I know what the 12volt battery powers but it still doesn't make sense to me why it's even needed, especially since it's a power source with such an unreliable limited life. This power can come from the main battery pack, powered down to 12volts. Obviously I just don't understand how it all works, maybe someone could explain it to me.
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Why is the 12volt Battery Needed?
- Thread starter Resist
- Start date
UncertainTimes
Active Member
Because Big 12V Battery Industry wants you to keep sucking on its teet
Oh kind of like how we still don't have good airless tires, even thought the technology exists.
Gauss Guzzler
Safety Score = 42
The 400V system needs to be able to *completely* disconnect from the car for safety in the event of a crash, fire, flood, overload, or GFCI incident. And when that happens you're really gonna want working headlights, power steering, power brakes, airbags, wipers, windows, and door handles. Imagine if a chafed wire somewhere allowed 400V to get onto the car body, how could the system protect you from electrocution while still providing brakes and steering if it didn't have a separate 12V battery?
Also, it takes a lot of current to run all that 12V stuff simultaneously (lights, windows, radio, steering, brakes, etc.) so it'd take a pretty beefy and expensive DC-DC converter to handle it all. And that big ol' converter circuit is going to waste a lot of power when it's just sitting there all day looking for your key signal. That's also why Tesla's system recharges the 12V battery in a crude on/off cycle rather than a constant trickle.
Note that fossil cars have this same redundancy for some of the same reasons. The alternator will power the car if the battery fails and the battery will power it if the alternator/engine fails.
Also, it takes a lot of current to run all that 12V stuff simultaneously (lights, windows, radio, steering, brakes, etc.) so it'd take a pretty beefy and expensive DC-DC converter to handle it all. And that big ol' converter circuit is going to waste a lot of power when it's just sitting there all day looking for your key signal. That's also why Tesla's system recharges the 12V battery in a crude on/off cycle rather than a constant trickle.
Note that fossil cars have this same redundancy for some of the same reasons. The alternator will power the car if the battery fails and the battery will power it if the alternator/engine fails.
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I understand what you said but, couldn't Tesla have just partitioned a small portion of the pack dedicated to the 12volt stuff and keep it separate from an accident disconnect?
CyberGus
Not Just a Member
They did. It's called "the 12v battery module"
I said the pack, the 12volt battery module is not part of the pack.
ucmndd
Well-Known Member
You think replacing a 12v battery is an inconvenience now? Careful what you wish for…
That will make it harder to manufacture, service, and the main battery is high voltage, can’t really used to power smaller sensitive computerrs
Gauss Guzzler
Safety Score = 42
Sure, they could tap into the pack to pull 12V off a small group of cells but then those cells would be subjected to many more charge cycles than the rest of the pack. And since the whole pack is only as good as its weakest cell, that could become a pretty significant compromise for the pack life span.
Also cell balancing is a wasteful process typically only done while charging as it's most critical when the cell approaches maximum voltage (hence the phenomenon of "recovering range" by occasionally charging to 100%). If they were to pull 12V from a group of cells they would need to implement some way to get that group back in balance while driving or else your usable main pack SOC would be limited to that of the 12V subset SOC.
Also cell balancing is a wasteful process typically only done while charging as it's most critical when the cell approaches maximum voltage (hence the phenomenon of "recovering range" by occasionally charging to 100%). If they were to pull 12V from a group of cells they would need to implement some way to get that group back in balance while driving or else your usable main pack SOC would be limited to that of the 12V subset SOC.
It's doubtful that it would be harder to manufacture, it's just changing a few connections at the end of the pack. Since you wouldn't using the whole pack, just enough cells for 12volts, high voltage wouldn't be an issue.
ucmndd
Well-Known Member
And what do you do when those “special” 12v cells inevitably fail?
I appreciate your response but still feel Tesla could have solved this issue and probably more about revenue. You include an item that has a known limited life and in a fairly uncommon form factor, people will then buy replacements from you. The downside for us is that these batteries often go bad at the worst time, and often. without much notice.
I have a Jeep and it's battery is 8 years old, haven't had a problem with the battery...yet. The fact that so many Model 3 12volt batteries are having short lives, considering they aren't exposed to heaving cranking loads, is concerning and probably why Tesla now uses Lithium Ion 12volt batteries.
Apparently you are an electrical engineer, manufacturing enginee, and robotic engineer
What do you do when the entire battery pack fails? I see no reason that there would be any early failures and certainly not as often the current 12volt lead acid batteries have been failing.
ucmndd
Well-Known Member
Replace it at great cost, labor, and inconvenience - orders of magnitude greater than replacing an easily accessible 12v lead acid battery every 4 years or so.
Lol, are you new to Tesla?
No need to be a sarcastic turd. I just don't believe setting aside a portion of the battery pack for 12volt needs would fail any sooner than the whole pack would. The fact is that the 12volt lead acid battery Tesla decided to use has a early failure rate. Granted they changed to a Lithium Ion 12volt battery but left those of us with the lead acid batteries out in the cold, by only replacing them with another lead acid battery should it failure during the warranty.Replace it at great cost, labor, and inconvenience - orders of magnitude greater than replacing an easily accessible 12v lead acid battery every 4 years or so.
Lol, are you new to Tesla?
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