E bike small battery

[SteveB21]

Hi,
I'm building a full suspension bike with a mid drive motor. I have a very limited area for the battery. Would a 36V 7Ah battery last long with a Bafang / Tongsheng 250w motor.
I weigh 12 stone, (about 75kg) and only ride off road. Assistance is only required on the hills. Thanks for any advice

 

[Nealh]

Depending on power mode used one should see 20 - 25 miles (may be a bit more) range from a 252wh battery.
I use a 44v 8.4ah battery with my TSDZ2 on a cargo bike towing a trailer, my 370wh's manages approx. 25miles in eco mode on relatively flat terrain on an old rail line route.

 

[sjpt]

If you really only use it on hills you can guestimate by energy needed; and you don't need to know what kind of hills, just total ascent.

how many kwh are needed to raise 75kg 100m?,
answer: approximately 0.0204 kWh of energy is needed to raise a 75kg object 100 meters
Allow for the weight of the bike raises that to 27wh or so, and allow for 70% efficiency and it may be 38wh
You have a 250wh battery, so you'll get around 6.5 times that 100m height, eg 650m.

More if you put in significant effort on the hills. Much less if you do use it on the flat after all.
If the hills aren't very steep and you go fast then air and rolling resistance will need to be allowed for,
I may well have forgotten some other factor that makes this wildly wrong.