How large does my battery need to be?

How large does my battery need to be?

Understanding Battery Capacity

Set of battery charge level indicator, three-dimensional rendering, 3D illustration

Unless you have an eBike stolen from a museum, we will assume that your battery type is lithium ion. These themselves come in various types, however, the key information when choosing a battery is the voltage and capacity, measured in amp hours (often written ‘Ah’)

The amp hours measurement tells you how many amps a battery will deliver for one hour. So for example, a 15ah battery will give you 15amps for one hour, or 7.5amps for 2 hours, or 5 amps for 3 hours. Hopefully you are getting the idea! 

When working out how long the battery will power your motor it is helpful to convert the voltage and amp hour figure to watt hours. This will tell you how many watts it will supply for an hour. It is easily calculated by multiplying the volts and amp hours together.

For example, a 15Ah, 48v battery would deliver  15 x 48 = 720W hours (often written 720Wh). Or a 17.5Ah 36V battery would deliver 17.5 x 36 = 630Wh.

You then need to remember that you should never completely empty a rechargeable battery. If you do, your battery life will suffer. As such, all devices that use batteries are designed to cut out before the battery is completely discharged. How much you can discharge them depends on the type of battery. Lead Acid batteries of the type that power caravans or start your car should not be discharged by more than 50%. The good news is that lithium ion batteries can be discharged by 80%, so that only 20% of their capacity is left.

The final complication is weather. All batteries loose capacity in the cold. At 0 degrees, the lead acid battery in your car looses half of it’s capacity – this is one of the reasons cars can be harder to start in cold weather. Lithium Ion batteries tend to loose 10-20% – lets say 15% on average.

With all of this information lets calculate the capacity of a battery. Lets take our best selling 48V 17.5Ah battery.

If we calculate 17.5Ah x 48V = 840watt hours. So the battery will in theory supply 840Watts for one hour.

Next, lets take into account the 20% of capacity we must not use.  Taking 80% of the capacity:

840WH x 80% =  720WH

This means that our battery in the summer can produce 720W for 1 hour. 

In the winter, if we drop this by 15% – we get 612Wh – so when its 0 degrees outside we are only going to get 612 watts for an hour.

How long will this power my motor?

Once you know the watt hour measurement for your battery, taking into account the time of year and ensuring you only use 80% of the capacity, you need to understand the power requirements of your motor.

All motors will come with a maximum power rating. This is usually 250W (BBS01b 250W motor), 500W (the BBS02 500W motor), 750W (the BBS02B 750W motor) or 1250W (the BBSHD motor).  Don’t forget this is the maximum usage of the motor. Unless you always travel flat out you will be using a lot less than this.

The actual amount of power that you use will depend on a few other factors. Typically these are:

1. How fast are you going to travel?  At 8MPH most of the power will be from your pedalling and the motor will do very little. At 30MPH most of effort will be your motor, which coupled with the extra wind resistance will use loads of power.

2. How heavy are you and your bike? If you are a larger person, or if you are using a motor to move a cargo bike (we have carried out several ice cream bike conversions) they are going to use far more power.

3. How hilly is it? There is no getting away from the fact that every uphill will use more power!

4. Are you using much throttle? Just like your car, being heavy handed on the throttle will use more power.

5. What mode are you in? All eBikes allow you to set a mode which will restrict how much power is used.

Taking all this into account, we usually suggest that for an average cyclist, traveling at a a medium speed of maybe 15MPH, with typical British rolling hill landscape, and not using much throttle, then the most they will use is around 50% of their motors stated maximum use. So a 750W will probably use around 375W per hour.  Of course, set your mode lower, pedal more, keep off the throttle and get fit, and you can easily drop that figure to only 25%. This means that a 750W will only use 187W per hour.

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