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12V Usage Info? Potentially avenue to adding a couple of miles of range?

Discussion in 'Model S: Battery & Charging' started by wk057, Aug 30, 2015.

  1. wk057

    wk057 Senior Tinkerer

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    Greetings,

    Not sure if this has been considered elsewhere, but figured I'd share a random thought.

    I was thinking about the Model S's 12V system and I'm curious: Does anyone have any data on how much power is used by the 12V components of the car while driving?

    I know the instrument cluster, the 17" screen, fan blower, lights, sound, windows, etc all use the 12V system. Based on some of the vampire drain numbers people have seen (easily > 100 watts) I'd be willing to bet the 12V portion of the car uses a decent amount of power while driving, and I'd say something like 200W wouldn't be too crazy and probably a low estimate.

    So, driving for 3 hours would be 600Wh used by the 12V systems. That's two miles of rated range.

    It doesn't seem like it'd be difficult to throw together a ~1kWh lithium-ion pack that would be used to offset this 12V power drain while driving so some or all of that the power didn't come from the DC-DC converter. I would think that if you keep the voltage of the 12V system high enough so that the DC-DC thinks no power is needed, then it wouldn't push any power. So the DC-DC from my aux 1kWh battery would need to just be a hair higher than the output of the car's DC-DC I'd think.

    In any case, probably not worth it, but I could see it adding something like 1% range with somewhat minimal effort if it actually worked and the 12V system uses as much power as I figure it does while driving. It could also probably be done without the car getting angry, too, since I doubt it keeps tabs on the 12V loads in detail and just would welcome the reduction in load on the DC-DC. Unlike some power augmentation on the HV side, where the car would likely get pretty mad when things didn't add up as expected, I think we could get away with this one.

    If this is something Tesla implemented themselves by replacing the 12V with say, a 1kWh lithium battery, then they could charge this battery when the car charges, then utilize it for 12V loads while the car is in use (falling back to the DC-DC if needed) and just charge it up again using the existing DC-DC when the car is charging for a ~1% range boost at a minimal cost.

    *shrugs* Random thought. :p

    Edit: er, title fail. Should be "Potential avenue for"
     
  2. scottf200

    scottf200 Active Member

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  3. Rocky_H

    Rocky_H Member

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    I'm really confused as to how you think replacing one type of battery with another type of battery can extend range and somehow cover those electrical loads without having that energy consumed from somewhere.
     
  4. wk057

    wk057 Senior Tinkerer

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    Replacing the 12V lead acid battery (maybe 0.2kWh?) with something with higher capacity (1.2kWh worth of 18650's?) to offset the 12V loads during a trip to gain a small range boost, then recover that battery during charging instead of leeching from the HV battery during driving.
     
  5. DCWitt

    DCWitt Member

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    @wk057...do you have a source for a BMS? I like your idea and might give it a try.
     
  6. WarpedOne

    WarpedOne Supreme Premier

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    • Funny x 1
  7. efusco

    efusco Moderator - Model S & X forums

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    Seems like some roof top PV panels would be a better way to buy a couple of miles (at least in the daytime). I'm not saying it would be a good choice for recharging, but would probably balance out some of the vampire drain when parked (in the sun) and offset some of the energy use by accessories. I'm not smart enough to know how to do the math on that, but I do know that if I had 5 more miles range it would have made a few trips a lot easier and I also know that the one time I killed my battery pack that the car was in the sun when I lost 8 miles of range to vampire drain.
     
  8. Mattias

    Mattias Member

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    I think it's a great idea. Also if you were to replace the lead-acid battery, a good size (lead) weight would also be saved!

    Every % counts.
     
  9. Rocky_H

    Rocky_H Member

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    You know how you could have gotten 5 more miles? Slow down 2mph.
     
  10. roblab

    roblab Active Member

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    EXACTLY! It is so ridiculous that people are stressed about range. The car tells you how many miles you can drive. People know how far the trip will be -- or they should. So what do they do?

    They Speed. They Go fast. They Zoom around. They Repeat the mantra "I didn't spend a hundred thousand to go slower" over and over. And then publish "I could have used an extra 5 miles." When all they have to do is slow down a couple miles an hour. Oh, the pain, the punishment! Maybe if I got a different 12 volt battery! Maybe if I got solar panels! Maybe if I lost 30 pounds. Maybe if I didn't carry all that junk around with me in the frunk. Maybe if I turn my radio and my AC down.

