Looking for battery advice

Hi there, I'm not super technical so please have a heart. I'm building up an ebike with this motor and a 40 amp Controller:
https://www.cycmotor.com/x1-stealth

My goal is to have a bike that goes 25 - 35 mph (I realize the size/weight of the bike will factor into this) and my friend who is helping me is recommending 2 7/8 x 3 7/8 x 6 1/4g a battery that is 8.4 amp, 52 volt and 50 amp max continuous Discharge Rate:

"Also one thing that's real important with batteries, is the maximum discharge rate. The batteries like I have are rated at 50 amp max continuous. The six amp hour batteries that I've seen are rated at 30 amp, which is probably adequate for your application, but right on the line."

The battery he is recommending is large compared to the frame (2 7/8 x 3 7/8 x 6 1/4) and im looking for something smaller. Range is not super important (15 miles or so) as i can tote an extra battery. Any recommendations?

Thanks!!!!! - Darren

Meepsh said:

Hi there, I'm not super technical so please have a heart. I'm building up an ebike with this motor and a 40 amp Controller:
https://www.cycmotor.com/x1-stealth

My goal is to have a bike that goes 25 - 35 mph (I realize the size/weight of the bike will factor into this) and my friend who is helping me is recommending 2 7/8 x 3 7/8 x 6 1/4g a battery that is 8.4 amp, 52 volt and 50 amp max continuous Discharge Rate:

Click to expand...

You probably don't need a 50A discharge rate from your pack to achieve your goal, so it would be easier if you relaxed that requirement. If you ever did discharge at that rate, you'd only have 10 minutes of riding before the battery is flat/dead. It will also be difficult to find cells that output at that rate, in an 8.4Ah pack, so would need to be built with specific cell types (e.g. Samsung 25R) to output 50A. If you ride at 35mph, then an 8.4Ah pack will likely be fully discharged after 15 miles, but a much greater range if you ride at 20mph, since wind resistance is a huge factor when riding over 20mph.

This is if the pack is comprised of lithium ion 18650 cells, like the majority of pack. If you used lithium polymer batteries, those discharge rates wouldn't be a problem, but you would need to follow a lot of safety precautions, since it's a more dangerous battery chemistry.

If you're sticking with a smallish battery, then a 30A discharge is more doable, but still get you 15 miles, if you ride slower or pedal harder.

Meepsh said:

My goal is to have a bike that goes 25 - 35 mph (I realize the size/weight of the bike will factor into this) and my friend who is helping me is recommending 2 7/8 x 3 7/8 x 6 1/4g a battery that is 8.4 amp, 52 volt and 50 amp max continuous Discharge Rate:

Click to expand...

What conditions (terrain, wind, road surface, etc) do you need to maintain those speeds on?

That will determine how much power (watts) you require to do that. The simulator at http://ebikes.ca/tools/simulator.html can help you see how everything works together to get the results you need under the conditions you have.

If it's all flat, well-paved roads, with no winds, then the only thing to worry about is the wind resistance of going that speed, but if you have winds or hills it may take a *lot* more power than the below:

A default configuration
https://ebikes.ca/tools/simulator.html?axis=mph
showing it takes about 650w out of a 36v battery at only 18A battery current, to go 25mph (but that's it's max speed).

A configuration to do 35mph
https://ebikes.ca/tools/simulator.html?axis=mph&batt=B7210_DT&cont=C40&autothrot=true&throt=80
takes about 1800w out of a 72v battery at 26A battery current.

So that means it would take, regardless of battery voltage/etc., around 650w to do the 25mph minimum you're after, on flat ground, no wind, and almost three times that power to do the 35mph max you want, again on flat ground, no wind.

For a 52v battery (14s), 650w would only be 12.5A. (650 / 52 = 12.5)
1800w would only need 35A. (1800 / 52 = 35)

So, in theory, you only need a battery capable of 35A continuous current. (I dont' recommend accepting "max continuous" because "max" is often used for peak (few seconds) current, and too many sellers simply conflate the two, and sell a battery (or other part) that can really only handle what they claim as "max continuous" for a few seconds, not actually continuously, because of that. Marketing is too often the art of lying as much as they can get away with to get people to buy things...and worse, most people selling things don't know anything about them; at least not enough to understand them or why different ratings mean different things).

However...most battery packs are marketed using the individual cell laboratory testing max ratings, multiplied by the number of cells in parallel, as if this is how it will really perform in the realworld application it's put into, which tends not to be true (or not for very long, since they'll age a lot faster used hard than they will used gently).

If that's not important for you, and only performance is, then you can use a battery that can "barely" meet your requirements. But if it matters to you how long the battery continues to work well for you, then using it as far from it's max ratings as is practical will help with that.

(Side note: cheap batteries may be marketed using whatever will get the buyer to pay for them, whether those ratings are real or not, and you can't even find in many cases what cells are really inside them, even if you have them in your hands, or if you do, you may not be able to find an actual spec sheet for that cell from it's actual manufacturer...assuming they are not recycled garbage cells, which is unfortunately not uncommon).

Meepsh said:

Range is not super important (15 miles or so) as i can tote an extra battery.

Click to expand...

Range...that's up to how many wh/mile you use, vs how many wh the battery has.

We'll use the 35mph case since it's the worst power eater, but we're still assuming flat paved roads with no wind; if your conditions are different, it could take a LOT more power and thus a much bigger battery to do the same range.


At 35mph, in the simulation previously linked, it takes about 50wh/mile.

