Is 72V Safe?

[hulk]

I have a cargo bike, a Yuba Mundo Classic, it's 7 feet long and I'm thinking of going with a rear hub 72V 2000 watt motor and 72V battery. I keep reading how some members are saying that 52V is safer than 72V.

What do they mean by that? How could 72V be less safe than 52V? I plan on using the bike as a car replacement, so I'll be using it to get groceries and take my 6 year old to school. I'm just wondering if I should go with a 52V instead of the 72V battery and motor combo for safety reasons, I won't want my kid to get shocked but I'm new to ebike building so I don't understand how the battery can malfunction.

 

[K PierreR]

Unless you want to go fast all the time, 72V is probably overkill. If you are going to be under 20 mph then I would rather have 52V

As far as safe, DC voltage is far more deadly than AC voltage once you exceed the resistance of your skin to electrical flow. I researched it and came to the conclusion that you have to be far more careful with 72 Volts about chaffed bare wires in wet conditions like rain, because there is a slight chance you might get shocked and DC is unforgiving.

In the end, Voltage controls ultimate motor speed, Amps gives torque. I did not plan on running my bike above 28 mph so I settled for a 52 volt. At the time I built my fat bike, I was concerned about not having enough power for everyday riding. I have more than enough with 52 Volts and a Bafang BBSHD and rarely exceed 600 watts. when I am pushing more than 400 lbs

 

[retiredNH]

Unless you want to go fast all the time, 72V is probably overkill. If you are going to be under 20 mph then the motor will be operating in an inefficient range.

As far as safe, DC voltage is far more deadly than AC voltage once you exceed the resistance of your skin to electrical flow. I researched it and came to the conclusion that you have to be far more careful with 72 Volts about chaffed bare wires in wet conditions like rain, because there is a slight chance you might get shocked and DC is unforgiving.

In the end, Voltage controls ultimate motor speed, Amps gives torque. I did not plan on running my bike above 28 mph so I settled for a 52 volt. At the time I built my fat bike, I was concerned about not having enough power for everyday riding. I have more than enough with 52 Volts and a Bafang BBSHD and rarely exceed 600 watts. when I am pushing more than 400 lbs

You might want to check your information. Dr. Google is far from perfect.

George Westinghouse and Thomas Edison first battled over the safety of DC vs AC, with evidence showing that AC is more dangerous, results that have been confirmed many times since. The issue is that AC is more likely to trigger heart fibrillation, an often fatal problem.

When it comes to safety, frayed insulation is not much of a problem, since it's likely to cause a short long before you have the opportunity to touch it. And decent connectors should make it hard to touch live parts.

Bottom line, it should make no difference in a decently designed and assembled system. Observe proper connector use. Avoid the chance to abrade insulation by properly locating and securing wires.

 

[K PierreR]

You might want to check your information. Dr. Google is far from perfect.



Bottom line, it should make no difference in a decently designed and assembled system. Observe proper connector use. Avoid the chance to abrade insulation by properly locating and securing wires.

Would you care to clarify the part that I have put in bold.

I redesigned and repurposed older machines for a living.
I used the following formula.
MH + M +CFM = QO That is Material Handling plus the Machine plus a Complete F'ing Moron = Quality of Output.

A CFM is a special kind of Moron. A CFM is a half way intelligent person whom tends of horse around or bypass safety and other material handling issues or skip maintenance or shut down procedures in order to contribute the least amount of effort on their part. In other words, they would do things you would least expect and get into trouble.

He has a kid whom is likely, to like the bike. His kid could easily be out poking around at stuff on the bike out of curiosity when he is not around. Again, can you define the bold text?
Go research where that threshold is on DC voltage safety and give us a solid answer. I could not nail it down and stay within my formula. In the above case, I could easily be the CFM, not paying much attention. A reason I don't like throttles or instant on power from the pedals.

 

[retiredNH]

Would you care to clarify the part that I have put in bold.

There's a long list of proper practices. I won't go into all of them here. The most important, IMHO, is avoiding degradation of insulation. Use proper grommets if wires are going through a hole with sharp edges, for example. Know what to do with sharp edges that are not part of a hole. Know when, where and how to secure wires. Know how to size wire. In power situations like this, know how to make safe connections. Know how to size and use power connectors.

The biggest risk for builds like this is damaging the equipment through a short, maybe the battery, or the controller. Shock risk is much lower (except for the charger) since there's no earth ground.

