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How far can I drive an (average sized) electric bicycle with a 250 watt engine using three serial connected 12 V motorbike batteries?

Trying to research the feasibility of this. I would like to serial connect three 12 V motorbike batteries and use them to run an electric bicycle. Maybe batteries similar to DYNAVOLT Motorcycle GEL Upgrade Mg5-3b Battery

enter image description here

So how far would I be able to go with this solution? A rough estimate would be good too. Would it work in practice? I have seen a YouTube video with a guy powering his electric bicycle with one single 12 V battery so it should at least work to some extent.

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  • $\begingroup$ Uphill, downhill or on the flat? $\endgroup$ – Solar Mike Nov 15 '18 at 12:38
  • $\begingroup$ Let's say level road (flat). $\endgroup$ – Vesa Nov 15 '18 at 12:57
  • $\begingroup$ Start by determining the total Watt-hours available from each battery. Then find out what wattage your engine will pull when running at a reasonable speed (hint, it will be a lot less than 250 W) $\endgroup$ – Carl Witthoft Nov 16 '18 at 15:35
  • $\begingroup$ Do you really mean "serial connect"? That would give 36V, which I suspect won't be good for the motor. Parallel connection is what you need. $\endgroup$ – Ray Butterworth Apr 18 '19 at 1:16
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As a first approximation.

Those batteries are rated at 5Ah (more on this later).

At full power a 12V 250W motor will draw about 21A so on the face of it one battery will give you about 15 minutes of operation at full power.

However standard lead acid vehicle batteries don't really like being used by this, they are designed to give short bursts of high current for starting and then be slowly recharged so a constant load doesn't do them much good nor does fully discharging them so you are never going to be able to use the full capacity and ideally you want batteries specifically deigned for deep cycle use.

In terms of estimating power requirement you are probably better off looking at data of the power output of cyclists rather than trying to estimate the actual loads on the bike. A professional cyclist can generate something like 200-300 Watts for hours on end so something like 150W+ is probably a reasonable working estimate for casual cycling.

Of course as you add weight you will also need more power, especially in stop/start riding and climbing hills, at a constant speed on the flat weight makes much less difference.

A lot of powered bicycles have a motor rated for the average expected load which is supplemented by pedal power for accelerating and climbing.

You may be able to extend the range a bit with a regenerative braking system but again that comes with a cost of added mass.

Estimating range is more difficult but this calculator may help obviously that is aimed at competitive cycling but it is at least real world data.

https://www.cyclingpowerlab.com/CyclingPowerOutput.aspx

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  • $\begingroup$ On the side of the image of the battery it says 12V5Ah. Would that be 12 V, 5 Ah? $\endgroup$ – D Duck Nov 15 '18 at 20:49
  • $\begingroup$ Yeah you are right I misread the details $\endgroup$ – Chris Johns Nov 15 '18 at 21:12
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In my opinion, your motor is 250W/12V as almost no 36V motor. If your motor is 250W/12V, then your battery connection will be parallel, and that will give you 12V/15Ah. By theory, your motor will finish your battery is 0.72hour or in 43.2 minutes.

Bear in mind that 12V/5Ah is mean, if you have device with 12V and 1A voltage and current requirement, then the such device will finish the power source (battery) exactly one hour). Then, the label 250W/12V in a device, mean that it needs 12V power source and it will finish 250W power in one hour. By so, the current it required is 250W/12V=20.8333A. So, if now your batteries are connected parallel, then they will have 12V/15Ah. With so, then the 250W/12V motor will finish the 12V/15Ah batteried (with parallel connection) in (12V*15A)/250W=0.72 hour or 43.2 minutes.

If your motor really 250W/36V, then your batteries need to be serial connection, and it will give you 12V/5Ah=60Watt. Then your 36V/5Ah will be finish (12V*5A)/250W=0.24H or 14.4 minutes.

But all of this just a theory. The fact, you have to consider loses of the motor. You have to consider loses of the battery. You have to consider drag coefficient of the the bicycle to the road. Also, the weigh of the the bicycle itself.

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How far you go depends upon your speed (assuming you're on a smooth, level road). Whitt and Wilson (page 186, 2nd ed) say 15 kcal/km at 16 km/h. You'll travel 3.4 km per battery. (Would take about 38 min for 10.2 km)

But that would more power than your 250 W motor can drive. You'll go slower for about 45 min.

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  • $\begingroup$ And it depends on the weight of bicycle and rider, on average something between 110-130 kg. $\endgroup$ – Sam Farjamirad Nov 15 '18 at 9:47
  • $\begingroup$ Not really. Wheel (dependent on mass) and aerodynamic (not dependent) losses are about equal at 16 km/h, and the initial acceleration isn't required as you would need to push off to start like an ordinary cyclist. $\endgroup$ – D Duck Nov 15 '18 at 9:56
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    $\begingroup$ Yes and no, the drag force is geometric dependent, how heavier the rider how thicker, taller the body. I used to bike to school, somedays i had to ride my friend on the back, i am still mad at that chubby ... . I can tell it makes difference, but as a rough estimation seems okay, the real problem is huge voltage drop after about 10 min. $\endgroup$ – Sam Farjamirad Nov 15 '18 at 10:24

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