    Come on, people. The absolute largest use of power is the drive train. Watch your buffer - that amount of charge that is more than what you think you'll need. If it starts to shrink, just slow down a mile an hour or two. And lose the stress.
     
  11. wk057

    wk057 Senior Tinkerer

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    See, this kind of post annoys the hell out of me.

    If range isn't your priority, fine. If you don't mind doing 50 MPH in 70+ MPH zones to get to where you need to go, fine. That's your prerogative.

    As for the car "telling you how many miles you can drive"....... do you own a Model S? The range meter is complete nonsense and is nearly *always* displaying more range than you can actually drive. I've never done a trip where I was able to maintain a 1:1 ratio with the dash's rated miles display for any extended period of real world driving. So, it's not all that helpful aside from a being a battery meter. I personally have it set to % now that it's an option.

    Slowing down "a mile an hour or two" in the real world is not always useful anyway. You have to drop speed significantly for a significant increase in efficiency. The difference between 65 and 70 MPH is not all that significant. The difference between 70 and 55 MPH is.

    And I'm in the group of people saying I didn't spend $100k+ to be inconvenienced. I'm not going to go slower than the speed of traffic on a trip that is already longer than normal due to charging times. And before you say it takes longer to charge because I drive normally you should investigate the numbers... you'll find that it's actually at worst equal time and at best a bit faster to charge at superchargers having used more energy and shaved time off of a trip leg. Net time is still faster driving quicker than if you drove slower and added 30 minutes to the drive and charged less (5 to 10 minutes off of a supercharge).

    If +5 miles of range opens up some more possibilities and less inconvenience, then that is a positive thing. Stop making it out like it isn't.

    Anyway, back on topic...
     
  12. islandbayy

    islandbayy Active Member

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    I touched on this idea 2 years ago. In fact, their are already drop in replacement "12v" lithium batteries that are drop in replacements for lead acid.
    While it might be ok in warmer climates, in the colder climates, a pack heater would then be required, as you cannot charge those when the temperature of the cells is at or below 32*F, as the internals of the batteries will electroplate themselves.
     
  13. wk057

    wk057 Senior Tinkerer

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    Well, I'd think it'd need to be more than a drop in replacement (currently existing) to do what I was describing. Basically would want the battery to be smart to the point where it would essentially trick the DC-DC into thinking that the DC-DC didn't need to output additional power to the 12V system by basically maintaining a higher voltage that a battery would normally.
     
  14. MsElectric

    MsElectric Active Member

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    But then you would need a BMS with heating/cooling for the 18650s wouldn't you? I though the reason they didn't do this to begin with is because the 12V chemistry is more durable with regards to not needing external heating and cooling to deliver reliable current...
     
  15. wk057

    wk057 Senior Tinkerer

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    the 18650s were just an example. Technically wouldn't need a very sophisticated BMS, especially if it had it's own DC-DC converter (which it would need to have to keep the voltage steady through discharge). Could theoretically just use cells in parallel and the DC-DC to bump the voltage instead of needing to balance cells in series.
     
  16. SirGoon

    SirGoon New Member

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    I think the best option without increasing weight significantly and adding capacity while using a reasonably safe battery chemistry would be LiFePo4 cells. The voltages (both fully charged and discharged) align reasonably well with a lead acid AGM battery in the 12v capacity. LiFePO4 is a very safe chemistry even when abused, punctured, etc. and offers moderately high volumetric energy density and good gravimetric energy density. Round trip charging efficiency is extremely high, low self discharge rates, but high costs per Wh are an inhibitor. A BMS is still a must to prevent over discharge/over charge which will damage the cells. Batteryspace (not a great supplier from a cost standpoint) offers 12V LiFePO4 batteries and they have been used in motorcycles as drop in replacements. I would size the discharge to at least match the fused 100A rating of the DC/DC on the Model S.

    Examples: Server Error
    NOTE: No BMS is included this is cells only

    Another example:
    Server Error

    Both of these examples are higher capacity then what I think the general consensus is a 33ah AGM battery currently in the Model S (note: AGM batteries will deliver significantly less capacity depending on load conditions then a typical LiFePO4 battery).