 

[theemartymac]

72V is quite common in hand-held power tools. A 72V Ebike battery is just a bit larger physically, but you're still only running a fancy wheeled cordless drill...

Don't fear the reaper - BOC

 

[AHicks]

Regarding "safe", I think you have to consider how well the battery has been built, no? If it's something that's been soldered up by an amateur and gorilla taped/glued in place, I'd consider that an accident looking for a place to happen!

If professionally built using the latest "best practices", I would think a 72v system could be as safe as any other.

 

[harryS]

I think 72V is safe when packaged up in a battery and in the harness of a well built ebike. If I were to put two of my 36V packs in series and bring out 84 volts (full charge) to two metal plates, no, you could not pay me to lay my hands on them.

Sixty years ago, we kids would build or own tube radios that were powered by 67 to 90 volt batteries. One mean kid once jabbed me with a 90 volter. Yow, did it bite.

 

[ki11a]

I think 72V is safe when packaged up in a battery and in the harness of a well built ebike. If I were to put two of my 36V packs in series and bring out 84 volts (full charge) to two metal plates, no, you could not pay me to lay my hands on them.

Sixty years ago, we kids would build or own tube radios that were powered by 67 to 90 volt batteries. One mean kid once jabbed me with a 90 volter. Yow, did it bite.


View attachment 86787

That old battery reminds me of this

LMAO

 

[CdnShaun]

I have a cargo bike, a Yuba Mundo Classic, it's 7 feet long and I'm thinking of going with a rear hub 72V 2000 watt motor and 72V battery. I keep reading how some members are saying that 52V is safer than 72V.

What do they mean by that? How could 72V be less safe than 52V? I plan on using the bike as a car replacement, so I'll be using it to get groceries and take my 6 year old to school. I'm just wondering if I should go with a 52V instead of the 72V battery and motor combo for safety reasons, I won't want my kid to get shocked but I'm new to ebike building so I don't understand how the battery can malfunction.

Here there hulk, hopefully I can help you a bit.

I have a Surly Big Dummy Cargo bike that I have converted to electric. I went with 2WD, 52V using 9C 1500 watt motors.

I also did some silly...installing 6 x 23.8Ah 52V batteries (hence the cargo bike choice in the first place for this build, to carry all that battery weight). I have around 130lbs of custom frame and batteries on the back, carry a 20-30lb bag on top and I'm currently 270lbs of rider.

Note I did look at the Mundo as well when I was shopping for the bike and it's a great choice. I just found the Surly was a better fit for me and my LBS is amazing and they carried Surly.

Things to consider:

-Way more selection out there for 52V setups. Motors, batteries, etc.

-Components like PAS and Torque PAS often are limited to 52V or less - not compatible with 72V.

-72V Batteries are much harder to find and a price premium per Wh compared to 52V options. This is important when you may consider perhaps having 2, 3 or 4 batteries for all day riding (carry the extras on your cargo bike or install multiple on the frame).

-52V Motors can easily maintain 28mph/45kmph. On a cargo bike with lots of weight out back, you really do not want to ever go faster than 30mph. The reason riders go 72V is to push motors up to 40mph+ usually.

Now, some real world riding insight.

I built my cargo bike up with 2WD from day one....because I could. LOL. Watching my CA3 computer screen I can tell you that with 400lbs of payload (me the rider plus everything out back) I am only using 400-1000watts 90% of the time and I'm cruising along at 25mph/40kmph all the time.

When do I use the 2WD? Hills of course. With both motors on tap I can climb even 10% grade hills at 25mph/40kmph which blew my mind when I first experienced it.

Yeah 2WD is overkill...but I was bored at home in covid lockdowns and decided to build this beast.

I do appreciate having the second motor so if I have a motor failure I still have the second in place to get me home. I also like hub motors for the safety of breaking a chain the hub motor still works.

With my 6 batteries I have a 300-350km range with my riding/fitness level. I also have a Burly trailer to test out this year making my 7ft bike (like yours) plus 6ft of trailer into a 13ft 'train' going down the highway.


If I was helping you with your build and you decided to stick with RWD and not consider 2WD (budget, complexity, etc - totally understand) I would suggest:

-1500watt Rear Hub Motor with cooling solution within - example - https://ebikes.ca/shop/electric-bicycle-parts/motors/rh212-std.html - $20 extra for 'statorade' is well worth it
-Controller, computer, PAS or torque sensor in the bottom bracket of your choice.
-2 (or more) 17AH down tube batteries or a large single triangle battery if it has enough Ah/Wh for your day to day needs.

