What battery capacity do I need for my e-cycle?

[Mad Professor]

Good day all.

I am looking for help and advice with buying or building my own battery pack for use with my e-cycle.

My current e-cycle had 3x 12v 12ah lead acid batterys, that I want to replace with lighter and more powerfull lithium cells.

But before I start buying a prebuild pack, or building my own, I am wanting to make sure I get one that will be suitable for my needs.

Now I am still quite new to things so please take it easy on me.

I have a standard pedal cycle that is fitted with a 36v 500w 3-phase brushless hub motor, so 500w / 36v = 13.888Ah.

I live in a little 20x25mile island, so even a return trip end to end would only be 50miles, but most of my return trips are only aprox 25miles.

I know the average speed of a cyclist is aprox 15MPH.

If I am working this out right, 25miles at an average speed of 15MPH would take 1.666hours.

So if I take the motor current draw of 13.888Ah and muply it by 1.666hours, I will need a battery pack of at least 23.137Ah.

Can someone please confirm my way of thinking, or am I cycling into an impossible headwind?

Thanks for your time.

Best Regards.

 

[Ykick]

This is wrong - 500w / 36v = 13.888Ah

The value isn't Ah it's A or Amps. In this example it's not a constant thing but with a power meter you'd be in better position to estimate your capacity requirements.

12Ah SLA is really only about 6-7Ah for EV high current use on it's best day. How far do you get with that now or when it was new?

As pure "guesstimate" for most lithium chemistry you might, might be able to cover 25 miles at 15 MPH with around 10Ah battery depending how much you pedal. 15Ah almost certainly would cover that distance at that speed no matter how much or little pedaling you do.

Best thing IMO would be to get a power meter and actually measure what your setup consumes before making large purchase of batteries.

 

[Mad Professor]

Ykick: Thanks for your reply.

Sorry I did mean A not Ah, I was in a rush to type up my post before I went out.

I know I will not be using the same power all the time, I am just looking at worst case figures.

I can try and use a amp meter, but my current sealed lead acid batterys are past had it, I don't know how well I would be able to trust any readings.

Thanks again.

Best Regards.

 

[pdf]

On different bikes and different routes, an average speed of around 15-16 mph (which includes stops, so not continuous) in a relatively hilly region gives me between around 14-18 Wh/mile. If you are in a flat area and/or travel a bit slower, you will see a little lower figure. Ykicks figures give around 14.4 Wh/mile, if you use an average voltage of 36V and use (P= volts x amps). I have ridden thousands of miles and this estimate for Wh/mile is pretty consistent. I figure you will get on the low side of this range, based on your set-up (relatively low power requirement so less heat losses). Using 14 Whr/mile, a nominal voltage of 36v and 25 miles, that is 14/36*25 = 9.7 Ahr. Remember, batteries will lose capacity as they age (I multiply by 1.3 to account for this, so a loss of about 25% of energy by end of life) and you need to allow for an 80% depth of discharge max (in my opinion). So it will be around 9.7*1.3/0.8 = 15.7 Ahr. I think I would go with 20 Ahr so you have no battery anxiety, but if you don't mind living on the edge and you don't anticipate any side trips adding to the mileage before charging, you could get away with 15 Ahr. A 15 Ahr 36v battery is a nice size. I have one on my daily ride but my trip is only 14 miles round trip I can get two rides out of it but I am pushing it on the second ride. When the battery ages more, I will probably get a bit less mileage and maybe won't be able to get two trips.

This is a heuristic approach, but since there are inefficiencies and losses that are only best estimates anyway, my experience is that it will work pretty well for estimating in most cases. Some people would use a higher depth of discharge factor so my estimate might be a bit on the high size, battery-wise.

 

[SamTexas]

25 miles at 15mph:

Less than 10wh/mi if you pedal assist (isn't that a requirement over there?). So at most 250Wh or 7Ah, 36V
Around 15wh/mi if the motor does all the work. So 375Wh or 10.5Ah, 36V.

But apparently you are getting by with your existing 36V 12Ah, right? Meaning that you are able to cover 25 miles. If that is true, then all you need is a 36V, 6Ah of lithium. To give yourself some breathing room, go for 10Ah.

 

[Chalo]

I have a standard pedal cycle that is fitted with a 36v 500w 3-phase brushless hub motor, so 500w / 36v = 13.888Ah.

As has already been pointed out, amp-hours are for battery capacity or total power consumption during a period of time, and amps are for instantaneous power.

500W is almost certainly the nominal continuous rating for your motor, not its maximum power demand. There is the matter of efficiency, which is generally between 50% and 75% on average in the real world. And then there is the matter of your motor's peak power, or more importantly your controller's peak power, as opposed to the nominal rated power.

The bottom line is that you'll need a lot more than 14A of current from your battery now and again. The controller probably has its maximum current rating printed on it somewhere. Find that number and you'll know how much you need from the battery. Then keep in mind that battery C ratings are quite optimistic, and cut them in half when reckoning their practical capabilities.

 

[PeteCress]

I live in a little 20x25mile island, so even a return trip end to end would only be 50miles, but most of my return trips are only aprox 25miles.

I know the average speed of a cyclist is aprox 15MPH....

If I am working this out right, 25miles at an average speed of 15MPH would take 1.666hours.

So if I take the motor current draw of 13.888Ah and multiply it by...

One thing to consider is what I would call "Electric-Motorcycle-vs-Electric-Assisted-Bicycle.

If somebody is just sitting on the bike, that's one thing. But if they're basically pedaling and just using the battery for assistance that's another thing altogether.

For the "Just Sitting" scenario, I would propose that a moped would be more cost-effective.

For the "Assisted" scenario, I would be riding that bike you already have and keeping track of how many AH you use - and size the new battery accordingly.

 

[docnjoj]

My figures pretty much agree with pdf and ykick. I do about 20 miles at about 15 Wh/mile on 36 volts of 20 ah A123. Actually I do a bit better as we ride recumbents and the wind resistance is a bit lower than a conventional bike. 20 ah seems about right but 15 may be possible, however that is the conundrum and if you get too small a battery you will have distance problems with wind and hills. Of course get too big a battery and you have to figure where to put it on the bike. Rear racks suck for all types of bikes and trikes. Good luck with your venture.
otherDoc

 

[dogman dan]

At 15 mph, I have gotten 35 miles from a 36v 20 ah lifepo4 pack. And I've gotten 22 miles many many times at 24-25 mph.

I'd say get a 36v 15ah or 36v 20 ah pack. You'd have to do a bit of charging along the way to get 50 miles though. I don't recomend you buy a gigantic pack to have 50 miles. 36v 20 ah is about as big a battery as you can comfortably carry on the bike. About the same volume as your existing pack, but only about 16 pounds.

You'll love a good lifepo4.