    The charge controller on the model S may not be suited to properly charge a LiFePO4 battery (intended for AGM lead acid only). But likely it is just a simple CC charge until a shutoff voltage or Imin is reached at a CV set point. It might be interesting to note whether the model S has a charge timer and whether a larger capacity would trip some internal monitoring circuit that would alert a likely failing battery condition.
     
  17. Rocky_H

    Rocky_H Member

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    @wk057:

    Quote: “If range isn't your priority, fine. If you don't mind doing 50 MPH in 70+ MPH zones to get to where you need to go, fine. That's your prerogative.”

    Nice textbook use of the false dichotomy logical fallacy. I see it often on this topic. When people suggest slowing down some from 80mph to not waste so much energy, people like you respond with the only alternative to 80mph being 50 or 55 mph, as if there is no speed anywhere in between, and the say how AWFUL and PAINFUL and DANGEROUS that is.

    Quote: “As for the car "telling you how many miles you can drive"....... do you own a Model S? The range meter is complete nonsense…”

    Yes, but that rated miles display in the front dash is not the display he’s talking about. If you haven’t found out about the projected miles and the trip meter in the energy app, you are really missing out. They show a projection of real miles that you can go based on your actual current consumption rate, not based on the optimistic rated miles thing. And the trip graph does take into account all of the elevation change and real speed limits along your route.

    Quote: “Slowing down "a mile an hour or two" in the real world is not always useful anyway. You have to drop speed significantly for a significant increase in efficiency. The difference between 65 and 70 MPH is not all that significant. The difference between 70 and 55 MPH is.”

    That has not been true in my experience. Besides, that’s why this was being brought up in this discussion. People are talking about substituting a different type of battery system in the car to get a tiny amount of range difference. The point was that if you’re going for that insignificant of a range gain, there is a very easy and painless way to get that small gain, rather than modifying your car. Also, how much difference scales with speed. At the higher speeds, the waste from wind resistance is getting really high, so toning it down from 80 to 75 makes a really big difference, whereas from 70 to 65 makes less difference. But I have found that any difference does matter over a long period of driving time.

    Quote: “And I'm in the group of people saying I didn't spend $100k+ to be inconvenienced. I'm not going to go slower than the speed of traffic on a trip that is already longer than normal due to charging times.”

    And yet you think the amount paid for the car makes it somehow immune to physics. You could pay way more for a Lamborghini or Bugatti, and they can go 200mph, but you know what? They will still, despite being really expensive, get supremely crappy gas mileage and have shorter driving ranges at those high speeds. Sorry (not sorry), but that’s life driving through air. You do always have the freedom to drive as fast as you want most of the time when you are within easy range of charging, but when you need to get the most distance out of a charge, speeding is not the way to accomplish that task.

    Quote: “Net time is still faster driving quicker than if you drove slower…”

    Now that’s a different topic. We were talking about total range and how you could get a few extra miles of total distance, not the balance of speed versus charging time.

    Quote: “If +5 miles of range opens up some more possibilities and less inconvenience, then that is a positive thing. Stop making it out like it isn't.”

    Sure, it’s a positive thing, but there are way easier and less questionable ways to do it than by changing the parts in your car. I extend range by more than 5 miles by going 73 instead of 78 on the highway.
     
  18. wk057

    wk057 Senior Tinkerer

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    #18 wk057, Sep 3, 2015
    Last edited: Sep 3, 2015
    Ugh... I feel like I'm arguing in favor of ICE vs EV here, but in reality I'm pointing out, well, reality... and apparently you don't get it.

    There is no fallacy here. I never said 80 MPH anywhere in my posts. My example was 70+ to 50 MPH, which would save a substantial amount of energy under perfect conditions (flat roads, no HVAC, no wind, etc). Unfortunately the reality of things are that slowing down is just not always the answer to more range.

    As some recent evidence, I crossed Fancy Gap four times in the past two weeks (mountain pass in southern VA). It's 7+ miles of steep grade. I climbed it at 45 MPH one time (stuck behind trucks) and at 78 MPH another time (in the left lane cruising past people). The energy difference was literally < 20Wh/mi more for a 33 MPH speed difference. Wind resistance is not the only thing we battle in the real world.

    On the flip side, if I were to come down that mountain at 45 MPH vs 78 MPH, maybe there would be a slightly better efficiency gain.

