311–320 of 504 posts
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Wouldn’t you just start the car if the HV was flat and the LV was charged? If they’re both flat I agree, I’m curious what you do.
There is no 12v starter motor, it always just uses one of the large motor-generator units to start the engine, and in the Prius drive chain, they require 600v (boosted from 200v from the HV battery). So if the HV battery is flat, you can't start the engine to charge it.
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We seem to stick with 7-smooth numbers, and really extra prefer highly composite numbers ( https://en.m.wikipedia.org/wiki/Highly_composite_number ) for things we chose before the age of calculators. 3 and 6 were the standard voltages for consumer stuff back in the days of people needing to buy multiple packs of AA cells every year
Are we not all still buying multiple packs of AA batteries every year? Or maybe it's just those with kids.
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Why is it inherently noisier?
Faster slew rate for all switch supplies in the system. Nothing directly runs 48v.
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I think another big part of it is that DC switches tend to get expensive above 12V. Cheap AC switches work fine at higher voltage, because the arc is self-extinguishing as it passes through zero twice per cycle, but DC doesn't do that so you can end up with an arc that doesn't extinguish itself, which, aside from not turning the thing off when you want to, burns out the electrical contacts.
Now we have MOSFETS and IGBTs that can switch DC without sparks. I suspect most DC switching in the Cybertruck is relayed through these. (Except the main contactor and pyro fuse of course.)
Which is to say, yeah, it's a non-issue for pretty much everything. Even in the 12v realm, new BCMs have so few relays anymore, almost everything's done with onboard FETs and software.
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We seem to stick with 7-smooth numbers, and really extra prefer highly composite numbers ( https://en.m.wikipedia.org/wiki/Highly_composite_number ) for things we chose before the age of calculators. 3 and 6 were the standard voltages for consumer stuff back in the days of people needing to buy multiple packs of AA cells every year
Are we not all still buying multiple packs of AA batteries every year? Or maybe it's just those with kids.
And non techies. They always have a few disposable battery flashlights hanging around it seems like.
I'm ignorant and curious: why 48V specifically? Why not 40V? Why not 50V? Is there some ideal ratio that makes DC-DC conversions more efficient or something? For comparison, Makita (and I think some other power tool makers) have augmented their 12V lineup with a newer 40V platform for tools that need a bit more oomph. I guess there's no need for them to interoperate with other DC voltages, so maybe that explains the…
For batteries it tends to be multiple of the battery cell voltages. So 48v is probably so you could wire 4x12v batteries in series but that is a guess. With tool voltages this tends to be a multiple of the nominal battery voltage (rounded for marketing).
Lithium batteries do not have 2.2v cells; typical li-ion cells are 3.7v. So there's no particular reason why you should stack them to 48v, except that there's already some amount of industrial capacity for 48v components due to the preponderance of 48v golf carts.
Of course, these systems don't work at exactly 12v or 48v or whatever; voltage on the bus fluctuates with state of charge (and state of charging).
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Dumb question but why does deciding to run at a higher voltage decrease the amount of wiring necessary? And why not increase it even further then?
60 volts DC is a safety threshold. You start to move current through tissue at higher voltages.
60 volts is nothing UNLESS you are completely wet or sweaty.
"It is estimated at 150 ohms for completely wet skin (in water), 1000 ohms for sweaty skin, and 100,000 ohms to 500,000 ohms for dry skin."
Assuming a worst-case scenario with dry skin providing a resistance of 100,000 ohms, fatality becomes a possibility if the current exceeds 50 mAmp.
Therefore, the lethal voltage would be above 0.05 (50 mAmp)×100,000=5000 Volts. [1]
So if you are wet or sweaty, it could be 7.5V to 50V that gets dangerous.
So it makes sense why 60 volts is a safety threshold, especially for those that live in Florida or Arizona.
[1] https://www.scienceabc.com/humans/how-many-volts-amps-kill-y...
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The reason to not keep going higher is purely practicality because stepping down bigger voltage gaps for lower voltage applications begins to become less efficient/more waste heat. If you design 100v computers, monitor, headlights, seat adjustment motors, window/windshield motors, pumps, etc. you could cut these 48v wires in half again but if you end up stepping everything down anyway it loses its utility. The safety…
Just don't try that with wet hands.
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No. For safety, hybrids and electric vehicles physically disconnect the high-voltage battery from everything when it's off with massive relays. So you can end up in the somewhat stupid situation where your high-voltage battery is fully charged, but you can't do anything because the tiny 12v motorcycle battery is flat. Starting simply requires enough 12v current to close a relay, the amps going though the "jumper pins…
That describes the situation where the 12V battery is flat but the HV battery is charged. But what happens if the HV battery is flat on a non-plug in hybrid? Can it be charged from 12V?
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Dumb question but why does deciding to run at a higher voltage decrease the amount of wiring necessary? And why not increase it even further then?
Watts = Amps * Volts (or, more scientifically, P = I * V) If something draws 48 Watts of power, you can either supply it with 4 Amps @ 12 Volt, or 1 Amp @ 48 Volt. It's the amps that determine how thick the wiring needs to be, so by lowering the amperage, you lower the amount of wiring needed. I believe the only reason cars are 12 V is because that's a practical voltage to build lead-acid batteries at (which is actua…