Estimating current battery usage

[Lovelock]

Hey, I have a fairly lightweight build using a roadbike with a q128c. I've broken the q hub but upgrading to a Bafang 270rpm.

My battery is a 48v 17.5aH GA cells from Insat Internation in the UK. The issue im having is that the extra weight from the battery is ruining the ride experience. Every bump is 10x worse and the rear derailer is clanging etc.

I will be keeping the battery but want to see if I can downsize for a new commuter im going to build. The issue is I don't know how small I can go as I don't know how to estimate my current usage.

My commute to work is 6 miles each way (<10km), on my LCD3 dsplay the battery indicator doesn't drop after both legs so 12 - 15 miles depending on the ride. I have a 20a controller but use PAS level 3 so use less than 500w when pedalling, sometimes on a hill i'll feather the throttle just to keep it easy for me.

I would love a battery that is way smaller and lighter, the requirement would be that it can handle 6 miles without a significant power drop as I can charge at work during the day.

I do however understand that a battery should be larger than you need it to be, but feel I can get away with something much smaller.

Is there some logic I can use to work out my current aH usage per mile? Is a 36v battery also smaller in physical dimensions than a 48v?

The q128c is 36v 201 rpm but I run at 48v to give me 22mph, for a commuting battery I could use a 36v smaller battery on the 270rpm 36v bafang and achieve the same speed.

 

[wturber]

The obvious thing to do is simply measure your usage on your current bike. The simple and direct way to do that is to wire a $15 dollar inline ammeter and measure what your usage is. If you have a voltmeter, you can measure your before and after battery voltage and and extrapolate from there. Ever drop of one volt represents about 10% of the battery capacity used. So if you start at 54v (full charge) and end at 50v, you probably used about 40% of your capacity or close to 330 watt hours from your 840 watt hour battery.
Failing that, you could go with typical averages which range between 15 & 25 watt hours used per mile. 20 watt hours is probably in the ballpark of what you are using if you are typical.

Given you are only going about six miles one way, my bet is that you could do a round trip with a 48v 6 amp hour battery. So something like a 48v 8 amp hour battery - something about half the size of what you have - will probably take care of you easily and provide plenty of buffer for longer trips or in case you forget to charge at the end of your commute.

 

[e-beach]

Get an rc watt meter from ebay or some place and meter your rides. Fully charge your current battery, connect the watt meter inline and see how many watts you burn on a normal ride to work. The rule of thumb around here is about 25 watts pre mile. That will change for you depending on all the things that effect an electric vehicle. Flat ride vs hills, air temperature, weight of vehicle, aerodynamics, stop and go vs non-stop etc.

Another rule of thumb around here is volts x amp hours = watt hours. So 48v * 17.5 = 840 watts. 6 miles * 25 watts = 150 watts used on your ride to work. From the rule of thumb rules around her you could get to work on a 4ah battery.

However, I recommend, like wturber, to get your self a proper meter and do a real test. In fact do it multiple times to make sure you are getting consistent readings. Once you have real information you will know what size battery to get.

 

[fechter]

The battery needs enough capacity to make the round trip (and have some reserve) and it also has to supply the maximum current the motor takes. You could cut the size of your battery in half and still have enough.

 

[Lovelock]

Thanks for the replies

I'm going to message Luna and also the UK battery builder and ask if they can build a replica of this:

https://lunacycle.com/batteries/packs/52v/52v-mighty-mini-cube-samsung-ebike-battery-pack-30q-6ah-3-pounds/

It should be enough to do what I want and the 52v would give me that extra bit.

 

[fechter]

Something like that should work, but it would be marginal if you couldn't charge at work. With enough pedaling and slow enough speed, you can get quite a bit of range.