Determine the amp hour rating of a home made battery pack
If you series connect the batteries the amp-hour rating stays the same. The voltage has increased though, so the power available has increased. That's why using watt-hours is more useful than amp-hours when comparing battery packs of different voltages.

So ...


If you're using 10 x 3.6 V batteries in a string you have a 36 V, 2.5 Ah battery.
That's a 36 V x 2.5 Ah = 90 Wh (watt-hour) battery.
Put three of those packs in parallel and you get a 36 V, 7.5 Ah battery.
That's a 36 V x 7.5 A = 270 Wh battery.


Now here's where watt-hours are so much more useful. Let's say you're going to need 135 W average power for your motor. Your battery duration will be 270 Wh / 135 W = 2 hours duration.

My electric bike uses about 150 W in normal cycling mode at almost 30 kph. I reckon my legs are putting in another 150 W.

Little technical point: In the SI system of units it's capital letters for abbreviated units named after someone but lower-case when spelled out: V for volt, A for amp, W for watt, etc. Yes, there will be exceptions ...