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YOUR BATTERY IS YOUR FUEL TANK. 

More Amp hours = bigger tank

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How far you want to go and for how long depends on the following main elements: your motor size, battery capacity, the level of ‘assist’ you choose coupled with your own effort, speed, the terrain, your weight, the weather and the temperature. In short, there are many variables; some of which you can control and others you cannot. 

 

The maximum output power of your motor is measured in Watts (W). Typically, this is 250W (maximum legally permissible rating for use on public roads in the U.K), 500W, 750W and 1000W.

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Different motors require batteries with different nominal voltages. Usually, the most common 250W motors will require a 36 volt battery, the larger ones, such as 750W or 1000W require a more powerful 48 volt battery (or more). 

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AMP HOURS (Ah) 

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The capacity of the batteries is shown in amp hours (Ah). The higher the Ah, the bigger the battery. When looking for a battery, the bigger the Ah number the bigger the 'fuel tank' for your e-bike.

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WATT HOURS (Wh)

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To calculate how long your battery will power your motor for, you need to work out how many watts your motor can deliver in one hour. This measurement is called Watt hours (Wh). When you see an e-bike advertised, you may see the numbers 400, 500 or even 625 on the sales label. What does this mean? (It's got nothing to do with how 'powerful' your bike is. That is determined by the motor size and the voltage of the battery used to power it). Well, these numbers refer to Watt hours (Wh).

 

Simply put: a 500Wh battery will fuel a 250W motor at full power for about two hours or a 250W motor at 50% power (125W) for 4 hours and so on.

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But (isn’t there always?) lithium-ion batteries should never be fully discharged as this shortens their life. They are designed to be discharged up to 80% of their capacity so in actual fact when in use, your battery will automatically cut off when it reaches approximately 20% of it's nominal capacity. This 20% needs to be subtracted from your Wh figure These are the numbers in brackets below and is the more realistic measurement of usable battery capacity in Wh:

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  Calculating Wh: voltage (v) x Amp hours (Ah) = Watt hours (Wh) 

 

36v x 14.5Ah = 522Wh (418Wh) 

36v x 17.5Ah = 630Wh (504Wh) 

48v x 14.5Ah = 696Wh (557Wh) 

48v x 17.5Ah = 840Wh (672Wh) 

48v x 19.2Ah = 921Wh (736Wh) 

 

So, at full power:   

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36v 250W motor with a 14.5Ah battery will propel you for approximately 1 hour and 40 minutes (418 / 250). 

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48v 750W motor with a 14.5Ah battery will propel you for about 45 minutes (557 / 750). 

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48v 750W motor with a 19.2Ah battery will propel you for just under an hour (736 / 750). 

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You get the idea... but (yes, there is another 'but') lithium-ion batteries, like all batteries, perform less efficiently in the cold so if it is freezing outside, you would have to subtract anywhere between another 10% or 20% from the usable Wh figure. A couple of examples of this, splitting the difference at 15%:  

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36v 250W motor with 14.5Ah battery 

(36 x 14.5 = 522Wh - 20% = 418Wh - 15% = 355Wh (355 / 250))

= 1 hour and 25 minutes. 

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48v 750W motor with 19.2Ah battery

(48 x 19.2 = 921Wh - 20% = 736Wh - 15% = 625Wh (625 / 750))

= 50 minutes

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Wow. That doesn't sound like a lot. Please bear in mind though that

very few people will use their e-bikes at full power in sub-zero temperatures for the above times.

 

Most people will set the pedal assist some way below the maximum to cope with typical British terrain, will cycle when the weather is above freezing and by doing so will increase the time they can use the battery and also the distance they can cover.

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That's great but how FAR can I ride... what's the range?

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This is the single most often-asked question and the question which is the most difficult to answer because of all those variables mentioned before. How long is a piece of string? 

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For example, at 50% power and travelling at 15 mph a 250W motor will use 125W per hour (418/125). Your battery usage now extends to about 3 hours and 20 minutes and in theory you could travel 50 miles in that time. If it’s freezing, (355/125), 2 hours and 50 minutes or about 42 miles. In theory.

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If you have absorbed all of the above and we won't blame you if you haven't, then please click on the link and take a look at the range table which assumes a reasonably modest average speed of 15 miles per hour and should serve as a rough guide only

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If you have read this far, well done. Perhaps it is time to go