Assist needed for 72v battery for e-motorcycle

[Rabadax]

Hello.
I am rebuilding an old Husqvarna light motorcycle (125 cc) from 1953 to an e-motorcycle.
I have purchased a QS mid drive 3000w and a kelly controller kls7245 for 72 v.

I am in the process of either buying a battery pack or building one (if i build i will buy a spot welder).

The motorcycle will be limited to max 80 km/h and if i will build the pack, my thoughts is to go with 20s7p, beacuse i do not know if i need more amps? I would like a compact battery (140mm thick instead of 70mm).
I have found that nkon in holland sells refurbished LG MJ1 batteries to a decent price, is refurbished batterys a good or bad idea?

I have searched this forum but cannot seem to find a ”pack schematics” for how to place the cells, and are unsure of how to do it in a compact design (folding). What make of BMS can be recomended and should it be a 50 or 60 amps out?
Is there any website that has pack build schematics or program / app for this?

Thanks in advance
Mattias

 

[amberwolf]

There's many ways to build packs, and it depends on your skills, tools, and budget, as well as what the pack needs to do, as to which is best.

I would recommend reading as many battery build threads as you can to see how theirs worked out for them, and what they did, as well as battery troubleshooting threads so you can see what *not* to do.

But before you go doing that, you first need to determine a few things to find out how big a pack you actually need.


How fast are you going to go?

How far do you need to go at that speed?

What terrain will you be riding on?

What wind/weather conditions will you be riding in?

How much will you and the bike and anything you must carry weigh?


With that info, we can make a guesstimate as to how many kWh your pack must be, which will tell you the minimum size of the pack.


Then you need to know the maximum current draw of your system, both peak (and how long a peak), and continous, so that you know the minimum current capability of the pack, and thus the minimum number of cells that you must have in parallel, as well as minimum interconnect sizes, and minimum wire gauges for external connections, and minimum BMS capabilities.


Once you know all these things, then you can begin working out where to put the battery on the bike (it may be too large to fit all in one spot), and how to connect it all together.



Regarding "refurbished" batteries, that depends on what they mean by that. If they mean they are used, then unless they guarantee a minimum capacity, and a maximum internal resistance, and a minimum lifespan, you'll end up needing to test and sort them (would be a good idea anyway), to build the most "even" pack you can. The more cells, the more complicated a pack build can be, with non-new cells.

If they don't have a precise definition of "refurbished", along with complete specifications for the cells in that condition, you might want to skip it, because you can't know anything about the cells other than whatever they do tell you.




Regarding controllers, remember that Kelly controllers are generally not rated as battery current, but rather for phase current. This means that if you get say a "45A" controller, it's 45A current to each of the motor phases. The battery current it is capable of would likely be much lower--but in any case would be different from phase current ratings. This makes it difficult to predict what your battery must be able to supply, or what performance you will actually get from the system.



Regarding the system as a whole, you'll need to make sure that your battery voltage is high enough, relative to your motor winding vs wheel size, to reach the speed you want it to reach.

To help understand how that works, you can go to https://ebikes.ca/tools/simulator.html and experiment with various setups (there aren't exact matches for your parts, but you can use "custom" settings to help match some of them).


BTW, which one of these bikes is the type you're converting?
https://www.google.com/search?q=1953+%22Husqvarna%22+125cc&tbm=isch

 

[Rabadax]

Thanks for the reply.

Max speed 80 km/h (50 mph) but usually 50 km/h
Range minimum 40 km (25 miles).
Flat surfaces (asphalt) with small hills.
Summertime only (cruising)

Total weight including me approx 170 kg (375 pounds).

The bike is a Husqvarna model 30 ”Rödmyra” ( Red ant), found this site with the model http://www.saarela.se/aktuellt/aktuelltbild11/110625.html

New LG MJ1 is about 0,5 Euro higher than refurbished, so its not that expensive with brand new ones.

If someone uses the same type of motor (QS mid drive 3000 w)with controller (Kelly kls7245h), and have built a functioning battery pack for that combination and could provide specs for the number of series connection and recomended parallell connections and also a blueprint of the cell positioning and type of bms would be very thankful.

/Mattias

 

[amberwolf]

Max speed 80 km/h (50 mph) but usually 50 km/h
Range minimum 40 km (25 miles).

Since you don't state it explicitly, we'll have to assume that the minimum range is at the maximum speed, continously with no stopping, slowing, accelerating, etc, so 25 miles at 50mph, or 30 minutes of riding time.

At 50mph, a typical motorcycle on flat ground with no wind of any kind could use up to 100-150wh/mile. We'll assume the worst case, to give the best margin for error (I'd guess it's likely to be slightly less than 100wh/mile, but you have to test under your specific conditions to find out). That means 25 miles x 150wh/mile = 3750wh, or 3.75kwh. At 20s, that's an average voltage of 20 x 3.7v = 74v. 3750wh / 74v = 50.7Ah. (if you only use 100wh/mile, that cuts the pack size by a third, too, for the same range).

Using the LG MJ1, a google search shows it rated about 3.5Ah, and capable of about 10A (about 2.5C). To get your minimum runtime, you'd have to have at least 50.7Ah / 3.5Ah = 15 parallel cells (you have to round up ). Theoretically that also ends up giving you a 150A current capability, if the cells, interconnects, wiring, and BMS (if any) can all handle that.

20s x 15p = 300 cells. Assuming the battery is built as a block of 20 flat packs of 15 cells, then just the cells will be 65mm (2.6") thick, and 20 x 18mm = 360mm (14,2") long, and 15 x 18mm = 270mm (10.6") wide. It'd be 300 x 46.8g = 14040g (14kg, 31lbs) in weight, just for the cells. That doesn't count enclosure, padding, interconnects, wiring, BMS, etc.

If you decide you want double that range at that speed, you would need a pack twice that size and weight. (same if you need twice the current capability).

If the range needed is at a different speed, or the speed varies a lot, or you have a lot of stops and starts, the power usage is completely different, and may be lower or higher.

Flat surfaces (asphalt) with small hills.
Total weight including me approx 170 kg (375 pounds).

What height and slope is a "small" hill, in your area? Makes quite a bit of difference, especially with that much weight. The more weight, the more power needed to maintain the same speed up the same hill, and the steeper the hill the more power needed as well. The longer the hill, the longer the system must maintain that power level, which generates more heat in the whole system. The closer you run the parts to their limits, the harder this will be on them, the longer they have to run that way.

If you want to learn more about interactions between system and environment, and various parts of the system, you can go to https://ebikes.ca/tools/simulator.html , read the entire page to learn how to use it, and then setup a system close to what you intend to use (it's meant for bicycles but has options that allow custom higher power setups), and play with various settings, to see which things cause what response.

Summertime only (cruising)

Don't know what this means in regards to riding conditions. Some places that means perfect weather, some means intense rains and winds, some means unbearable heat, etc.

 

[john61ct]

Even the MJ1/M36, as good as it is, will not last many cycles in a use case where over 2C is the continuous discharge rate.

Short bursts at 3C are fine, but truly continuous usage should be kept between 1-2C, and ideally at the lower half of that range.

same with VC7 and 35E other good choices in that high energy category.