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What Battery Size do I need?
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The capacity of the battery is found by bringing together volts, and the measure of current against time - amp/hours. By multiplying the two figures together, we get watt/hours - a measure of the energy content of a battery. Unfortunately, it isn't that simple... but you didn't think it would be, did you? In practise, you're unlikely to get results that match the stated capacity of a battery because battery capacity varies according to the temperature, battery condition, and the rate that current is taken from it. Lead/acid batteries are rated at the '20-Hour' rate. This is the number of amps that can be continuously drawn from the battery over a period of 20 hours. However, an electric bicycle will usually exhaust its battery in an hour or two, and at this higher load, the battery will be much less efficient. So the figures for lead/acid batteries tend to be optimistic. On the other hand, Nickel-Cadmium (NiCd) batteries are rated at a 1-Hour discharge rate, so although the stated capacity of a NiCd battery might only be half that of a lead/acid battery, performance will be much the same. Nickel-Metal-Hydride batteries (NiMH) are measured at the 5-Hour rate, so their performance tends to be somewhere between the two. The capacities of typical bicycle batteries vary from Powabyke's 504 watt/hour giant (36 volts x 14 amp/hours) to the tiny 84 watt/hour pack on the SRAM Sparc kit. It's best to choose a package that will provide twice your normal daily mileage. It's difficult to guess the mileage from the watt/hour capacity, because actual performance depends on the bicycle and motor efficiency, battery type, road conditions, and your weight and level of fitness. As a general rule, a big battery like the Powabyke's will give a range of between 10 miles (doing in all the work in quite hilly terrain) and 25 miles ( a joint effort in hilly terrain). This is enough for most uses, although it's a very big and heavy battery. Small units, such as the SRAM Sparc, give a maximum range of 5 - 10 miles. How can I measure the efficiency of an electric bike? We measure overall efficiency by dividing the watt/hours used by the battery charger by the mileage achieved, giving a figure of watt/hours per mile. This varies according to the terrain, the weight and riding style of the rider and the type of battery, but our figures are measured in exactly the same way, so they should be comparable, bike against bike. The best we've seen is 8 watt/hours per mile, and the worst is 32. |