I'm helping a buddy build a cargo bike of his own (used Big Dummy) with a single RWD motor and 3 x 17AH batteries to replace his car just like you are considering. He lives in Toronto during the warmer months and Florida in the winter so even if he has to go 'slower' up hills occasionally he will be fine with the 52V 1500Watt motor and with the 3 batteries he will have 120kms/75miles of distance per day estimated, little less with a lot of cargo loaded on.

Any questions let me know and I will help. I log in every few days so if I'm slow to reply I apologize in advance.

 

[hulk]

Shaun, thanks for the info, greatly appreciated it. What are your thoughts on placing the battery in the front basket on a cargo bike VS the down tube (triangle part of the bike).
I kind of like the idea of having two water bottles mounted in there and placing the 52V 20a battery in front basket and covering it, making it look totally stealthy. The bike has rear wheel covers so a kid's feet can't reach into the spokes so it would cover up like 95% of the rear hub motor.

The basket mounts to the actual frame of the bike, not the handlebars. Last night I filled up an old whey protein bucket with 14 pounds of water (about the the battery pack weighs) and placed it in the basket and rode around with it and it seemed to make the bike even more stable. Just wondering what your or other members thoughts were on this.

 

[CdnShaun]

Shaun, thanks for the info, greatly appreciated it. What are your thoughts on placing the battery in the front basket on a cargo bike VS the down tube (triangle part of the bike).
I kind of like the idea of having two water bottles mounted in there and placing the 52V 20a battery in front basket and covering it, making it look totally stealthy. The bike has rear wheel covers so a kid's feet can't reach into the spokes so it would cover up like 95% of the rear hub motor.

The basket mounts to the actual frame of the bike, not the handlebars. Last night I filled up an old whey protein bucket with 14 pounds of water (about the the battery pack weighs) and placed it in the basket and rode around with it and it seemed to make the bike even more stable. Just wondering what your or other members thoughts were on this.

Great idea. Because the basket is attached to the frame and therefore doesn't 'steer' with the handle bars, it's a solid place to install the battery in my opinion. A hardshell battery ideally (versus a shrink wrap type battery that offers less protection to the cells) and a bit of padding to further protect the battery along with 'wedge' it in place to prevent shifting around would be awesome.

As you may have noticed there is lots of choices for 52V batteries of all shapes, sizes and Wh's. 20AH is just over 1000Wh of juice for your cargo bike and will give you a 50-80km range on a single charge to 90%.

The batteries I went with are the EM3EV 23.8Ah 14S7P with Samsung 35E cells. 1200Wh per battery, hard shell, smart BMS, every cell is fused (for safety)...just a great choice all around. 4-8 week delivery time from China however so don't be in a hurry if you consider this option for yourself.

EM3ev 52V (14S7P) Hard Case Ebike Battery Pack | EM3ev

EM3ev 50V (AKA 52V, 14s7p) Rectangle Ebike Battery Pack uses top quality official Samsung and LG Chem cells. With safety features not found elsewhere.

em3ev.com

Here's my favorite photo of my in progress project of the cargo bike I described. You will see 2 of the 4 batteries I was using that day on the right side for example installed in the custom frame I built. I took this photo at 150kms into a 190km ride, ended with 20% capacity remaining and I have 2 more batteries now added in the build.

Small trick for water bottle mounts - take a pair and zip tie them together just behind the bike stem for an easy to grab location of 2 bottles. You can see one of my purple signature bottles in my photo for example.


Looking forward to your completed build story and photos! Good luck and hope you're riding electric soon.

Shaun

 

[tomjasz]

I just retired two EM3ev batteries purchased in 2014. And have one new 35E pack. I also bought a 21700 50E pack that totally outperforms the 35E pack. See @Jenny Mao She suggests 21700 cells are a better price too!

 

[tomjasz]

I just retired two EM3ev batteries purchased in 2014. And have one new 35E pack. I also bought a 21700 50E pack that totally outperforms the 35E pack. See @Jenny Mao She suggests 21700 cells are a better price too!

To be clearIm happy with my two new 35E packs but the 50E, are 5000mAh! 9.8A! I’m getting great mileage and consistent power. 48V and 20+Ah would be sufficient. IMO. I’m a Satiator user from their introduction. I recently added the latest version. I believe they have more than paid for themselves. One will be mounted on my trike build. Solar panel basket lid for charging lights and accessories battery. My new “car”.