If you have a 52v 6Ah battery, that's only 312Wh (52 x 6 = 312).

312Wh / 50Wh/mile = 6.24miles. (that's a maximum, assuming perfect battery condition, running it from completely full to completely empty, which is hard on the battery--and assuming your controller's LVC doesn't shut it off sooner, which for battery health it should be set to do...which probalby takes up to a half mile or more off the table).

A 52v 8Ah battery is 416Wh, which gets 8.32miles max range under the assumed conditions. (realistically, less).

This also assumes no stops or starts except at trip beginning and end...each one of those will eat up power, more of it the heavier you and the bike/etc are, and the harder you accelerate, and the more often you do it.


If you're only going 25mph, it only takes about half the Wh/mile, so it could double your range, all other things being the same.

There are way to many factors to consider as amberwolf has pointed out. We don't know what engine you use, what your weight is, what terrain we talk about and so on.

I can reach 25mph quite easily on my 48V battery. It's size isnt that big and it's very long lasting as well. a 52v 8AH battery won't last you 15 miles. That's wishful thinking at this point.

Also be aware that the range will decrease over the life of the battery. I add an extra 20-25% capacity so that I still have adequate range at "end of life". Also be aware that different battery chemistries have different life expediencies (and cycle limits). Examples: approximately 3 years for LiCo, 6 years for LiFePO4 (these weigh considerably more as well).

Also be warned returning an ebike battery to the vendor via common carrier is problematic. The shipping regulations for lithium batteries is very strict and requires special permits. Most ebike batteries are beyond the lower capacity limits that individuals are allowed to ship.

Using a rough speed and power calculation, and assuming a 6 foot, 200 pound rider, I get approximately 1850 electrical watts to maintain 35 mph. So that means about 14 minutes of runtime and a range of about 8 miles, full to empty, for the battery you specify. In reality, battery capacity will probably be less because of the high discharge rate.

Running slower than 35 mph will add to range, but stops and starts will subtract from it.

If you want that much power and speed available, I recommend using a much bigger battery. It's easier on the battery, and it gives you a more usable operating envelope.

Build a bike that cruises wide open at about 20 mph, and the battery you have there makes a lot more sense. Then you're talking a hypothetical 66 minutes of full speed runtime and 22 miles of range, full to empty.

If the picture is a mock-up of the battery, that’s a pretty good size, many people tend to use the hailong type which is a bit larger (but can pass for a factory ebike)

I’d be inclined to ask what cells are going to be used- you want Samsung/LG/murata/Panasonic ideally as they are reliable. But otherwise if your friend is an expert then just take his advice and you will have support for whatever he makes.

I have found a high current rating on the battery is good for cold weather as the voltage doesn’t sag much which helps.

Is that a brand x ht01 frame? Great choice if so

electric_nz said:

If the picture is a mock-up of the battery, that’s a pretty good size, many people tend to use the hailong type which is a bit larger (but can pass for a factory ebike)

Click to expand...

Physical size is fine; it's the capacity that might prove to be a problem. 50A discharge from an 8.5Ah battery is almost 6C. That's not feasible for most batteries and not healthy for most that can do it.

amberwolf said:

What conditions (terrain, wind, road surface, etc) do you need to maintain those speeds on?

That will determine how much power (watts) you require to do that. The simulator at Motor Simulator - Tools can help you see how everything works together to get the results you need under the conditions you have.

If it's all flat, well-paved roads, with no winds, then the only thing to worry about is the wind resistance of going that speed, but if you have winds or hills it may take a *lot* more power than the below:

A default configuration

Motor Simulator - Tools

Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...

ebikes.ca

showing it takes about 650w out of a 36v battery at only 18A battery current, to go 25mph (but that's it's max speed).

A configuration to do 35mph

Motor Simulator - Tools

Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...

ebikes.ca

takes about 1800w out of a 72v battery at 26A battery current.

So that means it would take, regardless of battery voltage/etc., around 650w to do the 25mph minimum you're after, on flat ground, no wind, and almost three times that power to do the 35mph max you want, again on flat ground, no wind.

For a 52v battery (14s), 650w would only be 12.5A. (650 / 52 = 12.5)
1800w would only need 35A. (1800 / 52 = 35)

So, in theory, you only need a battery capable of 35A continuous current. (I dont' recommend accepting "max continuous" because "max" is often used for peak (few seconds) current, and too many sellers simply conflate the two, and sell a battery (or other part) that can really only handle what they claim as "max continuous" for a few seconds, not actually continuously, because of that. Marketing is too often the art of lying as much as they can get away with to get people to buy things...and worse, most people selling things don't know anything about them; at least not enough to understand them or why different ratings mean different things).

However...most battery packs are marketed using the individual cell laboratory testing max ratings, multiplied by the number of cells in parallel, as if this is how it will really perform in the realworld application it's put into, which tends not to be true (or not for very long, since they'll age a lot faster used hard than they will used gently).

If that's not important for you, and only performance is, then you can use a battery that can "barely" meet your requirements. But if it matters to you how long the battery continues to work well for you, then using it as far from it's max ratings as is practical will help with that.

(Side note: cheap batteries may be marketed using whatever will get the buyer to pay for them, whether those ratings are real or not, and you can't even find in many cases what cells are really inside them, even if you have them in your hands, or if you do, you may not be able to find an actual spec sheet for that cell from it's actual manufacturer...assuming they are not recycled garbage cells, which is unfortunately not uncommon).

Click to expand...

For some reason even EM3EV uses "max continuous".