    In the real world slowing down 5 MPH from the flow of traffic is just not going to gain any significant amount of range.

    Put it this way. Let's say you're driving 200 miles. You do so at 65 MPH. It takes you 185 minutes. At 78 MPH it takes 154 minutes. It's 90 degrees out and you have a car with air conditioning that you're going to use because you're not an idiot. So, you'll be running the A/C for 31 extra minutes, along with every other base load in the car. For a low ball estimate, let's say that's an additional 1.2 kWh used by the base loads, or 4 miles range wasted by slowing down. In colder weather using heat that's even more significant since the heater uses much more power. So driving slower in the cold is stupid, IMO, if you want to stay warm.

    You mean the based-on-the-last-30-miles range estimate that has said 999 miles for me as well as 40 miles with 80+% charge? lol. Come on now. Just as useless as rated miles.

    And while the trip meter is cool, it's never correct and almost always adjusts downward when driving normally.

    I never said it was immune to physics. However, every other car (ICE, PHEV, EREV, etc) I've ever owned I could run the heat, air conditioning, etc and never have to worry about how much energy was being used or could be saved by dropping my speed 5 MPH. I could always get to where I needed to go without question.

    Why am I going to sacrifice basic features of every other vehicle in a car that costs more any other vehicle I've owned to-date? Basic things like keeping with the flow of traffic, not sweating or freezing my ass off because I have climate control, etc. These things are not optional in a vehicle, IMO, regardless of where or how far you're traveling.

    You miss the point of this particular note. I was pointing out one of many reasons slowing down is not the cure all for EVs.

    First, you wont extend range by 5 miles in real world condition by slowing from 78 to 73.

    Next, I don't think replacing the 12V battery is really something all that questionable. You mean to tell me if I made a drop in replacement for the Model S's 12V battery that extended range on long trips by up to ~5 miles and only cost a couple hundred bucks that people wouldn't use it? (Back on topic, I envision a device the same dimensions as the existing 12V battery that contains all of the cells, BMS, DC-DC, etc to do what I describe in the first post with zero user interaction that would be useful for the Model S or any other EV with a 12V battery).

    Overall, I really hate it when people assume slowing down is the answer to saving on range. It isn't always and can actually be detrimental in many cases, making it terrible generic advice from people who don't know and explain the caveats.

    - - - Updated - - -

    So, on topic, I think the idea boils down to this:

    A 12V battery drop in replacement that does everything needed to do what I described in the first post on its own without user interaction or any other connections to the vehicle besides the 12V +/- posts.

    The unit would be able to determine when the DC-DC is enabled based on power flow. The hard part would be getting it to know when the car was charging so it could charge from the car's DC-DC at this time and not while the car is in use. For this I'd figure an accelerometer/gyro/magnetometer could be used to figure out when the car has been stationary for an extended period with the DC-DC enabled and if needed charge itself at that time since the car is likely charging.

    The logic could get a little more sophisticated, like it could increase it's output voltage to the point where the DC-DC isn't being used, measure load, and guestimate if the car is being charged or not based on the load it sees then vs what it normally sees when in motion (unplugged).

    I figure it could have a single optional input wire that ties to the car's accessory system to know for sure when the car is on vs when the DC-DC is on while charging the main pack, as well as something like a bluetooth interface for definitively telling it to charge now before a trip.

    Should all be able to be done in a lighter package than a normal 12V lead acid battery too.

    Might be a fun project to add to my TODO list.
     
  19. deonb

    deonb Active Member

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    If slowing down is such a panache then why does Tesla have a 90kWh? Or for that matter, an 85? Just buy a 70 and drive slower. Or a 60 for that matter. Heck, bring back the 40.

    I know what, bring out a model with a 10kWh battery - it will certainly address the cost issue!

    Where does it end?


    And Tesla has shown us with the 3+ years it took to go from 85 to 90 that a 200 battery isn't just across the horizon, and might not be even in our lifetime. Every bit of juice that can be sopped up from anywhere, should be.
     
  20. Max*

    Max* Autopilot != Autonomous

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    Dunno about you, but based on the projected 5% battery growth every year (from the last telecon?), a projected 200kWh battery is in my lifetime (~17 years)

    I use projected, because Elon projects a lot of things, and most of them don't stick to the projected timeline. Maybe this is the outlier? ;)
     

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