drag bike. Expect lower numbers if you're heavy and /or less athletic or have a less efficient bike.
Did you notice how in the above table that two batteries with the same amp-hour rating (10Ah) have differing range numbers? If so, you might wonder what's going on here. What's going on is what we said before, batteries are more complex than little tanks for electricity. The next table will give you more food for thought in comparing batteries.
Battery Specifications
12 lbs,
5.5 kg |
about 100 Wh |
about 1400 watts |
about 400 watts |
6 |
32 to 100 F
0 to 38 C |
2 hours |
100/300 |
3 to 6 |
$0.10 |
17 lbs
7.8 kg |
about 200 Wh |
about 1800 Watts |
about 500 Watts |
5 |
14 to 100 F
-10 to 38 C |
2 hours |
100/300 |
3 to 6 |
$0.05 |
23 lbs
10.5 kg |
about 300 Wh |
about 2500 Watts |
about 700 Watts |
4 |
-4 to 100F
-20 to 38 C |
2 hours |
100/300 |
3 to 6 |
$0.05 |
10 lbs
4.5 kg |
about 380Wh |
about 4300 Watts |
about 2500 Watts |
1 |
14 to 130 F
-10 to 55 C |
20 minutes |
1000/2500 |
4 to 10 |
$0.06 |
11 lbs
5.0 kg |
about 530 Wh |
about 1400W |
about 1100W |
3 |
-4 to 130 F
-20 to 55 C |
6 hours |
500/1000 |
2 to 5 |
$0.07 |
16.5 lbs
7.5 kg |
about 800 Wh |
about 1600W |
about 1300W |
2 |
-4 to 130 F
-20 to 55 C |
9 hours |
500/1000 |
2 to 5 |
$0.07 |
(1) Weight exclusive of case, fuse, switch, and wiring.
(2) Useable Watt-hours is Amp-hours times nominal voltage times a derating factor. Battery Amp-hour ratings are commonly specified under ideal conditions and low discharge currents. These numbers correct for that. Note that this is what you can expect with a new battery. As batteries age useable capacity drops.
(3) Our BMC motor draws about 1100 Watts under worst case conditions. The Powerpack motor draws about 1400 Watts. If the battery cannot supply this much, motor power output drops.
(4) All batteries lose ability to supply power as charge is withdrawn. SLA batteries drop off faster than lithium.
(5) Performance ranked from highest (1) to lowest (5). Performance is top speed and acceleration and is a function of useable battery voltage. Higher voltage, lower impedance batteries score higher.
(6) Sealed Lead acid (SLA) batteries can be operated at higher temperatures, but each 8 C (15 F) rise in temperature roughly halves battery life. SLA Capacity drops rapidly at low temperatures, lithium less so. Smaller batteries become unusable sooner as temperature drops.
(7) Assuming a large enough charger, the minimum recommeded time to fully charge a 100% discharged battery. The standard charger EcoSpeed supplies will not charge at this high a rate except in the case of lithium polymer.
(8) Cycle life rises rapidly as discharge depth decreases. Thousands of cycles are possible at very shallow discharge depths. Note that for a given usage, a larger battery will have shallower cycles. Also, note that lithium batteries may be safely left in partially discharged states whereas SLA cannot.
(9) Calendar life is an educated guess because of the many variables involved. Relative life is accurate.
(10) Estimated based on typical 80% depth of discharge and current prices. Note that your cost may be much worse if the battery's calendar life expires before you use up all of the cycles. Also, there are many ways to kill a battery. The lithium batteries have an advantage in that respect because they use protective electronics. Even shallower average discharge cycles may lower your cost
As the above table shows there are a lot of factors to consider when choosing batteries. Worthy of note is that the cost per useable amp-hour doesn't really vary that much across different battery chemistries even though the up front cost can vary by a factor of up to 20.
Take a look at the relative performance column. Batteries at similar voltage levels can vary quite a lot in how much power you can actually extract. That's because all batteries have an internal resistance that causes voltage to drop proportional to current being supplied. A big voltage drop means a lower motor speed and less power. Also, the internal resistance increases as a battery is discharged. This is especially noticeable with the small SLA batteries. They can supply a lot of power when fully charged but drop off rapidly as they are discharged.
Take a look at the weight of each battery. Of course the lithium batteries are the lighest, but notice how the smallest SLA system is almost as light. That's because, surprisingly, SLA's have a power density (power they can supply per unit weight) on par with lithium polymer. That means that you can put together an SLA battery pack that's almost as light as a lithium pack. Of course, you won't have much range or capacity, but that may be just the ticket if light weight and low up front cost is important and you just want enough boost to climb a small steep hill or two on your regular ride or commute.
Conspicuous by it's absense in the above table is the popular Nickel Metal Hydride (NiMh) chemistry. EcoSpeed doesn't offer a NiMh pack because with nickel prices at current levels, a quality NiMh pack approaches lithium polymer in cost.
With NiMh, pack quality is very important. Because NiMh cell voltage is only 1.2V (vs 2V for SLA, 3.2V for LiFePO4, and 3.7V for lithium polymer), it takes 30 cells in series to make up a 36 Volt pack. With that many cells strung together it is very important that each cell have very low internal resistance. Otherwise, the pack voltage drops too much at high currents and you don't get good performance. Cheap NiMh cells either have high resistance or low resistance and low capacity.
If you have a source of good quality NiMh packs, it's perfectly OK to use one with your EcoSpeed system. Expect a "D" cell based pack to have about 280 Wh of useable energy at a weight of 13 lbs (6kg), or an "F" cell pack to have about 430Wh at a weight of 20 lbs (9kg). Performance will be intermediate between the SLA and lithium options.
