Safety of 72v vs 48v vs 36v

[leew]

Hi, I am considering building an eTrike (see separate thread) I was initially thinking of using 48v as I believe 50v DC is the max safe DC voltage which won't kill you if you accidentally touch it. In the other thread someone is suggesting I use a higher voltage for better performance for my parameters. I have since discovered that even at 48v nominal, the charging voltage is closer to 60v so could still theoretically kill you? (Of course I am aware that if you stick two needles in your chest then even a small voltage across them can kill you though I am talking about accidental contact with hands etc). So to be under 50v I believe I would have to use 36v nominal which would of course impact performance. Is anyone here aware of the dangers of 72v nominal on an ebike, I would mainly be worried about water getting into a connector or a wire chafing through and touching the frame something like that and causing a zap. Has anyone had or know anyone who has had a zap off 72v or a similar voltage from an e-bike? As you can see I am a bit weary of high voltages and am wondering if the extra benefit is worth the risk.

Many thanks, lee.

 

[LI-ghtcycle]

Do you have a link to the other thread you've referenced?

The specific application of your build would help, but generally speaking, as volts go up, so does efficiency depending, but 48V is really pretty good for most applications, in my own quest for efficiency, I will be going to 18s (72V) LiPo as it will give me considerably more efficiency even with nearly the same Watt Hours of battery VS 48V with a semi-recumbent cargo bike while climbing a long hill (2 miles, averaging 7% grade, with a gross weight around 450lbs) so it can make a big difference.

Reduce that gross down to just 330 lbs, and I can get by with 48V just fine, so if you're using a something like a stream liner in your pic, 36V is common as that is all that is needed for many who want to pedal along, and 48V might be just fine.

Be sure to use Justin's calculator on ebikes.ca, it has a lot of parameters to narrow things down.

http://www.ebikes.ca/tools/simulator.html

 

[hydro-one]

i have touched up to 90vdc without any sensation at all to my hands . i think the speeds you achive will be more dangerous by comparison to directly touching lmao

over 100 you start to feel it

disclaimer: i am not an electrician

now as for shorting high voltages i dont reccommend it ! https://endless-sphere.com/forums/viewtopic.php?f=2&t=31627

 

[Sunder]

Hi, I am considering building an eTrike (see separate thread) I was initially thinking of using 48v as I believe 50v DC is the max safe DC voltage which won't kill you if you accidentally touch it.

It's 50v AC, or 120v Ripple Free DC.

Unless you pierce your skin, it is EXTREMELY unlikely you will kill yourself using 72v.

https://en.wikipedia.org/wiki/Extra-low_voltage

 

[Eskimo]

I use 54V and it"s definitely no biggie. Of course you have to be careful. Limiting your DD on a velomobile to 45 km/h or thereabouts may sound reasonable, but in reality it"s not working that well. After your DD shuts down, you are not going much faster even on a downhill. DD-motor creates quite significant drag on high speeds. That"s why i was talking about high voltage. You don"t have to use it to accelerate you but it allows you to go fast on downhills and nice tailwind flats.

 

[wesnewell]

The likelihood of being electrocuted from an ebike battery is almost nil. I've been using 75V-100V packs for almost 5 years now. Electricity always takes the path of least resistance, which is close to 0 in operation. The human body is close to 100K ohms. The bike frame should not carry either a positive or negative charge from the battery pack.

https://www.youtube.com/watch?v=9iKD7vuq-rY

 

[dogman dan]

Fairly simple to avoid a situation that would result in your getting a shock from your pack.

But in general, unless you really need 72v for higher, more motorcycle like speeds, 48v is all you really need. Very typical to go to 72v once you want 3000 watts or more, just to keep wire sizes reasonable. But if you are building for more like 2000w max power, 48v is all you need. 48v 40 amps is cheap, the wires are only 12g, the controllers easy to find and affordable, the chargers tend to cost less.

But if you need the wattage, or the speed, then 72v makes total sense so you don't start needing 8g wire and such.

 

[Drunkskunk]

48 volts is often considered "Safe" because it's at the threshold were most people can begin to feel it.
What you're really feeling is the current passing through your muscles, causing small contractions. Your body is around 10k ohms resistance. It can be higher or lower depending on how dry the skin is, or how naturally conductive your body is, but 10k is a general average. You need 5 miliamps of DC passing through your body before you can sense a shock.
48v / 10k ohm = 0.0048 A = 4.8mA, ~5mA

So at around 48 volts, you can just hit the point were you feel a slight shock through your skin.

It takes at least 10miliamps passing through your body to cause you to lose control of a muscle. this is the threshold where it actually becomes dangerous, and can cause you to be burned, or have your heart stopped. If you do the math, 10ma X 10K ohm = 100 volts. So in theory, 100 volts is the danger point for most people. However, that can be higher or lower depending on your individual resistance. Since this number is twice what the minimum voltage you can feel was, you can test for it. At whatever voltage you can start to feel a shock, That is half the voltage that could be dangerous for you.

