Why do EV's have high voltage low amps?

[Niekname]

Hello all, first post here. I have always been fascinated with a self build EV's, and right now I am looking into building one myself. I don't know yet if I am gonna build a bike, skateboard, onewheel etc. I come from the RC world, and when looking at typical motor, ESC and battery combinations, I noticed they have way higher voltage and lower amps compared to R/C, why is this?

For example, my 1:8 R/C has a Team Corally branded Hobbywing Max 8 combo. The ESC has 120 A cont. and accepts batteries up to 6s. So ~22V*120 = 2640 W (continuous!). When comparing to EV conversion kits and builds I have seen here, that is enough wattage to power a E bike or different EV. But I have never seen someone build a EV with R/C electronics, so it probably wouldnt work to well, otherwise someone would've done it already. Or at least that is what I am thinking. So can you explain to me why my R/C electronics do have enough wattage, but not enough power? I am thinking it has something to do with the voltage and amps, because I generally se EV's have a higher voltage (36-96) and lower amps (15-100). That's just what I have observed so I am curious if there is a reason behind this.

 

[harrisonpatm]

Couple reasons. First, you're comparing to rc, 120amps continuous. Is it really continuous though? How long is that ESC actually drawing 120amps continuously? I would guess a minute at most, certainly for short bursts. But RC batteries and applications tend to not last that long, due to battery capacity. Whereas EVs are expected to be relied upon for continuous, regular transportation. 1hr, 2 hr, ect.

Another reason is amperage and wire gauge. Going higher volts, lower amps, means you can get away with thinner cables. And manufacturers especially like this, as thinner cables cost less.

There's lots of reasons. But that's not to say it's totally impossible to make an EV using RC components or ideas. You just need to decide what's best practice for your intended application.

 

[Niekname]

Couple reasons. First, you're comparing to rc, 120amps continuous. Is it really continuous though? How long is that ESC actually drawing 120amps continuously? I would guess a minute at most, certainly for short bursts. But RC batteries and applications tend to not last that long, due to battery capacity. Whereas EVs are expected to be relied upon for continuous, regular transportation. 1hr, 2 hr, ect.

It is rated for 120 A cont. and 180 A burst, I have never tested if this is really continious and also have never seen someone test it. I get that you can't use RC batteries if you want to have a decent runtime on an EV, but because it uses less cells, you could create a 6s5p, that uses the same space as a 10s3p. Because it draws more amps it should get the same runtime in the end, but I haven't done the math

Another reason is amperage and wire gauge. Going higher volts, lower amps, means you can get away with thinner cables. And manufacturers especially like this, as thinner cables cost less.

It just seems strange to me that EV manufacturers do this to save costs, but RC manufacturers don't. I can't think of a reason that doesn't aply to both use cases, but there has to be one, right?

 

[harrisonpatm]

I get that you can't use RC batteries if you want to have a decent runtime on an EV,

That's not necessarily true. Cells are cells, and if the cells work for your application, then you can use them. There are plenty of people still on this forum who have ebike packs made from RC lipo cells. (@neptronix , don't you have one?) It goes the other direction too: I briefly look into the FPV drone world, and I noticed that some users are now preferring to switch over to 18650 or 21700 high-power cells for their drones, because even though they experience more voltage sag and are less energy dense than some lipo packs, users are finding that they last longer (cycle life), are more durable, and are easier to build into packs. Just depends on what you want.

It is rated for 120 A cont. and 180 A burst, I have never tested if this is really continious and also have never seen someone test it.

I don't what to s*&% on products that I know nothing about. However, you'll find if you dig through this forum, there's a big difference between "rated for" and "can actually perform." We discuss controllers, batteries, chargers, etc on a regular basis, because even well-respected brands can put out a product that doesn't live up to specs.

To answer your question about higher voltage, we can refer to Ohm's law, and we know that at a lower voltage, you'll see more resistance. More resistance equals more wasted heat. I was thinking that perhaps that's not as important when it comes to RC, because when you're dealing with something so small all over, your cable lengths are never going to be very long anyway, so it's okay that they're lower voltage. But once you start stinging your cables down the length of a stakeboard, bike, car, resistance goes up, cables need to get thicker, cost goes up.

It just seems strange to me that EV manufacturers do this to save costs, but RC manufacturers don't.

RC battery cable lengths are less than a foot. The shorter a cable run is, the less heat buildup you'll get. I put about 20ft of battery cabling on my motorcycle build for the main battery positive/negative. Costs add up when you start making bigger things.

