Range calculation....am I doing this correctly .

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jonny jeez

Legendary Member
Hi everyone.

I recently wrote a post asking for some help with a newly purchased second hand E bike.

It turned out one of the three batteries was no good so I have replaced that one like for like.

All seems ok but I am trying to wirk out the correct range of the bike.

Just wondered if someone could confirm I am doing it right or could correct me if not?

So here is what I have done.

1/ Take the power of the motor and devide that by the battery voltage to give me the current consumed by the motor.

2/ Take the Ah rating of the batteries and divide the current cosumed by it. This should give me the total running time.

3/ Take this running time and multiply it by the running speed to give me the maximum distance.

I have assumed a couple of things here.

a) the throttle is fully open to running at max speed / motor power.

b) there is no pedalling so running purly on the batteries.

I guess, if I have done this correctly then the real figure will be slighly less due to efficiency of the motor and other external influences etc..

Thanks in advance everyone.

Best regards

Jonathan
I would have thought it impossible to calculate an exact expectation of range as the eventual range will be effected by wind, terrain, heat, stopping and starting (traffic) and even the rain.

Not to mention any potential modifications to chainring or sprockets (to gain more speed)

Don't the manufacturers suggest a range that you can check by just riding.
 

jonny jeez

Legendary Member
I do wonder if it is worthwhile spending much time trying to estimate the range by calculation, there are so many variables as to make the calculation little more than a guesstimate. e.g. gradients, surface rolling resistance, load carried on bike, undulations on different routes, battery ageing, wind speeds and direction.
Even the cycle manufacturers claimed range could be considerably different to the actual on the road range. I'm thinking of some car manufacturers claimed fuel consumption figures which have been proved to be rather misleading:whistle:.
You beat me to it!
 

fossyant

Ride It Like You Stole It!
Location
South Manchester
Don't forget to add into the equation if you had a dump this morning as it may affect the weight calculation especially if you had curry the night before. :whistle::tongue::angel:
 

snailracer

Über Member
...All seems ok but I am trying to wirk out the correct range of the bike...
You can estimate how much energy (in kCalories) your journey takes by using http://www.cyclestreets.net/. The energy is calculated at a certain default speed, which is typically slower than average for an electric bike, so recalculate how much energy any of the 3 suggested routes takes by hitting the "change" button and increasing the speed to 15mph - note that this will increase the amount of energy required. Also, the energy required will be different for outward and return journeys, due to elevation differences and one-way roads.

You can then estimate the amount of energy in your battery according to the following formulae:

Usable Battery Energy (in kCalories) = Voltage X Ah X 0.35​

To be clear, the amp-hour rating of, say, four 18Ah batteries in series, is still only 18Ah.

The "0.35" is a combined Joules-to-kCalories conversion and fudge factor that, hopefully, accounts for controller, motor and battery inefficiencies.

If the battery energy exceeds that required for both outward and return journeys, then you have a sporting chance of making the round-trip without running out of batteries. If there is a strong wind, or lots of stop-starts, old battery, etc. then allow extra margin.
 
Last edited:
Just picked up on this thread

I can only assume that you are using SLA batteries ( assuming its 36v and one battery is u/s )

If you are using SLA batteries then the rating at the 1 hour rate is about 55% that of the marked rating on the battery ie 12 ah is at the one hour rate about 7 ah where as Lipo , Lifepo4 and lithium have a longer life being able to use about 90% of the charge before the BMS cuts in and isolates the power.

hope this might clarify the situation more

regards emma ( twinkle on pedelec site )
 

raleighnut

Legendary Member
I'd also be aware that temperature has a big effect on batteries, the range on my trike is about 2/3 on a cold day compared to a warm one.
 
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