Choosing the Right Voltage for a Battery

[mcristiani]

Hello,

I have electrified 6-7 bikes and I have built 3 batteries. My goal is to get great range, constant exercise and really after 24mph I get out of my aerobic range for long distances so I am calling that cruising speed.

Assuming the same GS01 Bafang motor, a targeted speed of 22-24mph with constant pedaling and a relatively flat terrain profile, is it better to get a lower voltage battery with more duration - ah? Or a higher voltage battery.

Also, current Em3ev controllers are trimmed back to only pull 14-16ah.

So... Is there any inherent advantage between:
36v 18.8ah
48v 11ah
52v 7ah

I know those WH aren't all equal. My question is really around any fundamental advantage to go with one over the other for future projects.

Any help or inputs would be greatly appreciated.

Thanks in advance.
Mario

 

[dogman dan]

Gets complicated fast. By now you must know that any inherent efficiency is there because of the combination of many variables. Controller, motor type (geared hub, dd, mid drive) and wind (rpm spec), battery voltage and type (both c rate and ah size).

So the short answer is no, there is not much inherent advantage to one battery or the other, except for how exactly they interact with all the above variables.

However, its pretty clear what makes a good battery. High enough c rate, and size large enough to minimize sag under load. That's it. So if you need the smallest size, with the biggest c rate, so you nearly eliminate sag under load, its RC lipo, where even the highly exaggerated c rates greatly exceed those of the round cell chemistries.

But if your needs are different, like 12 amps continuous for 80 miles, then you can use a pretty weak type, since you will in any case be packing a whopping 2000 watt hours of it. Your c rate will be so low on a pack that big, that you never see much sag.

Bottom line here is same either way, big sag under load means your battery is getting hot. And heating that battery both costs watt hours lost to heat, but also causes premature low voltage cutoff. All bad for efficiency.

I have not answered your question though, how to pick your voltage. Primarily, its what max speed you need, for a particular motor system. On bike A, I might desire 40 mph top speed, just for crossing or riding briefly down one nasty 6 lane road. So likely that means 72v. On bike b, I might need only to poke along at 15 mph, like a delta trike that loses all steering at 20 mph. 36v plenty for that, or possibly even too much depending on the motor system.

Personally, for simplicity's sake, I have settled in on 48v, either 13 or 14s. The main reason for this is I have some very large hills to climb, and though I tend to ride 18 mph, vs the 25-30 mph top speed, I still need a full 1000w to get up those hills. So 48v, allows me 1000w with typical cheaper 22 amps controllers.

Back to the subject of range. that one is even simpler. Range is why I tend to cruise 18 mph. I can easily ride to town and back, climbing 1000 vertical feet to home, if I ride 18 mph. If I ride 25 mph, I make it to only the near side of town, not downtown. Its simply amazing how much more your pedaling provides when you slow down. Call your effort 100w, ( mine more like 50) so 18 mph takes 400w, and you provide one 4th of the total watts. At 25mph, you provide one 6th, or less. You just go a lot farther drawing 300w on your battery, vs 600-800w.

 

[quantumcloud509]

Dear dogman dan. That made a ton of sense. Thank you.

 

[mcristiani]

Thanks Dogman.

"Its simply amazing how much more your pedaling provides when you slow down." - I am going to say this is because of drag increasing at an ever faster rate, yeah?

I still don't quite get the seemingly "dead space" in batteries where voltage sag flips the controller off.

So a similar second question.

If I had two 13.4ah batteries and plugged them in one after another would this get the same range as connecting the two - making sure voltages are +-1v - in parallel and running them until lvc?

As for the speed comments in your response... While I like 40mph, it is not my goal right now. I want to achieve good range through making my 100w actually contribute and not just get wasted in drag. Personally, I am good at 22-24mph. On long distances, 10+ miles, I can't keep pace above that range.

Thanks.