There's also the rest of the infrastructure to consider. RC packs are frequently 2-6s, so the ESCs for RC are almost all made for 2-6s, so the motors are built for the voltage range of 2-6s. It's what most people do, therefore its what the market provides at the most common price point. You'll see that for the ebike world too: the highest voltage controllers you'll commonly see for high-powered motorcycle and ebike builds are in the 72-120v range, therefore it's harder and more expensive to get BMS's for higher than 120v. It's not that they don't exist, its just that they break into a price range that makes them less available for hobbyists. If I wanted to get a BMS and a controller for my motorcycle higher than the 120v that I used, I would have to start shopping in the range of products for electric car parts.

You said that you want to build an EV, but weren't sure about whether to make a bike, or skateboard, or onewheel... Once you start researching specifically which one you want to build, you'll find that there's a common voltage range that most people work with, and people who go higher or lower have to be more DIY about the build, because going outside of the usual voltage range changes the whole spectrum of battery, controller, motor that you'll need.

 

[stan.distortion]

There's components too, larger EVs tend to operate in the range of existing high current components. IGBT's for example, the industrial equivalent of the MOSFETs in most light EV controllers are usually rated for 600v or 1000v so it makes sense to work in that range and sourcing high capacity components for lower voltages can be difficult, welders work in the 20-100v range but solid state welding plants tend to use much higher voltage rated components because that's what's readily available.

 

[Chalo]

It just seems strange to me that EV manufacturers do this to save costs, but RC manufacturers don't.

RC toys don't have a lot of real estate to cover with long cables, and most can tolerate localized wire heating because they get decent ambient airflow.

When you're talking about a bike, scooter, car, or whatever practical vehicle, having to use big cables that run significant lengths costs you money, nonproductive weight, ease of wire routing, and disproportionately large and expensive connectors and controllers.

High amp RC controllers are known to be failure prone when coupled to heavy loads, e.g. a cargo bike rather than a big prop. That's a clue that their ratings are not transferable to all applications.

 

[E-HP]

I noticed they have way higher voltage and lower amps compared to R/C, why is this?

The high cost of copper, which is a big consideration for the design of many electrical systems. An example is solar PV systems where you can save a huge amount of money wiring panels in series to raise the voltage, so you can maximize power transfer with less loss and greater efficiency using smaller conductors. Utility lines run for long distances with voltages raised as high as 500kV for the same reason.

 

[wturber]

It is rated for 120 A cont. and 180 A burst, I have never tested if this is really continious and also have never seen someone test it. I get that you can't use RC batteries if you want to have a decent runtime on an EV, but because it uses less cells, you could create a 6s5p, that uses the same space as a 10s3p. Because it draws more amps it should get the same runtime in the end, but I haven't done the math

It just seems strange to me that EV manufacturers do this to save costs, but RC manufacturers don't. I can't think of a reason that doesn't aply to both use cases, but there has to be one, right?

It's a matter of scale. The larger cables are more significant at larger scales. It is also easier to combine more cells at larger scales.

 

[YellowBuzzaroundEX]

My understanding is that higher volts, lower amps is a more efficient way to generate a given amount of watts (power).

For example:

24V x 50A = 1200W is my scooter that's a designed for AGM batteries. This 24V system could also work with (and be somewhat more efficient with) LiFePo4 (aka LiFePo) battery or batteries.

48V x 25A = 1200W would be a more electrically efficient way to have a 1200W scooter. Lithium batteries are more suitable (than AGM) for making 48V batteries.

96V x 12.5A = 1200W would be even more efficient. This requires lithium battery or batteries because AGM are not suitable for more than 48V.

192V x 6.25A = 1200W is even more efficient.

Notice that the amount of power (watts) is the same in each example above. With the use of rpm reduction gearing, the final drive rpm at the tires, speed, and torque can all be identical at the tires for each example. However, higher voltage, lower watts is more efficient.

More efficiency can be used to obtain more range, less heating of battery, motor, wires, physically smaller lighter motors and wiring. A lighter vehicle, and possibly a less expensive vehicle.

An often overlooked advantage of lithium battery or batteries is that lithium is better suited (than AGM) to making higher voltage batteries. Higher voltage batteries are more efficient at discharging and higher voltage electricity travels more efficiently through wires. This allows the creation of a more efficient system. Higher voltage batteries can be correspondingly lower amperage.

Higher voltage is also faster and more efficient for charging.

LiFePo (LiFePo4) version of lithium battery is slightly less energy dense than other types of lithium batteries, but LiFePo is much safer, which is important to avoid risk of fire.

I don't want to risk getting burned. So LiFePo4 is the version of lithium batteries I prefer. There's still some risks, but LiFePo is safer than other types of batteries (incl AGM) AFAIK.

I'm not an expert. I'm an enthusiast amateur.

 

[jonescg]

EV manufacturers using 800 V architecture are doing so for fast charging. 200 kW into a battery at 750 V is 266 A - manageable with 35 mm2 cables. Half the volts means twice the amps for the same power (hefty cables and connectors) or, accept half the charge rate. Other than charging, it carries no substantive benefit.