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Is 72V Safe?

Old 05-06-21, 05:51 AM
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The Big Wheel
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Is 72V Safe?

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.
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Old 05-06-21, 07:16 AM
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Higher voltage batteries have more cells in series and so the chance of failure is higher. I'm not sure they are talking about shock hazard, although in some circles 48v is considered the limit where voltages become lethal.
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Old 05-06-21, 08:28 AM
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I'm a Chemist, not electrical expert:
It's current (equals voltage divided by resistance), not voltage that is dangerous. However the resistance of the human body changes because of humidity etc, so voltage is the parameter that is prescribed. For ultimate safety 36V is the number many use for direct current, while 60V is where extreme care is necessary. You should receive more comments probably from those more erudite.
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Old 05-06-21, 09:37 AM
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You can't deliver the current unless you have enough voltage. Any voltage is safe if it can't deliver much current. But a bike battery can deliver a heck of a lot of current.

Like I said, as a rough rule, 48 v is the highest voltage that is considered safe. That was what telephones used and people think that's the origin of that rule of thumb. Although I have seen people say it's too high. It really depends on the circumstances. For example, if a good ground isn't available, then it will just raise your skin potential.

I don't think I would want to short 24 volts through my body if the battery could deliver a lot of current. Just because it wouldn't kill doesn't mean it wouldn't hurt.
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Old 05-06-21, 09:46 AM
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The basic limits of the human body, while they vary, are known; therefore any battery sold to power an e-bike has enough current potential (thought that was a given).
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Old 05-06-21, 10:56 AM
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Originally Posted by The Big Wheel View Post
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.
Below 50V, the resistance of your body will prevent a lethal shock from occurring. That's a guideline from most electrical safety stuff. A 48V battery is technically above that, 54.6V at full charge, but it's close enough. OSHA requires that electrical contacts be guarded for anything above 50V.
The biggest risk with any ebike battery is arc-flash, and the higher voltage does make it more likely to arc.

In summary, yes, a 72V battery is definitely more dangerous, but it's not drastically more dangerous. Large lithium ion batteries are scary, treat them with a LOT of respect. Look up arc-flash and batteries if you want to get a sense of just how dangerous ANY large battery is. You can get significant arc-flash burns from a 12V car battery.
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Old 05-06-21, 05:02 PM
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Electric cars have batteries with voltages in the 400 volts range. So no, it isn't like the voltage is going to jump out and bite your passenger.

Your concern probably came from a post I made some time ago. I thought 60V was some legal definition of low voltage. Turns out it was an ebike blogger who claims 60V is often considered the limit of what you can touch with your fingers and not get shocked. Voltages below 60V won't penetrate the resistance of your skin whereas greater voltages might do so.

Technically 10 milliamps through your heart is considered enough to stop it beating. How you get that to happen has too many variables to list. Suffice to say the higher the voltage, the greater the chance of it pushing that 10 milliamps through your heart.

I think the bigger worry with bike batteries is the higher voltage can create a much more substantial arc if something shorts out. Google pictures of Kentucky fried fingers

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Old 05-06-21, 08:59 PM
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What motor are you going to use. heat for the internals could become a problem.
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Old 05-06-21, 10:05 PM
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72V nominal is really 84V at full charge. You probably need to be careful. If you start pulling at your battery wires in a torrential rain, you could have hands wet enough to feel a jolt. With enough bad luck against you, you might get hurt.

.
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Old 05-07-21, 10:38 AM
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As I understand it, 60V is the point at which an e-bike becomes an e-motorcycle. Not even a moped, a full on motorcycle. A light electric vehicle in other words. Serious business. Besides the motor and battery there is a controller that must also be rated for 72V. I don't know ... unless sourced from some of the lowest of the low off brand take your life in your hands shady sources, the cost of a package like that should be pretty daunting. Like enough to dissuade casual consideration. 90lb women seem to manage their Mundo's well enough on 48V. 52V really ought to be satisfactory. Leave the stuff above 60V to the hooligans that build stealth motorcycles out of bike parts and vlog the carnage on YouTube as a side hustle.
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