That doesn't mean, say, 80 volts is going to be fun to touch.

 

[Dogboy1200]

Here's a great article that covers your questions https://www.electricbike.com/52v-battery-3077-fet/

 

[Noamsal]

higher voltage = higher efficiency, you could get same performance with less Amps running through your controller
to keep things safe you could allways get a controller with safety options like traction control and soft start... adapto has these features

 

[wesnewell]

Just for the heck of it I just grabbed the ends of my 10ah 24s rc lipo pack before charging it. Voltage measured 92V. Care to guess what happened? My wife called the doctor. Not for me, but to set up her normal appointment. Me, nothing. Not even a tingle. Exactly what I expected. Body resistance is just too high to allow current flow from an unloaded battery source. Now short the ends together is another story. Well, actually you won't be able to since the explosion will quickly evaporate the connectors and leave a few cauterized burns on you. Disclaimer: In no way am I suggesting you try this. This is just my personal experiences.

 

[Deafcat]

Here's a great article that covers your questions https://www.electricbike.com/52v-battery-3077-fet/

Solid info here.

 

[leew]

Here's a great article that covers your questions https://www.electricbike.com/52v-battery-3077-fet/

Thanks for the info

 

[leew]

Just for the heck of it I just grabbed the ends of my 10ah 24s rc lipo pack before charging it. Voltage measured 92V. Care to guess what happened? My wife called the doctor. Not for me, but to set up her normal appointment. Me, nothing. Not even a tingle. Exactly what I expected. Body resistance is just too high to allow current flow from an unloaded battery source. Now short the ends together is another story. Well, actually you won't be able to since the explosion will quickly evaporate the connectors and leave a few cauterized burns on you. Disclaimer: In no way am I suggesting you try this. This is just my personal experiences.

Would that happen even if there is a fuse in the battery, would the fuse not pop before the connectors evaporate?

 

[wesnewell]

Of course the fuse would blow if you had one on the battery pack. My fuse is not on the battery packs, but in the wiring going to the controller.

 

[ScooterMan101]

Reading that article, it looks like an Infineon 3077st-4 ( 12 fet ) controller could then not only use a 14 s battery pack fully charged to 4.2 volts per cell ,
But
a 16 s pack as well , if, it was charged to a max of 4.05 volts per cell . ? ? ?

So could the 16s pack be the next best thing , after, the newest best thing of a 14s ( 52) volt pack ?

Here's a great article that covers your questions https://www.electricbike.com/52v-battery-3077-fet/

Solid info here.

 

[kdog]

Grab your DMM and take a few readings of body resistance hand to hand. Do it after exercise, when you're wet, dry etc, kids have many times less resistance due to thinner moister skin etc, so take this into account. When I was building my 20s pack, normally I had no probs except after washing the dishes ( wtf was I doing washing the dishes!) nice wet wrinkly hands gave me quite a tingle- uncomfortable but ok. Still a long way off dangerous.

 

[dogman dan]

And, like I said, fairly easy to avoid it. Use a fuse good, but if you have a short before the fuse on your wires, you still get a nice spark, which can set something flammable on fire, burn your house down, etc.

So mostly, just make sure you have sound, protected enough wiring so you never get that short, never get that shock. Protect the battery itself, so it doesn't rub till it shorts. Voltage matters none for this aspect of it. In 1976, I had a truck burn because I got a nice short on a 12v wire from the alternator to the battery. All metal trucks in those days, so the result was just every wire melted. Modern wire harnesses do have built in fuses all over btw. But not on the big wire I had short.

But still a disaster. Avoid the short is the main thing. Build it smart. Then you have close to zero chance of a shock from touching the frame.

I wouldn't hesitate to run 72v because 48v is safer. But I would hesitate to run 72v if a 48v battery is cheaper, the charger is cheaper, etc.

 

[leew]

I seem to remember reading somewhere that it was recommended you put your main fuse in the middle of the battery, so if you had a 16 cell battery for example, the fuse would go between cells 8 and 9, is that a good idea?

 

[Drunkskunk]

I seem to remember reading somewhere that it was recommended you put your main fuse in the middle of the battery, so if you had a 16 cell battery for example, the fuse would go between cells 8 and 9, is that a good idea?

Put it as close as practical to the battery. Between cells is great but not always practical.
The best place is someplace close to the battery, since it's function is preventing a short, and a short is probably going to be in the wiring. If the battery is has 3 feet of wire to the fuse, and the short happens 2 feet out along the wire, the fuse does no good. If the fuse is the first thing on the wire after the battery, it can protect the battery from a short 2 feet out along the wire.